What is known about the relationship between obesity and cancer? How might obesity increase the risk of cancer? How many cancer cases may be due to obesity? Does avoiding weight gain or losing weight decrease the risk of cancer? How does obesity affect cancer survivorship? What research is being done on obesity and cancer? What is obesity? 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.