In 1997, the World Cancer Research Fund (WCRF) and the American Institute for Cancer Research (AICR)

Maintenance of a heathy weight to protect against cancer and other chronic diseases

Obesity is commonly linked to a number of chronic diseases such as coronary heart disease and stroke, but many people are unaware of its link to cancer. The WCRF/AICR Second Expert Report judged that maintaining a healthy body weight throughout life may be one of the most important ways to protect against cancer.

Weight gain, overweight, and obesity are now more common than in the 1980s and 1990s. Rates of overweight and obesity doubled in many high-income countries between 1990 and 2005. Chronic diseases including obesity are now more prevalent than nutritional deficiencies and infectious diseases in most countries in Asia and Latin America, and some in Africa. And overweight in childhood and early life makes overweight and obesity in adulthood more likely.

The evidence linking overweight, obesity, and cancer is now judged to be even stronger than in the mid-1990s, when the evidence for the first Expert Report was assessed.

The evidence is convincing that greater body fatness is a cause of cancers of the oesophagus, pancreas, colorectum, breast (postmenopause), endometrium, and kidney; and that greater abdominal fatness is a cause of cancer of the colorectum. Greater body fatness is probably a cause of gallbladder cancer, and greater abdominal fatness is probably a cause of cancers of the pancreas, breast (postmenopause), and endometrium.

One key mechanism for the link between obesity and cancer is the influence of body fatness on levels of a number of hormones and growth factors. Insulin-like growth factor 1, insulin, and leptin are all elevated in obese people, and can promote the growth of cancer cells. Plus obesity increases insulin resistance, leading to overproduction of insulin. Sex steroid hormones are also likely to play a role: adipose tissue is the main site of oestrogen synthesis in men and in postmenopausal women.

Obesity is also characterised by a low-grade chronic inflammatory state, which can promote cancer development. Fat cells produce pro-inflammatory factors, and obese people have elevated concentrations of tumour necrosis factor-alpha, interleukin-6, and C-reactive protein, as well as of leptin, which also functions as a inflammatory cytokine.

While the evidence is convincing that greater body fatness is a cause of postmenopausal breast cancer, it probably protects against premenopausal breast cancer. The role of oestrogen in the cancer process may be the reason for this striking difference. Before the menopause obese women tend to have anovulatory menstrual cycles and, as a result, reduced levels of oestrogen; after the menopause they have higher levels. Nevertheless, for practical reasons, and because premenopausal breast cancer is much less common than postmenopausal, and because of other health benefits, there is no specific recommendation on weight for premonopausal women.

The Second Expert Report also judged the evidence on causes of weight gain, overweight, and obesity. The evidence is convincing that physical activity protects against (and sedentary living promotes) weight gain, overweight, and obesity, so it would be expected to reduce risk of cancers linked to these factors. Sedentary living comprises both high levels of physical inactivity and low levels of physical activity. Sugary drinks, fast foods, and other energy-dense foods – those that provide more than about 225-275 kcal/100g of food – and television viewing probably increase weight gain, overweight, and obesity. Conversely low energy-dense foods probably promote a healthy weight, so would be expected to reduce the risk of obesity-related cancers.

Being breastfed probably protects against excess weight gain in children. Exclusively breastfed children show different growth patterns to those of formula-fed infants, and they consume less total energy and protein. The recommendation for body fatness is to be as lean as possible within the normal range of body weight. This refers to appropriate ranges issued by national governments and the World Health Organization. The recommendation is related to a number of the others, such as those for physical activity and for foods and drinks that promote weight gain.

The WCRF/AICR public health goals are for median adult body mass index (BMI) to be between 21 and 23, depending on the normal range for populations. Also that by 2017, the proportion of the population that is overweight or obese should be no more than current levels, or preferably lower.

The personal recommendations are to ensure that body weight through childhood and adolescence projects towards the lower end of the normal BMI range at age 21. Also that people should maintain body weight within the normal range from age 21; and to avoid weight gain and increases in waist circumference throughout adulthood.