Am Fam Physician. 2003;68(12):2422-2425
This statement summarizes the U.S. Preventive Services Task Force (USPSTF) recommendations on routine vitamin supplementation to prevent cancer and cardiovascular disease and the supporting scientific evidence. Explanations of the ratings and of the strength of overall evidence are given in Tables 1 and 2, respectively. The complete information on which this statement is based, including evidence tables and references, is available in the articles, “Routine Vitamin Supplementation to Prevent Cardiovascular Disease: A Summary of the Evidence for the U.S. Preventive Services Task Force,”1 “Routine Vitamin Supplementation to Prevent Cancer: A Summary of the Evidence from Randomized Controlled Trials for the U.S. Preventive Services Task Force,”2 and “Routine Vitamin Supplementation to Prevent Cancer: Update of the Evidence from Randomized Controlled Trials, 1999–2002,”3 which can be obtained on the USPSTF Web site (http://www.uspreventiveservicestaskforce.org) and through the National Guideline Clearing-house (http://www.guideline.gov). The recommendation statement and summaries of the evidence on these topics are also available from the Agency for Healthcare Research and Quality Publications Clearinghouse in print through subscription to the Guide to Clinical Preventive Services, Third Edition: Periodic Updates. To order, contact the Clearinghouse at 800-358-9295 or e-mailahrqpubs@ahrq.gov.
| The USPSTF grades its recommendations according to one of five classifications (A, B, C, D, or I) reflecting the strength of evidence and magnitude of net benefit (benefits minus harms). | |
| A. | The USPSTF strongly recommends that clinicians provide [the service] to eligible patients.The USPSTF found good evidence that [the service] improves important health outcomes and concludes that benefits substantially outweigh harms. |
| B. | The USPSTF recommends that clinicians provide [the service] to eligible patients.The USPSTF found at least fair evidence that [the service] improves important health outcomes and concludes that benefits outweigh harms. |
| C. | The USPSTF makes no recommendation for or against routine provision of [the service].The USPSTF found at least fair evidence that [the service] can improve health outcomes but concludes that the balance of benefits and harms is too close to justify a general recommendation. |
| D. | The USPSTF recommends against routinely providing [the service] to asymptomatic patients.The USPSTF found at least fair evidence that [the service] is ineffective or that harms outweigh benefits. |
| I. | The USPSTF concludes that the evidence is insufficient to recommend for or against routinely providing [the service].Evidence that [the service] is effective is lacking, of poor quality, or conflicting, and the balance of benefits and harms cannot be determined. |
This statement was first published in Ann Intern Med 2003;139:51–5.
Summary of Recommendations
The USPSTF concludes that the evidence is insufficient to recommend for or against use of supplements of vitamins A, C, or E; multi-vitamins with folic acid; or antioxidant combinations for the prevention of cancer or cardiovascular disease. I recommendation.
The USPSTF found poor evidence to determine whether supplementation with these vitamins reduces the risk for cardiovascular disease or cancer. The available evidence from randomized trials is inadequate or conflicting, and the influence of confounding variables on observed outcomes in observational studies cannot be determined. As a result, the USPSTF could not determine the balance of benefits and harms of routine use of supplements of vitamins A, C, or E; multivitamins with folic acid; or antioxidant combinations for the prevention of cancer or cardiovascular disease.
The USPSTF recommends against supplementation with beta carotene, either alone or in combination, for the prevention of cancer or cardiovascular disease. D recommendation.
The USPSTF found good evidence that beta carotene supplementation provides no benefit in the prevention of cancer or cardiovascular disease in middle-aged and older adults. In two trials restricted to heavy smokers, beta carotene supplementation was associated with higher incidence of lung cancer and higher all-cause mortality. The USPSTF concludes that beta carotene supplements are unlikely to provide important benefits and might cause harm in some groups.
Clinical Considerations
The USPSTF did not review evidence regarding vitamin supplementation for patients with known or potential nutritional deficiencies, including pregnant and lactating women, children, the elderly, and persons with chronic illnesses. Dietary supplements may be appropriate for persons whose diets do not provide the recommended dietary intake of specific vitamins. Individuals may wish to consult a health care provider to discuss whether dietary supplements are appropriate.
With the exception of vitamins for which there is compelling evidence of net harm (e.g., beta carotene supplementation in smokers), there is little reason to discourage patients from taking vitamin supplements. Patients should be reminded that taking vitamins does not replace the need to eat a healthy diet. All patients should receive information about the benefits of a diet high in fruits and vegetables, as well as information on other foods and nutrients that should be emphasized or avoided in their diet (see 2002 USPSTF evidence summary on counseling to promote a healthy diet).4
Patients who choose to take vitamins should be encouraged to adhere to the dosages recommended in the Dietary Reference Intakes of the Institute of Medicine. Some vitamins, such as A and D, may be harmful in higher dosages; therefore, dosages greatly exceeding the Recommended Dietary Allowance or Adequate Intake should be taken with care while considering whether potential harms outweigh potential benefits. Vitamins and minerals sold in the United States are classified as dietary supplements, and there is a degree of quality control over content if they have a U.S. Pharmacopeia seal. Nevertheless, imprecision in the content and concentration of ingredients could pose a theoretical risk not reflected in clinical trials using calibrated compounds.
The adverse effects of beta carotene on smokers have been observed primarily in persons taking large supplemental doses. There is no evidence to suggest that beta carotene is harmful to smokers at levels occurring naturally in foods.
The USPSTF did not review evidence supporting folic acid supplementation among pregnant women to reduce neural tube defects. In 1996, the USPSTF recommended folic acid supplementation for all women who are planning or are capable of pregnancy (see 1996 USPSTF chapter on screening for neural tube defects).5
Clinicians and patients should discuss the possible need for vitamin supplementation when taking certain medications (e.g., folic acid supplementation in patients taking methotrexate).
The Scientific Evidence and Recommendations of Others sections that usually are included in USPSTF recommendation statements is available in the complete Recommendation and Rationale statement on the USPSTF Web site (http://www.ahrq.gov/clinic/uspstfix.htm).
Discussion
The findings of this review must be placed in context because the review focused only on vitamin supplements and their role in preventing cancer and cardiovascular disease. The value of taking vitamin supplements for other purposes, such as folic acid supplementation by women capable of pregnancy, has stronger scientific support. Although the health benefits of vitamin supplementation remain uncertain, there is more consistent evidence that a diet high in fruit, vegetables, and legumes has important benefits; other constituents besides vitamins may account for the benefits of such diets.
| The USPSTF grades the quality of the overall evidence for a service on a three-point scale (good, fair, or poor). | |
| Good: | Evidence includes consistent results from well-designed, well-conducted studies in representative populations that directly assess effects on health outcomes. |
| Fair: | Evidence is sufficient to determine effects on health outcomes, but the strength of the evidence is limited by the number, quality, or consistency of the individual studies; generalizability to routine practice; or indirect nature of the evidence on health outcomes. |
| Poor: | Evidence is insufficient to assess the effects on health outcomes because of limited number or power of studies, important flaws in their design or conduct, gaps in the chain of evidence, or lack of information on important health outcomes. |
Furthermore, dietary supplementation with folic acid, vitamin B6 (pyridoxine), and vitamin B12 (alone or in combination) appears to lower plasma homocysteine levels, and higher levels of homocysteine may be an independent risk factor for cardiovascular disease.6 However, definitive evidence of the role of vitamin supplementation on altering cardiovascular outcomes is lacking. The results of a secondary prevention trial will be available within the next few years.