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.^8-15 Adult 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.