Cortisol — research review 2

...and this idea of "stress-busting" supplements really needs to be looked at through an evidence-first lens. What are the clinical trials actually showing?

Exactly. A great example is cortisol, often called the "stress hormone." Chronically elevated cortisol levels are associated with lower heart-rate variability, which is a marker of our body’s resilience and ability to adapt. That’s well-established.

So, the theory is, if you lower cortisol, you improve that resilience. And many supplements claim to do just that.

They do. But when you look for robust human clinical trials, especially randomized controlled trials, the picture gets complicated. For many popular "cortisol-reducing" ingredients, the evidence for a significant, sustained reduction in cortisol in healthy humans, or a direct improvement in heart-rate variability, is often either weak, inconsistent, or just non-existent.

So we're talking about a lot of null results, or very small effects that might not even be clinically meaningful.

Precisely. A systematic review in Nutrients in 2021, for example, highlighted how few ingredients have strong evidence to support direct cortisol reduction in stress-responsive human trials. Many studies use animal models or in vitro work, which doesn't always translate.

And crucially, even if a supplement does transiently lower cortisol, we don't fully know if that translates into long-term health benefits or improved resilience. The causal chain from supplement to cortisol to health outcome isn't firmly established for most.

Right. We know high cortisol is problematic, but whether a supplement reliably fixes that in a lasting way, or if simply reducing stress through lifestyle changes is more effective, is still largely unproven for many of these products. It's a huge gap in the evidence.