Thin people can eat both muscle-building powder and protein powder for fitness. Protein powder can be divided into animal protein and plant protein. The best pure animal protein is whey protein, and the best pure plant protein is soybean protein. In contrast, the absorption rate of animal protein is higher than that of plant protein, so it is suggested to buy whey protein, followed by egg protein and soybean protein. In addition to these three kinds, there are ovalbumin and collagen, which are not enough for thin people to keep fit.