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There’s no question that the more muscle you build, the easier it is to get lean. For decades it was assumed that this was due to the fact that muscle burns more calories than an equivalent amount of body fat. And while that is true, the difference is not significant enough to really make a huge difference in fat loss. While there may be many reasons why more muscle means less fat, one plausible mechanism involves the hormone leptin.

Leptin is a hormone released by fat cells that increases metabolic rate and decreases hunger. So you burn more calories and consume fewer. Yet a recent 12-week study from Spain combining weightlifting and aerobic exercise found that as you lose body fat, your leptin levels decrease. This makes sense, given that leptin is released from fat cells, yet is counterintuitive to being able to continue losing body fat. However, an earlier study from the same Spanish researchers shows why muscle growth is so important for continued fat loss. They reported that in the dominant arm of male pro tennis players, which has 15% more muscle mass than the opposite arm, it also has higher levels of leptin receptors. This means that the muscles have a higher sensitivity to leptin, so they likely get a bigger spike in metabolic rate despite having lower levels of leptin.

This finding supports the notion that a program combining weight training and cardio is far superior for fat loss than cardio alone. After all, the fat loss lowers leptin levels, so you need the strength training to boost muscle growth and leptin sensitivity for continued fat loss.

Another Reason to Trim the Fat

Being Fat Causes Greater Muscle Damage Compared with Being Lean

Need another reason to stay lean? How about the fatter a person gets, the more muscle damage that occurs with exercise. Researchers from the University of Houston reported that when obese mice were exposed to strenuous exercise, they experienced more muscle damage than lean mice. The researchers suggested that obesity alters the biochemical properties of the muscle fiber membrane, which makes it more susceptible to mechanical stress. So if you are a trainer, keep in mind that an obese person may not be able to recuperate from exercise as quickly as a leaner counterpart.