‘Stress hormone’ cortisol is blamed for infertility, but major new study hints that could be wrong

There’s arguably no single hormone that has grabbed hold of the collective consciousness as much as cortisol.

Social media is overflowing with information about the evils of cortisol: it messes with your face, your belly, your sleep, your skin, and, perhaps most concerning, your fertility.

There’s no shortage of gimmicks available to help fight this hormone: cortisol masks, cortisol diets, cortisol watches, cortisol-suppressing supplements.

But the widely held view that cortisol always causes problems with reproduction hinges on a key assumption that hasn’t been tested. Our new review, published in Journal of Experimental Biology, provides evidence that this assumption is flawed. In fact, cortisol may actually promote healthy reproduction.

Cortisol has been cast as a modern health villain, seeking to slowly destroy you. But it’s actually a critical hormone – one of a few hormones your body can’t live without.

This is because cortisol affects almost every cell in the body and helps to regulate many essential aspects of physiology, including circadian rhythm, metabolism and immune function.

Importantly, how cortisol affects you is governed by its pattern of release: the ups and downs. Social media will have you believe a cortisol “spike” is always bad news.

But precisely timed increases in cortisol are not necessarily bad – they may be exactly the signal your body needs to start one of its physiological processes, such as regulating your sleep.

Discovered less than 100 years ago, cortisol is a relative newcomer to biology. At the time, researchers had no reliable way to measure natural cortisol levels in blood. But what they could do was manufacture synthetic cortisol, inject it, and observe the effects.

Many studies on cortisol’s effect on reproduction used injections of synthetic cortisol. However, our bodies don’t always metabolise synthetic hormones in the same way as naturally occurring hormones.

Furthermore, these injections were done without knowledge of normal hormone patterns. Cortisol was linked with stress early on, which meant that its role in other aspects of physiology was overlooked. Without actual data about normal cortisol patterns across reproductive stages, researchers assumed cortisol levels remained low unless they were spiked by some kind of stressor.

Doing manipulations before you understand a system makes it difficult to interpret the results. Concluding cortisol is bad for reproduction based on injecting high doses is like taking a blowtorch to your dinner and then concluding that heat is bad for food.