When young guys burn more calories than they eat and work out hard, their stress hormone (cortisol) changes—and that’s linked to shifts in muscle and fat. But it doesn’t explain much about why people’s bodies change differently.

What we've found so far is that cortisol levels during intense exercise and low calorie intake appear to be linked to changes in muscle and fat in young men [1]. Our analysis of the available research shows this connection is present, but it doesn’t fully explain why individuals respond differently to the same conditions. We looked at the evidence and found 51.0 supporting assertions and no studies that refute this link [1]. This means the data we’ve reviewed leans toward a connection between elevated cortisol—often seen when young men are in a calorie deficit and doing intense training—and shifts in body composition. Cortisol is a stress hormone that can influence how the body stores or loses fat and whether it holds on to or breaks down muscle tissue. When energy intake is low and physical stress is high, cortisol tends to rise, and this pattern coincides with changes in muscle and fat [1]. However, what we’ve also seen is that cortisol doesn’t tell the whole story. While it’s part of the picture, it doesn’t explain much about why some young men lose more muscle or fat than others under similar conditions. Other factors—like sleep, overall diet quality, training history, or genetics—might play important roles that aren’t captured just by measuring cortisol. Our current analysis shows a consistent link, but not a complete explanation. We’re still building a clearer understanding of how much cortisol matters compared to other influences on body composition. Practical takeaway: If you're training hard and eating less, your body’s stress hormone levels may shift, and that could affect muscle and fat changes—but don’t assume cortisol is the main driver of your results.