# What causes obesity?
Experts have concluded that the chief causes of obesity are a sedentary lifestyle and overconsumption of high-calorie food (4):
* Sedentary lifestyle—Researchers have found a strong correlation between lack of physical activity and obesity (4, 5).
* Diet—A diet high in calories and/or fat appears to be an important factor in obesity (6).
# 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 (4).
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 (4).
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 (4).
# 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 (7).
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 (
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# Does obesity increase the risk of breast cancer?The effect of obesity on breast cancer risk depends on a woman’s menopausal status. Before menopause, obese women have a lower risk of developing breast cancer than do women of a healthy weight (4, 9, 10, 11, 12). However, after menopause, obese women have 1.5 times the risk of women of a healthy weight (9, 10, 13, 14).
Obese women are also at increased risk of dying from breast cancer after menopause compared with lean women (4, 11, 15, 16). Scientists estimate that about 11,000 to 18,000 deaths per year from breast cancer in U.S. women over age 50 might be avoided if women could maintain a BMI under 25 throughout their adult lives (16).
Obesity seems to increase the risk of breast cancer only among postmenopausal women who do not use menopausal hormones. Among women who use menopausal hormones, there is no significant difference in breast cancer risk between obese women and women of a healthy weight (4, 9, 11, 17).
Both the increased risk of developing breast cancer and dying from it after menopause are believed to be due to increased levels of estrogen in obese women (18). Before 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 (14). Estrogen levels in postmenopausal women are 50 to 100 percent higher among heavy versus lean women (11). Estrogen-sensitive tissues are therefore exposed to more estrogen stimulation in heavy women, leading to a more rapid growth of estrogen-responsive breast tumors.
Another factor related to the higher breast cancer death rates in obese women is that breast cancer is more likely to be detected at a later stage in obese women than in lean women. This is because the detection of a breast tumor is more difficult in obese versus lean women (12).
Studies of obesity and breast cancer in minority women in the United States have been limited. There is some evidence that, among African American women, the risk associated with obesity may be absent or less than that of other populations (19, 20, 21). However, a recent report showed that African American women who have a high BMI are more likely to have an advanced stage of breast cancer at diagnosis (22). Another report showed that obese Hispanic white women were twice as likely to develop breast cancer as non-obese Hispanics, but the researchers did not detect a difference in risk for obese Hispanic women before and after menopause (23).
Weight gain during adulthood has been found to be the most consistent and strongest predictor of breast cancer risk in studies in which it has been examined (4, 10, 11, 24).
The distribution of body fat may also affect breast cancer risk. Women with a large amount of abdominal fat have a greater breast cancer risk than those whose fat is distributed over the hips, buttocks, and lower extremities (13, 25, 26). Results from studies on the effect of abdominal fat are much less consistent than studies on weight gain or BMI.
# Does obesity increase the risk of cancer of the uterus?Obesity has been consistently associated with uterine (endometrial) cancer. Obese women have two to four times greater risk of developing the disease than do women of a healthy weight, regardless of menopausal status (4, 27, 28, 29, 30). Increased risk has also been demonstrated among overweight women (28, 30). Obesity has been estimated to account for about 40 percent of endometrial cancer cases in affluent societies (31).
It is unclear why obesity is a risk factor for endometrial cancer; however, it has been suggested that lifetime exposure to hormones and high levels of estrogen and insulin in obese women may be contributing factors (4, 27, 28, 29, 32).
# Does obesity increase the risk of colon cancer?Colon cancer occurs more frequently in people who are obese than in those of a healthy weight (4, 33, 34, 35, 36, 37). An increased risk of colon cancer has been consistently reported for men with high BMIs (34, 37, 38). The relationship between BMI and risk in women, however, has been found to be weaker (4, 34, 38) or absent (39).
Unlike for breast and endometrial cancer, estrogen appears to be protective for colon cancer for women overall (40). However, obesity and estrogen status also interact in influencing colon cancer risk. Women with a high BMI who are either premenopausal or postmenopausal and taking estrogens have an increased risk of colon cancer similar to that found for men with a high BMI. In contrast, women with a high BMI who are postmenopausal and not taking estrogens do not have an increased risk of colon cancer (41).
