Summary of a review by Trommelen et al.
Muscle mass in adults is reasonably stable with muscle tissue constantly being built up and broken down. Normally this breakdown and build-up are balanced, therefore the muscle mass remains stable. One of the purposes of this continual breakdown and build-up of muscle tissue is to replace damaged muscle proteins or to adapt the composition of the muscle tissue to training. If the breakdown and build-up are not in balance, this results in an increase in muscle mass (build-up > breakdown) or a decrease in muscle mass (build-up < breakdown).
- The intake of 20 grams of rapidly digestible protein isolate results in almost maximal muscle protein synthesis, both at rest and after training. Protein synthesis is another 10-20% higher when twice that amount, or 40 grams, is consumed.
- The intake of 40 grams or more of slowly digestible protein is recommended to promote protein synthesis when the period between meals is somewhat longer, for example during periods of 6 or more hours of sleep.
- Dietary recommendations for the stimulation of muscle protein synthesis should be adapted to the individual athlete taking into account age, sex, body composition and training—type, intensity and duration of training.
Training stimulates muscle protein synthesis (= build-up) and, to a lesser extent, muscle tissue breakdown. However, the net balance between muscle protein synthesis and breakdown will remain negative as long as no food is ingested. The intake of protein stimulates muscle protein synthesis and inhibits muscle tissue breakdown. Therefore, consuming protein after training leads to a positive net balance, which can, in the long term, lead to an increase in muscle mass and muscle strength. Various factors, such as the quantity of protein, the type of protein and the timing of protein ingestion, play a role in this. In addition, other factors also influence these processes, for example, general eating habits and activity levels, body composition, age and sex.
A recent review by Trommelen et al. (2019) provides an overview of the various factors which influence muscle protein synthesis after strength training. These factors can be divided into three categories:
- Direct effects of diet on muscle protein synthesis
- Eating habits in the long term
- Other factors (not related to nutrition)
1. Direct effects of diet on muscle protein synthesis
- Protein: quantity and type of protein and timing of ingestion
Various studies have shown that the ingestion of 20 grams of rapidly digestible protein isolate results in almost maximal stimulation of muscle protein synthesis, both at rest and after training. Muscle protein synthesis rates increase further by another 10 to 20% when 40 grams of protein are consumed. These studies measured this parameter during a period of 4-5 hours following training and used quickly digestible protein isolates (e.g. in the form of a supplement). Because of this, it is still not clear whether these findings can be directly translated into recommendations regarding the quantity of protein to be consumed per meal. The average meal, generally speaking, consists of more slowly digestible proteins in combination with other nutrients. This may have an impact on muscle protein synthesis.
Vegetable protein is normally less effective than animal protein at stimulating muscle protein synthesis. This is probably due to the fact that vegetable proteins contain fewer essential amino acids, lower quantities of the essential amino acid leucine and are less easily digestible. This may be compensated for by ingesting greater quantities of vegetable protein (> 20 grams) and/or by combining vegetable protein from various food sources, which results in the ingestion of a greater variety of amino acids.
The speed with which protein is digested and absorbed differs significantly depending on the type of protein. Whey protein, for example, is quickly digested and absorbed, while casein protein digests more slowly. When comparing their effects on muscle protein synthesis, whey protein is better at stimulating this synthesis than casein protein when measured over a period of 6 hours following training. This is due not only to the fact that whey protein is absorbed relatively quickly but also due to its higher concentration of the essential amino acid leucine. When the time between meals is longer, for example after 6 hours or more of sleep, consuming 40 grams or more of slowly digestible protein (e.g. casein) is recommended in order to promote muscle protein synthesis throughout this entire period.
- Protein as part of a meal
Up until now, most studies have looked at the effect of the intake of protein isolates on muscle protein synthesis instead of looking at the effect of eating a complete meal consisting of proteins, carbohydrates, fats and micronutrients.
Carbohydrates slow down the digestion and absorption of protein. Nonetheless, it appears that carbohydrates do not reduce muscle protein synthesis at rest or after strength training. Research in this area has, up until now, been limited to quickly digestible carbohydrates with a high glycemic index (e.g. carbohydrate-rich drinks), while carbohydrates in a meal are, generally speaking, complex, more slowly digestible and often contain fibre. Consequently these carbohydrates are expected to slow down the digestion and absorption of proteins even more.
Very little research has yet been done into the possible effects of fat on muscle protein synthesis after the ingestion of protein. A number of studies seem to show that the intake of fat does not have any influence on the digestion and absorption of protein. Yet, there are some indications that fat may inhibit muscle protein synthesis, although more research is needed to confirm these findings.
In addition to carbohydrates and fat, recent studies have shown that the intake of micronutrients may have an effect on muscle protein synthesis. Micronutrients, such as vitamin A, vitamin D, vitamin E, zinc, selenium and cholesterol, may stimulate muscle protein synthesis. Moreover, a recent study has shown that the intake of amylopectin and chromium improves muscle protein synthesis when suboptimal quantities of protein are consumed. However, supplementation with certain individual nutrients is, in fact, not recommended for athletes. For example, high doses of antioxidants (e.g., vitamins C and E) can actually inhibit the adaptation to training.
