Obesity and Cancer

National Cancer Institute
Tuesday, 25 September 2001

Rising numbers of overweight and obese people have led to concern about an obesity epidemic and its effects on public health. Researchers are learning that, in addition to the more widely recognized effects on coronary heart disease, stroke, and diabetes, obesity can also increase the risk of cancer. Several cancers are associated with obesity: those of the breast, endometrium, colon, kidney, esophagus, stomach, and gallbladder. Some studies have also reported links between obesity and cancers of the prostate, ovaries, and pancreas.

Obesity is an increase in weight that reduces life expectancy. A frequent measure of obesity is the body mass index (BMI), which is the ratio of weight (in kilograms) to height (in meters) squared. Guidelines established by the National Institutes of Health (NIH) place adults 20 years and older into four categories based on their BMI:

NIH BMI Guidelines (1):

<18.5 underweight

18.5 - 24.9 healthy

25.0 - 29.9 overweight

> 30.0 obese

At all BMI levels above the healthy range, weight has a negative effect on health. The following chart can be used to determine where individuals fall in relation to these guidelines. Click on the link below to view the chart.

http://newscenter.cancer.gov/pressreleases/bmi3.jpg

According to these guidelines and preliminary results of the 1999 National Health and Nutrition Examination Survey, 61 percent of adults in the United States are overweight or obese. This figure has increased 5 percent since the previous survey, conducted in 1988-1994 (2). The following chart shows the spread of obesity in the United States since 1991. Click on the link below to view the chart.

http://newscenter.cancer.gov/pressreleases/OBES.MAPS2.jpg

Surveys have also found increases in the number of overweight children and adolescents in the United States. In 1999, 13 percent of children ages six to 11 and 14 percent of adolescents ages 12 to 19 were categorized as overweight. These numbers were up from 11 percent for both children and adolescents in the 1988-1994 survey (3). A panel of experts from the International Agency for Research on Cancer (IARC), a part of the World Health Organization that coordinates and conducts cancer research, concluded in 2001 that preventing weight gain can reduce the risk of many cancers. The panel recommended that habits of healthy eating and physical activity be established 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 five percent to ten percent of total weight can provide health benefits (4).

Obesity and Breast Cancer

The effect of obesity on breast cancer risk depends on a woman's menopausal status. Whereas prior to menopause obese women are at lower risk for breast cancer than women of a healthy weight (5, 6, 7), after menopause obese women have 1.5 times the risk of women of a healthy weight (5, 6, 8, 9). The increased risk after menopause is believed to be due to increased levels of estrogen in obese women (10). Prior to menopause, the ovaries are the primary source of estrogen. However, estrogen is also produced in fat tissue and, after menopause, when the ovaries stop producing hormones, fat tissue becomes the most important estrogen source (9). This is believed to be why obesity increases breast cancer risk only after menopause.

Obesity only increases the risk of breast cancer among post-menopausal women who do not use hormone replacement therapy (HRT). Among women who use HRT, there is no significant difference in breast cancer risk between obese women and women of a healthy weight (5, 7). Studies of obesity and breast cancer in minority women in the United States have been limited; however, there is some evidence that among African-American women the risk associated with obesity may be absent or less than that of other populations (11, 12, 13).

Some studies have found that weight gain during adulthood is more important in determining breast cancer risk than BMI (6, 7, 14). Conversely, weight loss has been found to decrease risk (14, 15, 16).

The distribution of body fat may also affect breast cancer risk. Increased abdominal fat appears to confer a greater risk than fat which is distributed over the hips, buttocks, and lower extremities (8, 16, 17).

Obesity and Endometrial Cancer

Obesity has been consistently associated with endometrial cancer, with obese women having two to four times the risk of developing the disease compared to women of a healthy weight (18, 19, 20, 21). Increased risk has also been demonstrated among overweight but not obese women (19, 21). It is unclear why obesity is a risk factor for endometrial cancer; however, it has been suggested that high levels of estrogen and insulin in obese women may be contributing factors (18, 19, 20).

