Writting2
a159357hl
SmilPeopleandFood.pdf
milk (even in countries with high rates of lactase defi-
ciency), red meat, and poultry has risen.
Higher demand for animal foodstuVs has brought
not only significant increases in protein intakes, but also
in the consumption of lipids. As a result, in most rich na-
tions fats and proteins now supply more food energy than
do carbohydrates. Higher average consumption of ethanol
is also generally correlated with rising aZuence, but the
kinds of consumed alcoholic beverages vary a great deal
among diVerent countries.
Basal Metabolism Determination of an individual’s basal metabolic rate
(BMR) requires a body at complete rest, in postabsorptive
state (the last meal eaten hours ago), and in a thermoneu-
tral environment: most of us would qualify when in our
beds at 3 a.m. Physiologists have accumulated thousands
of BMR values for both sexes, mostly by measuring gas
exchange by indirect calorimetry in respiration chambers
(O2 consumption, CO2 production).
As it does in other warm-blooded species, thermoregula-
tion keeps the core human body temperature within nar-
row bounds, but this high degree of similarity does not
extend to our metabolic rates. Human diversity, demon-
strated in so many fascinating physical and psychical ways,
is very evident in surprisingly large individual diVerences
in basal metabolism, in energy costs of pregnancy and
lactation—generally much higher in aZuent nations than
in poor countries—and in metabolic expenditures on la-
bor and leisure.
Balanced intake of essential food nutrients is indis-
pensable for normal human growth,maintenance of body
tissues, and physical and mental activity. Throughout the
human evolution most of food energy has come from car-
bohydrates and, in turn, most of these nutrients came
from cereal and leguminous grains. Cereals have been
eaten in a great variety of foodstuVs, but none of them
has been of such importance in the Western civilization
as bread. Rising aZuence has transformed typical food
intakes: consumption of coarse cereal and leguminous
grains has declined, demand for processed cereals, sugars,
3
PEOPLE AND FOOD
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This huge data base is not unbiased: it includes statis-
tically significant numbers for all ages as well as values for
adults of diVerent stature and of diVerent weight for
height, but it is made up overwhelmingly of healthy indi-
viduals from Western nations. Measurements from poor
countries—where inadequate nutrition more commonly
prevents people to express fully their physical growth po-
tential—are underrepresented.
Body size, composition (share of metabolizing tis-
sues), and age determine complex variations of BMRs.
Relationship between BMR and body weight is best cap-
tured, for both sexes and for all ages, by simple linear
equations. Their fits remain unimproved by inclusion of
body surface area or height, but their predictive power is
Best fit lines of equations predicting basal metabolic rates for males.
10 -18
ye ar
s of
a ge
18 -3
0 ye ar
s o f a
ge
30-6 0 ye
ars of a ge
>6 0 ye
ar s o
f a ge
110805020
4
5
6
7
8
9
Ba sa
l m ee
ta bo
lic (M
J/ da
y)
Body weight (kg)
Evolution of specific basal metabolic rates in boys and men.
50 kg 65 kg
80 kg
Age
2.8
2.6
2.4
2.2
2.0
1 .8
1 .6
1 .4
1 .2
1 .0
0.8 0 10 20 30 40 50 60 70
Ba sa
l m ee
ta bo
lic ra
te in
m al
es (W
/k g)
80
CHAPTER 3
very high only for children and adolescents. Correlations
between body weights and BMRs range from impressive
0.97 for children less than three years old to 0.9 for teen-
age boys. In contrast, adult fits are poor: for men between
thirty and sixty, the group forming bulk of the economi-
cally active population, the correlation explains only about
adult neonate
kidneys heart
liver brain muscles other
Relative metabolic shares in newborns and in adults.
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claims typically more than 40 percent of BMR; liver, with
less than 5 percent of total body mass, needs a fifth of
BMR. Even in adulthood, when muscles are well devel-
oped, the four metabolically most active organs account
for two-thirds of the BMR.
Whatever its individual departures from large-scale
means, basal metabolism follows a well-understood life-
time course: it starts with a brief spike in infancy, from
about 2.3 watts per kilogram of body weight at birth to
around 2.7 watts three to six months later, then it follows
a steep decline to about half of the peak value by the eigh-
teenth year before stabilizing for the next four decades.
Resuming the decline at around the age of sixty, it falls to
just around one watt per kilogram during the seventies. A
final reminder: an individual’s BMR should not be mis-
taken for the minimal survival requirement. Metabolic re-
sponse to food, and energy needed to maintain basic
personal hygiene will increase it by at least 15–20 percent.
Thermoregulation DiVerences in basal metabolism—typical rates are some-
what higher in Northern Europe and among North Amer-
ican Indians and Inuit—have little to do with human
adaptation to cold. Highest rates of basal metabolism
were recorded among Arctic hunters, but that was due
more to diets high in lipids and protein than to adaptive
adjustments. Traditional multilayered Inuit clothes create
an almost tropical microclimate, making it a greater chal-
lenge to dissipate heat during hard work rather than to
conserve it.
Vasoconstriction is a useful adaptive response to cold
nights in warmer climates. Naked Australian Aborigines
could sleep without elevating their BMRs by reducing
blood flow into skin and extremities, but this would be
insuYcient in colder climates. In humans, conservation of
body heat has always been primarily a matter of intelligent
a third of the variance. In addition, these equations, de-
rived mostly from European observations, tend to over-
estimate the basal metabolism of many non-European
populations.
Clearly, BMRs are highly individual, but the key
question asked by Elsie Widdowson, an eminent British
physiologist, in the late 1940s—“Why can one person live
on half the calories of another, and yet remain a perfectly
eYcient physical machine?”—is yet to be answered satis-
factorily. Inevitably, the answers will lie in finding the
causes for substantial disparities in the functioning of in-
ternal organs which account for most of every individual’s
basal metabolism.
In relative terms, kidneys are metabolically the most
active organs, followed by the heart, brain, and the liver.
In absolute terms, the liver uses the largest share of BMR
in adults (at least one-fifth), the brain in children. A new-
born brain, accounting for only a tenth of total body mass,
81
PEOPLE AND FOOD
Huge differences in energy cost of encephalization in chimpanzees and in humans.
0 300 600 900 1200
0.0
0.5
1.0
1.5
2.0
2.5
Cu m
ul at
ive e
ne rg
y re
qu ire
m en
ts (G
J)
Brain size (g)
chimpanzees
humans
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extrasomatic solutions (clothes, shelter, fire) rather than
one of biophysical adaptations.
Not so with human response to heat. Of course, pro-
tective microenvironments have been also important,
ranging from counterintuitively dark-colored clothes
(loosely worn, they absorb a large share of incoming radia-
tion, which they then lose by convection without heating
the body) to various passive cooling arrangements in tra-
ditional buildings. A highly eVective extrasomatic adapta-
tion to heat came only with the post-1950 diVusion of air
conditioning—but outdoors we still have to rely on our
innate thermoregulatory adjustments.
