The connection between exercise and reduced body fat is not 100 percent clear at this time, as the major source of estrogen in post-menopausal women may also be playing a role, according to the study.
The report also stated "Further studies are needed to confirm these novel findings, and to evaluate similar relationships among premenopausal women … if found to be causally related to breast cancer, physical activity would have a substantial public health effect on the prevention of this disease, along with its other positive health benefits."
Another unrelated study published in the same journal stated that people who slim down by cutting calories may also be losing bone density -- something that does not happen if weight loss comes through exercise. The study -- carried out by doctors from the Washington School of Medicine -- looked at 48 adults involved variously in weight loss by caloric restrictions or exercise.
With bone loss possibly leading to fractures and osteoporosis later in life, the second study stated that "A common explanation given for the bone loss induced by weight loss is reduction in mechanical stress on the weight-bearing skeleton (the hip and spine)." When muscles pull on bones during exercise, it is thought to produce strains in the skeleton that stimulate new bone production.
Hence, exercise may be responsible for triggering the body into producing new bone mass. The Washington study went on to say "These findings have important implications in designing an appropriate weight-loss therapy program in middle-aged adults, particularly in the subset of patients who may already be at increased risk for bone fracture."