Nutrition Paper 2-3 pages

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HUN 1201 Chapter 4 The Carbohydrates

Meet the carbohydrates

Carbohydrates

The kitchen name of carbohydrates is 'sugars'. However, more formal names are used in the specialty literature. Let's analyze them one by one.

CARBOHYDRATE: what is the meaning of this name?

Think of having a silent 'n' as part of this name, like this: carbo(n)hydrate So, take this word apart into its 2 components:

carbo(n) - well, this is self-explanatory, it is 'carbon' from the carbon atom (C).

hydrate - this means 'water containing' or having H2O in it

So, then what is 'carbohydrate'? It means 'carbons that had water added to them' or 'hydrated carbons'.

What scientists noticed is that (almost) all sugar formulas can be simplified down to one common formula, which is this: Cn(H2O)m

Let's see an example: take glucose, which has this formula C6H12O6

Do you notice a pattern in the formula?...

The lowest common denominator across all atoms inside the glucose formula is 6. So, you can divide with 6 across the board...and what do you get?

C6H12O6 :6 --> CH2O Ta-dah! Now you know the meaning of 'carbohydrate'.

A note of CAUTION: please do not mix up 'carbohydrates' with 'hydrocarbons' - they sound somewhat similar, but they are completely different types of compounds. Hydrocarbons are gasoline and you obviously cannot eat gasoline.

Names of Carbohydrates: Saccharides and Polysaccharides Another name for carbohydrates is 'saccharides'. This originates from the word saccharine which means excessively sweet.

We will also see variants of this name, such as mono-saccharides, di-saccharides and poly-saccharides.

Here mono- means 'one', di- means 'two', and poly- means 'many'.

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Monosaccharides: glucose, galactose, fructose As the name implies monosaccharides have only one sugar unit in them, and so these are the simplest types of sugars.

There are 3 monosaccharides of importance:

- GLUCOSE: blood sugar, the most important physiological sugar and the main fuel for the brain. It forms a 6 atom (hexagonal) ring structure. Another name for glucose is DEXTROSE, and this is often the name used on IV saline packages.

- FRUCTOSE: fruit sugar as the name implies. It forms a 5 atom (pentagonal) structure.

- GALACTOSE: milk sugar. It forms a 6 atom (hexagonal) structure similar to glucose.

These 3 monosaccharides can be used as the building blocks to build higher level sugars.

It is very important to note that only monosaccharides can be absorbed in the gut, which means that all other sugars need to be broken down into monosaccharides before they can enter the blood stream.

Disaccharides: sucrose, lactose, maltose As the name implies these sugars have 2 sugar units linked up in them. The 3 monosaccharides (glucose, fructose, galactose) are the building blocks.

There are 3 disaccharides of notable importance:

- SUCROSE: table sugar; this is made of 1 glucose linked to 1 fructose

- MALTOSE: malt sugar (found in brewing malts - not all sugars are sweet tasting); this is made of 1 glucose linked to 1 glucose

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- LACTOSE: milk sugar; this is made of 1 glucose linked to 1 galactose

Polysaccharides: starch, glycogen and fiber What is polymerization?

Individual compounds (the monosaccharides this time) can be linearly attached to each other to form very long macromolecules called polymers. The analogy is building a string of pearls from individual pearls: the pearls are the monomers, and the string is the polymer.

Polysaccharides (='many sugars') are polymers built out of monosaccharides.

The function of polysaccharides is storage of sugars for 'lean days'; the polysaccharide can be broken up to get the monosaccharides out of it. (exception: fiber is not for storage of sugars, and cannot be digested by humans)

Starch is broken down by amylase enzymes. Remember, we have salivary amylase and pancreatic amylase.

We will be looking at 3 types of polysaccharides in this course: starches, glycogen, and fiber.

Starches

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Starches are plant based polysaccharides. Typical examples are potatoes, bananas, beans, and grains - and other products made of these.

Meat and dairy have no starch in them.

The starch chain could be linear or branched, and based on this we have to distinguish between 2 starch types:

- amylose type starch (linear)

- amylopectin type starch (branched)

The type of starch has important physiological and health implications, as we'll see shortly.

PS - Please pay attention and do not mix up the words 'amylose' and 'amylase'.

Amylose starch

This is the linear starch:

Amylose is found in white potatoes ,

long-grain rice, or non-sticky, rice

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Bananas also contain amylose type starch.

Amylopectin starch

This is the branched starch:

And it is usually found in potatoes ,

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in short-grained, sticky, rice (Sushi rice):

Glycogen

Glycogen is the polysaccharide found in animals and humans. It is stored in the muscles and in the liver. When the body needs glucose, the glycogen reserves are raided to release the glucose.

