(CBS) Teen obesity is expected to lower the average life expectancy in this country.

According to a 2004 National Institutes of Health report, U.S. teens are the most overweight teens among 14 countries studied. In the U.S., 12.6 percent of 13-year-old boys and 10.8 percent of 13-year-old girls were overweight. As kids get older, the problem worsens: 13.9 percent of 15-year-boys, and 15.1 percent of 15-year-old girls were overweight.

"Perhaps the biggest issue for teenagers is the body image," Dr. Harvey Haakonson tells The Early Show co-achor Rene Syler. He is the co-author of "Slow Carb For Life." "It's such a difficult time in their lives anyway in terms of the peer pressure and the attitude of their other teenagers about how they look that if you're overweight, it just really increases the peer pressure for them."

Dr. Haakonson says diets are not the answer for hungry, growing teens, and that most restrictive diets don't work for teens - or anybody for that matter.

He explains, "A wiser way to go is to change the thinking about how you deal with food. That means changing your attitude. That means changing your lifestyle. And if you're going to do all of that, you have to understand about the choices you're making."

He an dhis wife have filled his book with advice to help readers make better choices. He notes the root of the problem for teens lies in the fact that most have grown up with fast food, poor nutrition and little physical activity.

According to recent findings by the Children's Hospital in Boston, 75 percent of adolescents eat fast food at least once per week. The Centers for Desease Control estimates the annual hospital tab for juvenile-obesity-related illnesses at $127 million.

Dr. Haakonson says to change a teen's lifestyle requires the active participation of the teen.

The key to weight loss for kids who have already fallen into bad eating habits is to make gradual changes, Dr. Haakonson says. His advice for parents is to encourage kids to eat fewer bad foods or drink fewer soft drinks rather than try to stop consuming them all at once.

"We suggest [that you]include the teenager in the food buying and the food preparation," he says. "If they understand what goes into the purchase of the food and what is required in the preparation of it, and have some control over that process, it makes it a whole lot better for them. But I must say, even the fast food world, if you're going to have to indulge occasionally in fast food, there are some better choices there today. And I know people don't think of going to fast food to get a salad, but some are quite good, actually."

To achieve a balanced diet, it is important to include whole grains, fruits, and vegetables, he says, "Particularly choosing the ones that turn slowly to blood sugar. That's really the key."

Dr. Haakonson and his wife, Patricia, provide a wealth of teen-tested favorite recipes in "Slow Carb for Life" and their latest book "All New Easy Low-Carb Cooking."

An example is creamy garlic cauliflower, which provides the taste without the carbs. Snacks are also essential, and lunch boxes should be prepared so that friends will fight over them. Some suggested lunch box foods are cheese strings, jerky, trail mix, homemade protein bars, muffins and cookies.

Kids need to splurge, but it's possible to provide healthy splurges like dark chocolate or homemade trail mix, he says. The worst foods are French fries, chocolate bars, soda pop, fruit juices, white bread, and commercial bakery products.