Diet and Cancer: What You Need to Know—and Eat—to Feel Your Best While Fighting Cancer

Not if you consume it within recommended limits. Cancer cells do use sugar for energy, just like every other cell in your body, but a normal sugar intake has no direct effect on the growth of tumors. (1) What’s normal? The dietary guidelines published by the U.S. Department of Agriculture and the U.S. Department of Health and Human Services recommend limiting added sugar to 10 percent of your daily calories, or approximately 4 tablespoons. (2)

Animal studies suggest that too much sugar can have a negative impact on cancer progression. One study in the February 2016 issue of Nature found that feeding young mice enough sugar to cause hyperglycemia (high blood sugar) and insulin resistance also promoted more tumor growth in their livers than in mice fed a low-sugar diet. (3)

Another mouse study, published in the June 2015 issue of the Journal of Translational Medicine, found a typically Western, high-fat diet with sugar supplements in the form of high fructose corn syrup led to cell damage, inflammation, and tumor development. (4) A high sugar intake also promotes weight gain, and while study results have been mixed, excess weight may increase your risk of cancer and cancer recurrence. (5)

Some studies indicate that being overweight increases the risk of several types of cancer, particularly in the breast, endometrium, colon, esophagus, kidney, pancreas and gallbladder, and possibly other types of cancer as well. Although the connection is not well understood, it may be that excess fat in the body causes increased hormone production, chronic inflammation, or poor regulation of tumor cell growth which, in turn, promote the development of different types of cancer. (6)

If and how excess weight affects cancer risk and cancer recurrence may depend on many factors, including your sex, when you gain the weight, and the type of cancer involved. For instance, weight gain is associated with breast, endometrial and ovarian cancer in postmenopausal women, colon cancer in men, and kidney cancer in all adults. (7)

If you’re overweight, losing weight may reduce your risk of cancer and recurrence, and will certainly improve your health in other ways, but the most promising benefits come from avoiding excess weight gain in the first place.

A ketogenic diet is a very high fat, moderate protein, very low carb medical diet originally developed to treat rare and difficult cases of epilepsy. Variations on the diet have been promoted for facilitating weight loss, but most are not endorsed by health experts.

In recent years, studies focusing on the diet’s effects on cancer found reduced tumor size and delayed tumor growth in laboratory animals in which cancer of the colon, gastrointestinal tract and prostate had been induced. Limited studies and individual case reports in humans found that a ketogenic diet may help improve a patient’s response to radiation and chemotherapy while he or she undergoes these treatments.

A long list of potential side effects accompany a ketogenic diet, especially in children but also in adults. These include nausea and vomiting, hypoglycemia (low blood sugar), mineral deficiency and bone loss, kidney damage and stones, and increased blood cholesterol. For these reasons, a ketogenic diet should only be followed if it's medically prescribed and supervised. (8)

While it’s a good idea to focus primarily on a plant-based diet — one that is rich in fresh vegetables and fruits, whole grains and legumes (beans and lentils) — it's true that high-quality lean meats and poultry in reasonably sized portions can also contribute valuable nutrients to your diet.

But red meat and processed meats are another story. Many studies have consistently linked red meat and processed meats (cold cuts, sausages and hot dogs, as well as processed poultry products) with colorectal cancer. (9) The more meat consumed, the higher the risk. A review of studies specifically looking at meat consumption and colorectal cancer risk found that the risk increased 36 percent for every 3½ ounces of red meat and 28 percent for every 1½ to 2 ounces of processed meat consumed each day. (10)

Cooking any meat products at high temperatures (above 300 degrees F) and exposing them to smoke — by grilling, frying, or broiling, for instance — produces chemicals known as heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs) that are known to increase tumor development throughout the bodies of laboratory rodents. No direct link has been found between the formation of these chemicals on meat products and cancer in humans, but researchers around the world continue to investigate possible links between meat consumption, cooking methods, and cancer. (11)

For most people, that’s a big YES, particularly when it comes to prevention of and survival with colorectal cancer. (12,13) Although studies show mixed results, most support the current recommendation of getting approximately 25 grams of fiber each day from fresh fruits and vegetables, whole grains and legumes (beans and lentils) in order to maintain a healthy gut, reduce inflammation, and prevent chronic disease.

