Updated: Feb 18
A common complaint from my more competitive patients, who find themselves at the less desirable side of 50, is that they have to train double as hard to perform at their previous levels. They usually dread taking even a few days off training as they feel that their fitness levels decline much faster than in their younger days. Why do older adults lose muscle strength despite training hard and can anything be done about it?
(If you like facts and details please continue to read. If, however, you would like a quick summary and succinct advice, skip to the end of the post)
A recent review by Dickinson et al. (2013) looked at the causes of muscle loss in older adults and offers some insight into the processes that may partly be to blame for this decline in performance. What follows is essentially a summary of their article with some of my own thoughts interspersed.
It is important to note that all of this research was conducted using older adults (above the age of 50), who are mobile but not following a regular exercise regime. How these findings relate to physically fit older adults, who train on a regular basis, is unknown. I was unable to find any research that specifically compared older adults who trained regularly with young adults who trained regularly. There are also other factors than muscle strength that could influence performance in older adults namely a decreased aerobic capacity.
It has long been known that aging is characterised by the gradual loss of skeletal muscle. I use the word ‘aging’ very loosely here since it has been shown that muscle mass may start decreasing as early as the ages of 25 to 30. Interestingly it appears that the contractile properties of muscles remains unchanged and that it is thus the gradual loss of muscle size that is to blame for the decreases in strength and function.
In their review, Dickinson and colleagues, focused on the role exercise and nutrition can play in reversing or slowing down the process of muscle loss.
Background: Skeletal muscle
The body continuously breaks down and rebuilds skeletal muscle. The process of rebuilding muscle is called muscle synthesis. Changes in muscle size occur when one of these two processes (breakdown vs. synthesis) dominates the other.
Nutrition and exercise have previously been shown to be strong stimuli for muscle synthesis. One of the main factors thought to contribute to muscle loss in older age is an impaired ability of the muscle tissue to respond to the stimuli that cause muscle protein synthesis.
Building new muscle: Effects of nutrition and aging
It has been established that the balance between skeletal muscle breakdown and synthesis (building new muscle) is strongly influenced by the levels of amino acids (the building blocks of protein) circulating in the blood. The exact science involved in the process is still unclear but results thus far suggest that there is an intricate relationship between amino acid availability, skeletal muscle synthesis and aging.
Ingesting small quantities (7-10g) of essential amino acids (protein) is capable of stimulating skeletal muscle synthesis in young individuals but not in older adults. However, ingesting large quantities (20-40g) of amino acids stimulates muscle protein synthesis to the same degree in both age groups.
The authors conclude that the data indicates that the ‘threshold’ at which amino acids stimulates muscle synthesis is increased with age and that it could be overcome by ingesting adequate quantities.
A specific type of amino acid has been identified to be the most important for muscle synthesis, namely leucine. Leucine has previously been shown to be a potent stimulator for muscle growth and in studies where they increased the leucine concentrate in the protein consumed by subjects, they found that young (28/30yrs) and old (66/70years) subjects showed the same level of muscle synthesis despite the small dose of protein (7g but more than 2g leucine) ingested.
Thus increasing the amount of leucine in a meal may promote muscle synthesis in older adults.
NB: As with all processes in the body there is not just one factor involved in the synthesis of skeletal muscle and this is demonstrated by the conflicting results reported in the literature. As an example of this, see our post regarding Vitamin D and Athletic Performance for the influence Vitamin D can have on muscle synthesis.
Pennings et al. (2010) stand in contrast with the above findings in that they found no difference in muscle synthesis in response to 20g amino acid ingestion between age groups. This may be due to cultural differences(1). (Could the subjects in the Netherlands' study have a different lifestyle which impacts on their ability to maintain muscle mass in later life?)
Verhoeven et al. (2009) does not seem to agree with the above recommendations either. They found no increases in muscle strength or mass in healthy elderly men after taking a 2.5g leucine supplement for 3 months.
Possible differences in study design that could have contributed to these results are: The subjects may have already been taking above threshold levels of protein in their normal meals and therefore showed no further increase in muscle synthesis. Also, in contrast with the subjects in the previous studies, the subjects in this Verhoeven’s study consumed mixed meals which may have influenced how their muscles responded to the leucine(see section on insulin).
A recent randomised control trial conducted over 3 weeks looked at the effect of 20g protein supplement on muscle mass post total knee replacement surgery. They found that the control group showed attenuated muscle atrophy in all the muscles of the operated as well as un-operated leg.
