It raises the prospect that taking cholesterol-lowering drugs called statins could prevent cancer.
The work, published in the journal Science, helps explain why obesity is a major risk factor for the disease.
However, cancer charities cautioned that it was too soon to advise women to take statins.
Obesity has been linked with many cancers including those of the breast, bowel and womb.
The results from this early study are promising”
Dr Hannah BridgesBreakthrough Breast Cancer
The fat in overweight people can pump out hormones, such as oestrogen, which drive the growth of cancers.
A team at Duke University Medical Centre, in the US, showed that cholesterol was having a similar effect.
Cholesterol is broken down by the body into 27HC, which can mimic oestrogen and produce the same effect as the hormone in some tissues.
Experiments on mice showed that a high fat diet increased levels of 27HC in the blood and led to tumours that were 30% larger than in mice on a normal diet.
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