Vigorous exercise can protect women against breast cancer, new research claims.

Normal-weight women who exercise are approximately 30 per cent less likely to develop breast cancer than those who don't, scientists claim in a report published in the Breast Cancer Research journal today.

Researchers followed 32,269 women for 11 years and found that vigorous activity may protect against breast cancer, independent of body weight control.

"Notable strengths of our study include its large sample size, prospective design, high follow-up rate, and availability of relevant known or suspected breast cancer risk factors. These features enabled us to minimise any effects from other factors apart from exercise," Michael Leitzmann from the National Cancer Institute of the US National Institutes of Health said.

In contrast, the authors found that non-vigorous activity such as light housework and light exercise did not protect against the cancer.

"Possible mechanisms through which physical activity may protect against breast cancer that are independent of body mass include reduced exposure to growth factors, enhanced immune function, and decreased chronic inflammation, variables that are related both to greater physical activity and to lower breast cancer risk," Mr Leitzmann added.

"An alternative explanation for the stronger apparent effect of vigorous activity among lean over heavy women is that heavier women may misreport non-vigorous activities as vigorous ones."

"The relationship was much stronger among leaner women," he added.

Interestingly, non-vigorous activity such as light housework, walking, hiking and easy jogging, did not seem to offer any protection against breast cancer, the team reported in BioMed Central's Breast Cancer Research journal.

The researchers did not look at why exercise may help but Leitzmann noted other studies have shown that working out reduces estrogen levels -- a known risk factor for the disease -- and protects the body's general immune system.

(Reporting by Michael Kahn; Editing by Maggie Fox)

大强度锻炼可抵御乳腺癌

（路透社）——研究人员提到大量运动可以使健康年老女性的乳腺癌发病危险性降低30%。

研究人员说，对30000位停经后的妇女进行研究发现，重体力作业——从家务劳动如擦地板到跑步——都有助于抵御乳腺癌，即使对那些没有更高发病危险的人也适用。这种效应在偏瘦的女性中最明确。

Leitzmann及其同事用问卷的形式来决定女性剧烈运动的频率。研究开始时这些女性都是健康的。

十一年以后研究人员发现所有的志愿者中锻炼最多的人乳腺癌发病率会降低13%。
有趣的是，非大强度的活动，如轻的家务劳动，散步，徒步旅行和慢跑，这些对抵御乳腺癌都没有帮助，这个小组在生物医学中心的乳腺癌研究杂志发表了这项研究。研究人员没有研究为什么运动有益，但是Leitzmann说，其它研究已经表明大量运动可以减少雌激素水平，雌激素是共知的乳腺癌危险因素，并且可保护身体的免疫系统。