Is the Protein RDA Too Low?

 
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For many years there have been suggestions from a growing number of researchers and clinicians that a protein intake greater than the current recommended dietary allowance (RDA of 0.8g protein/Kg body weight) would help maintain lean body mass (LBM) and help prevent the loss of strength and function in older adults that leads to sarcopenia. In recent years a growing number of physicians have been advocating the use of exogenous testosterone even in older men whose levels are within the normal range to help prevent or reverse sarcopenia.

Study Examines Impact of Extra Protein With and Without Testosterone

To determine whether older men who already were experiencing some physical function limitations and a usual protein intake close to the current RDA level (mean 0.83g/kg body weight) might benefit from an increasing their dietary protein intake well above their current below average intake level, researchers in Boston carried out a 6-month study. In this study half the men were randomized to follow two isoenergetic diets with either 0.8 to 1.3 g of protein per kg of body weight. Half of the men were also randomized to a weekly injection of either testosterone (100mg testosterone enanthate) or a placebo.  Researchers looked for changes in lean body mass (LBM), muscle performance, physical function, fatigue, and well-being over 6 months in these 4 groups of older men. To assure compliance, subjects received pre-specified energy and protein content diets provided through custom-prepared meals and supplements. The primary outcome was change in LBM. Secondary outcomes were changes in muscle strength, power, physical function, health-related quality of life, fatigue, balance, and well-being.

The 92 men (mean [SD] age, 73.0 [5.8] years) in the 4 study groups did not differ in their baseline characteristics. Over the next 6 months, the researchers found no significant changes from baseline in LBM, muscle strength and power, walking speed and stair-climbing power, health-related quality of life, fatigue, and well-being between the men assigned to 0.8 vs 1.3 g/kg/d of protein regardless of whether they received testosterone or placebo. Researchers concluded that protein intake exceeding the RDA did not increase LBM, muscle performance, physical function, or well-being measures, nor did it even augment anabolic response to exogenous testosterone in older men who initially had physical function limitations and whose usual protein intakes were initially close to the current RDA level. The RDA for protein is sufficient to maintain LBM, and protein intake exceeding the RDA does not promote LBM accretion or augment anabolic response to testosterone.

A two-year randomized double-blind placebo-controlled clinical trial examined the effect of extra protein in a group of 196 older women (age 70 to 80). Half the women were randomly assigned to consume a high-protein shake daily that supplied an extra 30g of whey protein in addition to their normal diet. A second group received a similar shake in which carbohydrate calories replaced nearly all the protein calories. Compared to the subjects’ baseline diets, the group consuming the whey protein shakes ended up consuming about 20g more protein per day than those consuming the largely carbohydrate-based shake (which contained only 2g of protein). This difference in protein was confirmed by measurement of urinary nitrogen excretion. Measurement of muscle mass, body weight, and measure of physical function were taken after one and two years. While muscle mass and strength declined in both groups, the results of the study showed that those consuming the high-protein shake experienced no slower loss of muscle mass or strength than those consuming the low-protein shakes.

It has been suggested that, to reap the full benefits, extra protein must be consumed just before or shortly after resistance exercise training to stimulate the growth of more muscle tissue or to increase gains in strength. The effectiveness of providing a 20g protein supplement or a placebo supplement without protein to a group of 26 healthy older men (mean age = 72) who participated in a 12-week resistance training program showed that both groups increased their strength by about 30%. However, the increase in strength was no greater in those who consumed the extra supplemental protein either before or just after their exercise bouts 3 times per week. The researchers concluded that timed protein supplements did not increase muscle size or strength any more than the placebo shake. Clearly resistance training increases muscle mass and strength in older people, but increasing protein intake well above the RDA has no significant benefit at enhancing the gains in muscle mass and/or strength.

If It’s Not Low Protein Levels, Then What Causes Sarcopenia?

Researchers may have worked out why there is a natural loss of muscle in the legs as people age, and that it is due to a loss of nerves rather than inadequate protein intake. It has long been known that as people get older, their leg muscles tend to become smaller and weaker, leading to problems with everyday movements such as walking up stairs or getting out of a chair. This is called sarcopenia. Sarcopenia is something that affects everyone eventually, but why it happens is not well understood. Professor Jamie McPhee, from Manchester Metropolitan University, said that young adults usually had 60-70,000 nerves controlling movement in the legs from the lumbar spine. However, these nerves declined in old age with a typical loss of 30-60%. Aging appears to result in nerve fibers wasting away. 

In tests on 168 men, researchers found that nerves controlling the legs decreased by around 30% by the age of 75. This made muscles waste away. However, they that there was a better chance of being 'rescued' in older, fitter athletes, so resistance training may help nerves reconnect with muscle fibers and help slow the development of sarcopenia.

Muscles must receive nerve signals to contract from the nervous system. This is obvious to anyone who does not move their arm or leg that has been in a cast for several months. The loss of muscle mass and strength, even in young healthy people, can be quite dramatic. The good news is that with regular exercise, healthy muscles can regain some mass and strength as the surviving nerves can send out new branches that can help prevent muscles from wasting away. It has been shown that resistance training is an effective treatment strategy for enhancing the growth and function of muscles in both young and older subjects.

Bottom Line: Sarcopenia is largely the result of aging and inactivity. There is no credible evidence that a higher dietary protein intake than the current RDA helps prevent or treat sarcopenia. Those concerned about preventing or treating sarcopenia would be better advised to do regular resistance training rather than trying to increase their intake of protein-rich foods or protein supplements. It seems more than likely that increasing dietary protein above the current RDA level will likely do nothing to prevent sarcopenia. However, higher protein intake well in excess of the current RDA may very well result in metabolic changes known or suspected to accelerate aging and/or disease. More on this next month.

By James J. Kenney, PhD, FACN

References: 

  1. Bhasin S. et. al. Effect of Protein Intake on Lean Body Mass in Functionally Limited Older Men. JAMA Intern Med. doi:10.1001/jamainternmed.2018.0008. 
  2. Zhu K, Kerr DA, Meng X, et. al. Two-year whey protein supplementation did not enhance muscle mass and physical function in well nourished healthy older postmenopausal women. J Nutr 2015;145:2520-6.
  3. Verdijk LB, Jonkers RAM, Gleeson BJ, et. al. Protein supplementation before and after exercise does not further augment skeletal muscle hypertrophy after resistance training in elderly men. Am J Clin Nutr 2009;89:608-16. 
  4. Kooperman R. van Loon L. Aging, exercise, and muscle protein metabolism. J Appl Physiol 2009l106:2040-8.
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