Being thin and being fit both protect against cardiovascular disease. Being either fat or unfit increase your risk of a heart attack, but which carries the greater risk?
In a medical sense, fitness is usually determined by how much oxygen someone takes in during a treadmill exercise test. Being unfit is roughly equivalent to being out of shape, as when someone quickly becomes short of breath during physical activity. A number of previous studies have linked fitness with fewer deaths from all causes, including cardiovascular disease.
A recent study measured fatness, fitness, and 18 different risk factors for cardiovascular disease in 135 healthy men. The fit men did have fewer risk factors. However, three different measures of fatness -- body mass index (BMI), percentage of body fat, and waist circumference -- were all more strongly associated with the risk of developing cardiovascular disease than was being unfit. These results suggest that losing weight is a more important goal than being fit for overweight and obese individuals who want to protect their heart.
These findings should not discourage anyone from keeping their heart healthy by exercising vigorously to become fit. Fitness may also reduce the risk of type 2 diabetes because physical activity and being fit increase the body's sensitivity to insulin. Of course, being fat can lead to other health problems even if you are fit.
I agree with the study's authors, who conclude that sedentary men should increase their physical activity to "improve their aerobic fitness, regardless of their body fatness."