Constitution is a complex concept, including the comprehensive influence of individual genetic factors, metabolic rate, eating habits, lifestyle and other environmental factors. Although genetic factors have certain influence on physical fitness, they are not decisive factors.

There are several possible reasons why some people are not fat:

1. metabolic rate: everyone's metabolic rate is different, that is, how much energy the body consumes at rest. Some people may have a higher metabolic rate, the body consumes energy more effectively, and it is difficult to gain weight.

2. Eating habits: Some people may have healthy eating habits, take proper nutrition and avoid overeating. A balanced diet and moderate exercise help to maintain health and weight.

3. Lifestyle: Lifestyle differences will also affect weight. Some people may have a more active lifestyle, engage in manual labor or often take part in sports, which helps to consume extra energy.

It should be noted that eating fat does not mean health. Some people may be thin, but other health problems may be ignored. Moreover, weight control still requires a balanced diet, moderate exercise and a healthy lifestyle.

In a word, the formation of physique is the result of many factors, including heredity, metabolic rate, eating habits and lifestyle. Although there may be some natural differences in personal physique, most people can maintain a healthy weight by improving their diet and living habits.