Researchers found that the obesity is caused by the higher proportion of fast twitch muscle fibers in the skeletal muscle. There are two main skeletal muscle fibers, one is the fast twitch muscle fibers (also called white fibers), and the other is the slow twitch fibers (also known as red muscle fibers).The energy source of fast twitch muscle fiber is glucose, and the energy source of slow twitch muscle fibers is fat. According to researches, the proportion of these two types of fibers is varied on different people. The quadriceps, for example, the proportion of slow twitch fibers can be as high as 96% and also be as low as 13%. Research shows that the proportion of fast twitch muscle fibers in obese people’s bodies is higher than slim people. The thinner the person is, the larger the proportion of the slow twitch fibers is. Those who have slow twitch muscle fibers of a dominant amount consume more fat than people with dominant fast twitch muscle fibers. That’s why some people gain weight fast while others are keeping their slim figure easily.