Sports nutrition: a training strategy for protein consumption
There is more to protein intake than simply eating the right amount
There’s more to protein nutrition than just eating the optimum amount; the timing of consumption and the type of protein selected can both impact on nitrogen balance; and there are a number of nutritional ‘co-factors’ that are either essential or useful in promoting optimum protein metabolism within the body.This is especially true where carbohydrate is concerned, because building or even maintaining lean tissue mass is an ‘energy-intensive’ process. Increasing protein intake at the expense of carbohydrate can be a bad strategy for athletes engaged in heavy training, because without sufficient carbohydrate the body simply switches to other fuels for energy, and amino acids from protein (particularly the branched chain amino acids, leucine, isoleucine and valine) provide a ready source of energy!
Muscle tissue is a relatively rich source of branched chain amino acids (BCAAs), and tends to undergo breakdown during periods of highenergy demand, when carbohydrate and/or the amino acid pool becomes depleted. Furthermore, carbohydrates stimulate the release of insulin, a highly anabolic hormone, which helps to drive both glucose and amino acids into muscle cells. Any athlete seeking to optimise his or her protein metabolism should therefore ensure a carbohydrate intake commensurate with training volume.
The role of carbohydrate in enhancing endurance during long events and accelerating post- exercise recovery is undisputed, and new research (highlighted in PP 194, March 2004) indicates that carbohydrate feeding before and during high intensity exercise can limit the amount of stress hormone release, thereby reducing the risk of post-exercise immune suppression(1,2). However, research suggests that protein has a role to play, too. A study on resistance training examined hormonal responses to water, carbohydrate, protein or a carbohydrate/protein mix, given immediately and then two hours after a training session(3). As expected, those fed the carbohydrate and carbohydrate-plus-protein mix drinks showed an increased insulin response.
However, those fed the carbohydrate-protein mix also showed a modest but significant increase in growth hormone levels, suggesting that protein combined with carbohydrate following resistance training may create a more favourable hormonal environment for muscle growth.
Post-exercise protein feeding seems to be beneficial for endurance athletes also. In a study on 40 swimmers given either water or water-plusglucose during training sessions and then either water, sucrose or a sucrose-plus-milk protein mix after training, the subjects receiving the posttraining sucrose-protein mix exhibited lower levels of creatine phosphokinase (a marker of muscle damage) than the others(4). Moreover, creatine phosphokinase levels returned to baseline levels more rapidly in this group, indicating that the ingestion of protein with carbohydrate accelerates recovery.
A study on ultra-endurance athletes, published just a few months ago, showed that a carbohydrate-protein mix maintained a positive nitrogen balance during and after a six-hour training session (five hours of cycling and one hour of running), while a straight carbohydrate drink did not(5).
The consensus of scientific opinion now is that, following intense exercise, athletes should ingest a carbohydrate and protein mix (around 1 gram per kg of body mass of carbohydrate and 0.5g per kg of protein) within 30 min of completing exercise, as well as consuming a high-carbohydrate meal within two hours. This nutritional strategy has been found to accelerate glycogen resynthesis as well as promoting a more anabolic hormonal profile that may hasten recovery(6-9).
Research carried out over a decade ago indicated that ingesting a light carbohydrate/ protein snack 30-60 minutes before exercise is also beneficial(10,11). In these studies it was shown that 50g of carbohydrate and 5-10g of protein, taken before a training session, could increase carbohydrate availability towards the end of an intense exercise bout and also enhance the availability of amino acids to muscles, thereby decreasing exercise-induced catabolism (breakdown) of protein.
This research appears to be backed up by a very recent study carried out on 15 trained cyclists, who cycled to exhaustion on two rides 12-15 hours apart, the first at 75% and the second at 85% of VO2max(12). During the test, riders were split into two groups and given either a 7.3% carbohydrate drink (1.8ml per kg every 15 minutes), or the same drink with protein added at 1.8%. After 7-14 days, the test was repeated and the drink protocol reversed.
The results showed that riders taking the carbohydrate-plus-protein rode for 29% longer than the carbohydrate-only group during the first (75% VO2max) ride and 40% longer during the second (85% VO2max) ride! Furthermore, peak levels of creatine phosphokinase were 83% lower when carbohydrate plus protein was taken. Since the carbohydrate plus protein drink contained 25% more calories overall, further studies are needed to see how much of this effect is due to higher energy intake. However, it seems reasonable to assume that a carbohydrate-protein drink taken during training provides for increased protein concentration outside the cell, which can potentially enhance protein synthesis and repair.
