Losing just over half a stone could dramatically lower the chance of developing breast cancer.
Research shows that overweight women who shed 5 per cent of their body weight are up to 50 per cent less likely to have the disease.
The study of 439 women aged between 50 and 75 provides evidence that loss of body-fat reduces the levels of hormones known to trigger cancer.
The women were split into four groups – one was put on a diet and told to exercise more, the second just went on a diet, the third just took more exercise and the fourth group did not change their lifestyles at all.
The research showed that a woman who lost 5 per cent of her body weight could be 50 per cent less likely to develop breast cancer
The scientists at the Fred Hutchinson Cancer Research Centre in Seattle found on average the women on the diet and exercise programmes lost 10 per cent of their body weight. They also measured the levels of certain types of the hormone oestrogen and testosterone that are known to increase the risk of cancers.
All levels dropped by between 10 and 20 per cent among the women who lost weight. All were either overweight or obese.
Lead researcher Anne McTiernan worked out that even if an overweight woman lost only 5 per cent of her body weight she would be between a quarter and a half less likely to develop the most common form of breast cancer.
That weight loss would be the equivalent of a 12 stone woman dropping half a stone. Dr McTiernan, whose research is published in the Journal of Clinical Oncology, said: ‘The amount of weight lost was key to changes in hormone levels.
‘The biggest effect was through diet plus exercise, exercise by itself didn’t produce much of a change in weight or oestrogen.
‘I recommend women both diet and exercise, because in the long run that should help keep weight down and therefore keep oestrogens down.’
The researchers said the benefits of losing weight applied to those breast cancers that are ‘oestrogen receptive’ – 70 per cent of all cases.
Eluned Hughes, of Breakthrough Breast Cancer, said: ‘We strongly advocate the benefits of maintaining a healthy weight as it can help reduce a woman’s risk of developing breast cancer.
Risk: The study examined 439 overweight or obese women, all of whom were less susceptible after losing weight
‘This study suggests that obese women who lose weight after the menopause could also see this reduction in risk.’
Experts have long known that certain lifestyle factors such as obesity, alcohol, smoking and lack of exercise increase the risk of developing many types of cancer.
Last year scientists at Cancer Research UK calculated that 134,000 cases of cancer in Britain a year were caused by these ‘preventable’ factors.
A study earlier this year revealed that women who are overweight or obese when diagnosed with breast cancer are more likely to suffer relapses and die from the disease than those who are thinner. It showed a 17 per cent increase in the risk of the disease returning after initial treatment and of death in obese women compared with those of healthy weight.
In overweight patients, the risk increase was 8 per cent.
The study found a 1.5 per cent increase in risk for every one unit rise in body mass index.
It ruled out under-treatment with chemotherapy drugs as a possible cause, ensuring doses were adjusted to take account of participants’ weights.
Researchers believe higher insulin levels occurring naturally in larger women may be responsible for the differences in outcome.
Dr Jennifer Ligibel, a medical oncologist at the Dana-Farber Cancer Institute in Boston, and colleagues studied data from 1,909 US patients who were enrolled in a study between 1997 and 1999.
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