Antioxidants can have risks as well. According to the National Institutes of Health, high doses of beta-carotene may increase the risk of lung cancer in smokers, high doses of vitamin E may increase risks of prostate cancer and one type of stroke, and antioxidant supplements may also interact with some medicines.
Since they can interact with medicines, you should discuss your supplement intake with your doctor,researchers say.
Antioxidants, including vitamins A, C and E, are plentiful in vegetables and fruits and may help protect against cell or DNA damage.
As a result, they've been touted for cancer prevention, heart disease prevention and even warding off dementia.
'There is good scientific evidence that eating a diet with lots of vegetables and fruits is healthful and lowers risks of certain diseases,' said lead author Annlia Paganini-Hill of the Clinic for Aging Research and Education at the University of California, Irvine.
'However, it is unclear whether this is because of the antioxidants, something else in these foods, other foods in people's diet, or other lifestyle choices,' Paganini-Hill told Reuters Health by email.
Most double-blind randomized clinical trials - the gold standard of medical evidence - have found that antioxidant supplements do not prevent disease, she said.
The researchers used mailed surveys from the 1980's in which almost 14,000 older residents of the Leisure World Laguna Hills retirement community detailed their intake of 56 foods or food groups rich in vitamins A and C as well as their vitamin supplement intake.
Two-thirds of the original group took vitamin supplements, most often vitamin C.
The authors note, though, that the participants' diets alone were generally more than adequate to meet minimum dietary requirements for vitamin intake.
With periodic check-ins and repeated surveys, the researchers followed the group for the next 32 years, during which time 13,104 residents died.
When Paganini-Hill's team accounted for smoking, alcohol intake, caffeine consumption, exercise, body mass index, and histories of hypertension, angina, heart attack, stroke, diabetes, rheumatoid arthritis and cancer, there was no association between the amount of vitamins A or C in the diet or vitamin E supplements and the risk of death.
Vitamin users may have different lifestyles or underlying disease states that are related to their risk of death, the authors write.
'In the general population, health-promoting habits often cluster; e.g. those who take vitamin supplements often exercise, do not smoke, and are not obese,' Paganini-Hill said.
'Thus, these factors may explain the observed association between longevity and vitamin supplements.'
On the other hand, the authors note, people with unhealthy habits might be more likely to take supplements.
For instance, they found that men who were current smokers were about twice as likely to take in high or medium amounts of vitamin C compared to men who had never smoked.
A similar pattern held for men's vitamin A intake and women's intake of both A and C.
Some large studies have found a connection between vitamin intake and risk of death, but most have not, the study team points out.
'We know quite a lot about how antioxidants act and what they, theoretically, can prevent,' said Sabine Rohrmann of the Institute of Social and Preventive Medicine at the University of Zurich.
'One of the critical issues is that we don't know very much about how antioxidants act at different concentrations and how they act in humans who have, or who do not have, sufficient vitamin/antioxidant intake,' Rohrmann told INNLIVE by email.
Participants in the new study were largely white, educated and well-nourished.
'We know that the most important factors that influence mortality are smoking and excess body weight,' Rohrmann said.
'Many studies support the notion that vitamin supplements are usually not necessary because our nutrient intake via a healthy diet is usually sufficient, she said.
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