Hello. Personally, we must first determine whether thinness is a normal phenomenon or a pathological state. People's obesity partly depends on genetic factors. If both parents are thin, their children are more likely to be thin. Such people are generally not fat since childhood. As long as their bodies have been relatively healthy, there is no disease, their appetite is normal and they are not picky eaters. They don't need special conditioning, just let it be. They can appropriately increase some physical exercises, especially strength training, to make their muscles more developed and look stronger and fuller. If you lose weight gradually without obvious changes in diet and exercise, you should go to the hospital for examination first to rule out the influence of some diseases. The common causes of weight loss are hyperthyroidism, malnutrition, diabetes, malignant tumor, tuberculosis and other consumptive diseases. Depression is also a problem that leads to weight loss, which needs attention. Find the cause and then deal with the symptoms. The universally applicable principle is to maintain a comprehensive diet, appropriately increase protein fatty foods, appropriately increase muscle strength training, and maintain a happy mood. I wish you health and strength.