The internet is chock full of recommendations of what to add or remove from your diet to stave off cancer. Eat broccoli. Drink green tea. Cut sugar. Don’t overcook your food. But how often do these claims hold water? Are there really superfoods that can prevent cancer or bad foods that can cause or worsen the disease?
Nutrition does play an important role in our overall health, and a poor diet can influence our chances of developing cancer. According to the American Cancer Society, about 1 in 5 cancers in the U.S. and about 1 in 6 cancer deaths can be linked to poor nutrition, being overweight, not exercising, or alcohol. The American Cancer Society recommends healthy eating habits, which include lots of vegetables, fruits, and whole grains, as well as limiting red meats, sugary beverages, highly processed foods, and refined grains.
But how does a specific food, or type of food, affect our risk of cancer? Here is the evidence -- or lack of evidence -- behind some of the most popular cancer-related diet claims.
The Claim: Sugar Fuels Tumor Growth
All cells in our bodies, including cancerous ones, use sugar molecules, also known as carbohydrates, as their primary source of energy. But that’s not the only source of fuel for our cells. Cells can use other nutrients, such as proteins and fats, to grow.
We have no evidence that simply cutting sugar from your diet will stop cancer cells from spreading. “If [cancer cells] are not getting sugar, they’ll start to break down other components from other energy stores within the body,” said Carrie Daniel-MacDougall, PhD, MPH, a nutritional epidemiologist at the MD Anderson Cancer Center in Houston and director of MD Anderson's Bionutrition Research Core.
Scientists are, however, investigating whether certain diets can help slow the growth of tumors. For instance, some preliminary evidence from trials in rodents and humans shows that the ketogenic diet, which is low in carbohydrates and high in fat, may help slow the growth of some types of tumors, such as those in the rectum, when combined with standard cancer treatments like radiation and chemotherapy.
Although they don't understand exactly how this might work, experts have some hypotheses.
Ketogenic diets are good at lowering levels of insulin, a hormone that helps our cells absorb sugar, and research in mice shows that high levels of insulin can weaken the ability of certain therapies to slow tumor growth, according to Neil Iyengar, MD, a medical oncologist at Memorial Sloan Kettering Cancer Center in New York City. “We and others are studying ketogenic diets for those types of tumors in clinical trials,” Iyengar said. “But a ketogenic diet is probably one of those types of diets that is not applicable to general cancer risk reduction. I think it's one of those diets that needs to be matched to the tumor biology.”
But what about cancer prevention? Christine Zoumas, a registered dietitian and director of the Healthy Eating Program at the University of California San Diego Moores Cancer Center, noted an indirect link between eating high amounts of sugar and cancer risk. “Anything that has a lot of added sugars is a source of a lot of calories,” Zoumas said. “When you look at the things that increase cancer risk the most, especially for women, it’s excess body fat.”
The Verdict: Cutting sugar won’t stop cancer from growing, but early evidence suggests that a low-carb diet could enhance the effectiveness of certain cancer treatments.
The Claim: Eating Overcooked or Burnt Food Causes Cancer
When cooked at high temperatures, some foods -- particularly carbohydrates such as bread or potatoes -- release a chemical known as acrylamide.
“Some studies have suggested that by [overcooking or burning food], you create carcinogens in the food that can potentially harm the body,” Iyengar said. “I would call it a hypothesis right now. I’m not convinced this is truly the case.”
Scientists have found that in rodents, high levels of acrylamide -- many times what is found in food -- can cause tumors to form. Human studies, however, have turned up little evidence that the acrylamide in foods raises the risk of cancer. When researchers have examined large groups of people to see if there is a link between acrylamide and cancers in various parts of the body, including the bowel, kidney, bladder and prostate, the majority have failed to find a clear link.
In some cases, even when a potential connection appears, such as between acrylamide and ovarian cancer, that link disappears after using more robust measurement tools, such as looking at acrylamide levels in blood.
Certain methods of cooking meat, such as pan frying, grilling, or smoking, can release other chemicals -- substances called heterocyclic amines and polycyclic aromatic hydrocarbons. As is the case with acrylamide, rodents exposed to high levels of these chemicals develop tumors in various organs. In humans, however, the evidence is much less clear. While some studies suggest eating chemicals from cooked meats can increase the risk for certain cancers, such as colorectal or pancreatic, others have reported no association.
