Aging as a Modifier of Metabolism

Background

The 1989 edition of the U.S. Recommended Dietary Allowances (RDAs) divides adults into two age groups: the younger adults ages 23-50 and older adults age 51 and older. In 1989, the United States RDA Committee concluded that the data were insufficient to establish separate RDA subdivisions of healthy older people (for example, those ages 51-70 and age 70 and older).

In the U.S. edition of RDAs, the only micronutrients that account for differences between the groups ages 23-50 and 51 and older are thiamine, riboflavin, niacin, and iron; that is, the RDAs for thiamine, riboflavin, niacin, and iron (in females) for people age 51 and older are lower than those for younger adults ages 23-50. The decrements for thiamine, riboflavin, and niacin are due to low observed intakes of calories and protein in older adults, and the fact that these nutrients participate in metabolic processes involving energy expenditure and protein utilization. In the case of iron, the decrement for females is due to the lack of menstrual periods in the older age group.

It is now recognized that the elderly cannot be regarded simply as older versions of young adults. Results of experiments performed on younger adults indicate that elderly people have distinctly different metabolic processes that do not allow for easy extrapolation of nutrient needs. For example, Roberts et al. have recently shown that elderly people continue to underfeed themselves after a period of enforced underfeeding, whereas the younger adult immediately increases his or her dietary intake after a period of enforced underfeeding.

This paper focuses on how age modifies metabolism and requirements for specific vitamins and minerals, since protein requirements do not appear to change with age (i.e., 0.8 mg/kg body weight is adequate for both younger and older adults).

Vitamins

Riboflavin

The riboflavin requirement has now been shown to be the same for both young adults and elderly people. Thus, the lower riboflavin RDA for elderly people is not warranted.

Folate

From a cross-sectional analysis of elderly participants in the Framingham study, the amount of dietary folate needed to ensure normal blood homocysteine values is approximately twice (400 fig) the present RDA. This implies a greater need for dietary folate for both younger and elderly adults.

Vitamin D

1. There is a decreased ability to form previtamin D3 in skin upon ultraviolet light exposure in elderly people versus young people. Further, there is decreased absorption of vitamin D with age and decreased synthesis of 1-25 dihydroxy vitamin D by the kidney upon parathyroid hormone stimulation.

It has been shown that significantly less spinal bone loss occurs in elderly women supplemented with vitamin D (400 i.u./day) than in placebo-treated volunteers. The present RDA for vitamin D (200 i.u.) is too low.

Vitamin B6

Vitamin B6 requirements for the elderly have been shown to be elevated, although the mechanism for this is uncertain. The evidence that vitamin B6 requirements are elevated in aging comes both from depletion/repletion design studies and from epidemiologic studies on the amount of dietary vitamin B6 needed to insure normal blood homocysteine levels.

Vitamin B12

Vitamin B12 requirements may be increased in a large number of elderly people who have atrophic gastritis. Although most digestive and absorptive functions are well preserved during the aging process, atrophic gastritis occurs among elderly people with a prevalence of 20-50 percent, depending on how the diagnosis is made. The increased vitamin B12 requirement with aging is due to impaired digestion of cobalamin from food protein from lack of acid pepsin digestion, as well as bacterial uptake of vitamin B12 in the proximal small intestine.

Given the potentially devastating effects of vitamin B12 deficiency on the nervous system, the new uncertainties about how best to define vitamin B12 nutritional status and the high prevalence of a condition (atrophic gastritis) that can affect vitamin B12 metabolism in elderly people, it seems imprudent to have lowered the 1989 RDA for vitamin B12 in those age 51 and older. Until more data are available, an RDA of 3.0 lug seems safer for elderly people.

Vitamin A

Vitamin A requirements may be lower in the elderly than in younger people because of deceased clearance of the vitamin by hepatic and other peripheral tissues, and possible increased absorption from the gastrointestinal tract. There is no evidence that carotenoid metabolism is affected by age.

Minerals

Calcium Absorption

Calcium absorption efficiency falls with advancing age. Studies on calcium
supplementation alone seem to show that calcium intakes of more than 800
mg/day will not result in preservation of bone mineral. However, in
combination with vitamin D supplementation, calcium intakes in the range
of 1-1.5 g/day, have been shown to be of benefit in both hip and spine sites
(in terms of preservation of bone mineral).

Other Minerals

There is no evidence that other mineral requirements (except for lower iron requirements in postmenopausal females) are different in elderly versus younger individuals.

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