The initial response is to dilate peripheral blood ves-
sels and to shift additional blood from internal to su-
perficial veins. Then, usually when skin temperature
Vasoconstriction helped Australian Aborigines to have an uninterrupted sleep outdoors, while white
men repeatedly woke up shivering.
Time
0
200
400
0
200
400
0
200
400
11 12 1 2 3 4 5 6 7 11 12 1 2 3 4 5 6 7
11 12 1 2 3 4 5 6 7 11 12 1 2 3 4 5 6 7
11 12 1 2 3 4 5 6 7 11 12 1 2 3 4 5 6 7
Ox yg
en c
on su
m pt
io n
(c m
3 / m
in )
white aboriginal
82
CHAPTER 3
The human blood-circulation system is essential for effective thermoregulation: Vasoconstriction and dilation
are its most important responses.
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nia in colder climates. Europeans, who initially respond
to high heat by dangerously rising core temperatures and
heartbeats approaching tolerable maxima, will start
matching sweating rates of highly acclimatized tropical na-
tives in only about ten days! Our ability to cope with heat
by sweating must be seen—together with bipedalism,
hairlessness, large brain, and symbolic linguistic ability—
as one of the key defining human traits. Without it we
could not have become either successful warm-climate
hunters and gatherers, or, much later, builders of far-
flung empires.
Pregnancy and Lactation Considering its usual outcome—the miracle of a new
baby—pregnancy is an astonishing energy bargain. Well-
nourished women of rich countries add, on the average,
about twelve kilograms of new tissues during the 280 days
of normal pregnancy. The baby’s birth weight (mean of
3.3 kg) and four to five kilograms of fat reserves for lacta-
tion account for most of the gain, with the rest in placenta
Substantial differences in temperature, heart rate and sweating rate between acclimatized and unacclimatized Europeans
working in South African mines.
approaches 35°C, we begin to sweat, and during heavy
work this response can be considerably more eVective than
in other mammalian species. A horse can perspire at a rate
of 100 g/m2 a hour, a camel can lose up to 250 g/m2—but
a man can average more than 500 g/m2 per hour. With-
out sweating average person would lose, about equally
through respiration and skin diVusion, 12 W/m2 of body
surface, or just over 20 W in total. In contrast, hourly per-
spiration rate of 500 g/m2 equates to heat loss of between
550–625 W for adults!
Best acclimatized individuals can perspire up to 1,100
g/m2 per hour, an equivalent of 1,390 W. Such thermoreg-
ulation suYces to prevent a dangerous rise of core body
temperature even in extremely hard-working individuals,
but they require adequate rehydration. Temporary partial
dehydration is common during heavy work or long foot-
races, and it causes no problems as long as the water deficit
is made up within the next day.
High sweating capacities are genetically encoded and
are retained even by populations that have lived for millen-
hours unacclimatized acclimatized
Re ct
al te
m pe
ra tu
re (o C)
36
37
38
39
40
rest 1 2 3 4 hours
He ar
t r at
e ( be
at s/
m in)
40
80
120
160
rest 1 2 3 4 hours
Sw ea
t r at
e ( lit
re s/
ho ur
)
0.0
0.4
0.8
1 .2
1 2 3 4
83
PEOPLE AND FOOD
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and in associated maternal growth. Further energy cost
comes from the rise in basal metabolism and from in-
creased cardiovascular and respiratory eVort, especially
during the last trimester. The total energy cost of such a
pregnancy is about 335 MJ, or just 185 MJ when the cost
of fat reserves is charged against the requirements of
lactation.
The larger total prorates to almost 27 kJ/g of weight
gain, a higher rate than the cost of synthesizing new an-
thropomass during the years of childhood and adolescent
growth. Still, the energy cost of a typical rich world’s preg-
nancy represents only a modest additional food require-
Full-term fetus in utero from Smellie’s anatomical atlas (1754).
ment. Basal metabolism of active, healthy young women
goes up by no more than 15–20 percent, an increase
equivalent to food energy in just four slices of toasted
whole-wheat bread a day!
Lactation costs vary considerably with the volume
and duration of milk production. Average energy content
of humanmilk is 3.2 kJ/g. Assuming a daily mean produc-
tion of 800 mL for six months of lactation, the total en-
ergy cost of milk would be 520 MJ. But a typical Western
woman would start her breastfeeding with some 150 MJ
of fat reserves which will be converted to milk. Conse-
quently, the actual postpartum energy cost of lactation
would be only 370 MJ, an increment equal to about 25
percent of the normal food intake.
But these changes are not a universal norm. Impecca-
ble studies in a number of Asian, African, and Latin Amer-
ican countries have shown that many women in poor rural
Stages of pregnancy.
1 2 3 4
1 5
6 7
8
2 9 10
11 123
13 14
15 16
4 17
18 19
20
5
21 222324
6
25262728
7
29 30
31 32
833
44 55
41 42
36
9
37 38
39 40 10
embryo
fetus untilbirth
prematuredeliv er
y te
rm po
st ter
mlunar months weeks
84
CHAPTER 3
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foodstuVs are transformed with admirable eYciency (max-
ima go up to 97 percent!) into the nourishing fluid supe-
rior to any substitute. Breastfeeding not only lowers the
frequency of several neonate diseases (including obesity,
rachitis, and vitamin deficiencies), but it is also immu-
noprotective and confers important mental and neural
advantages.
Human Growth Normal infants grow fastest during the first nine months
of life, when their weight gains are almost perfectly linear;
those with smallest birth weights grow, on the average, up
to 20 percent faster than the heaviest newborns. After-
ward the lines of body weight plotted against age start
curving gently for both girls and boys. By the end of their
first year healthy infants almost triple their weight. Repre-
sentative means diVer both among nations and within
countries over time. Standard North American charts
show boys growing from 3.3 to 10.2 kilograms, and girls
from 3.2 to 9.5 kilograms. Daily energy needs during this
growth spurt average 430 kJ (or 5 W) per kilogram. Then
the needs decline, first very slowly to around 400 kJ (4.6
W) by the age of four to five and afterward much faster to
about 260 kJ per kilogram for girls and 300 kJ for boys.
