From From the National Cancer Institute, A Conversation with Dr. Walter Willett about Diet and Cancer

October 17, 2017

NCI National Cancer Institute LogoMany people believe there is a connection between obesity and cancer. However, data from observational studies can be difficult to interpret and they cannot definitively confirm that obesity causes cancer. The reason is obese or overweight people may differ from lean people in ways other than their body fat, and it is possible that these other differences—rather than their body fat—are what explains their different cancer risk.

Despite the limitations of the study designs, there is consistent evidence that higher amounts of body fat are associated with increased risks of a number of cancers, including:

  • Endometrial cancer: Obese and overweight women are two to about four times as likely as normal-weight women to develop endometrial cancer (cancer of the lining of the uterus), and extremely obese women are about seven times as likely to develop the more common of the two main types of this cancer. The risk of endometrial cancer increases with increasing weight gain in adulthood, particularly among women who have never used menopausal hormone therapy.
  • Esophageal adenocarcinoma: People who are overweight or obese are about twice as likely as normal-weight people to develop a type of esophageal cancer called esophageal adenocarcinoma, and people who are extremely obese are more than four times as likely.
  • Gastric cardia cancer: People who are obese are nearly twice as likely as normal-weight people to develop cancer in the upper part of the stomach, that is, the part that is closest to the esophagus.
  • Liver cancer: People who are overweight or obese are up to twice as likely as normal-weight people to develop liver cancer. The association between overweight/obesity and liver cancer is stronger in men than women.
  • Kidney cancer: People who are overweight or obese are nearly twice as likely as normal-weight people to develop renal cell cancer, the most common form of kidney cancer. The association of renal cell cancer with obesity is independent of its association with high blood pressure, a known risk factor for kidney cancer.
  • Multiple myeloma: Compared with normal-weight individuals, overweight and obese individuals have a slight (10% to 20%) increase in the risk of developing multiple myeloma.
  • Meningioma: The risk of this slow-growing brain tumor that arises in the membranes surrounding the brain and the spinal cord is increased by about 50% in people who are obese and about 20% in people who are overweight.
  • Pancreatic cancer: People who are overweight or obese are about 1.5 times as likely to develop pancreatic cancer as normal-weight people.
  • Colorectal cancer: People who are obese are slightly (about 30%) more likely to develop colorectal cancer than normal-weight people.
  • A higher BMI is associated with increased risks of colon and rectal cancers in both men and in women, but the increases are higher in men than in women.
  • Gallbladder cancer: Compared with normal-weight people, people who are overweight have a slight (about 20%) increase in risk of gallbladder cancer, and people who are obese have a 60% increase in risk of gallbladder cancer. The risk increase is greater in women than men.
  • Breast cancer: Many studies have shown that, in postmenopausal women, a higher BMI is associated with a modest increase in risk of breast cancer. For example, a 5-unit increase in BMI is associated with a 12% increase in risk. Among postmenopausal women, those who are obese have a 20% to 40% increase in risk of developing breast cancer compared with normal-weight women. The higher risks are seen mainly in women who have never used menopausal hormone therapy and for tumors that express hormone receptors. Obesity is also a risk factor for breast cancer in men. In premenopausal women, by contrast, overweight and obesity have been found to be associated with a 20% decreased risk of breast tumors that express hormone receptors.
  • Ovarian cancer: Higher BMI is associated with a slight increase in the risk of ovarian cancer, particularly in women who have never used menopausal hormone therapy. For example, a 5-unit increase in BMI is associated with a 10% increase in risk among women who have never used menopausal hormone therapy.
  • Thyroid cancer: Higher BMI (specifically, a 5-unit increase in BMI) is associated with a slight (10%) increase in the risk of thyroid cancer.

Does avoiding weight gain or losing weight decrease the risk of cancer?

As with observational studies of obesity and cancer risk, these studies can be difficult to interpret because people who lose weight or avoid weight gain may differ in other ways from people who do not. Nevertheless, when the evidence from multiple observational studies is consistent, the association is more likely to be real. Many observational studies have provided consistent evidence that people who have lower weight gain during adulthood have lower risks of colon cancer, kidney cancer, and—for postmenopausal women—breast, endometrial, and ovarian cancers. 

