Vitamins: Useful or Harmful?

Millions of Americans take some kind of vitamin supplement. Some are beneficial — but many seem to have zero value. Here is a review of some of the vitamins most likely to be found in your medicine cabinet. Examine the evidence, and make informed choices in this controversial area in which so much misinformation circulates.

The Medical Letter on Drugs and Therapeutics, an independent source of drug information written for physicians and pharmacists, reviewed vitamin supplements in their July 18, 2005 issue. We last wrote on this topic in the November 1998 Worst Pills, Best Pills News. Since that time, more information has become available on the potential harms and benefits of taking vitamin supplements. Much of the following information is from the Medical Letter article.

The table accompanying this article gives food sources for some vitamins and their Recommended Dietary Allowances (RDA — the average daily dietary intake level that is sufficient to meet the nutrient requirements of most healthy adults).

VITAMIN E

The Medical Letter editors note that the type of vitamin E found in food is mostly gamma-tocopherol, which acts as an antioxidant. The form of vitamin E found in supplements is mostly alpha-tocopherol, which may block the antioxidant activity of gamma-tocopherol. Some laboratory experiments show that alpha-tocopherol may have a pro-oxidant effect.

High doses of vitamin E may interfere with vitamin K metabolism and platelet function, both of which are important in blood clotting. A type of statistical summary of clinical trials called a meta-analysis published in the Jan. 4, 2005 Annals of Internal Medicine found that using vitamin E in doses higher than 400 International Units (IU) per day may result in an increased risk of death.

A randomized controlled trial, the scientific “gold standard” for showing cause and effect, studied more than 9,000 patients with vascular disease or diabetes. The study found that long-term use of 400 IU of vitamin E per day did not prevent cancer or major cardiovascular events and may have increased the risk of heart failure. This trial was published in the March 16, 2005 issue of the Journal of the American Medical Association.

Another randomized controlled trial appearing in the July 6, 2005Journal of the American Medical Association involved almost 40,000 women 45 years of age and older. This trial found that 600 IU of vitamin E per day provided no overall benefit in preventing major cardiovascular events or cancer.

We have consistently listed vitamin E as DO NOT USE to prevent or treat any medical condition other than a vitamin E deficiency (which is extremely rare) since the publication of the first edition of Worst Pills, Best Pills in 1988.

VITAMIN A AND BETA CAROTENE

Vitamin A and beta carotene, a source of vitamin A, are antioxidants, but laboratory experiments hint they, too, may have pro-oxidant effects.

Research published in the Jan. 2, 2002 Journal of the American Medical Associationsuggested that a high intake of vitamin A from supplements and food is associated with an increased risk of hip fracture in postmenopausal women.  A study appearing in the Aug. 10, 1995 New England Journal of Medicine found that about 1 in 57 babies born to mothers who took more than 10,000 IU of vitamin A per day were malformed.

A clinical trial published in the April 14, 1994 New England Journal of Medicine found that in Finnish smokers, a daily 20 mg supplement of beta carotene increased the incidence of lung cancer by 18 percent. This result was statistically significant.

Another large clinical trial in smokers and asbestos-exposed workers was stopped early because no benefit was demonstrated. That trial found that 30 mg of beta carotene and 25,000 IU of vitamin A taken daily in combination was associated with an increase in the incidence of lung cancer, cardiovascular death and overall death. The results of this study were published in the May 2, 1996 New England Journal of Medicineand the Nov. 6, 1996 issue of the Journal of the National Cancer Institute, respectively.

There is conflicting evidence. A meta-analysis of seven studies, not randomized clinical trials, found no link between beta carotene intake estimated from dietary questionnaires and the incidence of lung cancer. This research was published in the journal Cancer Epidemiology, Biomarkers & Prevention in January 2004.

Statistical summaries generally a provide lower level of scientific evidence compared to controlled clinical trials. When a discrepancy exists in the research, consumers should err of the side of caution and refrain from taking supplements of questionable value.

VITAMIN D

The Medical Letter editors note that many older adults, “especially those with dark skin, receive inadequate amounts of vitamin D because of limited exposure to sunlight, decreased synthesis of vitamin D in the skin, and decreased absorption and activation of the vitamin.” The latest government recommendations for daily vitamin D intake, based on amounts that have slowed the rate of bone loss, are 400 IU for men and women 51 to 70 years old, and 600 IU for men and women more than 70 years old. Older adults who do not drink milk and those without dark skin who do not expose themselves to sunlight, will also need to take supplements to achieve this level of vitamin D intake.

