Leah Sullivan, MBA05.01.06
This article in a nutshell:
Methodology
Findings
Breast cancer
Colorectal cancer
Prostate cancer
Summary
Scientific research supports a relationship between diet and cancer risk. Most studies have shown that consuming a diet rich in fruits and vegetables reduces cancer risk. The FDA has approved health claims for fruit and vegetable consumption coupled with a low-fat diet to lower cancer risk1. However, most Americans do not consume a low-fat diet, and only 60% of men and 37% of women consume the requisite five servings of fruits and vegetables each day2. As a result, some consumers use dietary supplements to supply the specific nutrients missing from the diet in the hope of reducing cancer risk. Few studies are available on dietary supplement use and cancer risk. The purpose of this review is to identify recent studies involving diet and supplement use and cancer risk.
Methodology
All clinical studies related to dietary supplement use in adults were searched for in PubMed between January 2005 and January 2006. To find studies on a single nutrient, we entered such terms as "Vitamin C AND cancer" and "Vitamin D AND cancer." In addition to identifying nutrient groupings, including those found in a multivitamin, we used "supplements AND cancer." Of the 331 articles retrieved, we eliminated those that were not conducted in humans, review articles, and those that were not pertinent to the topic.
Findings
Thirteen articles met the inclusion criteria3-15 (download Table 1 as a PDF). Of these, only four showed a benefit of dietary supplementation to reduce cancer risk: one for breast cancer3, one for colon cancer7, and two for prostate cancer11, 12. The others showed no significant effect on cancer risk for breast cancer4, 14, 15, non-Hodgkin's lymphoma5, colorectal cancer6, 8, 14, 15, lung cancer9, 14, 15, esophageal cancer10, 15, skin cancer13, 15, and prostate cancer15. In these articles, none of the supplements were deemed to be harmful. What follows is a review of the studies that showed supplement use had a significant effect on cancer risk reduction.
Breast Cancer
A case-controlled study was conducted in 250 women with breast cancer and 219 age-matched women without cancer in Taiwan3. Those with cancer used fewer supplements of any kind compared to those who did not have cancer. The results indicated a protective effect of dietary supplements (OR: 0.40, 95% CI: 0.3-0.7) and a harmful effect of dietary fat (OR: 2.6, 95% CI: 1.4-5.0) for the highest versus lowest quartiles in all women. Women 40 years of age and younger who were taking multi-nutrient supplements had a 60% reduction in the risk of breast cancer. For women over 40 years of age using supplements, the risk of breast cancer was reduced by 50%.
Specifically in the younger cohort, those with the highest intake of vitamin A and C (about 400% of RDA for each) had a reduction in breast cancer risk of 40%. Vitamin E intake among the quartiles had less of an effect on breast cancer risk. This is likely because the intake of vitamins was so low; the highest quartile of vitamin E was only 86% of the RDA. In the older group, the same protective effects were seen for vitamins A and C with 50% reduction for both. Vitamin E was also significant at reducing cancer risk (by 40%).
The study had several limitations. The study results were based on the dietary intake at the time of entry into the study. It took three years to accrue all of the subjects, and the investigators assumed that the participants had the same dietary intake and supplement use over these three years, which may not have been the case. Since the study was a case-control study, the effects of supplementation are simply observations but not causal. In addition, the study was conducted in Taiwan, so the results may not be applicable to American women due to differences in family history, diet, and cultural habits. Nevertheless, taking a multivitamin supplement is reasonable for women in light of these early promising effects on reducing breast cancer risk.
Colorectal Cancer
In an epidemiological study, a group of 40,000 women in the U.S. (average age of 60 years) were followed for over 8.5 years to determine the risk of colorectal cancer and calcium use7. The women who ingested the most calcium through diet and supplements (>1,270 mg/day) had a 25% lower risk of colorectal cancer compared to women whose total intake of calcium was below 600 mg per day. The study concluded that older women who get close to the RDA of calcium, regardless of whether it is from diet or from supplementation, have a lower risk of colon cancer compared to those who consume 50% of the RDA of calcium from both of these sources.
Although this review was intended to include articles only up to January 2006, one large and highly publicized study was released in February 200616. These findings showed that supplementation of 1000 mg calcium in carbonate form and 400 IU of vitamin D for seven years had no effect in reducing the risk of colorectal cancer in postmenopausal women. This study had several problems. First, the women were already taking high amounts of both nutrients (1151 mg of calcium and 367 IU of vitamin D), thereby limiting the effect of the slight increase in both nutrients. Second, the intervention doses of calcium and vitamin D may be insufficient to reduce colon cancer risk. Other data support a role in cancer risk reduction of much higher amounts of vitamin D17. Third, the study may not have lasted long enough to detect a benefit because colon cancer may be latent for 10 to 20 years.
