Obesity is caused by the kind of food you eat. Mounting evidence has shown that diet is the primary cause of cancer in the world today and that an obese person is more likely to develop cancer. A study, published in the journal CMAJ Open, analyzed the incidence of cancer attributed to lifestyle and environmental factors. The researchers estimated summary population attributable risk estimates for 24 factors: passive and active smoking; overweight and obesity; lack of physical activity; inadequate fruit and vegetable intake, inadequate fiber intake, excess red and processed meat intake, salt intake, inadequate calcium consumption, and vitamin D intake; alcohol; oral contraceptives and hormone therapy; Epstein-Barr virus, hepatitis B and C viruses, human papillomavirus, and Helicobacter pylori; air pollution; natural and artificial ultraviolet radiation; radon; and water disinfection by-products.
The findings of the study revealed that about 41 percent of cancer cases in Alberta, Canada in 2012 were caused by lifestyle and environmental risk factors. Most of the cancer cases were greatly caused by tobacco smoking, lack of physical activity, and excess body weight. Moreover, results showed that women were more likely to develop cancer because of these factors compared to men. (Related: Obesity causes cancer, says American Cancer Society.)
The researchers suggested that reducing the prevalence of these factors can possibly reduce the cases of cancer.
More studies prove that excess weight is the cause of most cancers. In the United States in 2012, being overweight or obese caused around 28,000 new cases of cancer in men and 72,000 in women according to a population-based study that used body mass indexes and cancer incidence data from the GLOBOCAN project. Moreover, the study revealed that the percentage of cancer caused by obesity was highest for gallbladder cancer in women with 54 percent and 44 percent for esophageal adenocarcinoma in men. Furthermore, a 2016 study revealed that the U.S. had the greatest number of cancer cases for colorectal cancer, pancreatic cancer, and postmenopausal breast cancer attributable to being overweight or obese in comparison with other countries worldwide.
According to an entry by Cancer.gov, there are some possible reasons why obese people are more prone to some cancers. Obesity causes people to have chronic low-level inflammation in their body which can damage the DNA and eventually lead to cancer. Overweight and obese individuals also have more fat tissues or adipose tissue than normal-weight people. High levels of estrogen produced by fat tissue have been linked to higher risks of breast, endometrial, ovarian, and some other cancers. Obesity increases the blood levels of insulin and insulin-like growth factor, which is a condition known as hyperinsulinemia or insulin resistance that precedes the development of type-2 diabetes. These may stimulate the development of colon, kidney, prostate, and endometrial cancers. Another factor is that fat cells produce adipokines which are hormones that may trigger or prevent the growth of cells. For example, leptin, an adipokine, increases body fat, while adiponectin, which is less seen in obese people, may have anti-proliferative effects. In addition, fat cells can directly and indirectly affect other cell growth regulators such as mammalian target of rapamycin (mTOR) and AMP-activated protein kinase. Other ways that obesity can promote cancer include altered mechanical properties of the scaffolding around breast cells, changed immune responses, effects on the nuclear factor kappa beta system, and oxidative stress.