Researchers are also looking at associations between different sources of fiber and other cancers. For instance, the European Prospective Investigation Into Cancer and Nutrition found that high total fiber intake is associated with a reduced risk of liver cancer. (14) A high intake of bean and grain fiber has also been associated with a 20 to 38 percent reduction in breast cancer risk. (15) It may be especially important to adopt a high-fiber habit starting early in life. (16)

Alcohol can increase the risk of some types of cancer, especially when combined with other risk factors, such as smoking. Even light alcohol consumption is associated with an increased risk of oral cancer, cancer of the pharynx, and breast cancer in women, when compared with nondrinkers and those who drink only on occasion.

Moderate to heavy (but not light) drinking is associated with colorectal cancer and cancer of the larynx, while studies show that heavy (but not light or moderate) drinking increases the risk of liver, stomach, lung, pancreas and gallbladder cancers. (17) The U.S. Dietary Guidelines define moderate drinking as up to one drink a day for women, two for men. (18)

Cancers that have not been linked to alcohol consumption include Hodgkin’s and non-Hodgkin’s lymphoma and those of the brain, cervix, ovary, small intestine, endometrium, and bladder. Interestingly, some studies have found that light to moderate drinking may actually lower the risk of Hodgkin’s and non-Hodgkin’s lymphoma, as well as kidney and thyroid cancers when compared with rates in nondrinkers and occasional drinkers. Researchers don’t yet know why this may be true. (17)

According to the American Cancer Society, the amount of alcohol (ethanol) you drink over time has more of an effect on cancer risk than the type of alcoholic beverage you choose. Some of the ways ethanol, and its byproducts that form within your body, may increase cancer risk include damaging tissue, blocking nutrient absorption, increasing estrogen production, and causing potential changes in DNA. (19)

When researchers reviewed more than 50 observational studies looking for links between diet and cancer, they found significant differences in cancer occurrence, recurrence, and death rates among people who did and did not follow a traditional Mediterranean-style diet.

Overall, those who follow a similar diet appear to be at lower risk of dying from any type of cancer. When the researchers looked at specific types of cancer, they found that people who followed a Mediterranean diet pattern had lower rates of colorectal, breast, gastric, prostate, liver, and head and neck cancer, while no significant association was found between this type of diet and other cancer locations. (20,21) Another study that looked at the Mediterranean diet for prevention of heart disease found that the diet also appears to protect specifically against breast cancer in postmenopausal women. (22)

The ingredients of a Mediterranean-style diet — fresh fruits and vegetables, fish, whole grains, legumes, nuts, and olive oil — are not all individually associated with a reduced risk of cancer. It’s not possible to analyze every individual food in the context of its relationship to every type of cancer. Rather, together, and as part of an overall approach to healthy living, these foods form the basis of a plant-based dietary pattern that may help prevent many forms of cancer. Other aspects of the diet and associated lifestyle, such as eating less meat and fewer high-fat dairy products, using flavorful seasonings other than salt, getting more exercise, and enjoying meals in a celebratory fashion with family and friends, also contribute to its healthfulness. (23)

Researchers point out, however, that although there seems to be an association between following a Mediterranean-style diet and some reduced cancer risks, the evidence for some cancers is stronger than for others, and there is no “one-size-fits-all” plan to help prevent and treat different types of cancer. Other global cuisines that incorporate similar types of food, eaten in similar ways, can be equally healthful.

And just as different stages of cancer are subject to different treatment recommendations, individual diet guidelines are also advised. (21) For instance, if you’re undergoing radiation or chemotherapy treatment, you may have side effects that include gastrointestinal symptoms like nausea, diarrhea, or constipation that will affect the way you eat. (21) You may need to adjust your diet, and even cut back on some of the healthier foods you normally eat, such as some high-fiber foods, until these symptoms subside.