Healthy but obese adolescents display abnormal responses to tests of blood vessel function, but after a short course of exercise their test results match those of lean youngsters, even though they remain heavy, according to a new study
in the May 19, 2004 issue of the Journal of the American College of Cardiology
"We were able to essentially normalize endothelial function in a short period using the exercise training program that we administered. That's the exciting part. It's a good news story," said Daniel J. Green, PhD, from the University of Western Australia in Crawley, Australia. "This paper adds another piece to the jigsaw puzzle which says that exercise is beneficial not only because it may improve your risk factors, like lipids and blood pressure, but it also probably has a direct effect on your blood vessels that makes them more fit."
The study involved 19 obese adolescents (age 12 to 15 years, average weight 96.4 kg, 212 lbs.), who were apparently healthy nonsmokers with normal blood pressure and cholesterol, and who weren't on any medications. Compared to a matched group of 20 lean controls, the obese adolescents displayed impaired flow mediated dilation, which is a measurement of how blood vessels respond to changes in blood flow. Impaired flow mediated dilation is considered an early warning sign of atherosclerosis.
But after an eight-week course of one-hour exercise sessions three times per week that used stationary cycling and strength training with light weights, the blood vessel responses of the obese youngsters matched those of their lean counterparts. Dr. Green pointed out that although blood vessel response is increasingly seen as an early step in the development of artery disease, the adolescents in this study did not have overt signs of atherosclerosis. He also noted that researchers have not demonstrated that improving blood vessel responses will prevent heart disease.
Dr. Green said it is important to note that the obese subjects did not lose weight, but their body composition changed. He said it is time to rethink both the type of exercise recommended for obese individuals and how the results of an exercise program are measured. The researchers, including lead author Katie Watts, BSc(HONS), used an X-ray technique known as DEXA analysis (dual-energy X-ray absorptiometry), which can differentiate between body fat and muscle mass.
"We found a decrease in percentage body fat, particularly from the worst place to have fat, which is the viscera, around the internal organs. We found a decrease in abdominal obesity in this group. And the reason that they didn't lose weight, even though they lost body fat, is that their lean muscle mass went up. So we were really seeing a very beneficial change in the composition of the body, without changes in gross measures like body weight," Dr. Green said.
Dr. Green said the results suggest that focusing on weight alone may be a disservice to obese individuals. He said they may become discouraged when exercise doesn't produce rapid weight loss, even as beneficial changes are occurring. In this study, the obese participants did not change their usual diets.
Half the subjects in this study exercised for eight weeks and then resumed their normal activities, while the other group waited eight weeks before taking the training course. The results showed that while the benefits of exercise can be felt quickly, they may disappear just as quickly.
"The effect seems to be quite rapid, although a key finding coming out of this is that the group who exercised first and then had a layoff period of eight weeks, by the time we studied them again after their eight-week layoff, the beneficial effects of the exercise had gone away again," Dr. Green said.
Ori Ben-Yehuda, MD, FACC at the University of California at San Diego, who was not connected with this study, underscored the growing importance of links between obesity and cardiovascular diseases, including atherosclerosis.
"It is thought that endothelial dysfunction is one of the first steps leading to atherosclerosis. In this study the authors demonstrate the importance of exercise in obese adolescents to improve vascular health. Interestingly exercise in this short-term study led both to an improvement in endothelial function as well as to a redistribution of fat away from the abdomen where its presence is associated with insulin resistance. The study emphasizes the importance of incorporating exercise into the lifestyle of our children," Dr. Ben-Yehuda said.
Roger M. Mills, MD, FACC at the Cleveland Clinic in Cleveland, who also was not connected with this study, also remarked that this study convincingly demonstrated that exercise benefits obese adolescents even if they do not lose weight.
"From years of clinical practice, I sense that most overweight individuals abandon exercise because 'it doesn't help.' Well, it may not help with weight loss, since caloric restriction is absolutely required. However, the benefits to overall vascular health do accrue. This should, in a rational world, be a strong argument for gym class and other programs designed to increase physical activity in teenagers," Dr. Mills said.
The American College of Cardiology, a 29,000-member nonprofit professional medical society and teaching institution, is dedicated to fostering optimal cardiovascular care and disease prevention through professional education, promotion of research, leadership in the development of standards and guidelines, and the formulation of health care policy. http://www.acc.org