Today, 66% of American adults are either overweight or obese, according to a study from the National Health and Nutrition Examination Survey. About 17% of young Americans ages two to 19 years old are overweight, and scientists know that children who are obese during adolescence have a seven in 10 chance of being obese as an adult. Obesity raises the risk of many health conditions, including cancer. Although estimates vary, overweight and obesity may account for as many as one in every five cancer deaths in women and more than one in 10 cancer deaths in men in the United States.

About obesity

Obesity is having too much body fat in relation to lean body tissue, such as muscle. It is the result of taking in more calories than the body uses—sometimes referred to as an energy imbalance. Typically, a measurement called the body mass index (BMI) is used to determine whether a person is overweight or obese. BMI is a measure of body fat using a ratio of a person's height and weight. In general, a BMI of 25 to 29 indicates that a person is overweight, and a BMI of 30 or higher indicates obesity. Estimate your BMI with a calculator from the National Heart, Lung, and Blood Institute. Measuring your waist is another way to find out your risk of obesity. Women with measurements of more than 35 inches and men with measurements of more than 40 inches are at high risk for obesity.

The causes of obesity

The most obvious cause of obesity is eating too much food and not getting enough exercise. The U.S. Department of Health and Human Services estimates that 40% of American adults participate in no regular physical activity. Genetics, diet, lifestyle, and social and cultural factors may also influence a person's weight. In some cases, cancer treatment can affect weight. Results from the Childhood Cancer Survivor Study show that survivors who received radiation therapy to the brain at a young age are at higher risk for obesity. Other cancer treatments, such as steroid medications or hormones, may also cause weight gain.

Obesity's effect on cancer risk

Being obese increases the risk of leukemia, non-Hodgkin lymphoma, multiple myeloma, breast, cervical, colorectal, esophageal, gallbladder, kidney, liver, ovarian, pancreatic, prostate, stomach, and uterine cancers. In addition, some studies show that obesity negatively affects a person's prognosis (chance of recovery). For example, obese men are more likely to be diagnosed with advanced prostate cancer, have a recurrence of prostate cancer, or die of prostate cancer than men who are not obese. A study from the Journal of Clinical Oncology shows that women who are overweight before a breast cancer diagnosis or gain weight after diagnosis are more likely to experience a recurrence of cancer or die of breast cancer.

Researchers believe the following factors may explain how obesity affects cancer risk and tumor development:

• Obese people tend to undergo fewer cancer screenings, possibly because they wish to avoid seeing a doctor or because some screening tests, such as mammography, may be more difficult to perform. For many cancers, regular screening can help diagnose cancers early, when the chance of successful treatment is better.
  
  
• Often, people who are obese experience co-existing conditions, such as diabetes or heart disease, which may disguise signs and symptoms of cancer.
  
  
• In some cases, undergoing surgery or other types of cancer treatment may be difficult or impossible for patients who are obese.
  
  
• People who are obese have more fat tissue, which can produce more hormones, such as insulin and estrogen. These hormones may cause the growth and development of cancer cells.

Achieving and maintaining a healthy body weight

The following recommendations can help you lose weight and/or maintain a healthy weight:

• Be physically active. Talk with a doctor or nurse about beginning an exercise program, especially if you have been inactive or are recovering from cancer treatment.
  
  
• Eat a healthy diet that is low in fat, simple sugars (sugars that have only 1 to 3 sugar building blocks linked together into one molecule; they are digested much faster due to less nutrient molecules needing to be broken down), and calories, and high in fiber, fruits, and vegetables.
  
  
• Reduce portion sizes. Learn more about keeping an eye on portion sizes from the National Heart, Lung, and Blood Institute.
  
  
• If you need to lose weight, do so gradually—half a pound to two pounds per week. Ask a dietitian or doctor to provide personalized recommendations.
  
  
• Limit time in front of the TV and computer.
  
  
• Join a friend or family member in making lifestyle changes. Studies show people are more successful in maintaining such changes if they have the support and accountability of another person.

Additional resources

American Institute for Cancer Research

National Cancer Institute (NCI): Obesity and Cancer Fact Sheet

U.S. Dietary Guidelines

More Information

Healthy Living After Cancer

Physical Activity and Cancer

Weight Control