Update on some common vitamins (abstracted from the Medical Letter, 8/05)

Vitamin E: Vitamin E in food is mostly gamma-tocopherol, which acts as an antioxidant. Vitamin E in supplements is mainly alpha-tocopherol, which may block the antioxidant activity of gamma-tocopherol and have a pro-oxidant effect in vivo. High doses of vitamin E may interfere with vitamin K metabolism and platelet function. A meta-analysis of clinical trials using vitamin E dosages higher than 400 IU/day found an increased risk of mortality. A randomized controlled trial in more than 9000 patients with vascular disease or DM found that supplementation with 400 IU vitamin E per day did not prevent cancer or major cardiovascular events, and may have increased the risk of heart failure. A randomized controlled trial in almost 40,000 women > 45 years old found that 600 IU of vitamin E per day provided no overall benefit for major cardiovascular events or cancer.

Vitamin A and beta carotene: Both vitamin A and beta carotene are antioxidants, but may also have pro-oxidant effects in vivo. A high intake of vitamin A from supplements and food has been associated with an increased risk of hip fracture in postmenopausal women and with teratogenicity when taken in early pregnancy. A placebo-controlled intervention in Finnish smokers found that 20 mg/d of beta-carotene supplement increased the incidence of lung cancer by 18%. Another DB intervention trial in smokers and asbestos-exposed workers terminated early because no benefit was demonstrated, found that combined therapy with 30 mg of beta carotene and 25,000 IU of vitamin A daily was associated with an increase in the incidence of lung cancer, cardiovascular mortality and total mortality. A meta-analysis of 7 cohort studies found no association between carotenoid intakes estimated from dietary questionnaires and the incidence of lung cancer during 7-16 years of follow-up.

Vitamin D: Many elderly receive inadequate amounts of vitamin D; it is recommended that men and women aged 51-70 get 400 IU of vitamin D/day, and 600 IU/d for those over age 70. A meta-analysis of randomized controlled trials of vitamin D in patients more than 60 years old found a reduced risk of hip and other non-vertebral fractures wtih 700-800 IU/d, but not with 400 IU/d.

Vitamin C: Dietary levels of about 300-400 mg/d of vitamin C maintain maximal body pools of the vitamin. An 8 oz glass of orange juice contains about 100 mg of vitamin C. A meta-analysis of 3 small, relatively short trials in elderly people found no benefit in mortality (larger longterm studies have not been done). High doses of vitamin C (> 1000 mg/d) are poorly absorbed, cause diarrhea, and could increase urinary oxalate excretion to a level that might cause kidney stones in people with pre-existing hyperoxaluria.

Vitamin B12: Atrophic gastritis, which affects 10% to 30% of older people, results in the inability to absorb vitamin B12 bound to food, with absorption of crystalline B12 usually left intact. Elderly people, therefore, should take vitamin B12, either in the form of B12-fortified foods (such as cereals) or as dietary supplements.

Folate: Folate supplements are about twice as bioavailable as folate in food. All enriched cereals sold in the US contain 140 mcg of folic acid per 100 gm of grain. Even this amount may be inadequate to prevent neural tube defects; supplementing women of childbearing age with 400 mcg of folic acid per day has dramatically decreased the incidence of neural tube defects in their offspring. Low intake of absorbable folate has been associated with high serum concentrations of homocysteine and a higher incidence of cardiovascular disease and stroke; folate supplements can reverse hyperhomocysteinemia, but it is not known whether this reduces coronary disease. High doses of folic acid can mask vitamin B12 deficiency, permitting progression of neurologic disease.