Reviewing What is Known of Dietary Protein Intake in Aging

While aging is complex, and so is diet, the recommendations are fairly straightforward: practice calorie restriction and you'll find that most of the other pieces of a sane diet fall into place by themselves, as it is very hard to assemble a calorie restricted diet out of anything other than healthy food. Not that you'd think there is a simple solution from reading around the subject. Diet is a topic in which there ten thousand people eagerly overcomplicate the situation, building mountains from molehills, confusing the issue, and generally making a mess of things. Ignore them all. Still, there is an interesting dichotomy that arises as a result of research on calorie restriction on the one hand and on protein intake and muscle mass in older individuals on the other, and that is the subject of the open access review paper I'll point out today.

The practice of calorie restriction is demonstrated to produce considerable benefits to health, even when undertaken in later life, and that includes benefits to muscle metabolism. The immediate effects of a lowered calorie intake appear to be triggered by protein restriction, and there is a fair amount of research that examines the role of lowered levels of the essential amino acid methionine as the primary trigger. On the other hand, older people do not process dietary protein as well as younger people, a change thought to contribute to the progression of sarcopenia, age-related loss of muscle mass and strength, and there is evidence to suggest that this can be offset to some degree by higher levels of protein. There is some work along these lines that investigates supplementation of the essential amino acid leucine as a possible preventative treatment, based on what is known of how processing of leucine changes with age.

Protein Consumption and the Elderly: What Is the Optimal Level of Intake?

One of the major threats to living independently is the loss of muscle mass, strength, and function that progressively occurs with aging, known as sarcopenia. A loss or reduction in skeletal muscle function often leads to increased morbidity and mortality either directly, or indirectly, via the development of secondary diseases such as cardiovascular disease, diabetes, and obesity.

Traditionally, protein recommendations have been based on studies that estimate the minimum protein intake necessary to maintain nitrogen balance. However, the problem with relying on these results is that they do not measure any physiological endpoints relevant to healthy aging, such as muscle function. In the case of daily protein intake, the estimated average requirement (EAR) for dietary protein is 0.66 g/kg/day and the Food and Nutrition Board recommends a recommended dietary allowance (RDA) of 0.8 g/kg/day for all adults over 18 years of age, including elderly adults over the age of 65. Experts in the field of protein and aging recommend a protein intake between 1.2 and 2.0 g/kg/day or higher for elderly adults. The RDA of 0.8 g/kg/day is well below these recommendations. It is estimated that 38% of adult men and 41% of adult women have dietary protein intakes below the RDA.

Most published results, based on data from either epidemiological or short-term studies, indicate a potential beneficial effect of increasing protein intake in elderly adults. These data demonstrate that elderly adults, compared with younger adults, are less responsive to low doses of amino acid intake. However, this lack of responsiveness in healthy older adults can usually be overcome with higher levels of essential amino acid (EAA) consumption. The mechanism by which dietary protein affects muscle is through the stimulation of muscle protein synthesis and/or suppression of protein breakdown by the absorbed amino acids consumed in the diet. There appears to be an EAA threshold when it comes to stimulating muscle protein synthesis. Ingestion of relatively small amounts of EAA (2.5, 5 or 10 g) appears to increase myofibrillar protein synthesis in a dose-dependent manner. However, a larger dose of EAA (20-40 g) fail to elicit an additional effect on protein synthesis in young and older subjects. Similar results were observed after the ingestion of either 113 or 340 g of lean beef containing 10 or 30 g EAA, respectively. Despite a threefold increase in EAA content, there was no further increase in protein synthesis in either young or older subjects following consumption of 340 g versus 113 g of protein.

The consumption of dietary protein consistent with the upper end of the recommendations (as much as 30%-35% of total caloric intake) may prove to be beneficial, although practical limitations may make this level of dietary protein intake difficult. The consumption of high-quality proteins that are easily digestible and contain a high proportion of EAAs lessens the urgency of consuming diets with an extremely high protein content.

So if you are going to try to optimize, bearing in mind that once past the simple and obvious items optimizing diet is largely a fool's game, does that mean more protein for older individuals or less protein throughout life? It is interesting that both approaches show benefits in various different animal and human studies, though the weight of evidence leans towards calorie restriction at the present time. I'd be inclined to think that the right approach to this question is to keep practicing calorie restriction, adjust the proportion of protein upwards over the years, and support work to find and address the causes of age-related changes in amino acid processing. The SENS vision would expect these changes to be somewhere downstream of the standard list of forms cell and tissue damage that cause aging, following a chain of epigenetic cause and effect, most of which is yet to be mapped.

Ultimately, no lifestyle plan can help you do any more than live just a little longer than you were going to anyway - your life span will still be somewhere in the expected human range. That isn't a big improvement in the grand scheme of things. If lifestyle is all you think about with regard to health, then a great opportunity has been missed. The real determinant of life expectancy and health in old age is progress in medical science, and specifically in the development of rejuvenation biotechnologies that can repair the damage that causes aging. That is the road to very large gains in healthy life span, far beyond those achievable by any available method today.


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