A meta-analysis of randomized controlled trials published in the May 11, 2005 Journal of the American Medical Association of vitamin D use in people more than 60 years old found a reduced risk of hip and other types of clinical fractures with 700 to 800 IU per day, but not with 400 IU daily.

However, a randomized controlled trial appearing in the April 30, 2005 British Medical Journal and involving more than 3,000 women at a high risk of fracture found no evidence that 1,000 mg of calcium in combination with 800 IU of vitamin D per day reduced the risk of clinical fractures.

VITAMIN C  

The optimal intake of vitamin C is approximately 300-400 mg per day. A single eight ounce glass of orange juice contains about 100 mg of vitamin C.

A meta-analysis of three small trials in older adults published in the August 28, 1999 British Medical Journal found no beneficial effect of vitamin C on decreasing mortality

A 2004 review of short-term randomized controlled trials published by the Cochrane Collaboration, a highly respected non-profit organization, found that taking vitamin C does not prevent upper respiratory infections such as the common cold.

The Medical Letter editors caution that high doses of vitamin C — more than 1,000 mg — are poorly absorbed, cause diarrhea, and could increase the risk of kidney stones in people prone to develop kidney stones.

VITAMIN B-12

A condition known as atrophic gastritis, a deterioration of the lining of the stomach, affects 10 to 30 percent of older people. This condition results in an inability to absorb vitamin B-12 that is bound to food protein. However, the form of vitamin B-12 that is found in supplements can be absorbed by people with this condition. The recommendation is that older adults should take vitamin B-12 either in the form of B-12 fortified foods such as cereals or in a supplement that contains at least the RDA of 2.4 micrograms (mcg).

FOLATE AND FOLIC ACID

A normal U.S. diet provides between 50-500 mcg of folate (also known as folic acid) that can be absorbed by the body daily, but the ability of folate to be absorbed depends on the type of diet. Folic acid in supplements is about twice as absorbable as that in food. All enriched cereal grains available in the US now contain 140 mcg of folic acid per 100 grams (about 3.5 ounces) of grain. It is estimated that cereal fortification increases folic acid intake by between 215-240 mcg per day.  

However, even this amount of supplementation may not be sufficient to prevent a type of serious birth defect known as a neural tube defect. This type of birth defect can occur early in pregnancy even before most women know that they are pregnant. Providing women of childbearing age with 400 mcg of folic acid per day, the amount contained in most multivitamin preparations, has had a striking effect on decreasing the incidence of neural tube defects.

A low daily intake of absorbable folate has also been associated with high concentrations of the amino acid homocysteine in the blood. Too much homocysteine is linked to a higher risk of coronary heart disease, stroke and peripheral vascular disease (fatty deposits in peripheral arteries). Folic acid supplements can lower homocysteine levels, but it is not known whether this actually reduces coronary disease.

High doses of folic acid can mask vitamin B-12 deficiency, permitting progression of neurologic disease.

VITAMIN B-6 (Pyridoxine)

A study published in the Feb. 4, 1998 Journal of the American Medical Association based on dietary questionnaires found a lower risk of coronary heart disease in women who had vitamin B-6 and folate intakes above their respective RDAs of 1.3 mg and 400 mcg. Randomized controlled trials are lacking, and the optimal dose and effectiveness of vitamin B-6 supplements have not been determined.

The Medical Letter editors concluded their review of vitamin supplements by saying:

Supplements are necessary to assure adequate intake of folic acid in young women and possibly of vitamins D and B12 in the elderly. There is no convincing evidence that taking supplements of vitamin C prevents any disease. Women should not take vitamin A supplements during pregnancy or after menopause. No one should take high dose beta carotene supplements. A balanced diet rich in fruits and vegetables may be safer than taking vitamin supplements. No biologically active substance taken for a long term can be assumed to be free of risk.

We agree.

What You Can Do

Your best source of vitamins is a healthy balanced diet. Use the chart in this article for examples of vitamin-rich foods. Use vitamins only when they are necessary and in consultation with your physician.