Energy needs are always dominated by basal metabo-
lism: growth needs peak at only about a third of all food
intake during the first month of life, a storage rate of
around 8 W. By the end of the first year the share of energy
claimed by growth is sharply down to just between 5–6
percent, and by the end of the decade it is mere 2 percent
of all food energy. Growth spurt during the early teens
may briefly double that share, but by the age of sixteen or
seventeen the rate is back to just around 2 percent, and
falling fast. By the age of twenty the rate is down to a
small fraction of one percent and it stays there for de-
cades in order to repair and regrow adult tissues ranging
areas have extremely low energy costs of both pregnancy
and lactation. Compared to Western expectations, their
energy shortfalls are up to about 3 MJ/day even if they
would just sleep and rest, and up to 4 MJ considering their
heavy labor. These women, giving birth to healthy chil-
dren, maintain genuine energy balance on what seem to
be incredibly low levels of food intake, commonly 20–40
and even close to 50 percent below the expected require-
ment! And among the Kauls of New Guinea, British re-
searchers found no diVerence in average food-energy
intakes of nonpregnant and nonlactating and pregnant
and lactating women!
Higher metabolic eYciencies of these women, adap-
tations to limited food intake, are the best explanation for
these surprising realities. Equally notable is the relatively
small eVect of maternal nutrition on the quality of milk.
Often even its quantity is not much lowered as nutrients
in the plainest, and not infrequently barely adequate,
Breastfeeding woman from Peter Brueghel the Elder’s The Rich Kitchen (1563).
85
PEOPLE AND FOOD
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from hair and nails, to constantly shed skin and intestinal
lining.
Finding the eYciency of infant growth is not easy, be-
cause the inputs depend on the shares of stored protein
and lipids: proteins need always more energy. Average rate
of 21 kJ/g is the most commonly assumed energy cost of
growth in young children. As the newborns average about
14 percent of fat and 20 percent muscle, the average en-
ergy content of their tissues would be about 10 kJ/g, and
the overall eYciency of infant growth would be close to
50 percent. Other published growth rates range from lows
of just below 15 kJ/g for infants recovering from malnutri-
tion to more than 30 kJ/g for adults during overfeeding
experiments.
Adult growth is greatly influenced by human sexual
dimorphism exhibited in diVerent rates of fat (adipose tis-
sue) storage. After adolescence, the body’s fat content in-
creases steadily in both sexes, but the diVerence widens
with age. Lipids make up about 15 percent of body
weight in young Western adult males, but about 27 per-
cent in females; by the seventh decade of life this disparity
widens to 23 versus 36 percent. On the average, females
are adding fat at rates of 0.3–0.4 kg/year, men at only
0.15–0.25 kg/year. This trend is accompanied by the loss
of lean body mass.
Muscles are just over 50 percent of weight in young
men, 40 percent in women. After the third decade the
male’s greater lean mass is lost more rapidly (2–3 kg per
decade) than the female’s musculature (about l.5 kg per
decade), and people over seventy years average about 40
percent less muscle than they had as young adults. This
loss is an inexorable sign of physical aging even in those
men and women who are in excellent health and who had
avoided a significant fat increase.
Proportions of human growth from the age of two years to maturity, from a French publication of 1833.
86
CHAPTER 3
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Ag e a
t M en
ar ch
e (y
ea rs
)
12
13
14
15
16
17
199019601930190018701840
Better nutrition has resulted in steadily increasing heights of Japanese boys (average shown is for eleven years of age).
He igh
t (c
m )
130
125
135
140
145
150
1900 1930 1960 1990
Growth curves, shown as tenth, fiftieth and ninetieth percentiles of body weights, of Canadian boys during the 1960s.
Walking and Running Walking is a key human trait—and it is also the most fre-
quent activity requiring considerable energy inputs. More
studies of energy expenditure are available for various
modes of walking than for any other activity. Plots of
their results show that at slow speeds the power needed
for walking increases exponentially, but that the relation-
ship at higher speeds is almost perfectly linear. Power in-
puts also vary with sex, age, body weight, load, slope, and
surface quality.
Slim adult females have the lowest walking expen-
ditures, but the rate is only about 10 percent below the
male mean. Energy markups for lighter loads (up to about
We igh
t (k
g)
Age (years)
75
65
55
45
35
25
15 5 7 9 11
90
50
10
13 15 176 8 10 12 14 16 18
87
PEOPLE AND FOOD
Better nutrition has also lowered the age of menarche throughout the affluent world from about seventeen years in 1850 to less than thirteen years in 1990.
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15 kg) carried on a level are relatively small, similar to the
cost of carrying a bit of additional body weight. Energy
needs of walking uphill go up with both the gradient and
the speed of the ascent, and numerous measurements indi-
cate almost perfectly linear rise for a wide range of speeds
and inclines. Uneven surfaces increase the input by up to
one-third; loose sand or swampy ground can double it
compared to unencumbered level walking. Total energy
expenditures thus range from less than two hundred watts
for slow level strolls to almost one kilowatts for strenuous
walking uphill with a fairly heavy load, or two to ten
times the rate of basal metabolism.
Energy cost of walking displays a distinctly U-shaped
trend as it falls from maxima of close to 400 J/m for very
slow walk to the minimum of about 230 J/m at the speed
of 1.3 meters per second, and then it rises in an almost
perfectly mirror-like fashion. Consequently, human terres-
trial locomotion is most eYcient when walking at be-
tween 4.5 and 5 kilometers per hour—but most people
Photographs of a running man from Eadweard Muybridge’s pioneering studies of human motion published in 1887.
walk either much slower or considerably faster. People in
small villages often walk with speeds below three kilometers
per hour, while many hurrying pedestrians in large metro-
politan areas commonly exceed six kilometers per hour.
Although record race-walking speeds surpass four
meters per second, these feats are achieved by peculiar ro-
tations of pelvis about its vertical and horizontal axes.
Normal walking is transformed into running once the
speed reaches about 2– 2.5 meters per second, or 8.3 kilo-
meters per hour. Above that speed running has lower en-
ergy cost than walking, and while the cost of walking
keeps on increasing with speeds above 1.3 meters per sec-
ond, the cost of running does not vary significantly with
speeds between 2.3 and 6 meters per second. This uncou-
pling of energy cost from speed is a unique human ability
that can be explained by a combination of bipedalism and
eYcient heat dissipation.
Quadrupeds have optimum speeds for diVerent gaits
(for example, the horse’s walk, trot, and canter) because
88
CHAPTER 3
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Maximum running speed as a function of distance. Up to about three hundred meters, runners can rely on stored energy
converted anaerobically; above that level their speeds must decline to conform to the slower aerobic balance.
