Does a Cancer Prevention Diet Really Exist?

The thinking about how lifestyle and diet affect your risk of cancer has shifted. Here's what you need to know now.

This year, more than 1.6 million Americans will learn they have cancer. Researchers have been working for decades to bring that number down and-despite recent headlines about a study suggesting that a lot of cancer is just "bad luck"-the American Association for Cancer Research says at least half of all cancer deaths are, in fact, preventable if we don't smoke, keep a normal weight, exercise, use sunscreen, eat a healthy diet, etc. Straightforward, right? If we only knew what an anti-cancer diet looked like.

Related: 6 Cancer-Fighting Foods to Add to Your Diet

A generation ago, eating fruits and vegetables seemed like the answer, with estimates that they might lower your risk of 78 percent of all cancers. At one point, experts at the National Institutes of Health confirmed that the multimillion-dollar 5-A-Day campaign encouraging eating fruits and veggies to fight cancer and other chronic diseases was backed by a "diverse and convincing body of evidence." Now there's evidence that is more diverse-and less convincing.

The Nurses Health Study and the Health Professionals Study, which combined followed over 100,000 men and women for more than a decade, reported in 2004 that fruit and vegetable consumption did not affect cancer risk. That was followed by similar findings from Greece and Japan. And finally, a 2010 analysis of 500,000 Europeans had results so disheartening that some skeptics went so far as to declare the 5-a-day "promise" simply a myth perpetuated by the produce industry.

So, should we pass the bacon cheeseburgers and ditch broccoli and salads?

Well, no. Rather than negating the value of veggies, the underlying message is: you can't count on easy, singular solutions. Just like the disease itself, diet advice is complicated. When it comes to cancer prevention, "we may never fully understand all the individual components that make up a healthy diet," says Carrie R. Daniel, Ph.D., M.P.H., of the University of Texas MD Anderson Cancer Center. "It's incredibly complex."

Related: Is Eating Red Meat a Big Cancer Risk?

While experts still say plants are a major part of an overall pattern of eating that your body prefers, they now recognize that what you're not having when you help yourself to broccoli may be as important as eating the vegetable itself. "People who are eating fruits and vegetables are eating less of something else, often animal products," says Gary Fraser, Ph.D., of Loma Linda University, whose research has found that vegetarians are less likely to develop cancer, for reasons that aren't fully understood. Data published in March in JAMA Internal Medicine shows vegetarians have a 22 percent lower risk of colorectal cancer (one of the most common malignancies). The study is part of ongoing research following more than 77,000 Seventh-Day Adventists.

The Research Isn't Black & White

The enthusiasm of the early days-when we heard you could cancer-proof yourself with produce-arose largely from studies comparing the diets of people who had cancer with the diets of people who didn't. Problem is, these studies can be biased because volunteers who don't have cancer tend to be health-conscious (and eat more veggies).

Other tantalizing clues helped back up the initial findings that fruits and vegetables are protective. Take breast cancer. Japanese women who move to the United States tend to approach the higher risk level of their adopted home, suggesting that something about their new diet and lifestyle makes a big difference.

Newer studies (the ones that found little, if any, protection through produce on overall cancer risk) are stronger evidence because they started with a group of people with no illness, noting who developed cancer and who didn't, and seeing how their diets matched up. These so-called prospective studies take longer, but are seen as more powerful because participants aren't potentially biased coming in. But that type of study has drawbacks too. Researchers ask people to report total consumption of fruits and vegetables (but don't record much detail about which types of produce they eat). And the studies usually look at the occurrence of all cancers bundled together, so protection from any one particular kind of food against a particular cancer gets lost in the background. For example, if tomatoes really do guard against prostate cancer, bundling them with every other kind of produce and prostate with every other kind of cancer masks the correlation. Pooling data like that might also dilute areas where benefit may be stronger. "If you lump all cancers together, you get weaker evidence for any individual one," says Alice Bender, M.S., R.D.N, of the American Institute for Cancer Research.

What Not to Eat

Further, studies face limitations factoring out the effects of what you're not eating. People who eat little red and processed meat, and replace that space on their plates with fiber-rich plant foods (beans, broccoli, whole fruit, etc.), could cut their colorectal cancer risk by as much as 50 percent.

Investigate further and you'll find the harm may not come from just the meat itself, but how it's prepared, says Daniel. The smoke from meat on a barbecue has some of the same carcinogens as cigarettes and car exhaust, she says, and those compounds can work their way into your food and then your body. Char your rib-eye or singe it in the broiler, and that crispy crust likely contains heterocyclic amines and polycyclic aromatic hydrocarbons, carcinogens formed when meat is exposed to high temperatures. Curing, which preserves about one-quarter of the meat Americans buy each year, may also be problematic. Processed meats have been linked to a 20 to 50 percent greater risk of colorectal cancer, says a report in Nutrition and Cancer, perhaps because they contain more fat and potentially harmful additives like nitrites and nitrates.

Alcohol and sugar also make the list of potentially cancer-causing foods. Women who binge-drink regularly or have more than two drinks a day are thought to raise their risk of breast cancer. Among men, the evidence is less strong, but health experts still advise men not to have more than two drinks a day. That said, it is also true that people who drink a little bit of alcohol have a lower overall risk of some cancers than teetotalers. As for sugar, it's not clear yet if the problem is the sweet stuff itself-and therefore the body's insulin response-or the extra body fat (which can disrupt hormones, potentially upping cancer risk) that can come from eating too much sugar.

Obesity raises the risk of many cancers; for some, like endometrial and esophageal cancers, perhaps as many as 40 percent of cases are attributed to higher body weight. This explains partly why exercise may be beneficial in preventing cancer. But the benefits of exercise go beyond weight control, as physical activity also boosts the immune system and helps regulate certain hormones to healthy production levels.

None of this, however, means certain foods should be forbidden. As usual, moderation rules. "We need to think of filling most of our plate with plant foods," Bender says. "Get those colors in there."

It's also important to remember that cancer isn't the only threat against good health. There's plenty of evidence that eating a healthier diet lowers your risk of heart disease, diabetes, stroke and obesity. So dig into a bowl of blueberries and oatmeal and you'll be less hungry for a blueberry muffin (denser on calories and sugar, lighter on fiber and vitamins). "When you start making good choices, you're not as hungry for the bad choices," Daniel says.

Laura Beil is an independent journalist who writes about health and science. She lives near Dallas.

Photos by Leigh Beisch.