Cases of colorectal cancer in the United States are down by about half what they were in the 1960s and so are the deaths, points out Walter Willett, chair of the nutrition department at the Harvard T.H. Chan School of Public Health.
“We’ve known for a long time that lifestyle and perhaps other aspects of the environment can have a profound effect on the risk of this cancer.”
Here are some of the factors that may or may not matter.
Dietary fiber. Fiber was long believed to protect against colorectal cancer, notes Willett. “It just made sense that fiber would move carcinogens more rapidly through the colon, so it would dilute them.” And some health authorities, like the American Institute for Cancer Research, recommend fiber-rich foods to prevent colorectal cancer. But the evidence is mixed. “Some European studies find a link,” says Willett. “But interestingly, the U.S. data on fiber and colorectal cancer is not impressive.”
Calcium. “Calcium is pretty clearly a beneficial factor for lowering the risk of colorectal cancer,” explains Willett. For example, when researchers pooled data from 10 studies on more than 534,000 people, those who consumed about 1,000 milligrams of calcium a day had roughly a 20 percent lower risk of colorectal cancer than those who consumed less than 500 mg a day. “Dairy products have also been related to lower risk, likely through the calcium pathway.”
The Recommended Dietary Allowance for calcium is 1,000 mg a day, but it jumps to 1,200 mg for women over 50 and men over 70. “Most of the benefit comes from getting 800 or 1,000 mg a day,” according to Willett. “And you can basically get there with one or two servings of dairy on top of a healthy diet. Or you can take a calcium supplement. With their calories and saturated fat, some dairy foods are a very expensive way to get some calcium.”
Whole grains. “They’re not a game changer for colorectal cancer,” says Willett. “But the evidence is so strong for cardiovascular disease and diabetes that eating whole grains is still important.”
Sugar. “We’ve looked, but we don’t see anything big there,” Willett notes.
Constipation. In a study of more than 110,000 nurses and health professionals, neither the frequency of bowel movements nor laxative use was linked to cancer risk. “We didn’t see a relationship with bowel habits, which also argues against the fiber hypothesis,” says Willett.
Alcohol. “It’s a risk factor, but the risk only goes up when you get beyond two drinks a day.”
Folic acid. “If you’ve done studies in cells and in animals, as I have, you know that consuming too much folic acid can enhance the development of cancer,” says Joel Mason, director of the Vitamins and Carcinogenesis Laboratory at the Human Nutrition Research Center on Aging at Tufts University. “If you think about the role folate plays in the cell, helping to synthesize new cells, this makes eminent sense.”
But the evidence is strong that a higher intake of folic acid is linked to a lower risk of colorectal cancer in observational studies that track people for years, notes Willett. On the other hand, clinical trials find no lower risk of colorectal cancer in people who take folic acid than in placebo takers, possibly because those studies last only a few years.
In the past, some researchers feared that the folic acid that is added to white flour might raise the risk of colorectal cancer. “But we’ve seen a lower—not a higher—risk, so that hasn’t been a problem,” says Willett. “The folic acid added to white flour and multivitamins has very likely contributed to a lower risk.”
Smoking. “It’s very clear that smoking is a cause of colorectal cancer, and the decline in smoking in the United States is one factor that’s contributing to our decline,” says Walter Willett. “Smoking has gone up in many East Asian countries and in China, in particular, now that people can afford it.” At the same time, colorectal cancer rates are skyrocketing in those countries. Rates in Japan now actually exceed those in the U.S., whereas years ago it was the reverse.
Sources: Int. J. Cancer 129: 192, 2011; JAMA 294: 2849, 2005; N. Engl. J. Med. 342: 1149, 1156, 2000; Cancer Causes Control 24: 1015, 2013.
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