Fewer studies have examined possible associations between weight loss and cancer risk. Some of these have found decreased risks of breast, endometrial, colon, and prostate cancers among people who have lost weight. However, most of these studies were not able to evaluate whether the weight loss was intentional or unintentional (and possibly related to underlying health problems).

Stronger evidence for a relationship between weight loss and cancer risk comes from studies of people who have undergone bariatric surgery (surgery performed on the stomach or intestines to induce weight loss). Obese people who have bariatric surgery appear to have lower risks of obesity-related cancers than obese people who do not have bariatric surgery.

Nevertheless, the follow-up study of weight and breast cancer in the Women’s Health Initiative found that for women who were already overweight or obese at baseline, weight change (either gain or loss) was not associated with breast cancer risk during follow-up. However, for women who were of normal weight at baseline, gaining more than 5% of body weight was associated with increased breast cancer risk.

Weight Loss Strategy

If you decide to try to lose weight, this list of additives, nutrients, and other dietary components have been studied by scientists for possible associations with cancer risk. You may want to talk with your doctor or a nutritionist about what’s best to eat and avoid. 

  • Alcohol – Although red wine has been suspected of reducing cancer risk, there is no scientific evidence for such an association. Also, alcohol is a known cause of cancer. Heavy or regular alcohol consumption increases the risk of developing cancers of the oral cavity (excluding the lips), pharynx (throat), larynx (voice box), esophagus, liver, breast, colon, and rectum. The risk of developing cancer increases with the amount of alcohol a person drinks.
  • Antioxidants – Antioxidants are chemicals that block the activity of other chemicals, known as free radicals, that may damage cells. Laboratory and animal research has shown that exogenous antioxidants can help prevent the free radical damage associated with the development of cancer, but research in humans has not demonstrated convincingly that taking antioxidant supplements can help reduce the risk of developing or dying from cancer. Some studies have even shown an increased risk of some cancers.
  • Artificial sweeteners – Studies have been conducted on the safety of several artificial sweeteners, including saccharin, aspartame, acesulfame potassium, sucralose, neotame, and cyclamate. There is no clear evidence that the artificial sweeteners available commercially in the United States are associated with cancer risk in humans.
  • Calcium – Calcium is an essential dietary mineral that can be obtained from food and supplements. Research results overall support a relationship between higher intakes of calcium and reduced risks of colorectal cancer, but the results of studies have not always been consistent. Whether a relationship exists between higher calcium intakes and reduced risks of other cancers, such as breast and ovarian cancer, is unclear. Some research suggests that a high calcium intake may increase the risk of prostate cancer.
  • Charred meat – Certain chemicals, called HCAs and PAHs, are formed when muscle meat, including beef, pork, fish, and poultry, is cooked using high-temperature methods. Exposure to high levels of HCAs and PAHs can cause cancer in animals; however, whether such exposure causes cancer in humans is unclear.
  • Cruciferous vegetables – Cruciferous vegetables contain chemicals known as glucosinolates, which break down into several compounds that are being studied for possible anticancer effects. Some of these compounds have shown anticancer effects in cells and animals, but the results of studies with humans have been less clear.
  • Fluoride – Fluoride in water helps to prevent and can even reverse tooth decay. Many studies, in both humans and animals, have shown no association between fluoridated water and cancer risk. For more information, see the Fluoridated Water fact sheet.
  • Garlic – Some studies have suggested that garlic consumption may reduce the risk of developing several types of cancer, especially cancers of the gastrointestinal tract. However, the evidence is not definitive. For more information, see the Garlic and Cancer Prevention fact sheet.
  • Tea – Tea contains polyphenol compounds, particularly catechins, which are antioxidants. Results of epidemiologic studies examining the association between tea consumption and cancer risk have been inconclusive. Few clinical trials of tea consumption and cancer prevention have been conducted and their results have also been inconclusive. For more information, see the fact sheet on Tea and Cancer Prevention.
  • Vitamin D – Vitamin D helps the body use calcium and phosphorus to make strong bones and teeth. It is obtained primarily through exposure of the skin to sunlight, but it can also be obtained from some foods and dietary supplements. Epidemiologic studies in humans have suggested that higher intakes of vitamin D or higher levels of vitamin D in the blood may be associated with a reduced risk of colorectal cancer, but the results of randomized studies have been inconclusive. For more information, see the Vitamin D and Cancer Prevention fact sheet.