protein metabolism

Protein Metabolism : Eating and recovery should protein replenishment start before exercise?

When it comes to recovery from athletic endeavours, the notion that ‘it’s not just what you eat but when you eat it’ seems intuitively correct. For example, numerous studies have demonstrated that muscles are hungrier for refuelling after exercise than they are before, giving rise to the concept of the ‘post-exercise window of opportunity’. However, more recent research has suggested that this window of opportunity may be wider than anyone had imagined, writes Andrew Hamilton.

One of the problems with making definitive recommendations about the timing of meals and drinks to enhance post-exercise recovery is the multifaceted nature of the components required for recovery. In broad-brush terms, there are four major nutritional requirements during postexercise recovery:

  • Water – to replace fluid lost as sweat and to aid the process of ‘glycogen fixation’;
  • Electrolytes – to replenish minerals lost in sweat (eg sodium, chloride, calcium, magnesium);
  • Carbohydrate – to replenish muscle glycogen, the body’s premium grade fuel for strenuous exercise, and also to top up liver glycogen stores, which serve as a reserve to maintain correct blood sugar levels;
  • Protein – to repair and regenerate muscle fibres damaged during exercise, to promote muscle growth and adaptation, and to replenish the amino acid pool within the body.

Although even a small degree of water loss can impair performance, the process of rehydration to replace lost water and electrolytes is relatively straightforward. Our bodies always strive to maintain optimum water and electrolyte balance, so as long as we consume plenty of fluids after training and eat a reasonably balanced diet (which will contain electrolyte minerals), full rehydration will occur as a matter of course. Moreover, it’s quite easy to tell when we’re fully hydrated – a pale straw-coloured urine and frequent urination being the most obvious signs.

Refuelling muscles with carbohydrate is less straightforward. Regular PP readers will be aware of the importance of carbohydrate feeding after exercise and also during prolonged endurance events. One of the earliest landmark studies demonstrated that a typical diet (with about 45% of calories derived from carbohydrate) produced a steady depletion in muscle glycogen during three successive days of running training (10 miles per day) (1). However, when runners were given additional dietary carbohydrate, they achieved near maximal repletion of muscle glycogen within 24 hours.

Subsequent studies showed that, to maximise the rate of glycogen repletion, carbohydrate consumption should be a priority after exercise. In fact, a recent literature review concluded that the highest muscle glycogen synthesis rates occur when large amounts of carbohydrate (1-1.85g per kg of body weight per hour) are consumed immediately after exercise and at 15- to 60-minute intervals thereafter, for up to five hours (2). Conversely, delaying carbohydrate ingestion by several hours may slow down muscle glycogen synthesis.

We now know that there is actually a two-phase process of glycogen replenishment. In the first hour after exercise, a muscle transporter protein known as GLUT4 quite literally opens the gates to your muscle cells, allowing glucose to flood in thereby facilitating the rapid synthesis of muscle glycogen. This is supplemented by a slower but longer lasting process, whereby carbohydrate- hungry muscles become much more sensitive to insulin, the anabolic hormone that helps drive glucose into muscle cells.

Although animal studies have shown that this second phase of enhanced glycogen replenishment can last for up to 72 hours post-exercise (3,4), the general consensus is that athletes engaged in frequent training should aim to start glycogen replenishment immediately after each training session. This is because the training schedules of most athletes simply don’t allow for up to 72 hours of recovery time between sessions, so athletes seeking rapid recovery must take advantage of the rapid phase 1 GLUT4 process and consume carbohydrates immediately after training!

Carbohydrate metabolism is a very well researched area of sports nutrition. This is because dietary carbohydrate and the glucose/glycogen our bodies produce from it are premium grade fuels and absolutely pivotal to sports performance. But another, less obvious, reason is that muscle glycogen stores are relatively easy to test by means of muscle biopsy. Moreover, muscle glycogen levels change rapidly in response to exercise (depletion) or carbohydrate feeding (replenishment). These factors make it much easier to investigate the relationship between the timing of carbohydrate feeding and its effects.

Contrast this with protein metabolism: unlike with carbs, there’s no ‘protein store’ in the body, other than muscle tissue, and observing changes in muscle fibres in response to protein ingestion is difficult for two main reasons:

  • It can take many days to detect an increase in muscle fibre mass as the result of protein incorporation into muscle tissue, which makes it very difficult to deduce a link between timing of protein intake and the body’s response;
  • Proteins in the body are in a constant state of flux; if protein demand suddenly rises, muscle fibres can be broken down to provide the body with extra amino acids for the amino acid pool and then regenerated from recycled amino acids once this demand has subsided. This explains why many studies on protein intake and muscle growth/ recovery are conducted over weeks, not days.