There is some evidence that abdominal obesity may be more important in colon cancer risk (37, 38). In men, a high BMI tends to be associated with abdominal fat. In women, fat is more likely to be distributed in the hips, thighs, and buttocks. Thus, two measures of abdominal fat, waist-to-hip ratio or waist circumference, 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 fat (42).
A number of mechanisms have been proposed for the adverse effect of obesity on colon cancer risk. One of the major hypotheses is that high levels of insulin or insulin-related growth factors in obese people may promote tumor development (4, 43, 44).
# Does obesity increase the risk of kidney cancer?Studies have consistently found a link between a type of kidney cancer (renal cell carcinoma) and obesity in women (4, 30, 45, 46, 47, 48), with some studies finding risk among obese women to be two to four times the risk of women of a healthy weight.
Results of studies including men have been more variable, ranging from an association similar to that seen in women (30, 46, 49), to a weak association (48, 50, 51), to no association at all (45). A meta-analysis (where several studies are combined into a single report), which found an equal association of risk among men and women, estimated the kidney cancer risk to be 36 percent higher for an overweight person and 84 percent higher for an obese person compared to those with a healthy weight (52).
The mechanisms by which obesity may increase renal cell cancer risk are not well understood. An increased exposure to sex steroids, estrogen and androgen, is one possible mechanism (4).
# Does obesity increase the risk of cancer of the esophagus or stomach?Overweight and obese individuals are two times more likely than healthy weight people to develop a type of esophageal cancer called esophageal adenocarcinoma (4, 53, 54, 55, 56). A smaller increase in risk has been found for gastric cardia cancer, a type of stomach cancer that begins in the area of the stomach next to the esophagus (54, 55, 56, 57). Most studies have not observed increases in risk with obesity in another type of esophageal cancer, squamous cell cancer. An increased risk of esophageal adenocarcinoma has also been associated with weight gain, smoking, and being younger than age 59 (54, 57).
The mechanisms by which obesity increases risk of adenocarcinoma of the esophagus and gastric cardia are not well understood. One of the leading mechanisms proposed has been that increases in gastric reflux due to obesity may increase risk. However, in the few studies that have examined this issue, risk associated with BMI was similar for those with and without gastric reflux (56).
# Does obesity increase the risk of prostate cancer?Of the more than 35 studies on prostate cancer risk, most conclude that there is no association with obesity (4, 44, 58, 59, 60). Some report that obese men are at higher risk than men of healthy weight, particularly for more aggressive tumors (61, 62, 63). One study found 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 (64).
Studies examining BMI and prostate cancer mortality have had conflicting results (8, 65, 66).
Despite the lack of association between obesity and prostate cancer incidence, a number of studies have examined potential biological factors that are related to obesity, such as insulin-related growth factors, leptin, and other hormones. Results of these studies are inconsistent, but generally, risk has been linked to men with higher levels of leptin (67), insulin (68), and IGF–1 (insulin-like growth factor-1) (69).
# Is there any evidence that obesity is linked to cancer of the gallbladder, ovaries, or pancreas?An increased risk of gallbladder cancer has been found to be associated with obesity, particularly among women (70, 71, 72). This may be due to the higher frequency of gallstones in obese individuals, as gallstones are considered a strong risk factor for gallbladder cancer. However, there is not enough evidence to draw firm conclusions.
It is unclear whether obesity affects ovarian cancer risk. Some studies report an increased risk among obese women (73, 74, 75), whereas others have found no association (76, 77). A recent report found an increased risk in women who were overweight or obese in adolescence or young adulthood; no increased risk was found in older obese women (78).
Studies evaluating the relationship between obesity and pancreatic cancer have been inconsistent (79, 80, 81, 82). One recent study found that obesity increases the risk of pancreatic cancer only among those who are not physically active (80). A recent meta-analysis reported that obese people may have a 19 percent higher risk of pancreatic cancer than those with a healthy BMI. The results, however, were not conclusive (83).