Although the results of these two studies are conflicting, women should still consume at least the RDA for calcium and vitamin D to maintain bone integrity, and perhaps lower colon cancer risk. This can be readily accomplished by eating two to three servings of dairy products a day, which have the highest amount of calcium and vitamin D of all commonly consumed foods, and taking dietary supplements of calcium and vitamin D to make up the difference.
Prostate Cancer
In a prospective study, over 5000 middle-aged Canadian men were randomized to a placebo or a low-dose multi-nutrient supplement for over eight years11. The supplement included beta-carotene (6 mg), vitamin E at 25% of the RDA, vitamin C at 200% of the RDA, and selenium and zinc, each at 50% of the RDA. In the group taking the supplements, the risk of prostate cancer was reduced by 48% in those who had a normal PSA level at the onset of the study (< 3.0 μg/ L), but not in those who had an elevated PSA. Supplementation at modest doses showed promise in reducing the risk of prostate cancer in healthy men. However, those who already were at higher risk of developing prostate cancer did not seem to derive any benefit from supplementation at these doses. Perhaps higher amounts are needed to reduce risk. Higher doses of vitamin E have been shown to reduce prostate cancer risk in other studies18. Clearly, supplementation with this antioxidant and possibly others show promise in reducing risk in this type of cancer in men.
In contrast to the promising data surrounding vitamin E, some studies have shown that calcium supplements may increase the risk of prostate cancer. However, in a 10-year study in which 600 American men were randomized to take a placebo or 1200 mg of calcium daily during the first four years, no such effect was observed12. At the end of the study, no increased risk of prostate cancer was found in the calcium group compared to the control group. To the contrary, there were significantly fewer cases of prostate cancer in the calcium group beginning two years after the start of the calcium treatment and lasting until two years after treatment (rate ratio: 0.44, 95% CI 0.21-0.94). This finding suggests that calcium has a delayed effect in reducing prostate cancer risk and that in order to obtain maximum benefit, calcium supplements should not be stopped. Thus, both vitamin E and calcium hold promise for reducing prostate cancer risk.
Summary
Although we identified numerous studies purporting to identify a relationship between dietary supplement use and cancer risk, only a few were actually clinical trials, and even fewer identified a significant effect of dietary supplement use on reducing cancer risk. From these studies it was difficult to determine the effect on risk reduction from the diet compared to the impact of the vitamin and mineral supplements. This is because many studies did not monitor dietary intake and compliance with supplement use. All studies were funded by government agencies or private foundations, and no attempt to tease out the effect of dietary supplements was carried out. Had dietary supplement manufacturers funded these studies, it is likely that more attention would have been paid to the discrete effect of the supplements.
Despite the conflicting results in studies on dietary supplements, the public continues using them in the hope of improving health and reducing the risk of disease. One reason for this behavior may be that many nutritional studies are often contradictory, which creates a state of confusion. For example, in February 2006, three major studies on the merits of low-fat diets were published19-21. After eight years and spending over $400 million, data from nearly 50,000 women showed no benefit in consuming a low-fat diet and reducing the risk of colon cancer, breast cancer, and heart disease. However, Americans have always been told to adopt such a diet to reduce the risk of these conditions. Adding to the skepticism was the new finding from 35,000 women that supplemental calcium and vitamin D did little to reduce hip fractures, but did increase the risk of kidney stones22. Americans are left confused and wanting to manage their own health.
Based on the recent research, consumers concerned about cancer should do the following things:
Consume at least five servings of fruits and vegetables a day
Adhere to a low-fat or moderate diet (30-35% of the calories from fat)
Include at least two servings of whole grains each day
Take a multivitamin each day with a wide assortment of micronutrients at the RDA or higher
Individual supplementation to reduce cancer risk from the recent studies support a possible role for:
Breast cancer: vitamin A, C, and E
Colorectal cancer: calcium, vitamin D
Prostate cancer: multi-nutrients (beta-carotene, vitamin C, vitamin E, zinc, and selenium), and calcium
About the authors: Leah Sullivan, MBA, MS, is a dietetic intern studying at Eastern Michigan University. She can be reached at 781-729-5118; E-mail: landjsullivan@yahoo.com. Stacey Bell, DSc, RD, is a research scientist at Ideasphere, Grand Rapids, MI. She can be reached at 617-484-2951; E-mail: staceyjbell@yahoo.com. Greg Grochoski is the director of research and development at Ideasphere. He can be reached at 616-464-5100; E-mail: ggrochoski@isibrands.com.
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