“Cancer is complicated because each cancer has a different set of risk factors acting at different ages,” cautions Walter Willett, professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health.
A plant-forward diet can help you dodge at least one cause of one cancer.
In 2015, the International Agency for Research on Cancer (IARC) put processed meats—like bacon, hot dogs, ham, sausage, and lunch meats—in the same evidence category as tobacco smoking and exposure to asbestos.
“IARC had 22 experts from 10 countries evaluate over 800 studies, and they classified processed meats as carcinogenic to humans based on sufficient evidence for colorectal cancer,” says Marjorie McCullough, senior scientific director of epidemiology research at the American Cancer Society.
But that conclusion “does NOT mean that they [processed meats, tobacco, and asbestos] are all equally dangerous,” noted IARC.
Each daily serving of processed meat (2 oz., or roughly the size of a hot dog) increases the risk of colorectal cancer by about 18 percent, said IARC. In contrast, smoking raises the risk of lung cancer by 1,500 to 3,000 percent (that is, 15 to 30 times).
Burgers, steaks, pork chops, and other unprocessed red meats (beef, pork, lamb, and veal) are probable human carcinogens, concluded IARC, because the evidence was less certain than it was for processed meats.
The evidence behind IARC’s conclusions
Most of the evidence comes from studies that ask people what they eat and then track them for years to see who gets cancer.
“The data suggests that there’s no threshold below which you can say that an amount of processed meat poses no risk of colorectal cancer,” notes McCullough.
(Last year, a meta-analysis concluded that there is too little evidence to recommend eating less red or processed meat, but the authors failed to consider all the evidence on meat and cancer.)
IARC also relied on “strong mechanistic evidence,” says McCullough. “How these foods increase cancer risk is not known, but there are several strong possibilities.” Among them:
N-nitroso compounds
“When you consume processed meat, you may consume pre-formed N-nitroso compounds, which are carcinogens,” says McCullough.
“Other processed meats contain nitrates and nitrites, which can form N-nitroso compounds in the gut.”
Heme iron
Red meats are rich in heme iron, which is part of the hemoglobin in blood. “Heme iron can catalyze the formation of N-nitroso compounds in the gut,” says McCullough.
Here's how red meat's heme iron can help create carcinogenic N-nitroso compounds. (Don't worry about nitrates from vegetables or water.)
Almost 20 years ago, researchers put people on a diet that included either beef and pork or poultry and fish.
“When they ate the red meat, they had a greater output of N-nitroso compounds than when they ate the white meat,” says McCullough.
Mutagens
“Mutagens are formed when meats are cooked at high heat or when they’re exposed to smoke,” explains McCullough. “Mutagens can cause DNA damage, and that increases the potential for colorectal cancer.”
Beyond meat and colorectal cancer, the evidence on diet and cancer is intriguing but uncertain.
The evidence for fruits and vegetables
“Fruits and vegetables are related to lower ER-negative breast cancer,” says Willett. (ER-negative tumors are not responsive to estrogen.)
In a 24-year follow-up of roughly 182,000 nurses, the risk of ER-negative breast cancer was 12 percent lower for every two servings of fruits and vegetables eaten per day.
The links were clearest for orange or yellow vegetables (like carrots or winter squash) and berries.
“The association with ER-negative breast cancer is also seen in studies that look at blood carotenoid levels,” adds Willett. (Carotenoids like beta-carotene give many fruits and vegetables their orange or yellow color.)
And ER-negative tumors are more difficult to treat.
“It’s the most dangerous kind of breast cancer,” says Willett.
Photos: fotolia.com—© robynmac (hot dog, carrots, pork chop), © Elena Moiseeva (burger), © Stanislav Pepeliaev (cold cuts), © Spauln (water), © OlgaLIS (bacon), © arnowssr (spinach), © Artyshot (steak).