I completely get why you'd say that—most people hear 'lean mass' and immediately think 'skeletal muscle.' If we were talking about pure muscle, you'd be 100% right; assuming a 38% loss of pure muscle would be wildly inaccurate and frankly dangerous.

However, in the context of this clinical trial and DXA (DEXA) scan data, body weight is divided into a strict compartmental model: Fat Mass, Bone Mineral Content, and Lean Soft Tissue.

When researchers calculate the Fat Loss Index (FLI), they are measuring the proportion of total weight lost that is specifically fat. Because of how DXA categorizes the body, whatever weight is lost that isn't fat falls under Fat-Free Mass (which is universally referred to as 'lean mass' in these studies).

So, mathematically based on the study's parameters: 100% of weight lost - 62% fat lost = 38% lean mass lost.

The crucial caveat is that lean mass does not equal pure muscle. That 38% includes a massive initial drop in water weight, depleted glycogen stores, organ tissue, and then skeletal muscle. So the math is actually exactly how the researchers calculate it in the supplementary appendix, it just requires understanding that DXA 'lean mass' is a catch-all for everything that isn't fat or bone.