By Merritt McKinney

NEW YORK, Sep 19 (Reuters Health) - Low-fat diets that are high in carbohydrates lower levels of harmful cholesterol, but they also can raise levels of other blood fats linked to heart disease. However, moderate exercise seems to counteract most of the potentially harmful effects of a high-carb diet, researchers in the UK report.

Low-fat diets are usually recommended because they reduce the risk of heart disease by lowering levels of LDL ("bad") cholesterol, Dr. Adrianne E. Hardman of Loughborough University in Leicestershire, told Reuters Health.

Since foods that are low in fat tend to be rich in carbohydrates, low-fat diets are usually high-carbohydrate diets, noted Hardman, the lead researcher on the new study.

Such high-carbohydrate diets, she said, have been shown to increase blood fats called triglycerides, which are thought to raise the risk of heart disease. They can also lower levels of the "good" cholesterol HDL, Hardman added.

"Theoretically at least, these changes might offset any benefits for heart disease risk accruing from the decrease in LDL cholesterol," she said.

But in a study appearing in Arteriosclerosis, Thrombosis, and Vascular Biology: Journal of the American Heart Association, Hardman and her colleagues found that moderate exercise prevents the potential harmful effects of a high-carbohydrate diet.

"Our study shows that moderate exercise--brisk walking for most middle-aged and older people--negates at least some of the potentially negative effects of a high-carbohydrate diet," Hardman said.

"We studied people's responses to ordinary meals and found that, when they were doing daily exercise the increase in their blood triglycerides which was seen when they ate a low-fat, high-carbohydrate diet was entirely prevented," she explained.

The study included eight women in their 50s or 60s who went on a series of 3-day diets: a low-carbohydrate diet, a high-carbohydrate diet without exercise, and a high-carbohydrate diet combined with an hour of brisk walking each day. At the end of each diet, the women ate a high-fat meal and then had their triglycerides measured.

As expected, triglyceride levels were higher after the high-carbohydrate diet without exercise than after the low-carbohydrate diet. But the increase in triglycerides was significantly lower after the high-carbohydrate diet plus exercise than after the high-carbohydrate diet without exercise.

However, exercise did not improve the decrease in HDL cholesterol that occurred after the high-carbohydrate diet.

Still, Hardman said "there is no need to be concerned that low-fat, high-carbohydrate diets may have some negative effects on the levels of blood fats, provided that people engage in regular, frequent exercise."

She noted that moderate-intensity exercise appears to be sufficient to counteract the negative effects of high-carbohydrate diets.

In fact, according to Hardman, this diet and exercise combo "may be the best of both worlds for coronary heart disease risk."

SOURCE: Arteriosclerosis, Thrombosis, and Vascular Biology 2001;21:1520-1525