Women that lose five per cent of their body weight could halve their risk of developing breast cancer.
The research at the Fred Hutchinson Cancer Research Centre in Seattle followed 439 overweight women aged between 50 and 75.
They were split into four groups, with some asked to diet and exercise, others asked to diet, a quarter just had to exercise and the remaining participants asked not to alter their lifestyle.
It showed that a reduction in body fat through weight loss cuts the level of the hormones known to trigger breast cancer.
Study co-author Dr Anne McTiernan, director of the Prevention Center at the Fred Hutchinson Cancer Research Center, explained that a five to ten per cent reduction in body weight for women that are overweight or obese can cut the hormones by between 25 to 50 per cent.
However, she claimed that while "weight loss represents an additional option for long-term breast cancer risk reduction without significant or bothersome side effects", there is no proof that it will prevent breast cancer.
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