Sampling muscle tissue for protein

Fortunately, however, a chemical imaging technique called radiolabelling has enabled scientists to probe the uptake of ingested protein amino acids into muscle cells. In simple terms, one of the amino acid building blocks of protein is ‘labelled’ by removing a normal hydrogen atom from the molecule and replacing it with radioactive hydrogen. This means you can see what happens to this molecule using scanners when a subject consumes a protein drink or food containing it. If you take a sample of muscle tissue and detect the presence of radioactive hydrogen, you know that the body has incorporated the amino acid into muscle tissues – ie that protein synthesis has taken place.

One of the first findings to arise from using this technique was that the presence of amino acids in the bloodstream and their availability to muscle cells is vital for protein synthesis after exercise. In a study on six untrained men, American scientists infused them intravenously with a mixture of pure amino acids and studied the protein dynamics both at rest then for three hours after a leg resistance exercise routine (5). They used an infusion rather than oral supplementation because they wanted to be certain that the muscles had an immediately increased supply of amino acids (ie without the time delay that digestion would introduce).

The amino acid infusion produced an increase in protein synthesis even at rest. However, after the resistance training there was a further substantial increase in muscle protein synthesis of 30-100%! In other words, amino acid supplementation not only enhanced protein balance and synthesis at rest but also led to an interactive post-exercise effect, which resulted in around a two-fold increase in protein synthesis after exercise.

Although an infusion of amino acids before training is extremely effective at enhancing the body’s ability to increase protein synthesis, it is not exactly practicable, so the obvious question to ask is whether amino acids taken orally after exercise can produce a similar effect?

A subsequent study was designed to answer this question (6). A group of healthy subjects performed a leg resistance routine and were then randomly fed one of three drinks:

  • Placebo (no amino acids);
  • An essential amino acid drink;
  • A mixed amino acid drink containing essential and non-essential amino acids (see box below).

Essential and non-essential amino acids

Just as words are constructed from letters of the alphabet, so all proteins are constructed from amino acid building blocks chemically linked together. Our diets typically contain around 20 of these amino acid building blocks in the foods we eat. Once the plant or animal proteins we eat have been digested to release the amino acids, our cells reassemble them to produce human proteins such as hair, skin, muscle etc.

Of the 20 amino acid building blocks, some are considered absolutely essential because they can’t be manufactured in the body from other molecules. These include:

  • Arginine; Histidine; Isoleucine; Leucine; Lysine
  • Methionine; Phenylalanine; Tyrosine; Tryptophan; Valine

The other amino acids are classed as non-essential because they can be synthesised in the body from fragments of the essential amino acids and carbon residues from glucose metabolism. More recently, scientists have identified a third category of amino acids, known as ‘conditionally essential’. Conditionally essential amino acids can be synthesised in the body when demand is low, but when demand rises (eg at times of metabolic stress) synthesis can’t keep up with demand and dietary sources then become vital. The amino acid glutamine is thought to fall into this category, being non-essential at rest but becoming essential at times of severe metabolic stress.

After training, the subjects rested for 45 minutes then began ingesting 4-5oz of drink every 15 minutes. Analysis of the results clearly showed that, while protein balance was negative when the placebo drink was consumed (ie muscle protein was being broken down overall), it became strongly positive when the amino acid drinks were consumed, and the increase in protein synthesis was almost as great as after infusion. The researchers also found that protein synthesis was not enhanced by the addition of non-essential amino acids.

‘So what?’, I hear you ask. ‘Surely everybody knows that protein is required after training?’ Well they probably do, but there’s a big difference between amino acids and protein; although amino acid solutions don’t reach the muscles instantly, they are absorbed very rapidly by comparison with protein. That’s because the process of digesting proteins (consisting of long chains of chemically linked amino acids) to release the constituent amino acid building blocks is quite time consuming – even for rapidly digested proteins like whey. A post-workout high-protein drink or meal could take several hours to produce maximum amino acid concentrations around muscle cells, yet we know it is the presence and availability of high levels of amino acids that seems to stimulate growth, especially after exercise.

Fast and slow proteins

Research suggests that the key to stimulating maximal protein synthesis in exercised muscles is to raise the level of circulating blood amino acids as rapidly as possible after exercise – or, even better, beforehand. Whichever strategy is employed, it is clear that proteins that digest and release their amino acid building blocks rapidly are best suited to raising blood amino acid levels quickly.

There are four commonly used proteins in sports drinks; whey, casein, egg and soy. Of these, whey is digested most rapidly, taking only about two hours to release its amino acids. Soy and egg release their amino acids at a gentler rate – around five hours – while casein is a slow-releasing protein, taking up to seven hours to release its amino acids. All these figures are approximate, as there is a large degree of individual variability in digestion rates.

In many of the studies referred to in this article, pure free-form (unbonded) amino acids were used. No digestion is required to release these amino acids, which means they can cross from the gut into the bloodstream within minutes rather than hours. Free-form amino acids can be purchased and mixed with fruit juice to produce an extremely fast-releasing drink. There are two major drawbacks, however: firstly, gram for gram, pure amino acids are very expensive by comparison with ordinary protein; secondly, they tend to taste like old socks, making any drink potentially unpalatable!

