We analyse global research on how obesity and weight gain affect the risk of developing cancer.
Overweight and obesity, generally assessed by various anthropometric measures including body mass index (BMI) and waist circumference, are now more prevalent than ever.
Approximately 2 billion adults are living with overweight and obesity and, within this group, 800 million are living with obesity (defined as a BMI =>30).
There is strong evidence that:
being overweight or living with obesity throughout adulthood INCREASES the risk of
greater weight gain in adulthood INCREASES the risk of
being overweight or obese as an adult before menopause DECREASES the risk of
being overweight or obese between the ages of about 18 and 30 years DECREASES the risk of
There is strong evidence that:
The evidence also shows that, in general, the more weight people gain as adults, the higher the risk of postmenopausal breast cancer. In contrast, the evidence shows that, in general, the more excess weight people have as young adults, the lower the risk of breast cancer. Despite this finding, we recommend maintaining a healthy weight throughout all stages of life.
There’s also other evidence on being overweight or obese throughout adulthood that is limited (either in amount or by methodological flaws), but is suggestive of an increased risk of cervical cancer for women with a BMI of 29 kg/m2 or more. Further research is required, and we haven’t used this evidence to make recommendations.
The increase in the proportion of adults categorised as living with obesity has been observed both in low- and middle-income countries and in high-income countries. Although the rate of increase has begun to slow in some high-income countries, the prevalence of obesity has tended to accelerate in low- and middle-income countries.
These accelerations have occurred in tandem with considerable changes in food systems and dietary patterns, commonly termed the ‘nutrition transition’. Overweight and obesity is occurring at an ever earlier age, increasing lifetime exposure to the associated risks.
Excess weight and obesity have been linked to a number of other chronic diseases including cardiovascular disease, diabetes and other metabolic disorders.
Excess energy from food and drink is stored in the body as fat in adipose tissue. The amount of adipose tissue in the body varies more from person to person than any other type of tissue (such as muscle, bone or blood).
The size and location of these fat stores also vary considerably between populations, between people and over the course of a person’s life. Excess body fat is a cause of a number of chronic diseases and reduces life expectancy.
Of course, excess weight gain is not itself a behaviour, rather the result of many different behaviours. To build on our findings about how weight influences cancer risk, the Third Expert Report included a review of factors influencing excess weight gain.
In 2018, we produced the Diet and Cancer Report, the third in our series of major reports looking at the many ways in which our diets, and how active we are, affect our cancer risk. You can find out much more about weight gain and the risk of cancer by downloading a pdf of the relevant chapter in the 2018 report. Please note, however, that this webpage may have been updated since the report was published.