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A CLOSER LOOK AT HEADLINES LINKING VITAMIN SUPPLEMENTS AND LUNG CANCER |
Mar 4, 2008
Category: Health Information |
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On Friday, The Globe and Mail published an article, “Smokers who take Vitamin E are at risk,” based on a study in the American Journal of Respiratory and Critical Care Medicine.
Some statements in The Globe and Mail report — “smokers who take vitamin E may have an increased lung cancer risk” and “ those who take popular supplements are just as likely to develop lung cancer as those who don’t — are inaccurate conclusions to draw from this study.
For your information, we offer the following summary and interpretation of the key study findings:
Q: What was the objective of this study? Who participated in it?
The objective of this study was to “explore the association of supplemental multivitamins, vitamin C, vitamin E and folate with incident lung cancer.” The study was based on 77,126 eligible participants. The results, however, were based on a comparison between 521 individuals who developed lung cancer in the average four-year follow-up and 76,605 participants who did not.
Q: How did the researchers measure each subject’s supplement intake?
Using a 24-page, self-reporting questionnaire sent by mail, the researchers asked participants to answer questions about their diet, medical history, personal characteristics, cancer risks and use of multivitamins, vitamins C and E and folate over 10 years leading up to the study. The researchers then calculated the average 10-year supplement use from the survey and that number became a “continuous variable” in determining the future risk of lung cancer over an (average) four-year follow-up period.
Q: Is this an acceptable protocol for gathering data? Was the methodology sound?
Three key issues with the methodology undermine the validity of the study. The first issue is the self-reporting questionnaire. A scientific precedent for self-reporting exists, but it is unreliable to ask participants to recall by memory their dietary supplement intake over 10 previous years.
Another methodological flaw is the researchers’ decision to exclude many study participants — approximately 10 percent or nearly 8,000 — who had missing data regarding their supplement intake. These individuals may have neglected to complete the questionnaire because, for example, they didn’t take any supplements. If a large number of those missing participants got lung cancer, the study conclusions would have been altered in any number of ways. It is statistically unacceptable to arbitrarily dismiss these reports. All data should be included in the final analysis.
In addition, the researchers attempted to control for confounding factors associated with developing lung cancer, such as age, body-mass index, food intake, history and quantity of smoking, and family history of cancer, but they did not include these results in the final analysis. They noted no changes from the unadjusted findings, which means that the results are potentially confounded by all the important indicators for lung cancer.
Q: What are the findings of this study?
This study found no associations between lung cancer and multivitamin formulas, vitamin C, vitamin E or folate. No single-level (predefined dosage range) of any of these nutrients was statistically significant for increasing the incidence of lung cancer. This fact is undeniable, major flaws in the study notwithstanding.
A closer look at the trends, however, reveals a marginal protective effect for the multivitamin, vitamin C and folate groups as the dose increases. But again, when looking at each dose within a vitamin group there are no effects — no benefit or no harm.
Q: Why then has the study attracted so much attention?
The media has latched onto a detail in the study without examining the context. The researchers found there was a trend toward an increasing risk for lung cancer as the dose of supplemental vitamin E increased. But when you examine the high-dose group — not the trend itself — there is no increased risk.
An important fact to note is that when the researchers added the effect of vitamin E intake from food, there was no indication of increased risk of lung cancer. This means that a very small increased risk of lung cancer arises only if you don’t eat foods with vitamin E, you smoke and you take vitamin E supplements in excess of 322 mg/day. This conclusion stretches credulity given that in order to survive, people must eat and, therefore, will inevitably consume foods — vegetable oils, nuts, leafy green vegetables and fortified cereals — containing vitamin E.
The risk factor is narrowed further because this trend was only associated with one type of vitamin E source, d/l alpha-tocopherol, which does not represent all vitamins E products.
Q: What conclusions should I draw from this study?
Multivitamins, vitamin E, vitamin C and folate do not increase the risk of lung cancer in smokers. You should continue to eat the required daily amounts of fruits and vegetables, in addition to taking supplements. If you are a smoker, you are at an increased risk of lung cancer over your lifetime and should stop smoking to decrease your risk. Do not expect a multivitamin-mineral formula, vitamin C, vitamin E or folate to decrease your risk of lung cancer if you are a smoker, but you should not expect any increased risk from taking a Recommended Daily Allowance of supplements. If you are concerned about your individual risk, always consult a licensed health-care practitioner with extensive knowledge of natural health products before you take any product.
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