The idea that children who are overweight will grow up to be overweight adults isn't especially accurate. Many studies have found that body mass index in childhood is very often reflected in body mass index in adulthood. People who were thinnest as children and fattest as adults tended to have the highest adult risk of disease.
The study looked at the health records of 1,142 children. Two-thirds of the group were followed until the age of 15 years. The researchers collected data on adult lifestyle, including height, weight, and self-reported illness through questionnaires. It's true that overweight teenagers were more likely to become fat adults, but most fat adults were not overweight as children. That's important to know for people who treat childhood overweight.