Protein dynamics

I found this to be very interesting.

Understanding how protein dynamics can aid muscle and strength development


Protein metabolism is a dynamic process, with a constant flux between degradation (breakdown) and synthesis. Virtually all the body's tissues are broken down and replaced at some point during our lives. Both tissue and dietary proteins pass through what is termed the amino acid pool, but only a small percentage are held there at any point in time. The fate of amino acids entering the pool depends on the immediate needs of the body's systems. Amino acids may be reused in the following ways depending on individual nutritional status:
a. to form new proteins for body tissues
b. broken down for use as an energy source
c. to synthesise glucose via a process called gluconeogenesis, which uses non-carbohydrate sources
d. to synthesise ketones and fatty acids.

In normal, healthy stable-weight individuals, protein breakdown is outstripped by synthesis to maintain a positive protein balance. This balance is important because it dictates the rate of growth of muscle and other tissues. Various factors can affect this crucial protein balance, including dietary intake, nutritional status, age, gender and exercise habits(1). This article focuses on the role of nutrition and exercise in protein dynamics.

Lack of carbohydrates impacts on tissue growth in training

There are many aspects of diet and nutrition which impact on the body's exercise capacity, but in my view the most important of these is energy intake. Adequate energy intake is crucial, whatever the training or exercise goal. This is because intake in the form of carbohydrate has a major influence on protein metabolism and net protein balance during exercise training. Exercising muscle is able to use amino acids as fuel. If carbohydrate intake is insufficient to fuel exercise, then all-important muscle and liver glycogen reserves will be depleted. This, in turn, leads to increased utilisation of protein as an energy source, making it unavailable for tissue growth.

The number of people choosing to follow diets free from animal products is on the increase, but food choices made by athletes following such diets may impact on protein quality. The quality of dietary protein is dependent on both its digestibility and its composition of essential amino acids(2).'Essential' in this context means that the body lacks ability to synthesise that particular compound and it must therefore be found in food. There are nine such amino acids, namely:
isolecine
lysine
methionine
phenylalanine
threonine
tryptophan
valine
leucine
histidine (essential only in infants).

The value of a particular source of dietary protein, as far as the synthesis of new tissue is concerned, depends on the proportion or number of essential amino acids it contains. Foods that are said to have a high protein quality (ie those with high quantities of essential amino acids) are mostly animal-based, while vegetable proteins may lack one or more of the essential amino acids.

In an ideal world a complete or high-quality protein would provide all the essential amino acids in equal quantities and in such proportions that they meet all the body's requirements at that point. Unfortunately, this is not the case: the fact that some amino acids are present in foods in smaller amounts than others means that they will be used up faster. And once one or more of these amino acids is no longer present in a particular source of dietary protein, its use in the synthesis of new tissue is limited. It is the'limiting amino acid(s)' present in dietary protein that ultimately determine their use in protein synthesis.

Common dietary sources of protein, in order of protein quality, are as follows:
eggs
fish
beef
milk
brown rice
white rice
peanuts
potato

The problem for vegetarian athletes is that plant proteins are less complete in their essential amino acid composition than proteins from animal sources, making them less useful in the synthesis of new tissue(3). For this reason athletes following diets free from animal products need to combine different plant protein sources, such as beans, grains, pulses and vegetables in their diet to make sure it includes sufficient essential amino acids to meet their needs. Beans on toast is a classic combination, and other useful combinations include rice and peas, baked potato and beans and stir-fry vegetables with noodles.

There is a general consensus that athletes and sportsmen and women have higher protein requirements than their less active counterparts. Dietary protein or amino acid supplements stimulate protein synthesis after exercise because of increased transport of amino acids into the muscle. This process is generally mediated by the hormone insulin, but other anabolic compounds, including growth hormone, also play a role in protein synthesis. The secretion of these compounds is raised as a result of increased concentrations of certain amino acids. However, protein or amino acid intake seems to influence protein synthesis only up to a certain threshold; once this is reached, any further increase has no effect on the rate of protein synthesis, probably because of the counterregulatory role played by other hormones(4).

Current data suggests that daily protein intake should be in the region of 1.2-1.4g/kg body weight for endurance athletes and 1.2-1.7g/kg body weight for strength and speed specialists.

As a general rule, a well-balanced diet containing appropriate levels of the various nutrients will result in increased protein synthesis and consequent muscle growth. However, the timing of nutrient or supplement intake may be a significant factor in creating a favourable environment for this activity and so optimising muscle growth. Current research suggests that athletes wishing to increase muscle mass and strength might be advised to ingest high-quality protein and carbohydrate immediately before exercise in order to increase amino acid availability when uptake by the exercising muscles is elevated.

The impact of exercise

Exercise training has a major impact on protein metabolism, but this varies according to the type, duration and intensity of the workout and - often - the training status of the individual.

There is a huge variation in sporting and exercise pursuits, ranging from brief explosive activities like sprinting or weight lifting to long, low-intensity activities like marathon running. Where protein usage is concerned, the immediate response to exercise - protein breakdown followed by increased rates of protein synthesis - is similar across the whole spectrum of activities. However, the end result in terms of physiological adaptation shows distinct differences at each end of the spectrum.

The intensity of training plays a key role in determining levels of tissue growth. Naturally this intensity needs to be sufficient to challenge (overload) the body's systems, but there is a desirable middle ground between exercise which is not intense enough to cause overload and exercise which is so intense that it inhibits the metabolic responses responsible for synthesising new body tissues.

Increased rates of protein synthesis may be maintained for up to 48 hours after exercise. Thus the timing of recovery periods is crucial for stimulating optimum muscle and strength development. Inadequate recovery between exercise bouts is likely to lead to a situation where the rate of protein breakdown - the first response to exercise - exceeds that of the subsequent protein synthesis, resulting in reduced gains or even loss of muscle mass.

In summary, the protein in a meal becomes available for use by the body only after it has been broken down into its constituent amino acids. The quality of any dietary protein is determined by its amino acids - and particularly by the presence of the nine amino acids which are not synthesised by the body and are therefore'essential'. Once digested, amino acids enter the body's amino acid pool and are distributed around the body according to requirements.

Physical activity in the form of resistance training and, to a lesser extent, endurance exercise, increases the rate of both protein breakdown and synthesis. When combined with appropriately timed nutrient intake in the form of food or supplements, physical exercise has a major impact on growth of muscle tissue. Providing certain nutrients are included in the intake, net protein synthesis will occur and, with regular resistance or endurance training, an increase in muscle and other tissues will be the result.

Ian Carlton

References

1. Foods Nutrition and Sports Performance, William C & Devlin J, London, pp65-80, 1992

2. Advanced Nutrition and Human Metabolism, Goff J, Gropper S & Hunt S, USA, pp154-196, 1990

3. Exercise physiology: Energy, Nutrition and Human Performance, McArdle W, Katch F& Katch L, Philadelphia, pp 29-30, 1991

4. International Journal of Sports Nutrition and Exercise Metabolism vol 11(1), pp87-145, 2001