Overweight and obesity

Overweight is defined by WHO as having a body mass index (BMI) of 25 kg/m2 or more and obesity as a BMI of 30 kg/m2 or more.44 Waist circumference is considered to be a measure of abdominal fatness. The WHO reference values for waist circumferences of 94 cm in men and 80 cm in women (on a population basis) are based on their rough equivalence to a BMI of around 25.8 Estimates on the percentage of cancer attributable to overweight and obesity range from 4.5% of cancer cases in Europe16 to 20% in the United States.3 Globally, it is estimated that 3.6% of all new cancers in adults are attributable to excess bodyweight, representing a total of 481,000 cases.45  

Overweight and obesity in Australia

The prevalence of overweight and obesity continues to rise in Australia, from 56.3% of adults in 1995 to 62.8% in 2011–12.46 Around a quarter of children aged 2 to 17 years in Australia were overweight or obese in 2011–12.46

Overweight and obesity and cancer

Based on systematic literature reviews, the 2007 WCRF and AICR report Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective and subsequent tumour-specific updates identified convincing evidence that greater body fatness increased the risk of various cancers, namely colorectal, oesophageal (adenocarcinoma), endometrial, pancreatic, kidney, postmenopausal breast and liver cancers (see Appendix 1 for explanation of WCRF & AICR categories of evidence).8-15 Greater body fatness was identified as a probable cause of gallbladder, advanced prostate and ovarian cancers.8-15

A large UK population-based cohort study of 5.24 million adults published in 2014 investigated the association between BMI and the most common site-specific cancers.47 Each 5 kg/m2 increase in BMI was linearly associated with a large increase in risk of uterine, gallbladder, kidney, cervical, thyroid cancers, and leukaemia.47 Overall positive associations with higher BMI, while non-linear or modified by individual level factors, were also reported for liver, colon, ovarian, and postmenopausal breast cancers.47 An earlier (2008) large systematic review and meta-analysis examined the association between BMI and different cancers and investigated differences in the association between men and women.48 It was reported that a 5 kg/m2 increase in BMI was strongly associated with oesophageal (adenocarcinoma), thyroid, colon and renal cancers in men, and endometrial, gallbladder, oesophageal (adenocarcinoma) and renal cancers in women.48 Weaker positive associations were observed with leukaemia,  multiple myeloma and non-Hodgkin lymphoma for both genders, rectal cancer and malignant melanoma in men, and postmenopausal breast, pancreatic, thyroid and colon cancers in women.48

The 2007 WCRF and AICR report and subsequent updates identified convincing evidence that abdominal fatness (i.e. wider girth) increased risk of colorectal cancer and endometrial cancer, and was a probable cause of pancreatic cancer and postmenopausal breast cancer.8-15Adult weight gain was identified as a further probable cause of postmenopausal breast cancer.8-15

Reducing individual risk of cancer and staying healthy

The 2002 IARC Handbook of Cancer Prevention reviewed the evidence on weight control and indicated obesity as a cause of 39% of endometrial cancer cases, 37% of oesophageal cancer cases, 25% of kidney cancer cases, 11% of colon cancer cases and 9% of postmenopausal breast cancer cases.49 The Handbook identified that avoiding weight gain had a preventive effect for these cancers.49

Few data are available regarding the preventive effect of intentional weight loss on cancer. A 2011 review indicated that limited information from observational studies suggests a reduced risk of breast cancer and potentially other cancers due to intentional weight loss.50 Further research is required to determine the degree of weight loss and timeframe to reduce cancer risk.

The WCRF and AICR report states that, while obesity is a cause of some cancers and other diseases, it is also a marker for dietary and physical activity patterns that independently lead to poor health.8 Maintaining a healthy weight throughout life has clear health benefits and may have an important protective effect against cancer.8 Results from recent analyses of the large EPIC study indicated a significant association between higher BMI and risk of cancer and risk of death.28,29

The WCRF and AICR report recommends being as lean as possible within the normal range of body weight in order to reduce cancer risk.8 Avoiding weight gain and increases in waist circumference throughout adulthood is also recommended to reduce risk of cancer.8 Cut-off points for healthy waist circumferences were specified as 94 cm (37 inches) for men and 80 cm (31.5 inches) for women, based on general equivalence to a BMI of 25 kg/m2.8

The Australian Dietary Guidelines recommend achieving and maintaining a healthy weight (BMI of 18.5–24.9 kg/m2) through physical activity and healthy eating.51

Cancer Australia recommendations for individuals

Cancer Australia recommends achieving and maintaining a healthy body weight within a BMI range of 18.5 to 25 kg/m2 to reduce cancer risk and a waist circumference below 94 cm for men and below 80 cm for women.

Table 2: Summary of evidence for overweight and obesity and cancer sites

Risk factor Source Evidence Cancer site

Greater body fatness

WCRF/AICR 2007–20158-15

Convincing

 

Colorectum, oesophagus (adenocarcinoma), endometrium, pancreas, kidney, postmenopausal breast, liver

Probable

 

Gallbladder, ovary, advanced prostate

Abdominal (central) fatness

WCRF/AICR 2007–20158-15

Convincing

Colorectum, endometrium

Probable

Pancreas, postmenopausal breast

Adult weight gain

WCRF/AICR 2007–20158-15

Probable

Postmenopausal breast

Protective factor Source Evidence Cancer site

Avoiding weight gain

IARC 200249

Sufficient evidence (highest IARC classification for carcinogenicity)

Colon, postmenopausal breast, endometrium, kidney (renal-cell), oesophagus (adenocarcinoma)

See Appendix 1 for explanation of evidence.