York researcher finds our ability to burn fat declines after puberty

Children who exercise regularly are fat-burning machines, an ability they lose during puberty when the body turns to carbohydrates rather than fat to fuel a workout, says a leading York health researcher.

Headshot of Michael Riddell, associate professor of Kinesiology"Children seem to be oxidizing about double the amount of fat that adults do during exercise per kilogram of their body mass. So, as an energy source, fat is really important in childhood and adolescence," says Michael Riddell (left), a professor in the School of Kinesiology & Health Science in York’s Faculty of Health.

The body uses two main fuel sources during exercise: fat and carbohydrate. Children’s bodies tend to hoard carbohydrate stores for growth and development, leaving fat to fuel physical exertion. But that’s no excuse to encourage children to carry extra weight, says Riddell. "If you take a look at the average child, even if they’re thin, they probably have enough fat stored to exercise. It’s not like they’ll run out during a sports event."

Riddell’s research finds this ability to burn fat at dramatically higher rates drops, from the middle to the end of puberty, at about 13 or 14 years of age for the average child. While adult bodies still use both carbohydrates and fat for energy, as they exercise, the body tends to shift more to carbohydrate as a fuel.

The study followed five boys, aged 11 or 12, through puberty, periodically testing the rate at which their bodies burned fat throughout a range of exercise intensities. These results were compared to similar tests performed periodically over time on nine men, ranging in age from 20 to 26.

The so-called fat oxidation rate is determined using respiratory technology that measures the amount of oxygen a body consumes and the carbon dioxide it releases; the ratio pointing to whether the body is drawing energy from fat or carbohydrates.

A reduced ability to burn fat during exercise may lead not only to excess weight gain, but to a reduced sensitivity to insulin as the body makes the transition from adolescence to adulthood. This may predispose people to Type 2 diabetes and other chronic diseases, says Riddell.

"We don’t know if exercise at a certain age or certain intensity programs your metabolism to prevent diabetes, but we do know that exercise is certainly beneficial in adolescence to help prevent excess weight gain. In adulthood, even if you have diabetes, exercise is certainly helpful in lowering body fat content, particularly in the midsection, which helps to improve insulin sensitivity," he says.

As an acknowledged leader in the field, Riddell was invited to review his work in a special edition of the Journal of Applied Physiology, released yesterday, which focuses on the biology of physical activity in youth.

Riddell’s studies also have implications for the continuing debate about whether sports drinks are effective at providing additional energy for working muscles. Children and adolescents who perform sport regularly benefit from drinking sport beverages prior to, during and after competition, Riddell says, citing his own studies that show that sport beverages provide a huge fuel relief for those with limited internal stores of carbohydrate.

That’s good news for sports drink companies who have been less persistent in marketing to kids. But the case for adults is less clear cut. "They’re not that beneficial for adults unless the activity lasts greater than 90 minutes. But for children we’ve shown that even for exercise that lasts from 30 to 60 minutes, sports drinks are clearly beneficial in improving performance and sparing critical stores of endogenous carbohydrate," says Riddell.

Future studies will examine fat utilization in girls and women and will seek to determine the mechanisms behind the reduced ability to burn fat as we grow older.

Riddell developed a new sports camp for youth with diabetes this year, the first of its kind in Canada. Camp participants monitored changes in their blood sugar during exercise to experience its benefits firsthand. The initiative is part of York’s Physical Activity and Diabetes Initiative (PAD), which aims to bring knowledge about diabetes into the community on an active level.