A diet high in animal products has long been known to increase the risk of atherosclerosis and heart disease. This increased risk is believed to be mediated largely by the elevation of atherogenic lipoproteins (LDL-C & remnant VLDL and chylomicrons) in the blood, which are caused largely by the high saturated fat and cholesterol content of many meat, dairy, and egg products. More controversial has been the association between the intake of animal foods and the risk of developing cancer. There have been many potential mechanisms proposed by which animal foods might be promoting at least some types of cancer. Elevated levels of potentially cancer-causing substances such as heterocyclic amines and polycyclic hydrocarbons are formed when meat is cooked at high temperatures. Excessive body iron stores can be promoted from an increased intake of heme iron found in meats. Elevated iron stores have been associated with increased risk of some cancers. Fatty animal products also can increase the amount of bile acids released into the gut and alter gut microbes in ways that lead to the production of secondary bile acids and possibly other substances that may promote inflammation and cancer.
There is also growing evidence that higher levels of insulin-like growth factor-1 (IGF-1) in the blood may well promote the growth of cancer cells. IGF-1 levels are increased with insulin resistance that is often brought on by weight gain and inactivity. There is little doubt that inactivity and weight gain are associated with an elevated risk of at least several types of cancer and total cancer mortality too. Part of the association with obesity and cancer may also be related to altered levels of steroid hormones and/or their sex-hormone binding globulin, which appear to play a role in promoting the growth of breast, ovarian, and prostate cancers. However, animal protein intake elevates IGF-1 levels independently of calorie intake and so it may be promoting cancer even in people who maintain a lower BMI, do regular aerobic exercise, and eat mostly low-fat and nonfat animal products. Dr. Fontana and colleagues examined plasma growth factors and hormone levels in 21 people who had been following a low-protein calorie-restricted diet and compared them with those of 21 endurance runners who were equally thin. They concluded, "...our data suggest that a lower protein and calorie intake may have additional protective effects against some types of cancer, because it is associated with a decrease in circulating IGF-1 independent of body fat mass" (1).
More recently, researchers examined the relationship between the intake of various food components estimated from production data reported to the Food and Agricultural Organization of the United Nations and data from 87 countries with the most reliable data on cancer incidence in men and women reported to the World Health Organization. The data on food intake went back to 1980 because it takes many years of exposure to potential carcinogens and cancer-promoting substances before the cancers start to show up. Based on the amount of animal products produced over the past several decades, the researchers came up with an animal product index to estimate average per capita intake since 1980. They also developed a smoking index to estimate how much people in those 87 countries had been smoking over the last few decades. For men, who tended to have smoked much more than women in most countries, the smoking index explained about twice as much of the cancer incidence as the animal product index. However, for women, the animal product index was about twice as predictive of increased cancer incidence as was the smoking index. Together, smoking and animal product consumption appeared to account for about 70% of cancer incidence other than lung cancer. The types of cancer most closely associated with increasing intake of animal products in women included breast, uterine, colon, pancreatic thyroid, and multiple myeloma. For men, prostate and testicular cancers replaced uterine and breast as the ones most strongly associated with increased intake in animal product intake (2). The results of this recent analysis are not much different than those observed many years ago in a now classic study that also found that significantly higher rates of most of these same types of cancer were associated with the intake of greater amounts of animal products (3).
Bottom Line: While there is still much we do not know about how a diet higher in animal products may promote cancer, the evidence continues to mount that a typical modern, Western-style diet loaded with fatty animal products and refined carbohydrates likely is a major factor in most of the common cancer incidences. If it turns out that it is the protein content of animal products and not so much the fat content that is promoting cancer, then efforts to produce animal products with less fat may simply reduce the risk of heart disease while increasing the incidence and mortality from cancer. This is a trend that is already well-established in the United States and other developed countries.
By James J. Kenney, PhD, FACN
- Fontana L, Klein, S, O'Holloszy JO. Long-term low-protein, low-calorie diet and endurance exercise modulate metabolic factors associated with cancer risk. Am J Clin Nutr 2006;84:1456 -62
- Grant WB. A multicountry ecological study of cancer incidence rates in 2008 with respect to various risk modifying factors. Nutrients 2014;6(1):163-89
- Armstrong B & Doll R. Environmental factors and cancer incidence and mortality in different countries, with special reference to dietary practices. Int J Cancer 1975;15:617-31
Stephanie Ronco has been editing for Food and Health Communications since 2011. She graduated from Colorado College magna cum laude with distinction in Comparative Literature. She was elected a member of Phi Beta Kappa in 2008.