Sp ee
d (m
/s )
5
6
7
8
9
10
1 1
12
Distance (m) 10 2 10 3 10 4
Energy cost of walking has a clear U-shaped relationship with speed, while energy requirements for running remain basically
constant for speeds between 3 and 5.5 m/s. En
er gy
co st
( J/
m )
200
300
400
m/s 0 1 2 3 4 5 6
running
walking
their ventilation is limited to one breath per locomotor
cycle: their thorax bones and muscles must absorb the im-
pact on the front limbs as the dorso-ventral binding rhyth-
mically compresses and expands the thorax space. But
human breathing can vary relative to stride frequency,
giving us an option to run at a variety of speeds with
basically unchanged energy costs. As for the thermoregu-
lation, our ability to dispose of metabolic heat surpasses
that of any other mammal. Combination of variable-speed
running and excellent heat dissipation makes it possible
for people to outperform some of the fastest mammals.
This ability was exploited by many hunting cultures:
North American Indians used to run down deer and
pronghorn antelopes; Kalahari Basarwa chased duikers,
gemsbok, and zebras; and some Australian Aborigines
pursued kangaroos.
Energy expenditures of walking on the level increase linearly for speeds up to about 7 km/h (almost 2 m/s).
+
+ +
+
+
+
+
+
+ +
Po we
r (W
)
m/s
0
100
200
300
400
500
600
700
800
0.0 1.0 2.0 2.51.51.5
89
PEOPLE AND FOOD
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These endurance runs have been repeatedly surpassed
in classical marathons (42.2 km) and in modern ultrarun-
ning races (100–160 km). Since the 1970s the best ath-
letes have been running marathons at a faster pace—at
speeds of nearly 5.5 meters per second—than the record
ten-kilometer run of the early 1940s. Ultrarunning races
are now covered at speeds of three to four meters per sec-
ond. All long-distance speeds are limited by two organismic
rates: by the uptake of oxygen in the lungs and by the distri-
bution of the blood by the heart. In contrast, sprinters de-
rive most of their energy from anaerobic sources. Record
sprint speeds translate to a mean of 10.2 meters per second,
a feat achievable thanks to a rapid use of creatine phosphate
and anaerobic glycolysis. Recent speed improvements in
sprint races have been, obviously, very small in absolute
terms, but very similar to those of endurance races in rela-
tive terms. And although record speeds of female runners
are still considerably behind the best male achievements,
women’s rates of improvement have been appreciably faster.
Typical power inputs in running range between 700
and 1,400 watts, corresponding to metabolic scope of be-
tween 10 and 20. Training can raise these rates, mainly by
History of world records for the marathon.
4
3
2
Tim e (
ho ur
s)
1905 1920 1935 1950 1965 1980 1995
Violet Piercy, Britain (October 3rd 1926)
Grete Waitz, Norway
(April 17th 1983)
Ingrid Kristiansen, Norway
(April 21st 1985)
James Peters, Britain (June 26th 1954)
Belayneh Densimo, Ethoipia (April 17th 1988)
John Hayes, USA (July 24th 1908)
women
men
increasing blood volume (roughly one mL for each mL of
a aerobic capacity) and hence the cardiac output. Top rates
for elite athletes can go over two kilowatts, that is, about
twenty-five times the BMR. This metabolic scope is higher
than for rabbits but lower than for wolves and dogs, the
best mammalian runners.
Labor and Leisure Since the 1890s physiologists have accumulated thou-
sands of values for energy costs of specific tasks in scores
of occupations as well as in just about every recreation ac-
tivity. Most of them have been determined by simple res-
pirometry: The energy equivalent of one liter of
consumed oxygen averages about 20 kJ per gram of oxi-
dized nutrients. A simple way to express these costs is to
compare them to basal metabolism. The stimulating eVect
of the last meal and some tossing will push sleep’s energy
cost 5–15 percent above the basal metabolic rate (BMR).
Sitting and standing require the deployment of many
muscles to maintain various postures: typical markups for
sitting are 15–20 percent, while standing requires 1.3–1.5
times the BMR. The huge assortment of occupational
tasks is best reviewed by typical levels of exertion. This
approach brings many surprising results.
Thinking is an enormous energy bargain. The adult
brain claims about a fifth of the BMR, but even the hard-
est brainstorming makes little diVerence to that fixed rate:
it requires no more than about four watts, equal to around
5 percent of a typical BMR. All modern science is thus a
matter of light energy expenditures—excepting, naturally,
climbs into rain forest canopies or crawling through con-
stricted caves.
Expectedly, nearly all tasks in the service sector are
light exertions. This huge category embraces not only all
deskbound jobs, but also virtually all cleaning, retail, re-
pair, teaching, hospital, or restaurant tasks, as well as most
transport services: driving a truck may actually require
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CHAPTER 3
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the duration of individual tasks. Moderately taxing, steady
work may be thus more demanding than jobs requiring
occasional heavy exertion. Underground coal miners do
many tasks calling for power of 550–600 watts, but they
may spend half of their day getting to and back from the
coal face and in periodical resting, averaging no more than
three hundred watts, a decidedly light activity. In tradi-
tional farming such disparities have also a clear seasonal
flux: periods of intense heavy labor (planting, harvesting,
plowing, digging) are followed by weeks of moderate
work and extended rest.
Daily rates of energy expenditures range from as
little as 6 MJ for older housewives to over 30 MJ for
less energy than typing sixty words a minute! Modern
manufacturing—be it assembling computers or making
trucks—is also overwhelmingly a matter of light work, as
is nearly all mechanized farming. Machines have also
shifted all but a few modern construction tasks into the
light exertion class.
In contrast, traditional farming still involves a great
deal of moderate and heavy labor. The most common
heavy labor tasks are plowing, weeding, hoeing, trans-
planting, digging, canal cleaning, brush clearing, and
mowing. Similarly, traditional net fishing, tree felling, and
mining entail heavy—and often very heavy—energy ex-
penditure. Naturally, total daily expenditures depend on
Human muscles remained indispensable prime movers until the widespread adoption of engines and motors. In the first illustration
two men on a cleated treadwheel raise water buckets in a German mine in the sixteenth century. In the second eight
men rotate a vertical capstan drawing a gold wire in an eighteenth-century French workshop.
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PEOPLE AND FOOD
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lumberjacks. Many ergometric studies put the best muscle
eYciencies during extended, aerobic work at around 20
percent. Consequently, even a hard-working lumberjack
accomplishes daily useful work equivalent to just 6 MJ.
That much useful kinetic energy can be delivered, even
when the overall conversion eYciency of an inanimate
prime mover would be just 20 percent, by burning about
seven hundred grams of crude oil or one kilogram of good
coal. Clearly, human eVort, even at its best, is a rather un-
impressive source of mechanical energy!
The energy cost of leisure activities is highly depen-
dent on their intensity: Normally moderate exertions,
ranging from badminton and canoeing to swimming and
tennis, may be readily pushed into the heavy category. In-
dividual high-energy sports include many track and field
events, mountain climbing, cross-country running, row-
ing, and squash. Cycling is the fastest mode of human
locomotion, and it has energized all human-powered
record-setting machines on land, water, and in the air.