The concept of different glycaemic indexes (the rate at which digested carbohydrate is released into the bloodstream as glucose) for different carbohydrates is now well accepted. However, different proteins display different rates of breakdown into their amino acid building block constituents, and hence uptake into the body.
A study into whey protein and casein (two types of protein supplements that are popular with athletes and bodybuilders) examined the speed at which one of the amino acids (leucine) appeared in the bloodstream after ingestion of a meal of each kind of protein (containing identical amounts of leucine)(13). The researchers found that whey led to a dramatic but short-term increase in plasma amino acids, while casein induced a prolonged plateau of moderately increased levels.
They concluded that the differences were probably explained by the slower gastric emptying of casein. Whey protein is a soluble protein whereas casein clots into the stomach, so delaying its gastric emptying. Likewise, soy protein appears to be digested more rapidly than milk protein, resulting in a higher but more transient peak of plasma amino acids(14).
The implications are obvious: an athlete seeking to supply a post-training or mid-training boost to the amino acid blood pool would be best advised to consume a fast-release protein, such as whey or soy. However, when a prolonged period of recovery is in store (eg at bedtime) a slowerreleasing casein protein drink, such as milk, would be better. Another implication of this study is that, providing a meal or drink supplies the same quantity of the essential amino acids, one type of protein is not necessarily ‘better’ than another. Of more importance is that its release rate is matched to the timing of ingestion.
The situation also appears to be complicated by age. A recent study, which looked at the effects of protein retention in young men (mean age 25 years) fed protein meals containing either slow-releasing casein proteins or rapid-releasing whey proteins, found a greater retention (ie uptake into muscles) after casein(15). However, when the same researchers studied protein retention in elderly subjects (mean age 72 years), their findings were reversed, with whey protein producing a significantly higher uptake of amino acids than casein(16).
The researchers surmised that amino acid availability may limit muscle synthesis in older subjects, and that the higher amino acid peaks produced by whey prevented this from happening. The implication seems to be that ingesting fast-releasing proteins mid- or postexercise may be more important for older athletes than their more youthful counterparts.
‘Free form’ amino acids
The process of digestion releases the amino acid building blocks from ingested protein. However, as we’ve seen, this release rate is variable and the process of digestion itself actually consumes energy. This has prompted some investigators to ask whether the use of ‘free form’ amino acids before, during or after training could be a rapid method of providing athletes with optimum amounts of amino acids exactly when they’re needed.
Particular interest has been shown in the branched chain amino acids (BCAAs), which are readily oxidised for energy and therefore in greater demand when energy output is high. In theory, BCAA supplementation might help to minimise protein degradation, thereby leading to greater gains in fat-free mass, or at least minimise lean tissue loss when training volumes are high.
BCAAs and body composition
There is some evidence to support this hypothesis; for example, a study conducted on trekkers at altitude found that taking 10g of BCAAs per day during a 21-day trek increased fat-free mass by approximately 1.5%, while controls on placebo experienced no such change(17). Meanwhile, another study found that 30 days of BCAA supplementation (14g per day) promoted a significant increase in muscle mass (+1.3%) and grip strength (+8.1%) in untrained subjects(18).
These findings suggest that BCAA supplementation may have some impact on body composition. Moreover, some recent evidence suggests that BCAA supplementation can decrease exercise-induced protein degradation and/or muscle enzyme release (an indicator of muscle damage), possibly by promoting an anticatabolic hormonal profile(6,10,19). However, despite the persuasive rationale, the effects of BCAA supplementation on short- and long-term exercise performance are somewhat mixed, with some studies suggesting an improvement and others showing no effect(6). More research is needed, therefore, before firm conclusions can be drawn.
Having said that, there is good evidence that BCAAs administered during training can reduce the perception of fatigue, while improving mood and cognitive performance. A study on seven male endurance-trained cyclists with depleted glycogen stores examined the effects of BCAA supplementation (versus placebo) on mental fatigue and perceived exertion(20). The subjects exercised at a work rate corresponding to approximately 70% VO2max for 60 minutes, followed by another 20 minutes of maximal exercise.
During the 60-minute section, the subjects’ ratings of perceived exertion were 7% lower and mental fatigue 15% lower when they were given BCAAs. In addition, cognitive performance in the ‘Stroops Colour Word Test’ performed after exercise was improved when BCAAs had been ingested during exercise. Interestingly, however, there was no difference in physical performance in the final 20-minute segment of the ride between the placebo and BCAA groups; the amount of work performed during this section was the same regardless of which supplement was taken.