On a molecular level, glycogen is a branched polymer, similar to amylopectin.

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In humans the glycogen reserves can last approximately 8-12 hours. This means that the glycogen reserves are at their lowest levels during the morning, provided the person did not eat during the night.

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Fiber

Dietary fiber is the indigestible portion of food derived from plants. There are soluble ( inulin, pectin, fructans) and insoluble (cellulose, chiton, hemicellulose, beta-glucans) fibers.

Humans cannot digest fiber, which then passes through the GI tract. Therefore fiber has no calorie value. However, it still has the following important physiological functions:

1) It stimulates peristaltic motion in the intestines, so it lowers the passage time of waste materials. The sooner these leave the system, the better.

2) Fiber binds to bile and removes it from the system. We already mentioned (in chapter 3) that bile is made from cholesterol, therefore the removal of bile lowers the cholesterol levels.

This is why high fiber foods (eg. oats) are advertised as 'heart healthy' or 'lowering cholesterol'.

3) Helps retain water in patients suffering from diarrhea.

There is a lot of fiber in broccoli and cauliflower stems (or stalks). These should not be thrown away, but cooked/steamed until soft.

Oats also have high fiber content (notice the cholesterol reduction advert):

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Fiber is very filling and induces a feeling of satiety, while providing no calories. Because of this, every weight loss program should include high fiber content foods in it.

Fiber is also sold as a health supplement in powder or drink format (Metamucil, Benefiber, Psyllium, etc.)

Absorption of sugars

The human body can only absorb monosaccharides, which means that all dietary sugars first need to be hydrolyzed (aka broken down) into monosaccharides.

Starches and glycogen are broken down by amylases, as we have already seen.

The disaccharides are broken down by enzymes specific for each disaccharide. The rule of thumb for naming these enzymes is adding the -ASE suffix to the substrate's name.

Therefore...

Sucrose is broken down by Sucrase.

Maltose is broken down by Maltase.

Lactose is broken down by Lactase.

These enzymes are secreted by the brush border cells of the small intestine (chapt 3).

The absorbed monosaccharides are transported to the liver via the portal circulation.

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Lactose Intolerance Some people's bodies do not make enough lactase, or they do not make it at all.

This means that lactose cannot be broken down into glucose and galactose, and it cannot be absorbed. As the unused lactose moves into the large intestine, it will feed the bacteria instead, which will cause gas or even diarrhea. This is lactose intolerance: the body cannot handle lactose.

The solution is to avoid dairy products.

Alternatively, dairy products from which the lactose was removed can be purchased: lactose-free milk, lactose-free yoghurt etc.

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Another option is actually to take lactase in a pill format, and replenish what the body is missing.

The Glycemic Index (GI)

The glycemic index is a time measure, that shows how long it takes a carbohydrate food to appear as glucose in the blood stream. The carbohydrate will be broken down, and then absorbed - all this takes time. So, we measure the time from the moment of ingestion to the moment the sugar hits the blood stream.

The longer this time the better!

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In order to compare carbohydrates, we need to have some kind of reference to which everyone compares. The reference is glucose, and it is given the 100% value by definition.

Carbohydrates that digest faster get a higher score, those that digest slower get a lower glycemic index score. The lower the score the better.

The Fate of Excess Sugar

There are 2 distinct energy storage categories in the body.

First, sugars are stored as glycogen, but this storage capacity is not that big.

About 100 grams can be stored in the liver, and about 500 grams can be stored in the muscles. If you had a large steak and mashed potatoes, and perhaps washed that down with a soda, you'd probably be maxing out that storage capacity by this single meal.

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When the glycogen storage is exceeded, that excess will be stored as fat. Unlike the glycogen storage, the fat storage is limitless. One could store hundreds of pounds in fat form.

There is a major functional difference between glycogen storage and fat storage though.

Glycogen is connected to glucose metabolism via a '2-way street'. Glucose can be stored as glycogen, but it can be also regained from that glycogen.

Excess glucose is connected to fat via a '1-way street'. Once the excess glucose was transformed into lipids, it can never be recovered as glucose anymore. This is the fat trap and we'll be talking more about it in later chapters.

Hormonal Regulation of Blood Sugar: Insulin Vs Glucagon

The glucose level in the blood cannot be too high or too low.

It is strictly regulated, meaning if the glucose level is too low then glucose will be withdrawn from the glycogen reserves.

If the glucose level is too high in the blood, then it will have to be removed from the blood and placed into the liver and muscles (glycogen).

The healthy range is between 70 and 100 mg/dL. If a deviation occurs then the insulin-glucagon hormone pair will kick into action and push the level back to normal.

