April 30, 2012

Study: Shedding belly fat helps improve blood vessel function

Overweight people who shed pounds, especially belly fat, can improve the function of their blood vessels no matter whether they are on a low-carb or low-fat diet, according to a study presented by Johns Hopkins researchers March 13 at an American Heart Association scientific meeting focused on cardiovascular disease prevention.

In the six-month weight-loss study, Johns Hopkins researchers found that the more belly fat the participants lost, the better their arteries were able to expand when needed, allowing more blood to flow more freely. The researchers also found that participants in the study who were on a low-carb diet lost about 10 pounds more on average than those who were on a low-fat diet. Being overweight increases the risk of cardiovascular disease, especially if the fat is accumulated in the belly above the waist.

“After six months, those who were on the low-carb diet lost an average of 28.9 pounds versus 18.7 pounds among those on the low-fat diet,” said lead investigator Kerry J. Stewart, a professor of medicine at the Johns Hopkins University School of Medicine and director of Clinical and Research Exercise Physiology at the Johns Hopkins Heart and Vascular Institute.

Stewart and his colleagues studied 60 men and women who weighed an average of 215 pounds at the start of the program. Half the participants went on a low-carb diet, while the others followed a low-fat diet. All took part in moderate exercise, and their diets provided a similar amount of calories each day.

In order to evaluate the health of the participants’ blood vessels before and after the weight-loss programs, the researchers conducted a blood-flow test by constricting circulation in the upper arm with a blood pressure cuff for five minutes. With this type of test, when the cuff is released, a healthier artery will expand more, allowing more blood to flow through the artery. The researchers measured how much blood reached the fingertips before, during and after the constriction of the artery. Stewart says that this test can give an indication of the overall health of the vascular system throughout the body. The researchers found that the more belly fat a person had lost, the greater the blood flow to the finger, signaling better function of the artery.

“Our study demonstrated that the amount of improvement in the vessels was directly linked to how much central, or belly, fat the individuals lost, regardless of which diet they were on,” Stewart said. “This is important since there have been concerns that a low-carb diet, which means eating more fat, may have a harmful effect on cardiovascular health. These results showed no harmful effects from the low-carb diet.”

In the low-carb diet used in the study, up to 30 percent of calories came from carbohydrates such as bread, pasta and certain fruits, while 40 percent was from fat consumed from meat, dairy products and nuts. In contrast, the low-fat diet consisted of no more than 30 percent of calories from fat and 55 percent from carbs.

Stewart notes that participants on the low-carb diet lost, on average, more weight, and at a faster pace, a result that also has been seen in several other studies. He says that eating higher amounts of carbohydrates can slow down the rate of body fat loss while on a weight-reduction diet.

The findings were consistent with early results presented by Stewart in June at the annual meeting of the American College of Sports Medicine. That initial report was based on results after participants in the study had lost their first 10 pounds. These longer-term results show that weight loss, along with exercise, is important for improving vascular health, and suggests that following a low-carb diet rather than the conventionally recommended low-fat diet for weight loss is not a concern in terms of vascular health.

—Ellen Beth Levitt