Studies have consistently shown that dietary restriction (also called calorie restriction) reduces oxidative damage to mitochondrial DNA and increases maximum longevity. Only protein restriction is responsible for the decrease in oxidative damage; the restriction of carbohydrates or lipids does not reduce oxidative stress or increase maximum longevity. Some studies have looked at the amino acid components of protein and have found that reduced intake of the amino acid methionine plays a major role in the decrease in mitochondrial damage and increase in longevity.
Some researchers conclude that the intake of proteins (and thus methionine) of Western human populations is much higher than needed, and that decreasing the levels could reduce tissue oxidative stress and increase healthy life span in humans. While this recommendation seems to make sense theoretically, it also is in direct conflict with the statistical findings of nutritional surveys.
The recommended dietary allowance (RDA) for protein, established by the Food and Nutrition Board of the United States National Academy of Science, is 0.8 g protein/kg body weight/day for adults, regardless of age. The 1994-96 Continuing Survey of Food Intake by Individuals found that protein intake was significantly below recommended levels. The following table has the percentages of white males and females below 75% of the RDA and below 100% of the RDA. The percentages of deficient black Americans were even higher.
|Protein||Below 75%||Below 100%|
|60 and over||10.4||29.6|
|60 and over||15.8||35.9|
The statistics show that the deficiencies increased with age. A large proportion of senior citizens are seriously deficient in meeting their minimum essential protein requirements and suffer health problems and complications like:
- Sarcopenia (muscle wasting; weakness, poor balance)
- Osteoporosis (weak bones; fracture and hospitalization)
- Dementia (loss of mental function; loss of cognition)
- Immune dysfunction (vulnerbility to infectious disease)
Inadequate protein intake results in loss of body cell mass, decreased muscle function, and lower immune response. On the other hand, supplementing the diets of patients with hip fractures with 20 grams of protein decreased time in a rehabilitation hospital and reduced the rate of loss of bone mineral density. Higher protein intakes were associated with decreased risk for hip fracture in postmenopausal women. A study of 2066 men and women aged 70–79 years found that participants in the highest quintile of protein intake lost approximately 40% less lean mass than did those in the lowest quintile of protein intake. The study concluded that dietary protein may be a modifiable risk factor for sarcopenia in older adults.
Concerns about potential detrimental effects of increased protein intake on bone health, renal function, neurological function and cardiovascular function are generally unfounded. In fact, many of these factors are improved in elderly ingesting elevated quantities of protein. An intake of 1.5 g protein/kg/day, or about 15-20% of total caloric intake, is a reasonable target for elderly individuals wishing to optimize protein intake in terms of health and function.
There are some practitioners of Calorie Restriction with Optimum Nutrition (CRON) who are experimenting with various approaches for reducing protein. Besides lowering the proportion of protein in their diet, they may also select vegetable sources of protein which are generally lower in methionine than animal proteins. The consequences of misjudging the minimum protein requirements with advancing age can result in shorter life rather than longevity. Thus far, the evidence for greater health in old age seems to be on the side of higher protein levels, and let us not forget that methionine is considered an “essential” amino acid.
 Wolfe RR, Miller SL, Miller KB, Optimal protein intake in the elderly,
Clin Nutr. 2008 Oct;27(5):675-84. Epub 2008 Sep 25, PMID: 18819733
 Houston DK, Nicklas BJ, Ding J, Harris TB, Tylavsky FA, Newman AB, et al, Dietary protein intake is associated with lean mass change in older, community-dwelling adults: the Health, Aging, and Body Composition (Health ABC) Study, American Journal of Clinical Nutrition, Vol. 87, No. 1, 150-155, January 2008
 López-Torres M, Barja G, Lowered methionine ingestion as responsible for the decrease in rodent mitochondrial oxidative stress in protein and dietary restriction possible implications for humans, Biochim Biophys Acta. 2008 Nov;1780(11):1337-47. Epub 2008 Jan 18., PMID: 18252204