Other studies support the notion that exercised muscles need protein very rapidly. A 12-week study on elderly males on a progressive resistance exercise programme found that a post-training meal immediately after training produced bigger gains in muscle fibre thickness than when given two hours later (7). Some researchers have cautioned that this might be because the muscles of elderly people are ‘less sensitive’ to amino acids after exercise and that immediate post-exercise feeding of protein is of no benefit to younger people. However, a study conducted just two years ago throws doubt on this argument (8).

Scientists studied the effects of a fast-digesting protein (whey) and a slow-digesting protein (casein) in two groups of volunteers:

  • Nine elderly subjects (average age 72);
  • Six young subjects (average age 24).

Protein after exercise

They found that, irrespective of age, whey protein led to a faster rise in blood amino acids than casein, thereby producing a higher rate of muscle protein synthesis. While there was no exercise component in this study, these results mirror those of the infused amino acid study mentioned above (5), suggesting that consuming protein or amino acids as soon as possible after exercise is beneficial for muscle protein synthesis.

The conventional wisdom on recovery nutrition has tended to emphasise the importance of rapid carbohydrate replenishment, with little urgency about protein replenishment. However, the studies to date suggest that we neglect rapid protein replacement at our peril! In fact, it may be that, for optimum recovery and growth, protein replenishment should begin even before exercise!

This idea arose from a study on six healthy and active subjects (three men and three women), who participated in two exercise trials in random order. In one trial, they performed an intense 45-minute resistance training routine for the legs, then immediately consumed an amino acid drink containing 6g of essential amino acids (including radiolabelled phenylalanine) and 35g of carbohydrate (9). (The carbohydrate was added to generate an insulin response – something that is known to help drive amino acids into muscle cells (10).) Blood samples and muscle biopsies were then taken for two hours after training, and muscle protein synthesis measured. The other trial followed exactly the same protocol, but this time the amino acid/carbohydrate drink was consumed immediately before training.

Using essential amino acids before training

If you’ve never used free-form amino acids before training but would like to try, there are basically two options:

  • Capsules. These can be purchased over the counter at health stores. The problem is that often only single types of amino acids are available in capsule form, which means purchasing several different bottles then taking a capsule out of each to ensure you've consumed all the essential amino acids (see box above). This will be expensive as well as inconvenient!
  • Powder. A slightly cheaper and much more convenient option is to purchase a proprietary amino acid blend. These products generally come in a tub containing the premixed powder.

As far as dosage is concerned, there are no precise recommendations. However, for a full body workout, something in excess of the 6g used in the limited leg exercise studies described in this article would seem sensible – perhaps 10-15g. Free-form amino acids are best taken with fruit juice: this not only provides carbohydrate but also helps hide the unpleasant taste!

The results were surprising to say the least. In the two hours post-training, almost twice as much phenylalanine was taken up by the leg muscles and incorporated into muscle protein when the protein/carbohydrate drink was consumed before rather than after training. Even more surprisingly, while muscle protein synthesis increased dramatically and then declined after an hour when the drink was consumed after training, the boost in muscle protein synthesis was sustained for longer when the drink was consumed before training.

The researchers went on to conclude that consumption of a relatively small amount of amino acids (combined with carbohydrate) immediately before exercise is a very potent stimulator of muscle protein synthesis.

In summary, there is good evidence for carbohydrate feeding as soon as possible after training; not only does it facilitate the short-term (GLUT4) mechanism of glycogen synthesis, but it also allows for additional glycogen replenishment before the next training session (more often than not the following day). If you’re taking a break from training for a few days, however, immediate carbohydrate feeding may not be necessary.

More surprisingly, perhaps, the evidence also suggests that boosting blood levels of amino acids by consuming quick-releasing proteins (or free amino acids) as soon as possible after training is a good idea; indeed, if maximising muscle growth is your goal, it could be an even better idea to raise blood amino acid levels before training.

Andrew Hamilton BSc, MRSC, trained as a chemist and is now a consultant to the fitness industry and an experienced science writer

References

  1. Am J Clin Nutr 1981; 34:1831-1836
  2. Sports Med 2003; 33 (2): 117-144
  3. Am J Physiol 1989; 256:E494-9
  4. Am J Physiol Endocrinol Metab 2003; 285: sE729-36
  5. Am J Physiol 1997; 273(1 Pt 1):E122-129
  6. Am J Physiol Endocrinol Metab 1999; 276: E628-E634
  7. J Physiol 2001; 535(1):301-311
  8. J Physiol 2003; 549(2):635-644
  9. Am J Physiol Endocrinol Metab 2001; 281: E197-206
  10. J Appl Physiol 2004; 96:674-8

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