Dickinson and colleagues warn that it is important to remember that exercise has been shown to play an important role in muscle synthesis and that physical activity or base level fitness may be an important reason for differences in results. I fully agree with this statement since the subjects in Dreyer’s study were asked to administer their supplements an hour after physical therapy, while Verhoeven and colleagues did not report on the activity levels of their subjects. See the section on exercise below for an explanation of why the timing of food intake may be important.
Role of insulin:
There is evidence that the physiological response elicited by ingesting a mixed meal (carbohydrates and 40g protein) interferes with the ability of amino acids to elicit a protein synthesis response in older adults.
This puzzled researches since a meal containing carbohydrates causes an increase in circulating insulin and insulin is known to stimulate muscle growth. Further investigation suggested that this effect may be due to the inability of insulin to stimulate an increase in muscle blood flow in older adults, since artificially increasing the blood flow in the muscles of older adults caused the difference in muscle synthesis between age groups to disappear.
These results are supported by those of Fujita et al. (2007) who found that a bout of aerobic exercise performed before eating a mixed meal could restore the normal muscle protein synthesis reaction to insulin.
Building new muscle: Effects of exercise and aging
Protein synthesis in older adults has an impaired response to resistance exercise. It can increase muscle size and strength in older individuals but just not to the same level as in younger individuals.
There is evidence that more protein is incorporated into muscle if exercise is performed before the meal. Drummond et al. (2008) also found a similar response to ingesting 20g of essential amino acids after a bout of resistance exercise in young and old.
There is not a lot of research available that look at the ability of aerobic exercise to stimulate skeletal muscle synthesis in older individuals. Traditionally it has been thought to rather bring about metabolic changes in muscles than increase muscle size.
Recently it has been shown that a 12 week cycling programme was effective in increasing muscle mass in young and old individuals. The researchers think that the secret may lie in aerobic exercise’s ability to overcome age related insulin resistance. It is once again important to note that we are talking about untrained individuals, so it may have been a case of these individuals showed improvement because of their low baseline muscle mass.
Summary: Why do older adults lose muscle strength
Our bodies build new muscle when it receives a message telling it to do so. The two main events that sends this message to the body is when you eat protein and when you exercise a muscle.
Researches have found that one of the reasons that older adults' bodies does not build new muscles as readily is because it does not respond in the same way to the food they eat as when they were young.
These guidelines can help you optimise your muscle strength in older age:
Ingesting 20 to 30 grams of quality protein or 2g of leucine per meal is recommended to maintain muscle mass in older age.
Ingesting more than this will likely not have any beneficial effect and excessive leucine intake may have a very negative effect. A table containing natural sources of leucine can be found on Wikipedia.
The timing as well as the content of protein rich meals may be important, but the relevance of this is not yet fully understood for the older athletic population.
Protein ingestion combined with resistance exercise could overcome age related impairments in muscle growth.
Ingesting a mixed meal (carbohydrates and protein) may impair the ability of amino acids to stimulate muscle growth in older adults due to insulin resistance.
Insulin resistance in older adults can be overcome by a bout of moderate aerobic exercise before a meal.
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About the Author
Dreyer, H. C., Strycker, L. A., Senesac, H. A., Hocker, A. D., Smolkowski, K., Shah, S. N., et al. (2013). Essential amino acid supplementation in patients following total knee arthroplasty. Journal of Clinical Investigation, 123(11), 4654-4666.
Drummond, M. J., Dreyer, H. C., Pennings, B., Fry, C. S., Dhanani, S., Dillon, E. L., et al. (2008). Skeletal muscle protein anabolic response to resistance exercise and essential amino acids is delayed with aging. Journal of Applied Physiology, 104(5), 1452-1461.
Fry, C.S., Drummond, M.J., Glynn, E.L., Dickinson, J.M., Gundermann, D.M., Timmerman, K.L., et al. (2011). Aging impairs contraction-induced human skeletal muscle mTORC1 signaling and protein synthesis. Skeletal Muscle, 1, 11.
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Pennings, B., Koopman, R., Beelen, M., Senden, J. M., Saris, W. H., & van Loon, L. J. (2010). Exercising before protein intake allows for greater use of dietary protein–derived amino acids for de novo muscle protein synthesis in both young and elderly men. The American Journal of Clinical Nutrition, 93(2):322-31.
Verhoeven, S., Vanschoonbeek, K., Verdijk, L. B., Koopman, R., Wodzig, W. K., Dendale, P., et al. (2009). Long-term leucine supplementation does not increase muscle mass or strength in healthy elderly men. The American Journal of Clinical Nutrition, 89(5), 1468-1475.
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