BETHESDA, MD—Despite finding that some dietary supplements such as selenium may be useful in cancer prevention, an independent panel of experts assembled by the National Institutes of Health has warned against taking multivitamin and mineral supplements to prevent chronic diseases such as cancer. More solid clinical trial evidence is needed, the panel said. The exception is the combined use of calcium and vitamin D to protect postmenopausal women from osteoporosis.
“Half of Americans are taking multivitamins and minerals, and the bottom line is that we don't know for sure that they're benefiting from them,” said the panel's Chairman, J. Michael McGinnis, MD, MPP, Senior Scholar at the Institute of Medicine of the National Academy of Sciences. “In fact, we're concerned that some people may be getting too much of certain nutrients.”
The panel's report, written during a three-day state-of-the-science conference on multivitamin supplements held at the NIH, is independent and does not represent the position of the NIH. A related concern is that some supplements sold specifically for cancer prevention are either contaminated or do not contain the amount of active ingredient listed (see box).
Dietary supplement use is “increasingly common” in the general US population and can lead to excessive intakes, especially among cancer patients, said one of the speakers, Cheryl L. Rock, PhD, RD, Professor of Family and Preventive Medicine at the Cancer Prevention and Control Program at the University of California, San Diego.
Dr. Rock said that an analysis of data from the INTERMAP Study of the US population showed that supplement users had mean total intakes (from food and supplements) of vitamin E and vitamin C that were more than 700% of the average recommendation. National Health and Nutrition Examination Survey (NHANES) data from 1999 to 2000 show that .3% of American adults consumed daily levels of vitamin E that were at or more than 400 international units (IU), the recommended daily amount, she said.
Seeking to Prevent Cancer Recurrence
Cancer patients who want to prevent recurrence are at the forefront of supplement over-use, Dr. Rock said. She cited a study she coauthored showing that 81% of women who had been diagnosed with early-stage breast cancer surveyed in 1995–1997 reported regularly using at least one dietary supplement. In this study, 46% of women used multiple vitamin and mineral supplements, but they also took single supplements more commonly than those in the general population.
For instance, Dr. Rock said, 49% of the women used a vitamin E supplement, compared with just 13% observed in NHANES for 1999–2000. In sum, she said, “the distribution of micronutrient intakes of this [breast cancer] population indicates that a considerably higher proportion exceed recommended levels of intakes.”
Among the general public, 62% believe that vitamin/mineral supplements are effective, and 33% use them to treat medical conditions, according to data prepared for the panel.
Today, condition-specific supplements are the most rapidly growing segment of the dietary supplement business; last year 44% of all US dietary supplement sold emphasized a particular health condition, according to a source cited by the panel, Nutrition Business Journal. This specialty journal estimates that 14 condition-specific medical areas, including cancer, account for 84% of supplement sales.
“About 68% of our population reports taking more than one vitamin supplement—maybe not every day, but on most days; I think that's a worrisome trend,” said Suzanne Murphy, PhD, RD, Research Professor at the Cancer Research Center of Hawaii, University of Hawaii (Honolulu).
Dr. Murphy said it is critically important to define the upper levels of multivitamin and mineral supplements that are safe. She also noted that there is a wide variability in the nutrient content of supplements, as highlighted in a Hawaiian study on the nutrient composition of 1,246 supplements.
In this study, for each of 15 nutrients examined, at least 10% of the supplement products contained none of the nutrient at all, Dr. Murphy said. For three nutrients analyzed (thiamin, vitamin B6, and iron), “the level reported in products at the 90th percentile was 10 times higher than the level in products at the median.”
Suggestive Evidence of Lowered Risk of Cancer
Currently, there is suggestive evidence that certain nutrients can lower the risk of cancer, said Meir J. Stampfer, MD, DrPH, Professor of Epidemiology and Nutrition and Chair of the Department of Epidemiology at Harvard School of Public Health.
Dr. Stampfer, who is also Professor of Medicine at Harvard Medical School, cited the following nutrients as having possible benefit in reducing cancer risk: selenium to reduce the risk of prostate cancer, skin cancer, and colon cancer; folate (folic acid) to reduce the risk of colorectal cancer; and calcium/vitamin D to reduce the risk of colon polyps that could become cancerous.
But Dr. Stampfer cautioned that in most cases it takes so long for cancer to develop that it is very difficult to mount randomized clinical trials to show that a particular nutrient has a demonstrable effect on cancer risk. Definitive trials on these supplements have not been done, he noted.
In its report, the NIH-convened panel reached the following specific conclusions, among others:
▪ Beta-Carotene. Overall, there is no benefit from taking beta-carotene, and two large trials launched to test lung cancer prevention with beta-carotene found a paradoxical increase in lung cancer incidence and deaths in smokers and male asbestos workers. “We found no evidence to recommend beta-carotene for the general population and strong evidence to recommend that smokers avoid beta-carotene supplementation,” the panel said.
▪ Selenium. In two Chinese trials, selenium decreased liver cancer incidence in patients at high risk because of either a family history or hepatitis B exposure status. But these trials were considered incomplete. A third selenium trial, conducted in men and women with a history of skin cancer, found no decrease in skin cancers but did report reductions in total cancer mortality and in the incidence of lung, prostate, and colorectal cancers (outcomes the study was not designed to investigate). The panel concluded that “there is a suggestion that selenium may reduce the risk of prostate, lung, and colorectal cancers.”
▪ Vitamin E. Four trials investigated vitamin E; in one, there was a decreased risk of prostate cancer and a suggestion of decrease in colorectal cancer risk in male smokers. No other effects were found on other cancers. The panel concluded that vitamin E may decrease prostate cancer incidence in male smokers and cardiovascular deaths in women.
▪ Stricter Rules. Because of concerns about adverse reactions and interactions, the panel recommended stricter rules in the way dietary supplements are regulated. It called on Congress to expand the authority of the US Food and Drug Administration to require supplement makers to disclose adverse events; ensure quality production; and increase consumer reporting of adverse events by including reporting information on dietary supplement labels.
The FDA encourages physicians and patients voluntarily to report adverse events from dietary supplements to the FDA's MedWatch program (1–800-FDA-1088; www.fda.gov medwatch
Problems with Supplements Sold for Cancer Prevention
An independent testing laboratory not affiliated with industry, ConsumerLab.com, recently tested three dietary supplements that consumers use widely for cancer prevention: green tea, lycopene, and selenium. While all of the lycopene products passed their testing, there were problems in the green tea supplements and in a selenium supplement, said the lab's President, Tod Cooperman, MD.
Two of the green tea supplements tested were contaminated with lead and one lacked some of the key compound it was supposed to contain, epigallocatechin gallate (EGCG). In addition, a selenium supplement contained only 38% of its ingredient. Dr. Cooperman said that sales of green tea jumped 45% to $160 million in the United States in 2004, according to Nutrition Business Journal.
Like the NIH-convened panel, Dr. Cooperman warned that consumers need to be wary of dietary supplements containing vitamins and minerals. “This is an area where consumers really need to be sure of the products they choose,” said Dr. Cooperman.
“You won't know if your supplement lacks active ingredients since you can't ‘feel’ a preventive therapy; and contamination is compounded when a supplement is used daily for years.”