Can your diet protect against cancer?

Evidence has increasingly shown that following a plant-based eating pattern, such as a vegetarian, vegan, Mediterranean, or DASH (Dietary Approaches to Stop Hypertension) diet, helps manage cholesterol and lower the risk for heart disease, the No. 1 cause of death in America. These diets emphasize fruits, vegetables, whole grains, legumes, and low-fat dairy products, with no (or minimal) red meat, processed foods, and alcohol.

But what about cancer, the No. 2 cause of death? Can this dietary approach also protect you from cancer, or even slow its progression?

The short answer is yes, but it's unrealistic to think that one type of diet could reduce cancer risk across the board.

"Linking diet with cancer prevention is like putting together a jigsaw puzzle without all the pieces and the picture on the box," says Theresa Fung, an adjunct professor of nutrition at Harvard's T.H. Chan School of Public Health. "You can develop a sense of what the final image might be, but it remains incomplete and somewhat frustrating."

Why proof is challenging

One of the main reasons for the ambiguity is that diets don't lend themselves to clinical trials, the best types of studies. So, most nutritional research consists of observational studies that show associations but not cause and effect. Take blueberries, for example. Blueberries are high in antioxidants and some research shows a link between eating foods rich in these substances and reduced cancer risk, possibly because antioxidants can dampen inflammation and protect against damage to cell DNA.

But this doesn't mean eating a lot of blueberries will protect you from cancer. "The problem with association studies like this is that you can't say for certain whether blueberries are the cancer fighter, or if it's one of its nutrients, or some other factors," says Fung.

Some studies have shown cruciferous vegetables like broccoli, cauliflower, brussels sprouts, and kale have a cancer-blocking effect similar to fruit. But here scientists have cited the vegetables' high amounts of glucosinolates (sulfur-containing compounds). Still others have touted the phytochemicals in fruits, vegetables, and grains for their ability to protect cells from damage.

A study in the December 2020 issue of Nutrients reviewed 54 studies and found a link between eating whole grains and a lower risk of cancers like those of the colon, pancreas, and esophagus. The reason? Scientists believe the high fiber in whole grains dilutes possible carcinogens (substances that cause cancer) and speed up digestion so smaller amounts can be absorbed by the body.

Then there is the issue of serving sizes. How much is enough? Again, it's up for debate. "Observational studies highlight different ranges but don't provide solid information on specific amounts and servings," says Fung.

Keep eating simple

What does all this mean? Your diet definitely can help with cancer prevention, but Fung says people shouldn't obsess about eating certain foods or specific amounts. Instead, embrace an overall healthy diet that includes a variety of fruits and vegetables.

"Follow a Mediterranean or DASH diet or similar plant-based eating plan if you want more structure," she says. "Otherwise, keep it simple."

Focus on consuming various fruits and vegetables every day, along with legumes (beans, peas, peanuts) and whole grains (oatmeal, popcorn, whole-wheat bread, brown rice).

Also, cut down (or cut out entirely) red meat, processed foods, and sugar-sweetened beverages. A 2019 study in JNCI Cancer Spectrum found that consuming these foods was associated with higher risks of various cancers.

"If you are eating a lot of unhealthy foods, that probably means you are eating too few healthy ones, and vice versa," says Fung.

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