Obesity has a clear link to many cancers
The International Committee for Research on Cancer, through more than 30 years of investigation into cancer epidemiology, has concluded that obesity is strongly associated with many types of cancer.
It is estimated that in the United States, 1 in 7 men and 1 in 5 women have cancers associated with obesity. Similarly, in European Union countries, 4% of male cancer patients and 7% of female cancer patients are associated with obesity. The relationship between obesity and cancer is extraordinarily complex. Although obesity is associated with increased risk factors for many cancers, it can reduce the risk of premenopausal breast and lung cancers.
Obesity and breast cancer
In Europe, although breast cancer mortality has declined in recent years, there are still about 1,000,000 new cases in women each year. The contribution of obesity to the development of breast cancer in postmenopausal women is well established. The weight of the role of obesity in breast cancer is equivalent to the family genetic role.
It is estimated that for every 1 increase in body mass index, the risk of breast cancer increases by 3%; for every 5 kg increase in body weight, the relative incidence of breast cancer in postmenopausal women increases by 1.08; in the United States, obesity is responsible for 20% of postmenopausal breast cancer occurrences and 50% of postmenopausal breast cancer deaths.
Obesity and endometrial cancer
Case-control and cohort studies have shown a very definite association between uterine body cancer and obesity. For women who have not been treated with postmenopausal hormones, a weight gain of 25 kg or more is associated with a risk ratio of 5.00 for the development of endometrial cancer; therefore, either obesity or adult weight gain is associated with a significant increase in the incidence of endometrial cancer.
Obesity and colorectal cancer
The incidence of colorectal cancer is increased in both men and women who are obese. Case-control and cohort studies are consistent, and the increase in incidence is greater in men than in women. It has been hypothesized that altered waist/hip ratios may account for the gender differences in colorectal cancer incidence, but no large-scale study data are available to confirm this conjecture.
Obesity and renal cell carcinoma
Obesity, especially in women, can cause an increased incidence of renal cell carcinoma, but the mechanism of action is unknown and needs to be confirmed by large scale data.
Obesity and esophageal adenocarcinoma
It has been suggested that obesity may increase the incidence of esophageal adenocarcinoma by increasing the incidence of esophageal reflux disease and Barrett’s esophagus, which is a saprophytic precursor of esophageal adenocarcinoma. However, other studies have concluded that the increased incidence of esophageal adenocarcinoma due to obesity is not associated with reflux disease.
Cancers that have some relationship with obesity
Obesity increases the risk of pancreatic cancer formation by a factor of 2. The incidence of hepatocellular carcinoma is increased in obese patients, but the extent of the increased risk has not been demonstrated. Obesity also increases the incidence of cardia cancer, most likely associated with obesity-induced chemosis of Barrett’s esophagus. A few studies have suggested that obesity may be associated with increased ovarian cancer, cervical cancer, connective tissue cancers, and lymphoma. Additional epidemiologic studies are needed to fully understand the relationship between obesity and these cancers.
Some cancers are also negatively associated with obesity
Some studies have confirmed that obesity and lung cancer are negatively correlated, and this negative correlation can be related to the complications of smoking, which is one of the primary factors in the occurrence of lung cancer, and obesity reduces the occurrence of lung cancer and the decrease in body mass index of patients who smoke. Premenopausal obese individuals have a reduced incidence of breast cancer; it is hypothesized that anovulatory cycles in these women reduce their estrogen levels, thereby reducing the incidence of breast cancer.
Pathophysiology of obesity and cancer
The pathophysiology of obesity-associated cancer is multifaceted and complex, and thus current understanding is incomplete. There are currently two main mechanisms: endocrine changes caused by biochemical alterations such as insulin resistance; and the metabolic syndrome associated with obesity.