Is obesity a cancer risk in women?

Research points to overweight and obesity increasing women's cancer risk, reports Sanjay Tanday.

What is the story?
Women who are overweight are at a greater risk of developing cancer, according to media reports.

Latest research, funded by Cancer Research UK, found that being overweight causes one in 20 cases of cancer seen each year among post-menopausal women.

The findings follow warnings about the hidden dangers of obesity in a report by the World Cancer Research Fund.

National survey data from the UK indicate that around 23 per cent of women in England are obese and a further 34 per cent are overweight.

While the link between cancer and being overweight is not new, this research is among the strongest evidence yet gathered in support of it, say the papers.

What is the research?
The reports are based on the findings of the Million Women Study, the largest ever study of cancer risk in women.

Researchers examined the relationship between BMI, cancer incidence and mortality in 1.2 million UK women aged between 50 and 64.

At recruitment for the study, the researchers asked the women for their current weight and height and then used these measurements to calculate their BMI. Women with a BMI of 25-29.9 were defined as 'overweight' and women with a BMI of 30 or more were defined as 'obese' in accordance with the WHO's criteria.

Risks for all cancer and for 17 specific types of cancer, including stomach, pancreas, endometrium, ovary and kidney, were measured according to BMI. The women were followed up for an average of five years for cancer incidence and seven years for cancer mortality using NHS central registers.

Over the duration of the follow-up period, 45,037 new cancers and 17,203 deaths from cancer were recorded.

Increasing BMI was associated with an increased incidence of all cancers combined and for 10 out of 17 specific types of cancer examined; endometrial cancer; cancer of the oesophagus; kidney cancer; leukaemia, multiple myeloma; pancreatic cancer; non-Hodgkin's lymphoma; ovarian cancer; breast cancer and bowel cancer. The risk of cancer of the oesophagus and of the endometrium doubled in overweight women compared with women of a healthy weight.

Results were adjusted for other risk factors such as age, smoking status, alcohol intake and levels of physical activity.

What do the researchers say?
Lead researcher Dr Gillian Reeves, from the cancer epidemiology unit at Oxford University, said: 'We estimate that being overweight or obese accounts for around 6,000 out of 120,000 new cases of cancer each year among middle-aged and older women in the UK.'

Research found that the relationship between BMI and cancer also depended on a woman's stage of life, said Dr Reeves.

'Being overweight increases the risk of breast cancer only after the menopause, and the risk of bowel cancer only before the menopause.'

Sara Hiom, director of Cancer Research UK's health information, added: 'This research adds to the evidence regarding the impact of being overweight or obese on developing cancer.'

What do other researchers say?
Dr Eugenia Calle director of the American Cancer Society, said: 'The findings of the study are generally in agreement with accumulated evidence to date.'

The worldwide obesity epidemic shows no signs of abating, so insight into the mechanisms by which obesity contributes to the formation and progression of tumours is urgently needed, said Dr Calle.

BMJ Online 2007

Informing patients
  • Being overweight causes 6,000 cancers a year in postmenopausal women in the UK.
  • Increasing BMI was associated with an increased incidence of all cancers in women.
  • Age appears to be a factor because breast cancer risk increased in overweight women only after the menopause.

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