The Verdict: The evidence that eating overcooked or burnt food causes cancer in humans is inconclusive and not compelling.
The Claim: Eating Processed Foods Causes Cancer
The evidence linking processed meats, such as salami, beef jerky, and cold cuts, to the risk of certain cancers -- namely colorectal cancer -- is strong.
In 2015, the International Agency for Research on Cancer (IARC), part of the World Health Organization, classified processed meats as a Group 1 carcinogen, a designation reserved for cancer-causing substances. In a statement about the decision, made after 22 experts from 10 countries looked at hundreds of studies, the agency noted that this decision was based on “sufficient evidence in humans that the consumption of processed meat causes colorectal cancer.”
At the same time, the IARC also looked at the association between red meat and cancer. After examining hundreds of studies, the group concluded that while there were links to colorectal, pancreatic, and prostate cancer, the evidence was limited, and it classified red meat as a “probable carcinogen.”
Some studies that follow people over time suggest that other “ultra-processed” foods, such sodas, canned soups, and instant noodles might increase the risk of developing cancer. Such foods may contain potentially harmful chemicals, such as acrylamide, nitrates, heterocyclic amines, and polycyclic aromatic hydrocarbons, but they are also often high in added sugar, salt, and saturated fat.
According to Zoumas, it’s the nutritional composition of these foods that are the most likely cause for concern, since they come with a lot of calories, which means eating too much can lead to an increase in body fat. Zoumas also noted that it is important to distinguish between “processed” and “ultra-processed” foods. Cutting up fruit, bagging lettuce, or fortifying foods with iron or calcium are ways of processing food that don’t compromise nutritional value or add possibly carcinogenic compounds.
The Verdict: There is a strong link between processed meat and cancer risk. Red meat and ultra-processed foods may also increase cancer risk, but the evidence is not as strong.
The Claim: Some Superfoods Can Prevent Cancer
While experts say that a diet rich in plant-based foods, such vegetables, fruits, and whole grains, can reduce cancer risk, they caution claims of any single superfood that keeps cancer at bay.
“So far, there have not been robust enough data to suggest that one particular food or food product can in and of itself reduce risk of cancer or cancer progression,” Iyengar said. “Nutrition is very complex and strongly relies on the synergy within the total diet that you’re consuming, and also in the context of your general metabolic health, physical activity levels, and genetic predisposition.”
Another consideration when it comes to diets is whether you’re starting a diet before or after a cancer diagnosis. While a plant-based diet may help stave off cancers in healthy people, when it comes to cancer patients, there are other considerations to be made. Daniel-MacDougall noted, for instance, that she wouldn’t recommend that cancer patients begin vegetarian or vegan diets without talking with a cancer dietitian. “Cancer patients really need to think about supporting their immune system, so I don’t want to see a cancer patient start a [new] diet and become protein or B vitamin deficient,” she said.
In addition, not all cancers -- or people -- are the same, so a dietary change that is good or bad for one person may not have the same effect on everyone else. “The type of dietary intervention that is optimal for an individual is going to vary from person to person based on that person's biology, but also their type of cancer and what stage or setting they’re in,” Iyengar said. “While there are general recommendations we can make to lower an individual's risk of developing cancer, I envision a future where we will have the data to support much more personalized recommendations.”
Remember that diet is only one of several things to consider when it comes to cancer prevention, and even people who eat healthy can develop cancer, Zoumas noted. “If you get cancer and you have a healthy lifestyle, it’s going to be easier to go into a treatment and easier to recover -- and you don't know how much worse it could have been,” she said. “For those who choose a healthy lifestyle, it’s never a waste -- and for people who haven't had a healthy lifestyle yet, it’s never too late.”
The Verdict: Adding a single superfood to your daily foods won’t keep you from getting cancer. But eating a diet rich in plant-based foods such as vegetables and whole grains can help prevent the disease.
Diana Kwon is a freelance journalist based in Berlin. She covers health and the life sciences, and her work has appeared in publications such as Scientific American, The Scientist, and Nature. Find her on Twitter @DianaMKwon.