These findings on BCAA supplementation, mental fatigue and perceived exertion were replicated in a study on runners given carbohydrate-plus-BCAA drinks or carbohydrateonly drinks (placebo) during a 30k cross-country run(21). Subjects on BCAAs improved their postexercise performance in the above-mentioned Stroops test by an average of 3-7% compared with those on placebo. The BCAA group also maintained their performance in two more complex mental tasks (shape rotation and figure identification) after exercise, while the placebo group showed a 25% and 15% reduction respectively in these tasks.
Researchers believe that this cognitive effect may be due to the ability of BCAAs to compete with and therefore reduce the uptake of another amino acid, tryptophan, across the blood-brain barrier and into the brain. Tryptophan is the precursor to a brain neurotransmitter called 5- hydroxytryptamine (5-HT – more commonly known as serotonin), which is involved in fatigue and sleep and is believed to contribute to the development of central/mental fatigue during and after sustained exercise. During exercise, the concentration of tryptophan in the blood relative to other neutral amino acids seems to rise. But supplementing with BCAAs seems to help block this effect, which would, in turn, reduce levels of 5- HT in the brain.
Essential amino acids
The BCAAs comprise just three of the nine essential amino acids (EAAs), the other six being histidine, lysine, methionine, phenylalanine, threonine and tryptophan. As mentioned, essential amino acids have to be obtained from the diet because they can’t be synthesised in the body from other amino acids. Although the six ‘straight chain’ EAAs are not so readily utilised as fuel, some researchers believe that giving all nine EAAs in a free form (ie as a mix of separate amino acids, not as protein), and in ratios that reflect the amino acid composition of muscle protein, is more beneficial for muscle protein synthesis than giving BCAAs alone.
In recent studies, scientists in Texas have found that ingesting 3-6g of EAAs before and/or after exercise stimulates protein synthesis(37,38). Moreover, this stimulation appeared to increase in a dose-dependent manner until plasma EAA concentrations are doubled, and was maximised when EAAs were administered to maintain this doubled concentration over a three-hour period. Adding carbohydrate seemed to enhance this protein synthesis, probably through the anabolic effect of insulin.
Although there has been very little research on EAA ingestion by athletes, studies on resistance training in healthy adults seem to confirm the potential benefits of EAAs; for example, muscle protein synthesis was increased 3.5-fold when 6g of a mixture of EAAs was given along with 35g of carbohydrate after resistance exercise(39).
In another study, three men and three women resistance trained on three separate occasions and then consumed, in random order, one of the following:
- a 1 litre solution of mixed amino acids containing both essential and nonessential amino acids (40g);
- a solution containing only essential amino acids (40g);
Net muscle protein balance was negative after ingesting placebo but positive to a similar magnitude for both the mixed and essential amino acid drinks. The researchers concluded that: ‘it does not appear necessary to include nonessential amino acids in a formulation designed to elicit an anabolic response from muscle after exercise’.
A comprehensive protein strategy
Given the above findings, what reasonable steps can an athlete take to optimise his or her protein nutrition? Below is a ‘protein checklist’, which crystallises these findings into dietary recommendations:
- Ensure an adequate intake of dietary protein – ie a minimum of 1.5g of high-quality protein per kg of body weight per day. Power/strength athletes, or those engaged in intense training, should consider increasing this to 2g per kg per day;
- Ingest protein-carbohydrate drinks after exercise rather than protein alone. Ideally, consume a drink made up of about 1g per kg of carbohydrate and 0.5g per kg of protein within 30 minutes of training, and eat a high-carbohydrate meal within two hours;
- Consume a light pre-exercise snack: 50g of carbohydrate and 5-10g of protein taken before a training session can increase carbohydrate availability towards the end of an intense exercise bout and also increase the availability of amino acids to muscles. However, make sure your snacks are low in fat to allow for rapid gastric emptying!
- Use protein/carbohydrate drinks during very long events: a solution containing 73g carbohydrate and 18g protein per litre, consumed at a rate of 1ml per kg of body weight per minute, may delay the onset of fatigue and reduce muscle damage;
- Consume quick-digesting proteins such as soy and whey immediately after training: this may be especially important for older athletes;
- At other meals, consume a mix of proteins in order to promote a more sustained release of amino acids into the body;
- Adding BCAAs to your normal protein intake may be useful for athletes undergoing prolonged or heavy training, and this may be particularly true for events/sports requiring large amounts of mental agility and motor coordination;
- HMB supplementation, at 3g per day, may be a useful additional strategy for novice athletes, or those returning to training after a layoff;
- Essential amino acid blends taken 1-3 hours after training may promote additional muscle protein synthesis, although this hypothesis is not proven in athletes;
- Don’t forget to ensure that your overall diet is of high quality and as whole and unprocessed as possible: this will ensure adequate intakes of other nutrients essential for protein metabolism, such as zinc and the B vitamins.
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