OBESITY AND CANCER

Obesity is a condition in which a person has an unhealthy amount and/or distribution of body fat.

To measure obesity, researchers commonly use a scale known as the body mass index (BMI). BMI is calculated by dividing a person’s weight (in kilograms) by their height (in meters) squared (commonly expressed as kg/m²). BMI provides a more accurate measure of obesity than weight alone, and for most people it is a fairly good (although indirect) indicator of body fatness. 

Other measurements that reflect the distribution of body fat—that is, whether more fat is carried around the hips or the abdomen—are increasingly being used along with BMI as indicators of obesity and disease risks. These measurements include waist circumference and the waist-to-hip ratio (the waist circumference divided by the hip circumference).

The standard weight categories based on BMI for adults age 20 years or older are

Compared with people of normal weight, those who are overweight or obese are at greater risk for many diseases, including diabetes, high blood pressure, cardiovascular disease, stroke, and many cancers. Extreme or severe obesity is also associated with an increased death rate; heart disease, cancer, and diabetes are responsible for most of the excess deaths.

That is because obese or overweight people may differ from lean people in ways other than their body fat, and it is possible that these other differences—rather than their body fat—are what explains their different cancer risk.

Despite the limitations of the study designs, there is consistent evidence that higher amounts of body fat are associated with increased risks of a number of cancers (6), including:

• Endometrial (womb) cancer
• Esophageal (throat) cancer
• Gastric cardia (stomach) cancer: 
• Liver cancer: 
• Kidney cancer
• Multiple myeloma
• Meningioma
• Pancreatic cancer
• Colorectal (colon) cancer
• Gallbladder cancer
• Breast cancer
• Ovarian cancer
• Thyroid cancer

Several possible mechanisms have been suggested to explain how obesity might increase the risks of some cancers.

• Obese people often have chronic low-level inflammation, which can, over time, cause DNA damage that leads to cancer. Overweight and obese individuals are more likely than normal-weight individuals to have conditions or disorders that are linked to or that cause chronic local inflammation and that are risk factors for certain cancers Obesity is a risk factor for gallstones, a condition characterized by long standing gallbladder inflammation, and a history of gallstones is a strong risk factor for gallbladder cancer. Ulcerative colitis (a chronic inflammatory condition of the large bowel) and hepatitis (a disease of the liver causing inflammation) are risk factors for different types of liver cancer.
• Fat tissue produces excess amounts of estrogen, high levels of which have been associated with increased risks of breast, uterine, ovarian, and some other cancers.
• Obese people often have increased blood levels of insulin. (This condition, known as hyperinsulinemia or insulin resistance, is associated with the development of type 2 diabetes.) High levels of insulin may promote the development of colon, kidney, prostate, and uterine cancers.

Most of the data about whether avoiding weight gain or losing weight reduces cancer risk comes from studies of obesity and associated cancer risk, these studies can be difficult to interpret because people who lose weight or avoid weight gain may differ in other ways from people who do not.

Nevertheless, when the evidence from these studies is examined, the association is noted. Many observational studies have provided consistent evidence that people who have lower weight gain during adulthood have lower risks of colon cancer, kidney cancer, and—for postmenopausal women—breast, uterine, and ovarian cancers. 

Fewer studies have examined possible associations between weight loss and cancer risk. Some of these have found decreased risks of breast, endometrial, colon, and prostate cancers among people who have lost weight.

Stronger evidence for a relationship between weight loss and cancer risk comes from studies of people who have undergone bariatric surgery (surgery performed on the stomach or intestines to induce weight loss). Obese people who have bariatric surgery appear to have lower risks of obesity-related cancers than obese people who do not have bariatric surgery.

Nevertheless, the follow-up study of weight and breast cancer in the Women’s Health Initiative found that for women who were already overweight or obese at baseline, weight change (either gain or loss) was not associated with breast cancer risk during follow-up. However, for women who were of normal weight at baseline, gaining more than 5% of body weight was associated with increased breast cancer risk.

So keeping your weight as near normal as possible for your height is associated with a healthier lifestyle, and a much lower risk for cancers.