Group sports with the highest energy costs are basketball
and soccer.
Short (30–180 seconds) exertions are energized
mostly by anaerobic glycolysis; aerobic recharge is the
Food requirements vary with age, sex, and level of activity.
male, 65 kg, moderate activity
female, 50 kg, light activity
Fo od
re qu
ire m
en ts
(M J/
da y
ca pit
a)
Age
5
10
15
0 20 40 60 80
.
main, though not the sole, energizer of sustained exertions
(anaerobic breakdown of glycogen also contributes). Be-
cause the body’s oxygen stores can support a heavy eVort
for less than half a minute, subsequent energy needs call
for linear increases in oxygen uptake. The peak aerobic
power reaches six hundred to nine hundred watts in
mildly active adults, but it can top two kilowatts in elite
athletes; it is slightly lower in females than in males of the
same age, and in untrained individuals it declines steadily
after the adolescence.
Healthy adults can easily work or exercise for several
hours at 40–50 percent of their peak aerobic capacity, that
is, at rates of three hundred to five hundred watts. In terms
of total work accomplished, peak aerobic capacities are
equivalent to 1.5–3.5 MJ for healthy adults, surpass 10
MJ for good athletes and reach an astonishing 45 MJ for
best long-distance runners and skiers!
Nutrients Human life depends on the digestion of about fifty essen-
tial nutrients ranging from complex organic compounds
to mineral elements. Nutrients consumed daily in larg-
Gross energy expenditures of some activities expressed as multiples of basal metabolic rates for males.
sitting at desk driving a truck cooking fishing feeding animals cutting trees digging canals loading sacks hand sewing pedalling rickshaw
Energy costs as multiples of BMRActivities 1 2 3 4 6 8 90 5 7 10
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just 17 kJ/g. Unfortunately, most of the biosphere’s carbo-
hydrates cannot be digested by humans: we lack the en-
zymes needed to break down wood’s lignin, cellulose, and
hemicellulose. But since the 1960s we have come to real-
ize the nutritional importance of these indigestible carbo-
hydrates. These dietary fibres—richly present in whole
seeds, whole flours, fruits, and vegetables—are needed
daily only in small amounts, but they are a critical part
of proper nutrition: We consumed them in much higher
quantities during millions of years of hominid evolution,
and we still need them to prevent, or to reduce, the inci-
dence of many diseases.
Proteins, with 23 kJ/g, are about a third more energy-
dense than carbohydrates—but their principal role in
human nutrition is not as sources of energy but rather as
suppliers of essential amino acids needed to build body
0
300
600
IL LE LY MC PT TH TR VA
A m
in o
ac id
s (m
g/ g
N) A
m in
o ac
id s
(m g/
g N)
milled rice
0
300
600
IL LE LY MC PT TH TR VA
lentils
IL LE LY MC PT TH TR VA
white wheat flour
IL LE LY MC PT TH TR VA
milk
reference protein deficitsurplus
est quantities—carbohydrates, proteins and lipids—are
needed to supply energy for basal metabolism, growth,
and labor and leisure activities, but even these com-
pounds have qualitative roles that enhance, or even greatly
surpass, their energy contributions. Vitamins and minerals
have no energy value and, compared to the three macro-
nutrients, are needed only in minuscule amounts (ranging
from a few grams a day for alkaline elements to a few milli-
grams for some vitamins), yet their adequate intake is es-
sential for healthy life. Although it makes no sense to rank
nutrients as to their overall importance, there is no doubt
which ones make the greatest contribution to human en-
ergy requirements.
Carbohydrates not only ranked first, but they also ac-
counted for the bulk of digested food in every traditional
society; they still provide more than three-quarters of all
food energy throughout the poor world, but in the richest
countries their share has fallen below 50 percent. People
eat carbohydrates in a wide variety of processed cereal and
leguminous grains—most commonly as bread, pasta,
steamed rice, or various gruels, stews, and fermented
products—in tubers, fruits, and vegetables, or as sugar,
honey, or concentrated tree sap.
No matter if they come as complex starches (polysac-
charides made up of thousands of glucose molecules) or
simpler sugars (monosaccharides fructose and glucose,
and disaccharide sucrose), carbohydrate energy content is
carbohydrates 17.017.4
lipids 38.039.0
proteins 17.223.0
ethanol 29.329.3
nutrients energy content (kJ/g)
actually availabletotal
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PEOPLE AND FOOD
Energy content of nutrients.
Amino acid composition of rice, wheat flour, lentils, and milk.
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tissues. Normal human growth is impossible without di-
gesting eleven essential amino acids which are then reor-
ganized into protein in body structures (in muscles,
bones, and internal organs) as well as in a variety of irre-
placeable biochemical carriers, catalysts and protectors
(antibodies, enzymes, hormones). Proteins come com-
plete—that is, including all essential amino acids in req-
uisite proportions—in all animal foods as well as in
mushrooms, and incomplete (with one or more amino
acids deficient) in plant foods (mainly in leguminous and
cereal grains and in nuts).
Lipids—fats and oils—are by far the most energy-
dense nutrients with 39 kJ/g. They contain three essential
fatty acids whose digestion makes it possible to carry fat-
soluble vitamins (A, D, E, and K) and to synthesize pros-
taglandins needed to regulate gastric and smooth-muscle
Nutritional changes in post–World War II Japan.
An nu
al co
ns um
pt ion
(k g/
ca pit
a)
10 0
10 1
10 2
1950 1960 1970 1980 1990
cereals
fruits
tubers
milk
meat
oils and fats
activities, to release hormones, and to build cell mem-
branes. Concentrated lipids are eaten as various plant oils,
butter, and lard. Lipid content of meat, fish, and dairy
products ranges from trace amounts in skinless poultry
breasts and skimmed milk to much more than 50 percent
in fatty pork cuts or double-crème cheeses.
No macronutrients are completely digestible.
Healthy people on balanced diets can derive up to 99 per-
cent of available energy from carbohydrates, about 95 per-
cent from fats, and no more than 92 percent from
proteins; in addition, more than one-fifth of consumed
protein (about 5.2 kJ/g) is voided in urine. Net energies
actually available for basal metabolism, growth, and labor
and leisure activities are thus basically identical to the
gross energy density of carbohydrates, are only a bit lower
for lipids, but amount to less than three-quarters of the
gross value for proteins. This adjustment matters because
standard food composition lists convert energy content of
foodstuVs by using net metabolic factors, not gross en-
ergy contents.
Micronutrients—minerals and vitamins—provide no
food energy, but their importance in building healthy tis-
sues and maintaining the complex biochemistry of human
metabolism is irreplaceable. Consequently, diets with a
surfeit of energy may still lead to serious cases of malnutri-
tion. Intakes of more than forty micronutrients are re-
quired for healthy living. Minerals needed in largest
amounts are calcium, phosphorus, and iron; vitamin re-
quirements are highest for C, B complex, and D.
