Exceptional Proff only

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INTRODUCTORY REMARKS The letter P brings together two of the most talked about supplement families: proteins, which have been perennially popular since the time of the ancient Olympians and prohormones, which will be dealt with in a later issue. Both supplement families include products which range from simple and relatively inexpensive, to exotic, expensive and emotively marketed. Part 32 also includes informa- tion on proline, a non-essential amino acid which is marketed for growth and repair of soft tissue based on its importance in the make-up of collagen.

PROTEIN S M Phillips L Breen Skeletal muscle protein turnover rates are ~1%–2%/d and exist in dynamic, usually balanced, equilibrium between muscle protein breakdown (MPB) and muscle protein synthesis (MPS). For example, in the fasted state, MPB>MPS, whereas in response to ingestion of protein-containing meals, MPS>MPB.1 Thus, in healthy adults, mus- cle mass remains relatively stable due to ‘fed-gain’ being balanced by fasted-loss, so daily protein fl ux, while it may be 3–4 times greater than net intake and loss, is in tight balance. Fasted-state protein losses are typically about 40– 60 g/d for a sedentary person weighing 70–90 kg and it is debatable what the losses would be in athletes, be they aerobically or resistance trained. Dietary protein for athletic populations can serve as signal and substrate for MPS, resulting in protein accre- tion for hypertrophy, repair of damaged proteins or assisting the maintenance of lean mass. There are important messages for athletes, who differ from sedentary individuals, in terms of quantity, timing and quality of protein intake in relation to an athlete’s training stimulus. The molecular changes underpinning these adaptations are gene transcription and mRNA translational signalling and are highlighted in a review.2

The general consensus is that adults need no more than 0.8–0.9 g/kg/d of protein to meet their needs. However, the notion of consumption of ‘extra’ protein above these levels to cover the needs of increased physical activity is not con- sidered. Dietary guidelines for athletes typically recommend protein intakes of 1.2–1.7 g/kg/d,3 4 based on maintaining nitrogen (ie, protein) bal- ance. By all accounts, nitrogen balance is a fl awed method, measuring the minimum amount of

protein required to balance losses. Given the functional demands of training and performance, an optimal protein intake for athletes might exist beyond merely satisfying a minimal requirement and thus being in nitrogen balance. Indeed, pro- tein intakes of 0.86 g/kg/d have been shown to reduce whole-body protein synthesis rates in strength-trained athletes,5 suggesting that cur- rent recommendations for athletes may be insuf- fi cient if synthetic rates of proteins are adversely affected. Recently, Moore et al6 demonstrated a protein dose response following resistance exer- cise. Specifi cally, resistance exercise-induced MPS increased in a curvilinear fashion with ingestion of graded amounts of isolated egg protein, reach- ing a plateau at 20 g, with no further increase at 40 g of protein. The amino acids supplied beyond 20 g of postexercise protein were not assimilated into new muscle protein but instead were directed toward oxidation.6 Interestingly, 10 g of essential amino acids (EAA), equivalent to 25 g of most high-quality intact protein, has been shown to maximally stimulate MPS at rest also.7

There is no clear consensus as to whether pro- tein ingestion before, during or after exercise promotes the greatest adaptive response. With respect to pre-exercise feeding, acute8 9 and long- term studies,10 comparing pre- and post-training protein feeding have yielded equivocal results. Consumption of protein during exercise may serve to provide amino acids required to improve protein balance during and after exercise.11–13 However, in these studies,11–13 carbohydrate and amino acids were provided: given the profound impact of insulin for the suppression of MPB,14 it may be that the greater net balance is simply an artefact of energy intake suppressing MPB and not a protein-mediated rise in MPS. In addition to adaptation, ingestion of additional protein dur- ing endurance exercise does not improve perfor- mance, reduce proxy markers of muscle damage or hasten the recovery of muscle function.15

The potency of postexercise protein ingestion for potentiating MPS is unequivocal.16 After exer- cise, the energy status of the cell is returning to resting levels, signalling that pathways are still active, the muscle is prone to greater rates of MPS, and all of these effects are enhanced with feeding. Resistance exercise specifi cally targets a synthetic response of myofi brillar proteins, it is therefore not surprising that protein ingestion augments this response.17 Interestingly, protein ingestion

1Department of Kinesiology, McMaster University, Hamilton, Canada 2Department of Nutritional Sciences, Rutgers, The State University, New Brunswick, New Jersey, USA 3Australian Institute of Sport, Canberra, Australia 4Performance Infl uencers Limited, London, UK 5Green Templeton College, University of Oxford, Oxford, UK

Correspondence to LM Castell, Green Templeton College, University of Oxford, Oxford, OX2 6HD, UK; lindy.castell@gtc.ox.ac.uk

Received 22 February 2012 Accepted 22 February 2012

A to Z of nutritional supplements: dietary supplements, sports nutrition foods and ergogenic aids for health and performance—Part 32 S M Phillips,1 L Breen,1 M Watford,2 L M Burke,3 S J Stear,4

L M Castell5

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route, it is not known how much body proline is derived from the diet or made de novo. Since proline and hydroxyproline (formed post-translationally) comprise approximately 25% of the amino acids in collagen, proline is important to skin, bone, cartilage, tendons, ligaments and connective tissues.30 Proline is degraded via proline oxidase (dehydrogenase) to glutamate or ornithine,29 31 32 and the fi nal fates include polyamines, arginine and entry into the TCA cycle. Proline is an osmo- protectant, a source of superoxide (in the immune system), and plays a role in sensing both energy availability and main- taining protein homeostasis. Hydroxyprolines are present in proteins other than collagen where they play a role in oxygen sensing while hydroxyproline, released from protein degrada- tion, is an antioxidant.

