French researchers examined results from 37 studies involving more than four million women.
They found women of any age or size who exercise for an hour a day all get a similar benefit, including those with the most aggressive cancers.
Because the results are independent of body mass index (BMI), the protective effects must be due to more than weight control, they said.
The age at which sporting activity starts appears to be ‘immaterial’, say researchers, and they did not find the risk fell only when the physical activity started at a young age.
The gain seems to be cancelled out in women taking hormone replacement therapy (HRT).
Professor Mathieu Boniol, research director at the International Prevention Research Institute in Lyon, France, said: ‘These are all the studies looking at the relationship between physical exercise and breast cancer risk that have been published to date, so we are confident that the results of our analysis are robust.
Adding breast cancer, including its aggressive types, to the list of diseases that can be prevented by physical activity should encourage the development of cities that foster sport by becoming bike and walk-friendly, the creation of new sports facilities, and the promotion of exercise through education campaigns.
‘This is a low cost, simple strategy to reduce the risk of a disease that currently has a very high cost, both to healthcare systems and to patients and their families. It is good news both for individuals and for policy makers.’
The study focused on vigorous exercise, but previous research suggests even moderate activity in the form of walking an hour a day leads to a lower risk compared to women who were least active.
It is thought activity works partly by enabling women to stay slimmer, which cuts down on the amount of cancer-promoting hormones produced by body fat.
Heavier women have higher levels of cancer-promoting hormones, such as oestrogen and insulin, in their bodies which are more likely to be taken up by breast cells where they can stimulate cancer growth.
In the UK, around 50,000 women develop breast cancer each year – four out of five in the postmenopausal years after the age of 50 – and 400 men.
In the UK people are advised to do 150 minutes of moderate activity such as gardening, dancing or brisk walking, or 75 minutes of vigorous exercise including playing sport, running or aerobics every week.
Three out of four Britons fail to achieve this.
Dr Hilary Dobson, chairwoman of the conference’s national organising committee, said: ‘These findings are important for all women, irrespective of their age and weight.
‘Whilst the mechanism for the potentially protective effect of physical activity remains unclear, the analysis, which is presented here, provides women with a real impetus to increase their physical activity by even modest increments.’
Hannah Bridges, health information lead at charity Breakthrough Breast Cancer, said: ‘This is an exciting finding.
‘Breakthrough Breast Cancer recently looked into all the best studies on physical activity and breast cancer with experts across the world, and we also found that there is good evidence that women can reduce their risk of breast cancer by being regularly active.
‘Even better, moderate physical activity counts, which can be things like gardening, housework or a brisk walk – so there are many ways to fit the needed physical activity into your day-to-day life.’
SOURCE: Daily Mail 
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