Less Fit Teens More Likely to Have Precursor to Diabetes

March 2003 A child who is overweight and unfit may already be on the road to developing insulin resistance, an early sign of diabetes, researchers reported at the American Heart Association's 43rd Annual Conference on Cardiovascular Disease Epidemiology and Prevention.

Insulin sensitivity is a measure of how well the body responds to insulin, a hormone that transports carbohydrates from the blood into cells where they are turned into energy. High insulin sensitivity means the body is responding well to insulin. Low insulin sensitivity also called insulin resistance is often a precursor to diabetes.

"This is the first study to look at these questions in a large group of adolescents that includes both blacks and whites and males and females," says lead author Bernard Gutin, Ph.D., professor of pediatrics and physiology at the Medical College of Georgia in Augusta, Ga. The researchers found that race and gender were related to insulin sensitivity, which was highest among white girls and lowest among black girls. In addition, boys had higher cardiovascular fitness than girls, as well as lower percent body fat. When the researchers controlled for race and gender, they found that higher cardiovascular fitness and lower body fat were independently associated with greater insulin sensitivity. That finding suggests that improving fitness or reducing body fat could protect high-risk children, Gutin says.

"This is an important finding because some people think that body fat is the critical factor in the development of diabetes," he says. "We found that it was certainly very important, but that even after adjusting for it, fitness still made an independent contribution."

The mystery of the early course of diabetes is getting a lot of attention now because the last two decades have seen an explosion in the number of teenagers with type 2 diabetes, a condition once called "adult-onset" diabetes because it was so rare in young people, he says.

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Low insulin sensitivity also called insulin resistance is not diabetes, but is often a prelude to the disease. In people with low insulin sensitivity, the body makes insulin but can't use it efficiently to break down glucose into energy, requiring a greater-than-average amount of insulin to transport an average amount of glucose.

When the pancreas can't keep up with demands for extra insulin, glucose levels begin to creep up toward the diabetic range. That's why it's important to measure blood levels of both glucose and insulin to study how diabetes develops, Gutin says.

They studied 289 teenagers ages 14-18 recruited from high schools in the Augusta area. Insulin sensitivity was estimated using fasting blood tests for insulin and glucose (blood sugar).

Cardiovascular fitness was measured by calculating how much oxygen each subject used when a brisk, uphill walk on a treadmill brought a participant's heart rate to 170 beats per minute. The researchers also measured body composition (the amount of fat versus fat-free mass) and waist circumference an indication of abdominal fat.

The researchers chose the 14 to 18-year-old age group because they hoped to study the early development of diabetes, while avoiding changes in insulin sensitivity during puberty that might have skewed the results, Gutin says.

Gutin recommends parents schedule at least an hour of sweat-inducing exercise into their children's afternoon activities. He says the study findings are important because they indicate that "every child can benefit from higher fitness and lower fatness."

Co-authors are Zenong Yin, Ph.D.; Matthew C. Humphries, M.S.; William H. Hoffman, M.D.; Hyun-Sik Kang, Ph.D.; and Paule Barbeau, Ph.D. The research was supported by grants from the National Heart, Lung, and Blood Institute of the National Institutes of Health.

Source: American Heart Association