Meat Consumption and Cancer Risk
The World Health Organization has determined that dietary factors account for at least 30 percent of all cancers in Western countries and up to 20 percent in developing countries. When cancer researchers started to search for links between diet and cancer, one of the most noticeable findings was that people who avoided meat were much less likely to develop the disease. Large studies in England and Germany showed that vegetarians were about 40 percent less likely to develop cancer compared to meat eaters.1-3 In the United States, researchers studied Seventh-day Adventists, a religious group that is remarkable because, although nearly all members avoid tobacco and alcohol and follow generally healthful lifestyles, about half of the Adventist population is vegetarian, while the other half consumes modest amounts of meat.
This fact allowed scientists to separate the effects of eating meat from other factors. Overall, these studies showed significant reductions in cancer risk among those who avoided meat.4 In contrast, Harvard studies showed that daily meat eaters have approximately three times the colon cancer risk, compared to those who rarely eat meat.
A number of hypotheses have been advanced to explain the connection between meat consumption and cancer risk. First, meat is devoid of fiber and other nutrients that have a protective effect. Meat also contains animal protein, saturated fat, and, in some cases, carcinogenic compounds such as heterocyclic amines (HCA) and polycyclic aromatic hydrocarbons (PAH) formed during the processing or cooking of meat. HCAs, formed as meat is cooked at high temperatures, and PAHs, formed during the burning of organic substances, are believed to increase cancer risk. In addition, the high fat content of meat and other animal products increases hormone production, thus increasing the risk of hormone-related cancers such as breast and prostate cancer.
Carcinogenic Compounds in Cooked Meat
HCAs, a family of mutagenic compounds, are produced during the cooking process of many animal products, including chicken, beef, pork, and fish. Even meat that is cooked under normal grilling, frying, or oven-broiling may contain significant quantities of these mutagens.6,7,8 The longer and hotter the meat is cooked, the more these compounds form. In some studies, grilled chicken has formed higher concentrations of these cancer-causing substances than other types of cooked meat.9
The major classes of heterocyclic amines include amino-imidazo-quinolines, or amino-imidazo-quinoxalines (collectively called IQ-type compounds), and amino-imidazo-pyridines such as PhIP. IQ-type compounds and PhIP are formed from creatine or creatinine, specific amino acids, and sugars.10 All meats (including fish) are high in creatine, and HCA formation is greatest when cooking meat at high temperatures, as is most common with grilling or frying. Consumption of well-done meat and PhIP has been associated with increased risk of breast cancer and colon cancer, as discussed in greater detail below. A recent case-control study at the University of Utah that included 952 subjects with rectal cancer and 1205 controls found that men and women with the highest consumption of processed or well-cooked meat had an increased risk of rectal cancer.11
Polycyclic Aromatic Hydrocarbons
Grilling or broiling meat over a direct flame results in fat dropping on the hot fire and the production of polycyclic aromatic hydrocarbon-containing flames. Polycyclic aromatic hydrocarbons (PAHs) adhere to the surface of food, and the more intense the heat, the more PAHs are present.5 They are widely believed to play a significant role in human cancers.12 A fairly consistent association between grilled or broiled, but not fried, meat consumption and stomach cancer implies that dietary exposure to PAHs may play a role in the development of stomach cancer in humans.5
Meat and Breast Cancer
Countries with a higher intake of fat, especially fat from animal products, such as meat and dairy products, have a higher incidence of breast cancer.13,14,15 In Japan, for example, the traditional diet is much lower in fat, especially animal fat, than the typical western diet, and breast cancer rates are low. In the late 1940s, when breast cancer was particularly rare in Japan, less than 10 percent of the calories in the Japanese diet came from fat.16 The American diet is centered on animal products, which tend to be high in fat and low in other important nutrients, with 30 to 35 percent of calories coming from fat. When Japanese girls are raised on westernized diets, their rate of breast cancer increases dramatically. Even within Japan, affluent women who eat meat daily have an 8.5 times higher risk of breast cancer than poorer women who rarely or never eat meat.17 One of the proposed reasons is that fatty foods boost the hormones that promote cancer.
According to new findings from the Shanghai Women’s Health Study, soy food intake provides protection against premenopausal breast cancer when consumed during adolescence and as an adult. The usual dietary intake of 73,223 Chinese women during adulthood and adolescence was assessed after a mean follow-up of 7.4 years. Those with the highest intake of soy protein or isoflavone versus those with the lowest had about half the risk of premenopausal breast cancer regardless of age at time of consumption. No significant association with soy foods was found for postmenopausal breast cancer.18
The consumption of high-fat foods such as meat, dairy products, fried foods, and even vegetable oils causes a woman’s body to make more estrogens, which encourage cancer cell growth in the breast and other organs that are sensitive to female sex hormones. This suggests that, by avoiding fatty foods throughout life, hormone-related cancer risk decreases. A 2003 study, published in the Journal of the National Cancer Institute, found that when girls ages eight to ten reduced the amount of fat in their diet—even very slightly—their estrogen levels were held at a lower and safer level during the next several years. By increasing vegetables, fruits, grains, and beans, and reducing animal-derived foods, the amount of estradiol (a principal estrogen) in their blood dropped by 30 percent, compared to a group of girls who did not change their diets.19
Harvard researchers recently conducted a prospective analysis of 90,655 premenopausal women, ages 26 to 46, enrolled in the Nurses’ Health Study II and determined that intake of animal fat, especially from red meat and high-fat dairy products, during premenopausal years is associated with an increased risk of breast cancer. Increased risk was not associated with vegetable fats.20
In addition, researchers at the Ontario Cancer Institute conducted a meta-analysis of all the case-control and cohort studies published up to July 2003 that studied dietary fat, fat-containing foods, and breast cancer risk. Case-control and cohort study analyses yielded similar risk results, with a high total fat intake associated with increased breast cancer risk. Significant relative risks for meat and saturated fat intake also emerged, with high meat intake increasing cancer risk by 17 percent and high saturated fat intake increasing cancer risk by 19 percent.21
Several studies show meat intake to be a breast cancer risk factor, even when confounding factors, such as total caloric intake and total fat intake, are controlled.22,23 Part of the reason may be that meat becomes a source of carcinogens and/or mutagens, such as HCAs, that are formed while cooking meat at high temperatures. A review of HCAs showed that certain HCAs are distributed to the mammary gland and that humans can activate HCAs metabolically.24 As a consequence, frequent meat consumption may be a risk factor for breast cancer.22
Source: Dr. Neal Barnard