Grains Several energy advantages explain the dominance of cereal
and leguminous grains in human diets. Above all, grains
combine fairly high yields with relatively high energy den-
sity. Cereal yields have been always appreciably higher
(commonly at least two times) than the harvests of legu-
minous seeds, but legumes, associated with nitrogen-
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Moreover, the complementarity of cereals (deficient
in lysine) and legumes (low in methionine) makes it pos-
sible to consume all essential amino acids in grains. Tra-
ditional, overwhelmingly vegetarian societies practiced
this complementarity—millennia before we understood
its biochemical basis—by combining staple cereals with
soybeans and beans in East Asia; with lentils, beans, and
peas in India, the Middle East, and Europe; with peanuts
and cowpeas in West Africa; and with beans in the Ameri-
cas. Oils are present in most cultivated varieties in modest
amounts, but their content is high in corn, peanuts, and
soybeans.
Other advantages, nutritional and logistic, include
relatively easy harvesting and simple basic processing of
grain crops; low moisture content, which makes mature
cereals and legumes suitable for long-term storage; and
the culinary versatility of grains. The ease of harvesting
and processing—especially when compared to tubers,
roots, and nuts, which supplied large shares of food en-
ergy for all gatherers—was an important contributing fac-
tor in the emergence of grains as staples of virtually all
settled societies.
fixing bacteria, are easier to grow and require hardly any
fertilizer nitrogen. Energy density of mature grains
diVers by less than 10 percent among the main cultivated
genera: with moisture content at around 14 percent (the
maximum necessary for safe storage) they contain about
15 MJ/kg. This is roughly five times higher than for tu-
bers, and equal to moderately fatty cuts of meat.
All principal nutrients contribute to this rather high
energy content. Carbohydrates are always dominant, pres-
ent mostly as highly digestible polysaccharides (starches).
Protein content of cereals ranges widely, from just 7 per-
cent for some rices to 16 percent for the Andean quinoa.
A typical mean of around 10 percent is about five times
higher than that of tubers, and more than an order of
magnitude above the values for vegetables. Leguminous
grains average over 20 percent of protein and soybeans
over 40 percent.
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PEOPLE AND FOOD
The three great staples of European cereal cultivation: wheat, rye, and barley.
Cultivation of rice, Asia’s dominant staple, is shown here in a classical Chinese illustration.
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The versatility of grains as food is obvious from list-
ing main processing and preparation options. Grains can
be eaten whole (all legumes), crushed (oatmeal), whole
after removing the outside layer by milling (rice), milled
to various degrees of fineness in flours (wheat above all),
Corn, the New World’s most important addition to global grain harvests.
milled and reconstituted as solids (breakfast cereals), or
as fermented products (soybean-based bean curd, bean
paste, and soy sauce are the best known possibilities). Mill-
ing, the dominant mode of cereal processing, has little
eVect on their overall energy density; it increases palatabil-
ity, but it also decreases nutritional value by removing vita-
mins and minerals present in the seed’s outer layer, and it
reduces the beneficial intake of indigestible fibre.
As for the cooking methods, grains can be boiled in
rough or elaborate gruels (from Scottish oatmeal porridge
and Russian buckwheat kasha to satiny East Asian rice con-
gees with floating condiments); cooked in hot water as
Winnowing—separation of chaff from grain—from a seventeenth-century Chinese encyclopedia.
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addition of such coarse grains as oats, barley, and buck-
wheat. Lentils or cauliflower may be mixed into Indian
chapatis. Bananas and molasses, milk and eggs, nuts and
dried fruits can be used to enrich fancy breads, spices and
herbs to flavor even the plain ones. But wheat flour re-
mains the quintessential ingredient for the world’s most
important baked staple.
Two of its major proteins, glutenin and gliadin, are
unique— not nutritionally, but because of their physical
properties. When combined with water they form a gluten
complex that is suYciently elastic to permit stretching and
shaping of an unleavened and rising of a leavened dough,
and yet strong enough to retain carbon dioxide bubbles
formed during the yeast fermentation. Leavened bread—
a fermented, kneaded, and baked mixture of flour, water,
salt, and Saccharomyces cerevisiae yeast—was such a domi-
nant staple in traditional European societies that medieval
Latin writings commonly referred to all other foodstuVs
collectively as companagium, accompaniments to the ubiq-
uitous panis served with all meals.
Laborious grinding of flour was one of the principal
reasons for introducing first inanimate prime movers,
Wheat grain.
with the almost endless varieties of European and Asian
pasta; steamed; and baked as bread, for millennia the lead-
ing staple of Western civilizations, and as a huge assort-
ment of salty and an even wider variety of sweet pastries
crowned by French gateaux and Viennese Torten.
The regional or local dominance of particular grains
is largely a matter of environmental possibilities, agro-
nomic traditions, and taste preferences. Wheat spread
from the Near East to become the most widely cultivated
cereal; rice, originally from the Southeast Asia, is now the
world’s largest grain crop; corn was introduced from
North America to every continent, and it is now a leading
feed grain; soybeans are as well, having spread from
China.
Cultivation of many coarse grains—millets, barley,
rye, buckwheat—has been in a global decline. So has
been, except for soybeans and Indian lentils, the planting
of leguminous grains. Although high in protein, they
have often low palatability and are diYcult to digest. As
soon as societies get richer one of the most notable nutri-
tional shifts is their declining consumption of legumes.
Per capita consumption of cereal staples also declines
with modernization. In traditional societies they com-
monly supplied nine-tenths of all food energy. Now they
provide about three-quarters in China, less than half in
North America. But the total cultivation of grains in rich
societies has grown: instead of eating them directly, rich
countries now channel most of their cereals through ani-
mals to produce more animal foods. Globally nearly two-
fifths of harvested grains were fed to animals in the early
1990s, and the share is above 60 percent in rich countries.
Bread Several cereals, and many nongrain ingredients, have been
used to make bread. American tortillas contain ground
corn with lime water. In northern parts of Europe heavy,
dark bread was often made only from rye flour with the
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PEOPLE AND FOOD
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waterwheels and windmills. Flour mill operations (grind-
ing, separation, sifting, and bagging) were fully mecha-
nized only after 1800; by 1900 steel rollers had almost to-
tally displaced elaborately dressed millstones. Yeast, tradi-
tionally gathered from beer or wine vats, is now mass-
produced from cultures grown on molasses.
