Cancer Control

By Alice Park

Ask a doctor about the best ways to prevent breast cancer, and you're likely to hear about a number of things you can't easily control: being blessed with the right genes, hitting puberty later than age 12, having your first child before you're 30. It's not exactly a basis for action. Increasingly, though, physicians are mentioning a few things you can do that just might help reduce the frightening 1-in-8 odds of getting breast cancer. They include keeping to a low-fat diet, watching your weight, avoiding stress and getting plenty of exercise.

The strongest evidence that lifestyle can make a difference comes from studies of women who have already had breast cancer. A study published in May found, for instance, that patients who had switched to a low-fat diet, with no more than 20% of their daily calories derived from fat, had a 24% lower rate of breast-cancer recurrence within five years than women who did not alter their eating habits. Another 2005 study found a similar benefit from exercise. Breast-cancer patients who walked briskly for one to three hours a week had a 25% lower death rate from the disease than sedentary patients. The finding has oncologists urging patients to get moving.

Exercise and a low-fat diet are good for you in principle, but whether they cut the risk of breast cancer for women who have never had the disease is still being investigated. "I don't think we have enough evidence yet," says Dr. Therese Bevers, medical director of cancer prevention at MD Anderson Cancer Center in Houston. Cutting back on fat does, however, make sense in theory. From 60% to 70% of primary breast cancers are hormone dependent--they grow in response to estrogen. Excess dietary fat primes the ovaries to release more estrogen, which in turn promotes tumor growth. And yet studies don't consistently show that low-fat diets lead to less cancer. That may be because other dietary elements, including overall calories, also influence tumor development by affecting growth factors and hormones like insulin, which also promote cancer.

Weight is another risk factor related to fat that may affect breast tissue through its own routes. Before menopause, women who are overweight may enjoy a slightly reduced risk of breast cancer, most likely because they ovulate less often than do thinner women, so their breasts are exposed to less estrogen. After menopause, however, heavier women are at greater risk. That's probably because once the ovaries stop churning out estrogen, the body's fat cells become its primary source of the hormone. Losing excess weight is one concrete step that women can take to reduce the risk of developing breast cancer after menopause.

Even women with genetic mutations that give them an 80% risk of getting the disease in their lifetime can benefit from such weight loss. A recent study found that overweight women with BRCA1 mutations who dropped more than 10 lbs. before their 30th birthday cut their risk of developing breast cancer 65% over the next two decades.

Researchers have also begun to look at how lifestyle habits during childhood influence breast-cancer rates. This summer Karin Michels, an epidemiologist at Brigham and Women's Hospital in Boston, released a study in which the mothers of more than 2,000 nurses were asked to recall what their daughters ate as preschoolers. She found that each additional weekly serving of French fries consumed from ages 3 to 5 increased the risk of getting breast cancer as an adult 27%. Oddly enough, the same wasn't true of other high-fat foods, such as ice cream or hamburgers.

Michels is the first to admit that her study alone isn't enough to establish the importance of childhood diet in breast-cancer incidence, but other work seems to support the idea. Dr. Steven Narod, a preventive-medicine geneticist at the University of Toronto, studied a group of Filipino women and found that women whose mothers cooked extensively with coconut oil, high in saturated fat, had higher rates of breast cancer than women whose mothers did not rely so heavily on the oil.

Childhood diet also influences two known risk factors for breast cancer: the timing of a girl's first menstrual period and her height (taller women are at slightly higher risk of developing breast cancer). Last year, in a study of over 117,000 Danish women, researchers concluded that girls who experienced the largest growth spurts from ages 8 to 14, when breast tissue is maturing, had the greatest chance of getting breast cancer later. Narod theorizes that the developing breast cells become particularly sensitized to the effects of growth factors and hormones. "More studies are suggesting that life events around the time of breast development may be critical in impacting risk and prevention of breast cancer," he says.

Meanwhile, other scientists are trying to determine how exercise affects the development of breast cancer. The American Cancer Society recommends that women adopt a fairly intense program if they hope to protect themselves from breast tumors. Its data show that a minimum of an hour and 15 minutes of brisk walking each week can reduce the risk of cancer 18%, while logging 10 hours a week can have an even greater benefit.

What's not clear is whether physical activity helps directly--by influencing levels of estrogen and other body chemicals--or indirectly, by promoting weight loss. "We don't understand all the various potential mechanisms involved," admits Dr. Moshe Shike, director of clinical nutrition at Memorial Sloan-Kettering Cancer Center in New York City, "but taking exercise and diet factors together is helpful in reducing risk."

That may sound vaguer than women might ideally like, but at least it's a start. And it's a lot more honest than the big claims found in popular books touting anticancer diets. No diet and exercise program is ever going to completely eliminate risk, explains Michels, because "no one factor alone causes breast cancer." Eating French fries as a toddler, say, won't doom you, but the wrong childhood diet coupled with a sedentary lifestyle, weight gain and not giving birth could be a formula for disaster. Luckily, as researchers identify more ingredients in the formula, women are gaining more control of their fate. p