The effects of aging are evident for everyone to see...decreased skin elasticity, decreased mobility and flexibility, decreased recovery ability and the list goes on. But a fascinating series of recent studies has shown that the process of aging also results in changes to the way dietary protein intake stimulates muscle protein synthesis both in the rested state and in response to resistance exercise. If you are an individual over the age of 40, its vital that you are aware of this information, as it will help maximise the anabolic response to your training.
Aging and Muscle Loss
In the medical literature, sarcopenia is defined as "the age-associated loss of skeletal muscle mass, which results in decreased strength and aerobic capacity and thus functional capacity”1. Another way of putting it is that sarcopenia occurs as a result of a gradual net loss of skeletal muscle protein due to an imbalance between the synthesis and breakdown of skeletal muscle proteins2. Some studies have showed a reduced muscle mass of up to 50% between the ages of 20 and 80 years3. More specifically though, after 50 years-of-age, approximately 1–2% of muscle mass is expected to be lost per year3, while muscle strength decreases at an even greater rate4.
Anabolic Resistance in Elderly
Recent studies have proven that the response of muscle protein synthesis (MPS) to anabolic stimuli, such as resistance exercise5 and protein ingestion6-10, appears blunted in older adults compared with their younger counterparts. This apparent failure of older muscle to mount a robust ‘youthful’ response to protein ingestion and exercise has been coined ‘anabolic resistance’11.
Having established this phenomenon in aging skeletal muscle, researchers have been preocupied with trying to establish the exact mechanisms governing this observed 'anabolic resistance'. What they have found is pretty fascinating!
Leucine Threshold in Elderly
One of the key findings relates to an apparent leucine 'threshold' that must be passed for protein ingestion to stimulate MPS. As an example, one of the earlier studies compared the effect of a mix of essential amino acids (with a amino acid profile similar to whey protein) to a similar essential amino acid mix with a higher percentage content of leucine (i.e. 26% vs 41%) on MPS in young and elderly subjects. Only the essential amino acid mix with the higher percentage content of leucine resulted in a stimulation in MPS in the elderly compared to the essential amino acid mix with normal leucine content. However, in the young individuals, both essential amino acid mixutres resulted in similar stimulation of MPS12.
Another similar study conducted in the same year compared the effects of a normal diet versus the same diet supplemented with leucine on MPS in a group of elderly males. When subjects were feed the leucine supplemented meal, they had a signifcantly higher rate of MPS, which was supported by higer plasma values of leucine13.
Yet another study compared the effect of whey protein to casein and casein hydrolysate in the rested state. The study involved a group of 48 elderly men with an average age of 74. Consistent with previous studies; whey protein provided the strongest stimulus to MPS, followed by casein hydrolysate and casein14. As further confirmation of the leucine 'threshold' paradigm, researchers found a strong positive correlation between peak plasma leucine concentrations and MPS14.
Finally, another study compared the effects of three different doses of whey protein (i.e. 10g, 20g or 35g) on MPS in the rested state in 33 healthy, older men with an average age of 7317. Interestingly, 35g of whey protein provided a stronger stimulus to MPS than 20g17. This is in contrast with the above studies, which suggest 20g of whey protein is the maximal amount needed to optimise MPS. However, the measures of MPS used in this study were different from the above studies. Additionally, no specific details are given about the composition of the particular whey protein, namely, whether it was concentrate, isolate or hydrolysate. Nonetheless, the findings of this study still support the notion that the elderly require higher protein intakes to maximally stimulate MPS than there younger counterparts.
Protein Intake Post Workout and Muscle Protein Synthesis
All the studies cited above examined the anabolic response to protein intake in the elderly in the rested state. The other question for researchers is how MPS response is affected after resistance exercise in elderly with different levels of protein intake. The most informative study examing this issue involved 37 elderly men with an average age of 71 split into one of four groups. Each group received either 0, 10g, 20g or 40g of whey protein isolate (WPI) after performing single leg extension resistance exercises. Each group then had their MPS monitored in both the exercised and unexercised leg. This allowed researchers to determine the additional stimulus to MPS provided by resistance exercise. When examining MPS in the unexercised leg for each group, only those receiving 20g and 40g of WPI had increases in MPS above baseline levels. However, when it came to examine MPS in each group following resistance exercise every group showed an increase in MPS above baseline as expected. The groups receiving 20g and 40g of WPI post exercise exhibited greater increases in MPS than the groups receiving 0g and 10g of WPI. The main finding however, was that the group receiving 40g of WPI post exercise had greater MPS than the group receiving 20g of WPI post exercise. This is in contrast to indentical studies in young individuals, which show that 20g of protein (albeit egg protein) post exercise did not provide any greater increase in MPS than 40g15. As a result, the authors concluded: “in contrast to younger adults, in whom post-exercise rates of MPS are saturated with 20 g of protein, exercised muscles of older adults respond to higher protein doses2.”
In another study with a similar design, researchers compared the effects of micellar casein and WPI on MPS both in the rested state and post exercise in a group of elderly men with an average age of 72. Similar to the above study, the researchers used a unilateral model of leg resistance exercises to measure the independent effects of protein intake in the rested versus post exercise state16. WPI was found to stimulate MPS 65% more than micellar casein in the rested leg(s). Similarly, when comparing post exercise values, MPS was 69% greater in the WPI group versus micellar casein16. The superior effect of WPI on MPS coincided with higher blood levels of essential amino acids and leucine compared to micellar casein16.
In concludsion, the scientific evidence concerning 'anabolic resistance' in the elderly is very strong. Muscles of the elderly are resistant to normally robust anabolic stimuli such as protein/amino acids and resistance exercise. More specifically, ageing muscle is less sensitive to lower doses of protein/amino acids than the young and requires higher quantities of protein to acutely stimulate equivalent muscle protein synthesis.
In terms of the practical applications of these findings, its important that elderly individuals make an effort to consume adequate protein (i.e. at least 20g) at each meal so as to maximise the anabolic stimulus for MPS18. The other crucial factor is the type of protein. The studies highlighted in this article are unanimous that whey protein is the best supplement for the elderly. Therefore its important that elderly individuals consider regular use of a whey protein supplement if they want to minimise their muscle lose with increasing age.
Lastly, resistance exercise combined with protein/amino acid ingestion provides the greatest anabolic response in elderly and is the best known way of assisting the elderly in producing a ‘youthful’ MPS response provided sufficient protein is ingested following exercise18.
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