1. Repair muscle injury: During exercise, muscle fibers will be damaged to some extent. Protein is an important part of muscle tissue. Supplementing protein is helpful to repair damaged muscle fibers and promote muscle growth and recovery.

2. Improve metabolism: protein intake can increase basal metabolic rate and make the body burn fat more effectively at rest. This helps to reduce body fat and shape a healthier body.

3. Enhance immunity: protein is an important part of the immune system. Supplementing protein can improve immunity and prevent diseases.

4. Control hunger: protein is slow in digestion and can effectively control hunger. Supplementing protein after exercise can keep satiety for a long time and avoid overeating caused by hunger.

5. Help muscle synthesis: protein is the basic substance of muscle synthesis. Supplementing enough protein is helpful to increase muscle mass and improve sports performance.

Therefore, it is very important to supplement protein after fitness training. It is suggested to take about 20-30g of protein within 30 minutes after exercise to help the body recover and muscle growth. At the same time, it is necessary to ensure adequate intake of protein in the overall diet to achieve comprehensive health effects.