Obesity and cancer

What is obesity?

People who are obese have an abnormally high and unhealthy proportion of body fat. To measure obesity, researchers commonly use a formula based on weight and height known as the body mass index (BMI). BMI is the ratio of weight (in kilograms) to height (in meters) squared. BMI provides a more accurate measure of obesity or being overweight than does weight alone.

Guidelines established by the National Institutes of Health (NIH) place adults age 20 and older into one of four categories based on their BMI:

Use our tool to check your Body Mass Index (BMI).

Compared with people in the healthy weight category, those who are overweight or obese are at greater risk for many diseases, including diabetes, high blood pressure, cardiovascular diseases, stroke, and certain cancers. Obesity lowers life expectancy.

How common is overweight or obesity?

Results from the 1999-2000 National Health and Nutrition Examination Survey (NHANES) show that an estimated 64 percent of U.S. adults are either overweight or obese. This represents an increase of 8 percentage points compared with the estimates from an earlier survey.

Nearly one-third of all adults are now classified as obese. This reflects an increase of 7.6 percentage points since 1994. The data show that 31 percent of adults age 20 and older-nearly 59 million people-have a body mass index (BMI) of 30 or greater, compared with 23 percent in 1994.

In addition, the percentage of children who are overweight continues to increase. Among children and teens ages 6 to 19, 15 percent (almost 9 million) are overweight according to the 1999-2000 data, or triple what the proportion was in 1980

What have scientists learned about the relationship between obesity and cancer?

In 2001, experts concluded that cancers of the colon, breast (postmenopausal), endometrium (the lining of the uterus), kidney, and esophagus are associated with obesity. Some studies have also reported links between obesity and cancers of the gallbladder, ovaries, and pancreas.

Obesity and physical inactivity may account for 25 to 30 percent of several major cancers-colon, breast (postmenopausal), endometrial, kidney, and cancer of the esophagus.

Preventing weight gain can reduce the risk of many cancers. Experts recommend that people establish habits of healthy eating and physical activity early in life to prevent overweight and obesity. Those who are already overweight or obese are advised to avoid additional weight gain, and to lose weight through a low-calorie diet and exercise. Even a weight loss of only 5 to 10 percent of total weight can provide health benefits.

How many people get cancer by being overweight or obese? How many die?

In 2002, about 41,000 new cases of cancer in the United States were estimated to be due to obesity. This means that about 3.2 percent of all new cancers are linked to obesity.

A recent report estimated that, in the United States, 14 percent of deaths from cancer in men and 20 percent of deaths in women were due to overweight and obesity.

Does avoiding weight gain decrease the risk of cancer?

The most conclusive way to test if avoiding weight gain will decrease the risk of cancer is through a controlled clinical trial. At present, there have been no controlled clinical trials on the effect on cancer related to avoiding weight gain. However, many observational studies have shown that avoiding weight gain lowers the risk of cancers of the colon, breast (postmenopausal), endometrium, kidney, and esophagus. There is limited evidence for thyroid cancers, and no substantial evidence for all other cancers.

Does losing weight lower the risk of cancer?

There is insufficient evidence that intentional weight loss will affect cancer risk for any cancer. A very limited number of observational studies have examined the effect of weight loss, and a few found some decreased risk for breast cancer among women who have lost weight. However, most of these studies have not been able to evaluate whether the weight loss was intentional or related to other health problems.

One recent study that examined the effect of intentional weight loss found that women who experienced intentional weight loss of 20 or more pounds and were not currently overweight had cancer rates at the level of healthy women who never lost weight. However, unintentional weight loss episodes were not associated with decreased cancer risk.

Does regular physical activity lower the risk of cancer?

There have been no controlled clinical trials on the effect of regular physical activity on the risk of developing cancer. However, observational studies have examined the possible association between physical activity and a lower risk of developing colon or breast cancer:

• Colon cancer: In 2002, a major review of observational trials found that physical activity reduced colon cancer risk by 50 percent. This risk reduction occurred even with moderate levels of physical activity. For example, one study showed that even moderate exercise, such as brisk walking for 3 to 4 hours per week, can lower colon cancer rissk.
  A limited number of studies have examined the effect of physical activity on colon cancer risk for both lean and obese people. Most of these studies have found a protective effect of physical activity across all levels of BMI.
• Breast cancer: The pattern of the association between physical activity and breast cancer risk is somewhat different. Most studies on breast cancer have focused on postmenopausal women. A recent study from the Women's Health Initiative found that physical activity among postmenopausal women at a level of walking about 30 minutes per day was associated with a 20 percent reduction in breast cancer risk. However, this reduction in risk was greatest among women who were of normal weight. For these women, physical activity was associated with a 37 percent decrease in risk. The protective effect of physical activity was not found among overweight or obese women.

This information is produced and provided by the National Cancer Institute (NCI). References are available at their website at www.cancer.gov. The information in this topic may have changed since it was written. For the most current information, contact the National Cancer Institute via www.cancer.gov or call 1-800-4-CANCER.