Most standard breadmaking recipes call, in mass
terms, for a ratio of 1:1.7 for water and flour. Because the
flour itself has about 12 percent of moisture, fresh doughs
are roughly 45 percent water. Most of the wood, coal, or
electricity used in baking goes into evaporating part of
that water and reducing bread ’s moisture content to as
low as 30 percent in light baguettes, and up to 36 percent
in heavier whole-wheat and rye loaves. Fresh bread has
relatively high energy density, equal to about 75 percent
of the identical mass of flour. Leavened bread is now
mostly mass-produced in commercial ovens, but Asian,
African, and Latin American unleavened breads are still
Breadmaking became cheaper with the introduction of first automated mills. This is Oliver Evans’s pioneering design from
1785.
commonly home-made, baked stuck to the sides of clay
ovens, grilled (Mexican tortillas), fried in oil (Indian puri),
or steamed (Chinese mantou).
The energy cost of bread depends on the scale of pro-
duction. At the retail level, American white loaves from a
large commercial bakery using the standard sponge-and-
dough process will need around 7 MJ/kg, with the cost
split about 2:1 between production and delivery to stores.
Baking itself will need less than 20 percent of that total.
European costs are very similar, but shorter distribution
distances make for lower transportation costs. The speci-
fic energy cost of home baking can be up to five times
higher—but there are no indirect energy costs for packag-
ing and distribution. And, not surprisingly, the process
displays striking economies of scale: three breads in oven
Saccharomyces yeast.
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mass migrations of salmon yielded catch with about 15
percent of body mass in fat, or nearly three times higher
energy density than in cod.
In overwhelmingly vegetarian agricultural societies,
diets rich in lipids not only gave the satisfactory feeling of
satiety but also reflected the consumer’s high social status.
For most peasants in preindustrial societies, oils, butter,
lard, and fatty meat were only occasional luxuries, supply-
ing commonly less than 10 percent of all food energy, a
pattern that also characterized typical diets during the
early stages of industrialization.
Modern societies have clearly overcompensated for
this deficiency: lipids now make between a third and four-
fifths of total food-energy intake in the United States,
Canada, and most European nations, a clear nutritional
excess. The best dietary guidelines recommend that no
more than 30 percent of food energy should come from
lipids. In recent years there has been some reduction of fat
intakes throughout the aZuent world, especially because of
canola oil safflower oil sunflower oil corn oil olive oil soybean oil peanut oil lard beef tallow palm oil butterfat oil coconut oil
Fatty acid content of common dietary fats (%) 250 50 75 100
saturated polyunsaturated monounsaturated
Fatty acids in common dietary lipids.
need about 50 percent less energy per kilogram than a
single loaf.
Lipids and Meat Animal fats and plant oils have the highest energy density
of all nutrients. This attribute made them highly regarded
in every traditional society. Hunting societies preferred
killing the largest mammals not just because those animals
provided plenty of meat, but also because that meat, in
contrast to that of smaller creatures, was also uncommonly
rich in fat. Bison provided twice as much energy per unit
weight as an elk or a deer, and the same diVerence applied
to elephants compared to even the largest antelopes.
Similarly, the high fat content of Pacific baleen whales
and salmon provided the energetic foundation for settled
and relatively complex societies of the Pacific Northwest:
precontact settlements totaled up to several thousand
people, concentrations unattainable by hunting leanmeat.
The most commonly landed immature whales averaged
nearly twelve tonnes, with blubber rated at about 36 MJ/
kg, and mukluk (skin and blubber) at 22 MJ/kg, while
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PEOPLE AND FOOD
Variety of breads.
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the warnings about health perils of high fat diets in gen-
eral, and their eVects on higher incidence of premature
coronary heart disease in particular.
These warnings have turned lipids into a question-
able treat for many people—but lipids provide much
more than the satisfying feeling of a filling meal. Two of
their constituent fatty acids—linoleic and linolenic—are
essential macronutrients needed not for their high energy
content but for their critical biostructural functions. These
acids cannot be synthesized by vertebrates, but all cell
membranes are made largely of lipids, as are the myelin
sheaths around nerve fibers. Consequently, adequate in-
take of essential fatty acids is especially important during
infancy: about 60 percent of the brain mass increase after
birth is structural fat. Fatty acids are also precursors of
prostaglandins, which regulate gastric functions, release of
hormones, and the activity of smooth muscles, and they
carry vitamins A, D, E, and K, which are not soluble in
water.
Both linoleic and linolenic acids are polyunsaturated
compounds with, respectively, two and three double
Examples of unsaturated (curved) and saturated (straight) fatty acids.
oleic acid
elaidic acid
bonds that make their long carbon chains (both have 18
C) curved and keep the acids liquid. Most plant oils, rang-
ing from corn to peanuts and from sunflower to soybeans,
are high in polyunsaturated acids. In contrast, saturated
fatty acids—myristic, palmitic, and stearic, with, respec-
tively, 14, 16, and 18 carbons—have no double bonds,
and their straight chains form solid structures of animal
fats, mostly consumed as rendered lard and tallow, and
they are also abundant in coconut and palm oil. Monoun-
saturated oleic acid has a single double bond, enough to
keep olive oil liquid.
Hydrogenation has been used since the late nine-
teenth century to straighten carbon chains—to have their
cis double bonds changed to trans double bonds—and to
make mostly unsaturated oil into a solid fat. After 1950
this transformation seemed to be the best solution to rec-
oncile widespread human preference for fatty diets with
the need to limit intake of solid saturated animal fats asso-
ciated with higher incidence of coronary heart disease:
butter consumption in rich countries fell sharply as marga-
rine sales soared.
1910
1970
1940
1990
Per capita fat supply (g/day) 0 50 100 150
fats, oils meat, fish dairy other
Growth and transformation of America’s dietary lipid intakes, 1909–1990.
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dreds of millions of children around the world lose most
or all of the enzyme, and the lactase deficiency prevents
them from consuming lactose-rich diets without side
eVects ranging from unpleasant (intestinal cramps, flatu-
lence) to painful (diarrhea, vomiting, nausea).
Lactose malabsorption has a complex spatial pattern.
Its highest prevalence is among the people of traditional
hunter-gatherer societies (Inuit, all American Indians,
South African Basarwa) and in nonmilking farming cul-
tures of sub-Saharan Africa and Southeast and East Asia.
In contrast, people in Northwest Europe, North Africa,
and the Near East have very high lactose tolerance. Dis-
parities in the incidence of lactase deficiency are huge: well
over 90 percent among Japanese and Chinese, much be-
low 10 percent among Germans and Swedes.
Yet lactase deficiency does not prevent consumption
of dairy products. They can be taken either in small quan-
tities, or in larger amounts when fermented. Lactobacillus
fermentation reduces lactose in yogurt by at least 30 per-
cent. Unripe cheeses (ricotta, cottage cheese) lose com-
monly more than 70 percent, and the ripe ones—soft
(Camembert or Roquefort) or hard (Edam or Cheddar)—
contain merely a trace of the sugar.
