A longtime rule of thumb in the fitness community has been that people tend to gain the most muscle at 10-15 percent body fat. Beyond 15 percent, they tend to gain more fat and less muscle. The research backs up the connection between higher fat levels and decreased muscle gains. In the 1980s, Forbes showed that there is a logarithmic relation between fat gain and lean body mass gain. In short, the extent of LBM gain or loss depends on the initial body fat in humans and other species. This is called "Forbes' Theory," and it's still generally accepted today.