Relation between cancer and obesity

Obesity, which defined as excessive fat accumulation in the adipose tissue that may impair health. The Body Mass Index (BMI) is the most commonly used metric to measure obesity and overweight. It is calculated based on the body weight (in kg) divided by the square of height (in m2).


The Department of Health stated that for Chinese adults living in Hong Kong, BMI in the range of 23.0 to < 25.0 kg/m2 is classified as overweight and BMI ≥ 25.0 kg/m2 is classified as obesity. The Centre for Health Protection has also defined central obesity as a waist circumference of >80 cm for women and >90 cm for men.


In 2016, almost 40% of the population aged 18 – 64 were classified as overweight or obese, including 20.7% as obese. A higher proportion of males (48.2%) than females (30.5%) were classified as overweight or obese. People aged 55 – 64 had a highest rate (49.4%) of overweight or obesity. Moreover, the survey also revealed that more than a half of the interviewees’ physical activity level did not meet the recommendations from the World Health Organization (WHO) in the past 7 days before the interview.


Obesity has become more common in the past few decades. The most significant reasons include an increased intake of diet with high energy and a significant reduction in physical activity of the general population due to sedentary behavior, changes in transportation modes and engagement in screen-related works.


Obesity could increase the risk of many chronic diseases, including hypertension, cardiovascular diseases, high lipid level, diabetes, stroke, and certain types of cancer in particular colorectal, breast and prostate cancer (these three cancers will be named as “obesity-related cancers” thereafter).

The World Cancer Research Fund of the American Institute for Cancer Research has reviewed all the studies about the link between obesity and cancer.


For colorectal cancer (CRC), there is compelling evidence suggesting that obesity is associated with 30-66% higher risk of developing CRC.  Weight gain during adulthood was also shown to increase risk of CRC.


As for breast cancer, post-menopausal women with the smallest waist (compared with the largest) had 39% lower risk of getting breast cancer.  A 5-unit increase in BMI is associated with 12% increased risk of breast cancer, and this figure escalates to 20-40% for postmenopausal women.


For prostate cancer, obesity was positively related to advanced stage prostate cancer.  The relative risk is 1.09 for every 5 kg/m2 higher BMI.