The U.S. Preventive Services Task Force (USPSTF) grades its
recommendations (A, B, C, D, or I) and the quality of the overall evidence for a service (good, fair, poor). The definitions of these grades can be found at the end of the "Major Recommendations" field.
The USPSTF concludes that the evidence is insufficient to recommend for or against the use of supplements of vitamins A, C, or E; multivitamins 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 either 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 the use of beta-carotene supplements, 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 2 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 people with
chronic illnesses. Dietary supplements may be appropriate for people whose
diet does 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 people 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 recommendations on counseling to promote a healthy diet).
- Patients who choose to take vitamins should be
encouraged to adhere to the dosages recommended in the Dietary Reference
Intakes (DRI) of the Institute of Medicine. Some vitamins, such as A and D,
may be harmful in higher doses; therefore, doses greatly exceeding the
Recommended Dietary Allowance (RDA) or Adequate Intake (AI) 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 (USP) 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 those 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 for all women who are planning,
or capable of, pregnancy (see 1996 USPSTF information on screening for neural tube
defects).
- Clinicians and patients should discuss the possible need for vitamin
supplementation when taking certain medications (e.g., folic acid
supplementation for those patients taking methotrexate).
Definitions:
The Task Force grades its recommendations according to one
of 5 classifications (A, B, C, D, 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 [this 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.
The Task Force grades the quality of the overall evidence
for a service on a 3-point scale (good, fair, 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.