Scientists guess that 80% of a person's weight gain comes from genetics and 20% comes from the person's environment. By 1998，more than 130 genes and gene neighbors were found to have some link to weight gain or weight loss in people. These different genes can e work all together or work in groups. This means that different people may b up or lose weight with difficulty for different reasons.
One example of these different kinds of combinations is a family of genes that allows people to change extra fat into heat instead of s the fat in the body. Animals which have this same family of genes do not gain weight easily as other animals. Other genes may affect how quickly the s lets the brain know that it is full. If these genes slow down the passage of this information，people may eat more than they really need. Genes are also thought to be linked with a person's fidgets in a day. Fidgeting is small movements of a person's hands，feet，arms or legs usually when the person is n or excited. Fidgeting actually uses a lot of energy over a day and his energy usually comes from burning fat.
In one experiment about weight gain，researchers wanted to see how much energy different people used when they were resting. People in the experiment were asked to lie on a bed and not move for 40 minutes. Researchers then measured the energy the person used for breathing. They found that different people in the study used very different a of energy for this one job，breathing. Researchers calculated that some people in the study burned 1，067 calories per day w others burned up to 3，015 calories. Those people who burn fewer calories doing normal work or exercise have a slow metabolism （新陈代谢）.
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