People in the United States and other developed countries have a basic problem: Food is cheap, and exercise is expensive. But our friends fat and lazy are so closely linked that it's not clear which part of the equation is killing us. In two new studies published in the Journal of the American Medical Association, researchers say—well, both, but in different ways.

What the researchers wanted to know: How do body mass index and physical activity affect women's risk of heart disease (first study) and diabetes (second study)?

What they did: The heart disease information came from a study sponsored by the National Heart, Lung, and Blood Institute of about 950 women with heart problems, who were asked about their physical activity and risk factors and had a physical exam and tests done. The diabetes data were from the Women's Health Study of about 40,000 people, which is actually testing aspirin and vitamin E for prevention of heart disease and cancer. But the researchers also know the women's body mass index, how active they are, and who has diabetes.

What they found: First, heart disease: Women who were less physically active had worse hearts—they were more likely to have coronary heart disease and were at higher risk of having bad cardiovascular events (such as heart attack, stroke, or congestive heart failure). But obesity, as measured by body mass index, didn't predict heart trouble well at all. On the other hand, diabetes was much more likely to develop in women who were overweight or obese, while physically active women were only slightly less likely to get diabetes.

What the studies mean to you: Sorry: Both obesity and inactivity are bad for you. Put down the Twinkie and back—no, run—away.

Caveats: The heart study doesn't prove that inactivity causes heart disease; maybe people with coronary artery disease just couldn't exercise that much (although other data from the study suggest that's not the problem). Women in the second study reported their own diabetes, but the researchers say they're pretty sure it's right.

Find out more: The National Heart, Lung, and Blood Institute has a page to help you "Aim For a Healthy Weight", which includes a tool to calculate body mass index and a guide to physical activity.

Read the articles: Wessel, T.R., et al. "Relationship of Physical Fitness vs. Body Mass Index With Coronary Artery Disease and Cardiovascular Events in Women." Journal of the American Medical Association. Sept. 8, 2004, Vol. 292, No. 10, pp. 1179–1187.

Weinstein, A.R., et al. "Relationship of Physical Activity vs. Body Mass Index With Type 2 Diabetes in Women." Journal of the American Medical Association. Sept. 8, 2004, Vol. 292, No. 10, pp. 1188–1194.

Articles online: Cardiovascular disease: http://jama.ama-assn.org/cgi/content/full/292/10/1179

Diabetes: http://jama.ama-assn.org/cgi/content/full/292/10/1188