Overeating, a poor diet, and lack of physical activity are by far the most important causes of obesity in children. Rarely, a thyroid or other hormonal disorder is the cause for childhood obesity.

Children of parents who are obese are 10 times more likely to become obese than other children. However, these children are usually taught poor eating habits and a sedentary lifestyle from their parents. Super-size meals, TV, computers and video games all greatly contribute to overeating and inactivity.

In some cases a predisposition to obesity may be inherited. Some experts believe as much as one-third of the variability in body fat content may be partially due to genetics. This means the genes children inherit from their parents help to determine their weight. This may include factors such as how efficiently the body absorbs and stores fat and the metabolic rate.

However, genetics do not mean someone has to be obese. The difference in energy requirements is a small amount - usually less than a few hundred calories a day. A minor adjustment in dietary calories can easily adjust for this difference.

Regardless of the cause, if the total calories (food energy) taken in is greater than the energy spent, then weight gain will occur.