Cancer adds further urgency to prioritising obesity control

Abstract

he increasing incidence of obesity in Australia is set to impose a major additional cancer burden at a time when population ageing alone is projected to cause unprecedented growth in cancer incidence. While obesity is frequently associated with type 2 diabetes, hypertension, lipid abnormalities and death from heart disease and stroke, there is growing evidence for its role in causing cancer. A systematic review and meta-analysis, which included the Million Women Study, investigated the link between body mass index (BMI; calculated by dividing weight in kilograms by height in metres squared) and types of cancer. 1,2 In men, a 5k g/m 2 increase in BMI was strongly associated with adenocarcinoma of the oesophagus, and thyroid, colon and renal cancers, and in women with adenocarcinoma of the oesophagus, and endometrial, gallbladder and renal cancers. There were weaker associations for melanoma and rectal cancer in men, and postmenopausal breast, thyroid and colon cancer in women. Leukaemia, myeloma and Hodgkin disease were associated in both sexes. One postulated mechanism for the link between obesity and cancer is that chronic hyperinsulinaemia results in raised levels of free IGF-I (insulin-like growth factor), with higher mean concentrations in men compared with women. This alters the environment of cells to favour cancers developing. 3 In adipocytes, androgens are converted to oestradiol, and chronic hyperinsulinaemia also reduces sex-hormone-binding globulin, leaving more oestrogen to impact on oestrogen-sensitive tissues. Further, adiponectin, a protein hormone secreted by adipocytes, is an insulin-sensitising agent that is anti-angiogenic and antiinflammatory, inversely correlated with BMI, found in higher concentrations in men than in women and, in some studies, its level is inversely associated with cancer risk. 4 Obesity is linked to 11% of colon cancers and 9% of postmenopausal breast cancers 5 — both increasingly common tumour types in Australia as a result of population ageing. With high percentages of endometrial cancer (39%), oesophageal adenocarcinoma (37%), kidney cancer (25%) and gallbladder cancer (24%) attributed to obesity and overweight, these rarer cancers may become more common as Australia’s obese population ages. 5 The risk of an obesity-related increase in cancer burden in this country is amplified, with the link between BMI and cancer compounded by a 50% increase in the number of obese or overweight Australians over the past 15 years. An estimated 7.4 million Australians are obese or overweight, including a quarter of children aged between 5 and 16 years. 6