One of the reasons it's so difficult to study the relationship between diet and cancer is because many dietary behaviors are associated with non-dietary behaviors. For example, one of the reasons we used to think coffee-drinking caused cancer was because people who drink coffee are more likely to have a cigarette in the other hand, which led to spurious conclusions. When you factor in those considerations, we find that coffee consumption may actually decrease cancer risk, as I detailed in Coffee and Cancer.
The reason it was so difficult to study cancer among coffee-drinkers is the same reason it's so difficult to study cancer among meateaters. Historically, those eating vegetarian have been noted to have lower cancer rates, but maybe it's just because they exercise more, or smoke less, or inhale less diesel fumes because they all own a Prius :)
In my 3 min. video Vegetarians Versus Healthy Omnivores I profiled new data that attempts to control for non-dietary factors by effectively comparing vegetarians only to meateaters who are as slim as vegetarians, exercise as much, smoke as little, and even eat roughly the same amount of fruits and vegetables. After all, maybe the reason vegetarians have been shown to be healthier is less about eating less meat, and more about eating more plants. Though these rigorous controls undercut key vegetarian advantages, such as lower obesity rates, even when vegetarians were compared in effect only to healthy meateaters with healthier diets, researchers still found "the incidence of all cancers combined was lower among vegetarians."
The most striking difference between the dietary groups was in the risk for cancers of the blood and bone marrow, such as non-Hodgkins lymphoma, various leukemias, and myeloma (more details in my 2 min. video EPIC Findings on Lymphoma). Chicken consumption appeared the most hazardous, associated with up to triple the cancer rates for every 50 grams of daily poultry consumption--that's just a quarter of a chicken breast worth!
Why was there so much more lymphoma and leukemia risk among those eating just a small serving of chicken a day? As detailed in my 2 min. video Chicken Dioxins, Viruses, or Antibiotics?, the association between poultry and cancer may be explained by the presence in chickens' and turkeys' flesh of drugs that were fed to the birds, industrial carcinogens such as dioxins, and/or the presence of oncogenic (cancer-causing) viruses.
In Poultry and Penis Cancer I present the largest study to date on poultry workers and cancer mortality, which found a whopping 8-fold increased risk of dying from penile cancer compared to controls, a finding thought due to chicken virus exposure, raising broader food safety concerns. If chickens can be infected with viruses linked to cancer in consumers, then what about eggs? Find out in Carcinogenic Retrovirus Found in Eggs.
These cancer-causing viruses found in chicken and eggs are utterly destroyed by proper cooking, so the primary risk of infection would presumably be associated with cross-contamination, such as when mixing cake batter, handling raw poultry, or even just touching meat in the supermarket.
-Michael Greger, M.D.
Image credit: Kate Tomlinson / Flicker