Athletes may consume relatively high quantities of alcohol. Thus far, the effect of alcohol on muscle protein synthesis has been investigated in just one study. This study showed that muscle protein synthesis is inhibited when excessive quantities of alcohol (+/- 12 standard units) are consumed (after a training session in combination with protein intake). The effects of less alcohol, for example, one or two standard units, is not yet known.
2. Long-term effects of diet on muscle protein synthesis
Muscle protein synthesis is not only influenced by the consumption of food immediately after training but also by eating habits in the longer term, such as the ingestion of energy, protein and omega-3 fatty acids.
Athletes sometimes eat more than their energy requirements in order to build muscle mass, but the effect of this on muscle protein synthesis is still unclear. However, it is more common for athletes to eat less than their energy requirements in order to reduce fat mass. An energy deficit usually leads to a reduction in muscle mass and muscle protein synthesis. A reduction in muscle mass due to an energy deficit can be prevented by combining a high-protein diet with strength training.
Several studies have shown that supplementation with omega-3 fatty acids for a period of 8 weeks stimulates muscle protein synthesis. However, it is not yet clear how much omega-3 should be taken or for how long, and more research is needed to confirm these findings.
3. Other factors
- Activity and training
Training is a strong stimulus for muscle protein synthesis. As described above, muscle protein synthesis is maximally stimulated by ingesting 20 grams of rapidly digestible protein at rest and after training. Training in combination with the intake of protein not only increases muscle protein synthesis itself but also extends the period during which muscle protein synthesis is increased.
Muscle protein synthesis is also influenced by regular activity habits. Various studies have shown that when muscles are not used (e.g. during long-term bed rest), muscle protein synthesis is reduced. Unfortunately, it does not appear that this can be adequately compensated for by ingesting more protein.
Consequently, reduced activity levels and training schedules in athletes with injuries can contribute to a reduction in muscle mass. In those cases, the athletes consume less energy and will probably ingest fewer calories to prevent increases in fat mass. This can lead to a lower total protein intake. Although a higher protein intake probably cannot completely compensate for the loss of muscle mass due to reduced activity/training levels, it is advisable to maintain the protein intake at habitual levels throughout the injury period.
Sarcopenia, the loss of muscle mass, often occurs in conjunction with ageing. In sarcopenia, muscle protein synthesis after the consumption of food is reduced. What is interesting is that in spite of this reduced response, it appears that this may be compensated for (at least partially) by ingesting extra protein.
In young adults, a plateau is seen in muscle protein synthesis at the level of 20 grams of whey protein ingested at rest or after training. In older adults, however, no such plateau is observed upon the ingestion of 40 grams of protein at rest or after training. It is possible that even more than 40 grams of protein is needed to maximally stimulate muscle protein synthesis in older adults. And it is still not clear whether 40 grams of protein is required or whether a lower dose, such as 30 grams, would also be sufficient. This hypothesis is supported by one study which found that 30 grams of protein is optimal. Older athletes should, therefore, try to consume at least 30 grams of protein per meal for recovery after training.
Men have greater muscle mass and less body fat than women. Nevertheless, no differences are seen in the rates of muscle protein synthesis between young adult men and women at rest or after training. One possible explanation for this is that the greater muscle mass in men is a result of a greater growth spurt during puberty due to the increase in testosterone. After puberty, the muscle mass remains fairly constant until middle age in both men and women. That is the reason why dietary recommendations for the stimulation of muscle protein synthesis are the same for young adult men and women.
Muscle protein synthesis in older adults, on the other hand, does differ between men and women. It has been observed that women in the post-absorptive state (= fasting) have a higher rate of muscle protein synthesis in comparison with men, however there is reduced stimulation of muscle protein synthesis after the intake of food at rest. Higher quantities of protein in a meal may (partially) compensate for this.
- Body size
Protein recommendations are often expressed in terms of kilograms of body weight. Up until now, only one study has investigated the effect of body size on muscle protein synthesis. This study found no difference in muscle protein synthesis with the ingestion of protein after training between persons with high or low muscle masses. This suggests that the amount of muscle mass does not have a significant influence on the protein requirements during recovery from training. One possible explanation for this could be that only a relatively small quantity of essential amino acids is needed to stimulate muscle protein synthesis, even in individuals with greater muscle mass.
- Breakdown of muscle protein
In addition to muscle protein synthesis, the muscle mass is also influenced by the breakdown of muscle protein.
Training and diet, however, appear to have a much greater impact on muscle protein synthesis than on the breakdown of muscle protein. Therefore, muscle mass is largely determined by the degree of muscle protein synthesis, whilst the breakdown of muscle protein plays a much smaller role.