The same kind of international comparisons that sparked the dietary fat–heart disease hypothesis have also generated strongly held beliefs about a connection between dietary fat and cancer. Countries with lower average fat intakes (mostly developing or less aﬄuent nations) tend to have lower rates of breast, colon, or prostate cancer than countries with higher average fat intakes. But better, more direct evidence linking diet and cancer has weakened support for this connection.
Throughout the 1960s and 1970s, international comparisons pointed to a link between dietary fat and breast cancer. Keep in mind that many other lifestyle diﬀerences between women in traditional cultures and those in Western countries could be responsible, such as age of ﬁrst menstruation, physical activity, smoking, and other dietary factors like fruit and vegetable or ﬁber consumption. Case-control studies—those in which women who developed breast cancer were compared with women who did not—also tended to ﬁnd a connection, although weaker ones than those found in the international comparisons. Based on a limited number of studies, the U.S. National Research Council concluded back in 1982 that reducing the fat content of the diet from 40 to 30 percent of calories would reduce the number of women diagnosed with breast cancer. Two years later, the National Cancer Institute made this the focus of a major health-promotion campaign. These eﬀorts have borne little fruit in lowering breast cancer, perhaps because they were based on inadequate data.
learn more: Fats in the Bloodstream
Later, larger studies of cancer have not supported the dietary fat–breast cancer connection. In the Nurses’ Health Study, more than ﬁve thousand of the participants have developed breast cancer since 1980. So far we have not seen an increase in breast cancer with higher dietary fat. In fact, the rate of breast cancer among women who ate the most dietary fat was slightly lower than the rate among women who ate the least. An analysis of all the large cohort studies from around the world also found no connection between dietary fat and breast cancer, except for an unexpected increase among the small numbers of women with the lowest fat intake.
Most studies into the connection between dietary fat and breast cancer involved postmenopausal women. That makes sense, because breast cancer is more common in older women. But it also can strike younger women, who may be susceptible to diﬀerent dietary inﬂuences. So my colleagues and I examined information collected as part of the Nurses’ Health Study II, which includes women between twenty-six and forty-four years old. Over an eight-year period, 714 of the more than 90,000 women developed breast cancer. High intake of animal fat, especially fat from red meat and dairy products, increased the chances of having breast cancer. High intake of vegetable fat did not. I’ll talk more about unsettling ﬁndings about dairy products.
learn more: Building a Better Pyramid
The clearest and most consistent ﬁnding from both animal and human studies is that too many calories, regardless of food source, are far more important to the development of breast cancer than dietary fat.
Some early studies suggested a link between dietary fat and colon cancer, the third leading cause of cancer deaths in the United States. But this, too, hasn’t been bolstered by more detailed work. There are some strong hints that eating a lot of red meat increases the risk of colon cancer. This could stem from the types of fats in red meat or the cancer-causing chemicals generated by cooking red meat at high temperatures. So far, too many calories in relation to exercise levels is the strongest dietary link with colon cancer—people who are overweight are more likely to develop this cancer than people who aren’t. On the ﬂip side, regular physical activity decreases your chances of getting colon cancer. So does not smoking and getting plenty of folic acid, one of the B vitamins .
The situation for prostate cancer is murkier, partly because there have been relatively few studies in this area. International comparisons show that Asian men, who eat relatively low-fat diets, have substantially lower rates of prostate cancer than their Western counterparts. Although Asian men do experience some increases in prostate cancer when they move to the United States, rates in this group always stay lower than among Caucasians, suggesting that some genetic factors play an important role. If there is a connection between dietary fat and prostate cancer, it seems to be mainly related to animal fat or some other component of red meat. That’s good news, because it means that olive oil and other unsaturated fats that decrease the risk of heart disease would not increase the risk of prostate cancer.
It is impossible to prove that there’s absolutely no connection between dietary fat and cancer. If fat does inﬂuence the development of cancer, though, evidence from large cohort studies with many years of follow-up show that the eﬀect is small. Given the strong and consistent association that has been observed between type of fat and heart disease, I think it makes sense to focus on dietary fats for their proven impact on heart disease, not for their hypothetical connections with cancers that so far have not been supported by good evidence.