You’ve probably heard it your entire adult life—building lean muscle mass is one of the best things you can do for your health. After all, in addition to boosting physical strength and burning calories, lean muscle mass also pulls glucose from the blood and helps protect against diabetes. But now a new study published in The Journal of Clinical Endocrinology & Metabolism is challenging that conventional wisdom.
A team of nutritional and chronic disease researchers from Louisiana State University analyzed health data from close to 60,000 adults (between the ages of 37 and 73) enrolled in the U.K. Biobank study. They focused on those who had undergone MRI and low-level X-ray scans capable of determining their fat-free mass, a commonly used proxy for lean muscle mass.
The team then investigated the association between fat-free mass and adverse health outcomes, like developing a metabolic disorder and elevated glycated hemoglobin (for connoisseurs of diabetes pharmaceutical ads, this is the HbA1C they’re always singing about). They also assessed different measures of fat-free mass, like visceral adiposity (abdominal and organ fat) and fat infiltration within muscle tissue.
The results painted a messy picture. For example, higher total fat-free mass (and fat-free mass adjusted for height) was actually associated with a slightly higher likelihood of developing metabolic syndromes and elevated HbA1C. On the other hand, a higher proportion of body weight made up of fat-free mass was associated with a lower likelihood of developing these adverse outcomes.
“The findings suggest that muscle quantity alone may not provide a complete picture of metabolic health and that where fat is stored in the body is an important part of the story,” study author Sri Lakshmi S. Devarakonda said in a statement
Drilling down into the data, the team found that visceral fat seemed to have the strongest influence. Those with the same total fat-free mass but more fat around their abdomens and organs were more likely to experience the adverse outcomes. Fatty infiltration into muscle showed the same pattern, albeit slightly weaker.
“Our findings suggest that we need to look beyond the amount of fat-free mass a person has to better understand their metabolic health,” study author Candida Rebello explained. “Where fat is stored in the body matters, particularly fat around internal organs and fat that accumulates within muscle tissue. These different patterns of fat distribution can influence the relationship between fat-free mass and metabolic risk, underscoring the importance of considering the quality and composition of body tissues, not just their quantity, when studying metabolic health.”
Does any of this mean you should avoid building lean muscle mass? Definitely not. It’s still one of the best things you can do for your health. It’s just not a panacea. 
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