Cortisol and kava: what the studies actually measured

2022-08-11 · 6 min read

By the kava.fyi editorial team · Last updated: 11 August 2022

"Kava lowers cortisol" is one of the more common lines in articles about this drink. It sounds scientific, because cortisol is measurable. The trouble is that studies which actually measured cortisol after kava in humans are very few, small, and inconsistent in their results. This article explains what cortisol is, how it responds to exercise and stress, and then looks at exactly what has been measured.

What cortisol is

Cortisol is a steroid hormone produced in the zona fasciculata of the adrenal cortex. Its production is controlled by the hypothalamic-pituitary-adrenal (HPA) axis: the hypothalamus releases corticotropin-releasing hormone (CRH), which prompts the pituitary to secrete ACTH, and ACTH triggers cortisol synthesis in the adrenal glands. According to the StatPearls handbook, a healthy adult produces between 8 and 30 mg of cortisol a day, following a distinct circadian rhythm: levels rise in the last hours of sleep, peak around waking, and then fall throughout the day.

Cortisol is not a "bad hormone." Its roles include:

increasing gluconeogenesis in the liver and limiting glucose uptake by peripheral tissues, that is, keeping fuel available for the brain;
retaining sodium and enhancing vascular contraction, which raises blood pressure;
promoting the breakdown of muscle protein and inhibiting its synthesis;
suppressing the immune response, both innate and adaptive.

The problem starts with chronic excess. The same handbook lists the features of Cushing's syndrome, the extreme, pathological overproduction of cortisol: central obesity, muscle wasting, hypertension, glucose intolerance, as well as mood and sleep disturbances and structural changes in the brain. Everyday stress does not produce a picture like that, but the direction of the effect is the same.

Exercise and cortisol

The old-guidebook intuition that an evening workout can "wind you up" rather than calm you down does have support in the data, with one important caveat: intensity matters. In a 2008 study by Hill and colleagues, twelve moderately trained men exercised for 30 minutes at 40, 60 and 80 per cent of maximal oxygen uptake (VO2max), and rested in a separate session. The change in blood cortisol was, respectively, minus 6.6 per cent at rest, plus 5.7 per cent at 40 per cent, plus 39.9 per cent at 60 per cent, and plus 83.1 per cent at 80 per cent of VO2max. The increases at 60 and 80 per cent were statistically significant and were accompanied by a rise in ACTH. Interestingly, once changes in plasma volume and circadian rhythm were accounted for, low-intensity exercise lowered cortisol. For the HPA axis, a gentle walk and a sprint are two different events. It is also worth noting that in this study blood was drawn immediately after exercise; how long an elevated level persists after a hard evening workout, and whether it lasts into bedtime, this work does not settle.

How kava acts on the nervous system

Before turning to cortisol, it is worth separating what is well established about kava from what is still hypothesis. Its anxiolytic effect is well documented. A 2003 Cochrane review covered 11 randomised, placebo-controlled trials, 645 participants in total; a meta-analysis of six of them showed that kava extract lowered the score on the Hamilton Anxiety Scale by an average of 5.0 points more than placebo. The Australian KADSS study from 2009 tested an aqueous extract, closer to the traditional drink: 60 adults with heightened anxiety, 3 weeks, 250 mg of kavalactones a day, a crossover design against placebo. The Hamilton score fell by 9.9 points in the first phase, against 0.8 points on placebo. A 2011 review by Sarris and colleagues describes the mechanisms: modulation of GABA activity through changes to lipid membrane structure and sodium channel function, inhibition of monoamine oxidase B, and inhibition of noradrenaline and dopamine reuptake. More on this is in the article on kava and GABA and the page on kavalactones.

The connection to cortisol in this picture is indirect: if something reduces anxiety and arousal, you might expect it to eventually affect the HPA axis. Expecting is not measuring. So let's see who actually measured it.

The studies that measured cortisol after kava

We found three studies that report cortisol as an outcome, two in humans and one in animals.

Smokers, 7 days, 2020. A pilot study from the University of Minnesota and the University of Florida, whose main aim was the metabolism of the tobacco carcinogen NNK in active smokers. Cortisol was a secondary endpoint, used as a stress biomarker. After a week of taking kava, plasma cortisol and its urinary equivalents fell. Authors of a later paper, summarising this study, add that the effect was seen in people with elevated cortisol at baseline, with no change observed in those with normal levels.
Special-forces candidates, 2 weeks, 2024. A randomised, double-blind crossover trial (registration NCT05381025) in 15 healthy, fit young men on a very demanding training programme. Dose: 225 mg of kavalactones a day for two weeks, then a switch with placebo. Cortisol was measured in saliva four times a day. Result: no clinically significant differences between kava and placebo. Cortisol, sleep and mood were already within normal range in these men at baseline, and the authors acknowledge that saliva collection times varied, which limits the conclusions. They did, however, observe sustained motivation to train.
Zebrafish, 2020. In adult fish, a single exposure to kava and kavalactones lowered whole-body cortisol, but seven days of exposure no longer changed it. An animal model, and so a long way from a cup of the drink.

The honest summary is not a flattering one: in humans there are two small studies where cortisol was a secondary measure, one showing a decrease in people with elevated levels, the other showing no effect in people with normal levels. There is no randomised, placebo-controlled trial with cortisol as the primary endpoint in chronically stressed people. The statement "kava lowers cortisol" does not today rest on anything like the evidence behind the statement "kava lowers anxiety."

What this means in practice

If you struggle to fall asleep after an intense evening workout, physiology suggests two simple moves: lower the intensity of late sessions and give the body time to wind down. Kava can be part of that wind-down as an alcohol-free drink known to ease anxiety; whether it does anything to the hormone remains an open question. We cover what research says about sleep after kava in the article on kava and sleep, and its general effect profile on the page about kava's effects. Anyone with an endocrine condition, taking medication, or planning intensive training should keep in mind that this article is for information only and does not replace medical advice.