The answer to this question depends partly on genes but also on what a child eats. A high-calorie diet can make a child fat by altering his fat cells - not in number but in size.
By adulthood, each person possesses a fixed number of fat cells. But this number may not be set until after adolescence, and the final number varies wildly from person to person. Patricia Crawford, a research nutritionist at the University of California at Berkeley School of Public Health, estimates that the number of fat cells ranges from 20 billion to 160 billion per person. This number, however, does not determine weight.More fat cells simply means more storage places, according to pediatric endocrinologist Sonia Caprio of Yale University. Good eating habits and exercise prevent fat cells from getting filled to the max. In fact, someone with fewer fat cells who eats a lot can be heavier than someone with more fat cells. But by overeating, the person with fewer fat cells forces those cells to expand, and it's these bloated cells that make a person fat.
"Obviously, parents should always try to feed their kids healthy food," says Dr. Diane Barsky, director of the nutrition support service at St. Christopher's Hospital for Children in Philadelphia. "But they don't need to worry about obesity, meaning 20 to 30 percent over ideal body weight, until a child is at least 2. Even then, there's usually not much reason to worry that she will become a fat adult."