Some animals eat meat, some animals eat grass, and some animals can eat both meat and vegetables, called omnivorous foods. For meat-eating and omnivorous animals, the muscles of the whole body can still be understood, but why can herbivores (more accurately, herbivores) grow muscles after eating grass?
Of course, this cannot be understood simply as "what you eat to supplement what you eat." The main component of muscle is protein, so in order to produce muscle, the body must get enough protein nutrition. Meat, eggs, milk and other foods are of course the best choice because they are rich in protein. After these foods are eaten, they are gradually broken down into amino acids in the digestive tract and finally absorbed into the blood. After that, blood transports amino acids to muscle cells, where they are "reassembled" to synthesize muscle proteins, and muscles can grow.
However, none of the three foods: meat, eggs, and milk are easy to obtain. Among them, only milk is selflessly provided by the mother in the early stages, while the remaining two are obtained by hunting other animals, which requires your life to die. Therefore, only those fierce hunters can do it. And even superior predators, such as lions, tigers, and leopards, starve one after another. As a result, animals with weak abilities or large amounts of food have to find alternative ways to find more readily available food. On balance, plant foods such as leaves and grass roots are the easiest to obtain, so gentle herbivores choose a more prudent feeding strategy and they become vegetarian!
Although plant food is not as nutritious as meat, there is no shortage of protein, sugar, fat, minerals, vitamins and other nutrients. As long as you eat too much, it is completely enough. Therefore, we can see that herbivores such as cattle, sheep, deer, and horses are always eating grass, while lions can go without hunting for a few days as long as they eat a full meal. In addition, the nutritional composition of a plant is often relatively simple and the amino acid types are incomplete, so many vegetarian animals need to eat multiple plants to achieve balanced nutrition. The ancients said that people should eat grains and grains, which is the same reason.