Obesity and Colon Cancer

Colon cancer occurs more frequently in people who are obese than those of a healthy weight (22 - 26). Many researchers believe the increased rate of colon cancer can be attributed to lifestyle patterns that are known to escalate colon cancer risk, particularly inactivity and a diet that is high in fat and low in fruits and vegetables (22, 23). Others have suggested that high levels of insulin in obese individuals may promote tumor development (27, 28).

An increased risk of colon cancer has been consistently reported for men with high BMIs. The relationship between BMI and risk in women, however, has been found to be weaker (23, 29) or absent (30). Some researchers have interpreted this to mean that overall body mass is less important in determining colon cancer risk than the distribution of body fat (29, 26). In men, a high BMI tends to be associated with fat that is distributed abdominally, whereas fat is more likely to be distributed in the hips, thighs, and buttocks in women. Thus, waist-to-hip ratios, a measure of abdominal adiposity, may be better predictors of colon cancer risk. Few studies have yet compared waist-to-hip ratios to colon cancer risk in women, however.

One study that did find an increased risk of colon cancer among women with high waist-to-hip ratios found that the association was present only among inactive women, suggesting that high levels of physical activity may counteract the effects of increased abdominal adiposity (31).

Obesity and Kidney Cancer

Studies have consistently found a link between a type of kidney cancer (renal cell carcinoma) and obesity in women (21, 32 - 35). Obese women are at two to four times the risk of women of a healthy weight for developing kidney cancer. Some studies have found that women who are overweight but not obese also have a greater risk of kidney cancer than that of leaner women (33).

Results of studies including men have been more variable, ranging from an association similar to that seen in women (21, 33), to a weak association (35, 36), to no association at all (32).

Obesity and Cancers of the Upper Gastrointestinal Tract

Significant increases in risk of a type of esophageal cancer (esophageal adenocarcinoma) have been found in overweight and obese individuals (27 - 40). Smaller increases in risk have been found for gastric cardia cancer, a type of stomach cancer adjacent to the esophagus(38 - 41).

Obesity and Gallbladder Cancer

An increased risk of gallbladder cancer has been found to be associated with obesity, particularly among women (42 - 44). This may be due to the higher frequency of gallstones in obese individuals, as gallstones are considered a strong risk factor for gallbladder cancer.

Obesity and Cancers of the Prostate, Ovaries, and Pancreas

The relationship between obesity and prostate cancer is still unclear. Some studies find little or no association between BMI and the risk of developing prostate cancer (28, 45), whereas others report obese men to be at higher risk than men of healthy weight, particularly for more aggressive tumors (46, 47). Studies looking at BMI and prostate cancer mortality have also had conflicting results (48, 49). One study found no association between BMI and incidence, but did find an increased risk among men with high waist-to-hip ratios, suggesting that abdominal fat may be a more appropriate measure of body size in relation to prostate cancer (50).

It is unclear whether obesity affects ovarian cancer risk. Some studies report an increased risk among obese women (51 - 53), whereas others have found no association (54, 55).

Several studies have considered the relationship between obesity and pancreatic cancer, with inconsistent results (56 - 59). One recent study found that obesity increases risk of pancreatic cancer only among those who are not physically active (57).

Current NCI Studies

Several NCI-funded studies continue to investigate the relationship between obesity and cancer. The Four Corners Breast and Endometrial Cancer Study is focusing on the effects of obesity and weight changes on breast and endometrial cancer risk among Hispanic, Native American, and non-Hispanic white women (60). A study of white, African-American, and Latina women is investigating whether phytoestrogen consumption can modify the risk of endometrial cancer associated with obesity (61). The Black Women's Health Study is considering the effect of risk factors, including obesity, on breast cancer risk (62).

For more information, or to contact National Cancer Institute, see their website at: www.cancer.gov