Scientists had more bad news for couch potatoes saying they had found that exercise can lower the levels of a "fat hormone" in the body -- no matter how fat a person is. Their findings add to a growing body of evidence that nothing works better than hitting the pavement to prevent diabetes and heart disease. The team, at the Harvard School of Public Health in Boston, found that, fat or thin, men who exercised the most had lower levels of leptin in their blood.

If you get more exercise, you can lower your leptin levels, even if your BMI stays the same. BMI stands for body mass index, and is used by researchers around the world to calculate obesity. Scientists have been intrigued since the hormone leptin was discovered, and became excited when they found that injecting leptin into rats could cause them to lose weight.

Sadly, the same is not true for humans. In fact, overweight people have higher levels of leptin than lean people do, which suggests the hormone, which controls appetite in rodents, is being overproduced by the body for some reason. Five years ago, it was believed that leptin research might offer a miracle solution to obesity and the many health problems associated with being overweight.

As you get fatter, your body makes more leptin. But when you exercise, the amount of leptin in the blood decreases. Most of the time, you also lose weight, which means your risk of heart disease decreases too.

The researchers looked at the dietary and lifestyle habits of 268 men aged 47 to 83. None of the men had heart disease, diabetes or cancer. Men whose leptin levels were the highest weighed more, exercised the least and ate more foods high in saturated fat and cholesterol. The researchers also found evidence that the fattest men were starting a tendency toward diabetes -- which in the early stages is marked by an increased production of insulin.

Men whose bodies produce more insulin also tend to have increased leptin levels. This would suggest that leptin might play a role in diabetes. It is too early to say whether leptin levels could be used to predict a person's risk of heart disease. But obesity raises a person's risk of a heart attack and stroke, and our findings provide evidence that physical activity may reduce the chronic disease risk in men through changes in leptin levels.

Meeting of the American Heart Association in San Diego March 3, 2000