With declining breastfeeding caused by urbanization
and rising female employment, milk and dairy products
Comparison of human and cow milk.
energy (kJ/g) 2.73.2 water (%) 8785 protein (%) 3.51.1 fat (%) 3.54.0 Ca (mg/100 g) 11833 vitamin A (IU) 140240
milk cowhumanTrans fatty acids now supply as much as 15 percent
of all energy from fat (or more than 5 percent of all food
energy) for some people in Western countries, but grow-
ing evidence has showed that at that level of intake they
may have a number of adverse eVects. Among the most
worrisome ones is the lowering of high-density lipopro-
teins (“good” cholesterol), reduction of fat content in hu-
man milk, increased risk for diabetes, and alterations in
the activities of the enzyme system metabolizing chemical
carcinogens and medications. Consequently, the best di-
etary advice seems to be to reduce intake of all lipids and
to eat moderate quantities of both saturated animal fats,
and mono- and polyunsaturated plant fats—as long as
they are not hydrogenated.
Milk For an emulsion containing more than 85 percent of wa-
ter, milk has an unusually high energy density. Human
milk, whose energy content is almost 20 percent higher
than that of cow’s or goat’s milk, has not only more di-
gestible food energy than all common temperate climate
fruits, but it also just edges white potatoes. Much like
grains, every milk derives this high energy density from a
nutritionally balanced combination of principal nutrients.
With the exception of reindeer milk, which is exceedingly
rich in both protein and lipids, all commonly drunkmilks
have less than 5 percent fat. Human milk has only about
a third of cow’s milk protein (just over 1 percent), but it
is nearly twice as high in carbohydrates.
Breastfeeding gives undoubtedly the best nutritional
start to a new life: lactation produces a virtually perfect
food for infants. But animal milk is not necessarily an
ideal food for older children and adults. Its sugar is lac-
tose, a disaccharide composed of glucose and galactose.
Digesting lactose is easy for all normal infants: they have
enough lactase to hydrolyze it. But after weaning, hun-
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have made surprisingly strong inroads even in societies
where milk drinking was traditionally absent. Most no-
tably, Japan’s per capita consumption rose from nothing in
1945 to more than forty liters by 1990, and more milk is
now drunk every year in China’s large cities. Cow’s milk
accounts for nearly all of this expansion. Even avid drink-
ers of that smooth, sweetish emulsion do not find other
Changing Holstein cow milk production with advancing lactation.
Weeks of lactation
milk (kg)
fat (%)
protein (%)
M ilk
(k g/
da y)
Pe rc
en t
5
10
15
20
25
30
35
0 0
1
2
3
4
5
6
7
0 8 16 24 32 40
Milk sugar, lactose, is composed of glucose and galactose.
carbon oxygen hydrogen
kinds of milk to be such gustatory delights. Acquired
tastes include not only goat and sheep milk (in much of
the Old World and in Latin America), but also water
buValo (mainly in monsoonal Asia) and camel (Arab
world) milk. Mare and yak milk drinking is limited to
parts of Central Asia, and reindeer milk is drunk in the
northernmost reaches of Eurasia (from Scandinavian Lap-
land to Siberia).
Ethanol Fermentation of alcohol from a variety of carbohydrates
(most commonly from cereal grains and sweet fruits) has
been done since antiquity—and the intoxicating eVect,
rather than any nutritional gain, is its main goal. Never-
theless, ethanol ’s food-energy density compares favorably
with values for the three main food nutrients. Its 29.3
kJ/g are some 70 percent above the density of digestible
carbohydrates and proteins, and are equal to nearly 80
percent of the energy content of lipids. Curiously, usual
servings of common alcoholic beverages accompanying
food have nearly identical ethanol content regardless of
their variety.
One glass of good beer (350 mL, 4 percent ethanol),
table wine (120 mL, 12 percent ethanol) or a fortified
aperitif or dessert wine (75 mL of sherry, Marsala, Cin-
zano, or Dubonnet containing 18 percent ethanol)—all
have about 14 mL (or 11.5 grams) of alcohol. But their
energy contents will not be identical. Because of diVerent
H20
CO2
alcohol acetaldehyde acetyl coenzyme
A NAD NADH
alcohol dehydrogenase
NAD NADH
aldehyde dehydrogenase
alcohol metabolism
Alcohol metabolism in human body.
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other liquids, ingesting particular foods, or engaging in a
strenuous exercise.
This rather sluggish constancy of ethanol ’s metabolic
breakdown means that while alcoholic beverages may sup-
ply a large share of everyday energy needs for habitual con-
sumers, they can never cover the total daily requirement.
An active, fifty-year old, healthy and not overweight (70
kilograms) French farmer will have to metabolize about
10.5 MJ of food energy to get through the day. His daily
bottle of vin ordinaire will give him exactly one-fifth of that
demand, his liver will easily process the ethanol load
amounting to just over two-fifths of its maximum clearing
rate—and the wine, unlike other alcohols, may give him
significant cardiovascular protection so that his chances of
heart attack will be lower than those for his distillate-
drinking counterpart in Scotland.
Effects of alcohol consumption on mortality.
cancers
cardiovascular diseases
total mortalityHa
za rd
ra te
ra tio
s
0.5
1.0
1.5
2.0
2.5
0 2 Drinks per week
4 6 10 12 148
carbohydrate shares—less than 4 percent in beer, up to 8
percent in cream sherries—a glass of beer will have about
540 kJ, a glass of table wine 350 kJ, and a glass of sweet
sherry 470 kJ.
Complex or simpler sugars in these alcoholic bever-
ages will be metabolized as any other carbohydrate—but
ethanol can be transformed to acetaldehyde, the first step
of its breakdown to acetic acid and one of the compounds
responsible for hangover, only by alcohol dehydrogenase.
This enzyme is present in the liver, and it is needed to
handle small volumes of ethanol produced by normal di-
gestion of carbohydrates and larger volumes generated by
intestinal bacteria. The rate of this reaction is just around
0.1 gram of ethanol per kilogram of body weight per hour,
that is between 150–260 kJ/hour (42–72 W) for most
people. This rate cannot be speeded up either by drinking
Alcohol content of common beverages.
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PEOPLE AND FOOD
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In contrast, a terminal alcoholic of the same weight
and age leading a collapsing life of minimum activity can
get by with as little as 7.5 MJ of food energy per day. His
maximum metabolic clearance of about 170 grams of
ethanol per day corresponds to two-thirds of a bottle of
80-proof whiskey and supplies 5 MJ, still one-third short
of his minimum energy needs. He may, of course, finish
the whole bottle of the liquor every day, but that would
only speed up his physical and mental deterioration with-
out supplying more energy.
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