As a medical oncologist specializing in gastrointestinal cancers at a research institution, every day I see patients suffering from terminal colorectal cancer who thought they could ignore their doctors’ advice and protect themselves from cancer by taking vitamins and other supplements. The medical literature has repeatedly proven them wrong.
March is Colorectal Cancer Awareness Month, my 31 days to tell people who aren’t already my patients that colorectal cancer is often preventable. But not by taking vitamin supplements. Your first act in prevention should be to get a colonoscopy when your doctor tells you to; the procedure can find tumors and pre-cancerous polyps before they become deadly. The other best prevention for colorectal cancer is exercise.
Unfortunately, most Americans don’t follow this advice. Only one in five of us meet U.S. physical activity guidelines, and one in four have a colon screening test done when it is recommended. Yet more than half of Americans take dietary supplements, a habit that notably has not been shown to prevent cancer.
Most people know that exercise prevents heart disease, diabetes, and obesity. But exercise’s ability to reduce death from colorectal cancer is just as well established by research. People are getting fatter and idler, which puts them at risk of many diseases of the sedentary. There is an 84 percent increased risk of death from colon cancer for the morbidly obese. A recent study of more than 2.5 million people found that having diabetes increases your risk of dying of colorectal cancer by 30 percent.
Here in New Jersey, my colleagues and I have noted in our oncology practice more young patients and more obese patients. Demographic data support our observations: The rate of diagnosis of colorectal cancer in people in their early 40s is increasing faster than in any other age group.
For all these risks, the antidote is exercise. In a large, long-term project called the Nurses’ Health Study, women who exercised the most had their risk of colon cancer drop almost by half. People diagnosed with colorectal cancer are less likely to die of cancer if they begin to exercise. Exercise has also been shown to reduce the risk of developing breast and prostate cancer, although these cancers are hormonally mediated, so the effect of exercise may be more complex.
Some of exercise’s protective effects may come from reducing fat around the midsection. Exercisers and other people with less belly fat are at lower risk for developing diabetes. Weight loss through diet alone isn’t enough because muscle mass is lost. In contrast, exercisers lose more fat than dieters and decrease their burden of central adiposity. In layman’s terms, they’re not as soft at the middle.
But what about those vitamins? Although vitamins can sometimes stop cancer growth in a Petri dish, inside people they are far less effective. Ironically, Vitamins A, C, and E and beta-carotene all increase the risk of death and have no effect on preventing gastrointestinal cancer. Vitamin D has garnered a lot of attention lately as a possible exception to its neighbors on the supplement aisle, partly because Vitamin D levels in the blood are correlated with a lower rate of cancer. However, Vitamin D levels may simply be an indication of overall health. For example, people who spend more time outdoors naturally have higher Vitamin D levels, and low Vitamin D is correlated with obesity and diabetes. In a recent meta-analysis, the benefit of taking Vitamin D for cancer was a paltry 1 percent. So while it is a good sign if you have healthy blood levels of Vitamin D, taking supplements to get there may not do much to prevent cancer. Compare these minimal benefits with a recent New England Journal of Medicine study showing that a screening colonoscopy cuts your risk of death from colorectal cancer by more than two-thirds.
So, be aware of which of your habits does genuine good. As food journalist Michael Pollan puts it, “Be the kind of person who takes supplements—then skip the supplements.” To be that kind of person, get off the vitamins and onto the treadmill. That way maybe I’ll see you at the gym, and not in my clinic.