If the glucose level is below normal, we talk about hypoglycemia.

If the glucose level is above normal, we talk about hyperglycemia.

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In diabetics this hormone system malfunctions and both hyper- and hypoglycemia could occur.

Hormones are chemical messengers, and they literally carry 'messages' or 'instructions' on what the body should be doing at the moment.

Insulin's message is always this: 'The blood glucose level is too high. All cells (but especially the muscle and liver cells) must take in glucose'.

Glucagon's message is the opposite: 'The blood glucose level is dangerously low. All cells (but especially the muscle and liver cells) must release glucose from the glycogen reserves'.

Glucagon is secreted by pancreatic alpha cells.

Insulin is secreted by pancreatic beta cells.

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Diabetes

Diabetes I Diabetes II

When the insulin system is broken blood glucose levels cannot be regulated and diabetes occurs.

Based on the underlying molecular defect, there are 2 types of diabetes.

Insulin is secreted by the pancreatic beta cells (endocrine function - see chapt. 3).

In certain individuals a poorly understood autoimmune disease occurs. The immune system does not recognize the beta cells as part of the body, and attacks and kills them as if they were 'intruders'. Since no beta cells are left, no insulin is made anymore.

The insulin recognition system functions, but simply there is no insulin. The only cure is to provide artificial insulin via injection.

Diabetes I usually develops in childhood. Most importantly, the autoimmune disease is not lifestyle based.

In Diabetes-II insulin is being made, however the signaling mechanism is broken.

Even if there is insulin present, the cells are 'deaf' and cannot hear the 'message' being sent. Higher and higher levels of insulin are required to get the message through: this is called insulin resistance.

Diabetes-II is a lifestyle disease and it develops in adulthood. Unfortunately, younger and younger people are getting the disease nowadays; some of them are even teenagers.

Diabetes-II is often preventable, and in most cases it is also reversible.

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Dangerous Side Effects of Diabetes Too high glucose levels in the blood will ruin the blood vessels, especially the capillaries. Therefore, many side effects of diabetes are tied directly to the malfunctioning of capillaries. The organs/tissues that heavily depend on capillaries are the retina, the kidneys, and nerve endings in the feet. When these tissues become sick, we observe the following typical pathologies:

- Diabetic Retinopathy: the retina slowly dies out, and this results in blindness.

- Diabetic Nephropathy: the kidney's filtration units, the nephrons, slowly die out and the kidneys cannot filter the toxins anymore. This will require peritoneal dialysis or machine driven dialysis (hemodialysis) on a regular basis.

In peritoneal dialysis the patient carries a permanent catheter, which is then connected to the dialysis unit at night.

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In hemodialysis, the patient must periodically visit a dialysis center where a machine will filter his/her blood:

- Diabetic Neuropathy: the nerve endings, especially in the feet, degenerate leading to a loss of feeling. As the patient cannot feel his/her feet, accidental wounds will occur. However wound healing requires a good capillary flow, which is missing in these patients. So, their wounds barely heal.

Eventually the wounds gangrenate, and the limb must be amputated. In the 1980s the amputation rate was 8% among diabetic patients, but due to improved care that dropped to around 3% these days.

Hierarchy of glucose use among organs

There is a hierarchy between organs, where some 'give orders' while others 'take and follow orders'. Glucose is used in a hierarchical order.

The king/queen at the very top is the brain.

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It takes orders from no one and it sacrifices or changes almost nothing in the way it operates. Rain or shine it goes about its business as if nothing bad happened. It is the lower level organs who must adapt and sacrifice function to serve the brain. Basically every other organ serves the brain.

The brain is the most picky organ in terms of what it consumes. It only consumes glucose, and this is why blood glucose level must always be maintained at the levels we have seen.

The liver has manager-level duties, and it is doing everything it can to satisfy the brain.

If 'brain food' (glucose) is scarce it will manufacture it and send it to the brain. Also, the liver will stop consuming glucose to allow the brain to have its fill.

The muscles are at the bottom of the hierarchy. They will take orders from everybody, including the liver.

The muscles will get to use blood glucose only during the best of times. The muscles will put it into their own glycogen storage.

If there is a slight dip in the blood glucose level, they are ordered to stop consuming glucose. When muscle glycogen is used up, they switch to using fats.

They will refuse precious glucose reserved for the brain, if ordered to do so.

If things get really bad (as in starvation), muscles are ordered to self-exterminate and have themselves turned into glucose to feed the brain. The brain will not go without glucose, and everyone else must sacrifice themselves for that purpose.

There is no democracy in physiology!

Energy content of sugars

The average energy content of sugars is 4 kilocalories per gram. (You must absolutely know this number)

Of course, fibers have a 0 calorie content.