Given the importance of proline in growth and wound repair, including the muscle hypertrophy of training, it has been pro- posed that proline may be conditionally essential. Indeed, proline has been marketed as a supplement for bodybuilders and weight lifters, and for recovery after strenuous exercise. However, there is no direct evidence to support these claims. It is notable that, while circulating proline concentrations decrease during burn injury, dietary supplementation with proline has no effect on plasma proline levels in such patients. Very few studies have looked directly at proline supplementation,29 though in patients with gyrate atrophy, supplements of up to 488 mg/kg/d are well tolerated. It is, however, not possible to make any claims about the safety or even effectiveness of proline supplements due to an almost complete lack of data.29 33 An alternative approach to increase proline availability would be to provide proline pre- cursors (glutamine, ornithine, arginine) as dietary supplements but again there is little evidence that these are effective or even result in increased proline synthesis.30

Concluding comments Proteins are clearly here to stay, although there is still some debate about whether it is best to give protein supplementation pre- or postexercise. Our authors have summarised the effects of protein on performance in a useful strategy table. In particu- lar, they emphasise the importance of consuming proteins as soon as possible after exercise. High-quality proteins include soya, milk and eggs, which means that the vegetarian athlete is also able to access a good source of protein. There is little or no evidence to support the claims that proline is helpful to weight- lifters. In addition, almost nothing is known about the safety of proline supplementation. The studies undertaken so far on proline have been almost exclusively in clinical situations.

Competing interests None.

Provenance and peer review Commissioned; not externally peer reviewed.

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also potentiates the acute muscle protein synthetic response to endurance exercise.18 Surprisingly, despite acute increases in mitochondrial protein synthesis with endurance exercise,19 protein ingestion following a prolonged cycle did not poten- tiate this response, but instead increased the synthesis of myofi brillar proteins.20 Thus, protein ingestion may assist in maintaining muscle structural integrity and power-generating capacity, rather than infl uencing muscle aerobic capacity.

High volume resistance exercise appears to sensitise the muscle to amino acid provision21 beyond the so called ‘window of opportunity’; a period thought to induce the greatest muscle anabolic effect.22 Thus, while there is some debate about the critical nature of the timing of postexercise protein consump- tion, we recommend that the sooner athletes consume pro- tein after exercise the better. In addition, relatively frequent protein ingestion (ie, every 3–4 h) over 24 h after exercise to sustain the elevation in MPS is also recommended.

A protein digestibility corrected amino acid score close to 1.0 is defi ned as ‘high quality’. This includes animal protein sources such as milk (composed of whey and casein protein), eggs, isolated soya protein and most meats. Habitual con- sumption of high-quality protein sources has a pronounced effect on muscle recovery and adaptation. For example, milk proteins result in a pronounced increase in MPS after resis- tance exercise, compared with equivalent amounts of isolated soya protein23 which, over time, promotes greater hypertro- phy24 and is likely to be due to the whey protein constituent in milk. Whey proteins stimulate greater rates of MPS over isonitrogenous amounts of casein and soy protein at rest and after exercise.25 The mechanisms underpinning the anabolic advantage of whey protein are not entirely clear, but maybe due to the relative amount of the branched-chain amino acids, in particular, leucine. Leucine occupies a position of promi- nence in that it alone can act as a stimulatory signal for MPS.26 Milk proteins and whey, in particular, are highly enriched with leucine. More importantly perhaps, the rapid absorption kinetics of whey proteins (or hydrolysed ‘slow’ digested pro- teins) induces a greater rate of leucine appearance in the circu- lation than soy and casein proteins and may be important for stimulating MPS.27 28 Thus, although rapid leucinemia may be important in activating MPS, provision of other EAAs may be required to sustain the anabolic response.

Summary Based on the current evidence, the following strategies are pro- posed which should be very effective at allowing repair, remod- elling and adaptation, and gains in lean mass in athletes:

Daily intakes higher than the RDA (1.2–1.6 g/kg/d). ▶ Emphasise dairy source proteins enriched in leucine. ▶ Consume protein in doses of 20–25 g/serving to maximise ▶ adaptive responses. Equally spaced protein meals throughout the day. ▶ Consumption of protein immediately after exercise. ▶

PROLINE M Watford L M Castell Proline is not considered essential in adult humans, although early work demonstrated a potential benefi t of proline when arginine was limiting. Proline is readily available in dairy, meat and eggs, and most plant proteins but can also be synthe- sised endogenously by two pathways, one arising from orni- thine and arginine, or one from glutamine and glutamate.29 Although the glutamate pathway is considered to be the major

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