Evidence Inconclusive on Vitamins, Health Risks

Evidence Inconclusive on Vitamins, Health Risks

ROCKVILLE, Md.There is insufficient evidence available torecommend for or against using vitamins A, C or E, multivitamins with folicacid, or antioxidant combinations to prevent cancer or cardiovascular disease,according to the U.S. Preventive Services Task Force (USPSTF), which published asummary of its recommendations in the July 1 issue of the Annals of InternalMedicine (139, 1:51-5, 2003) (www.annals.org).Researchers noted they based their conclusion on the fact that trials theyreviewed provided either inadequate or conflicting results, or the effects ofconfounding variables on study outcomes could not be determined.

The Task Force did a good job of reviewing the emerging evidence relatingto cancer and heart disease, said Annette Dickinson, Ph.D., president of theCouncil for Responsible Nutrition (CRN). They cited some positive studies andsome negative ones and concluded that on balance we need more research. No onecan disagreebut preventing cancer and heart disease are not the only benefitsfrom using vitamins.

Specifically, the research team covered the research behind the routinesupplementation of several vitamins and its link to reducing the risk ofcardiovascular disease and cancer. Vitamin A was determined to be ineffectivefor preventing cancer in men but inversely associated with the risk of colon andbreast cancer in women, as well as ineffective for reducing the risk ofcardiovascular disease mortality.

As for vitamin C, observational studies were not seen to reduce risks ofbreast, prostate, colon or lung cancer, and observational cohort studies showedinconsistent results for the antioxidants effects on cardiovascular disease.

The team noted vitamin E supplementation was not protective against lungcancer, but may have been beneficial against prostate cancer in a randomized,controlled trial. However, researchers also reviewed observational studies thatshowed no protection against prostate, lung or breast cancer from vitamin E,when it may have protected against colon cancer. In terms of heart disease,vitamin E was said to not have a statistically significant benefit inprevention, and only one of seven trials of vitamin E demonstrated a reductionin cardiac events.

As for antioxidant combinations, researchers uncovered mixed results. Theystated a combination of vitamin E, vitamin C and beta-carotene was not shown tohave a significant effect, and some studies showed an adverse effect ofcombination supplements on cancer. However, researchers also noted results fromthose studies were confounded by the inclusion of betacarotene. In terms ofheart disease, some observational studies suggested a benefit, while randomizedcontrolled trials did not duplicate that effect. The research team evenmentioned one trial that showed an increase in allcause mortality among womentaking antioxidant supplements.

The B vitamins were also discussed by the research team. They noted folicacid supplementation has been linked to a decrease in colon cancer risk,although this potential effect should be re-examined in prospective trials.Further, the researchers noted that among cohort studies evaluating multivitaminintake, one reported a reduction in coronary events, two showed no significanteffect on mortality, and another demonstrated an increase in all-cause mortalityin men.

The findings of these USPSTF reviews must be placed in context becausethey focused only on vitamin supplements and their role in preventing cancer andcardiovascular disease, USPSTF wrote. The value of taking vitaminsupplements for other purposes, such as folic acid supplementation by womencapable of pregnancy to prevent the birth of babies with neural tube defects,has stronger scientific support. The researchers also suggested thosechoosing to take vitamins closely follow dosage recommendations from theInstitute of Medicine.

In addition, the team recommended against taking beta-carotene because it isunlikely to provide important benefits and might cause harm in some groups,such as smokers. And, they noted vitamin A has been linked with a reduction inbone mass, and vitamin D can be harmful if taken in high doses.

Aside from vitamin A, vitamin D and beta-carotene, the USPSTF did not placeany restrictions on vitamin intake, noting further research into the realm ofpotential adverse events from vitamins is required, but there is little reasonto discourage people from taking vitamins. The team added its research did notevaluate the benefit of supplements in populations where nutrient deficienciesmight be present, such as in pregnant or lactating women, the elderly, children,people with chronic disease, and those taking pharmaceuticals that may causedepletions. They suggested a health care provider be consulted to determinewhether supplements are necessary.

The bottom line is that individuals have to make their own decisions aboutwhat makes sense for their own healthy lifestyle, Dickinson said. But manyscientists believe, as does CRN, that the regular use of dietary supplements,with a multivitamin as the foundation of a smart nutrition program, makes goodsense for the overall promotion of good health and prevention of disease.