A 30-minute walk each day is not only good for the heart, it also may help prevent diabetes, researchers say in Wednesday's issue of The Journal of the American Medical Association.

Vigorous exercise is known to improve the body's ability to use insulin, the hormone needed by muscle cells to absorb blood sugar to make energy, said the researchers, led by Elizabeth J. Mayer-Davis of the University of South Carolina in Columbia.But whether moderate activity helps people avoid diabetes has been unclear.

To find out, researchers analyzed the relationship between insulin sensitivity - the body's capacity to use insulin - and exercise among 1,467 men and women ages 40 to 69 in four cities. The subjects ranged from normal in insulin-using capacity to mildly diabetic.

They found that subjects who exercise moderately and regularly were significantly less likely to have impaired insulin-using capacity than sedentary subjects.

Diabetes occurs when the body is insulin deficient, becomes impaired in its capacity to use insulin, or both.