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Cortisol: what is true and what is being sold to you
Research review · PubMed
✍️ Автор: Катя
Cortisol has been cast as the villain behind weight, belly fat, fatigue and feeling awful. Tests, courses and supplements are sold on that story. We took the main claims to their primary sources — including Mendelian randomisation, the method built to separate cause from coincidence. Some facts held. The industry's central conclusion did not.
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What cortisol genuinely does
It is a steroid hormone from the adrenal cortex, and several of its effects are firmly established. It **raises blood glucose** by driving production in the liver. It **accelerates muscle protein breakdown**. And it **acutely suppresses insulin secretion** — shown directly: even a modest rise sharply brakes secretion. A detail usually left out: 4–6 hours later insulin resistance develops and persists for **over 16 hours**.
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First inaccuracy: "it supports immunity"
This turns the physiology upside down. Cortisol's classic role is **anti-inflammatory and immunosuppressive** — it lowers circulating lymphocytes. That is precisely why synthetic analogues are prescribed to *dampen* inflammation. "Modulates the immune response" is accurate; "supports" is not.
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"It helps you focus" — only half true
The relationship is an inverted U: moderate levels genuinely improve working memory, while **high levels impair** both it and recall. Presenting it as uniformly positive is wrong.
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The main event: "cortisol belly" does not exist
It is not a clinical term and appears in no guideline. There is a real phenomenon behind it — a characteristic fat redistribution, facial puffiness, fluid retention. But those are signs of **Cushing's syndrome** or of glucocorticoid treatment, where doses are orders of magnitude above anything a stressful job produces. Extending that to a person with deadlines has no basis.
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And now the surprise: the causality runs backwards
Mendelian randomisation uses genetic variants to separate cause from coincidence. A bidirectional analysis found that genetically higher cortisol is associated with **reduced** obesity risk, and that severe obesity is associated with **lower** morning cortisol. A second study found **no** causal effect on body mass index at all. The mechanism supports this: in ordinary obesity blood cortisol **is not elevated** — what changes is local enzyme activity inside fat tissue.
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Hair cortisol testing: what it actually shows
The method is validated — as a **research** tool, not a clinical one. It reflects accumulated exposure over months. What it does **not** show is how stressed you feel: a meta-analysis of 66 studies covering 10,289 people found **no consistent relationship** with subjective stress. Results are distorted by hair colour (darker hair reads higher), washing frequency and dyeing. There are no reference ranges for healthy people.
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And that same correlation with weight is microscopic
The largest study (2,527 people) did find a link between hair cortisol and weight: a coefficient of about 0.10. That is roughly **1% of the variance explained** — statistically significant only thanks to sample size, and practically negligible. The authors state outright that they cannot draw conclusions about causation.
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"Adrenal fatigue" is a diagnosis that does not exist
A systematic review of 58 studies is titled, literally, *Adrenal fatigue does not exist*. Fatigue, irritability and weakness are non-specific symptoms. Genuine adrenal insufficiency is real, but it is diagnosed with a stimulation test, not a single morning blood draw. And the guideline thresholds are 140–150 nmol/L, not the "100" that circulates online.
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Stress supplements: the number moves, the person does not
The most popular is ashwagandha. Its meta-analysis is titled, verbatim, "significant reduction in cortisol **but no effect on perceived stress**". In other words the figure on the lab report changes, and the thing people actually buy it for does not. There is no evidence that lowering cortisol in healthy people leads to weight loss or a smaller waist.
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What to take from this
Cortisol is a vital hormone with a daily rhythm, not an enemy. Its deficiency is dangerous, and an "excess" in a healthy person is not a diagnosis. Instead of hunting a number, the boring things work: **sleep, regular meals with enough protein, resistance training, limiting alcohol and late caffeine**. And if there are genuine red flags, that is a conversation with an endocrinologist, not with a jar of adaptogens.
❓ Часті запитання
Does stress put fat on your belly through cortisol?
There is no direct evidence, and genetic data point the other way: higher cortisol is associated with reduced obesity risk, while severe obesity is associated with lower morning cortisol. «Cortisol belly» is a simplification, not a clinical entity.
Should I test my cortisol just in case?
No. A single measurement means little: the level follows a daily rhythm and reacts strongly to the blood draw itself. Diagnosis uses a stimulation test and only when there is clinical suspicion.
Does a hair test show my stress level?
No. A meta-analysis of 66 studies found no consistent relationship with subjective stress. Results depend on hair colour, washing and dyeing, and there are no reference ranges for healthy people.
Does ashwagandha help with stress?
Meta-analysis shows it lowers blood cortisol but does not change perceived stress. The number on the report moves; how you feel does not.
What actually reduces stress load?
Enough sleep, regular meals, physical activity, limiting alcohol and late caffeine. Dull, but it is the part with evidence behind it — unlike «cortisol detox».
📚 Дослідження, згадані в статті (9)
- KEY: Mendelian randomisation — higher cortisol linked to reduced obesity risk (JCEM, 2022) — джерело →
- Second MR study: no causal effect of cortisol on BMI (Scientific Reports, 2024) — джерело →
- Hair cortisol and obesity: correlation about 0.10, causation unknown, 2,527 people (Obesity, 2017) — джерело →
- Meta-analysis of 66 studies: hair cortisol does not track perceived stress (Psychoneuroendocrinology, 2017) — джерело →
- «Adrenal fatigue does not exist» — systematic review of 58 studies (BMC Endocrine Disorders, 2016) — джерело →
- In ordinary obesity blood cortisol is not elevated (JCEM, 2001) — джерело →
- Cortisol acutely suppresses insulin secretion (Am J Physiol, 1996) — джерело →
- Small hippocampus as a predisposition rather than a consequence: twin study (Nature Neuroscience, 2002) — джерело →
- Ashwagandha: cortisol falls, perceived stress does not (meta-analysis, 2025) — джерело →
Це не медична порада. Маєш діагноз чи обмеження — звернися до лікаря.
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