In the United States, about one in every eight women will develop invasive breast cancer during the course of their lifetime. That's around 12 percent of the female population in the country. In 2014, it is estimated that 295,240 new cases of invasive and noninvasive breast cancer will be diagnosed.

Growing older is one of the most significant risk factors for the disease but a study has shown that middle-aged women who engage in at least 30 minutes of vigorous activity in a day have lower risks of developing breast cancer.

Carried out by Oxford University researchers, the study entailed three years' worth of data from 126,000 women who have reached menopause. The women also filled out questionnaires, detailing how often they engage in exercise, their diet, their weight, and their smoking and drinking habits.

According to the study, those who did three hours of vigorous activity in a week (about 30 minutes of exercise in a day) had 21 percent lower risk of developing breast cancer compared with women who did little to zero exercise. However, not just any kind of exercise will do. To reap the benefits of lowered breast cancer risk, women had to make sure they engage in intensive physical activity. In the gym, this means dance or spinning workouts.

It has long been established that being obese or overweight ups a woman's risk of developing breast cancer, believed to be caused by the rise in estrogen brought about by fat cells. Estrogen encourages tumor growth, after all.

However, researchers suspect that the benefits of exercise in curbing breast cancer risk extends beyond preventing weight gain, a move which keeps fat cells at bay. Rather, intensive physical activity itself lowers levels of estrogen in the body.

Middle-aged women who are fit are also likely to have been trimming most of their lives, highlighting the possibility that they have been enjoying the cancer-protecting benefits of physical activity for years.

Still, researchers pointed out that losing weight or simply starting exercise in middle age can still help in reducing risks of breast cancer.

"What's really interesting about this study is that [reduction in breast cancer risk] does not appear to be solely due to the most active women being slimmer, suggesting that there may be some more direct benefits of exercise for women of all sizes," said Professor Tim Key, a Cancer Research UK scientist from Oxford University's cancer epidemiology unit.