A 24-Hour Fast Shifts Cortisol and DHEA Rhythms in Obese Adults: What the Research Shows

A 24-Hour Fast Shifts Cortisol and DHEA Rhythms in Obese Adults: What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

Cortisol and DHEA (dehydroepiandrosterone) both follow a daily rhythm, rising and falling at predictable times based on the body's internal clock. Researchers in Poland wanted to know whether a single 24-hour water-only fast — a protocol similar to one full fasting day within an alternate day fasting schedule — would disrupt or shift that rhythm, and whether the effect differed depending on a person's sex, age, or natural chronotype (whether someone is a "morning person" or "evening person"). The study focused on adults with obesity, a population in which stress hormone regulation is often already altered.

Who Was Studied

Participant profile: Adults with obesity, BMI ranging from 32.2 to 67.1 kg/m², roughly balanced between men and women. Chronotype was assessed using the validated Horne and Östberg morningness-eveningness questionnaire.

How the fasting protocol worked in this study: Participants were hospitalized on a structured calorie-restriction diet for three weeks. Within that period, they completed a single 24-hour fast consuming only water. Researchers collected saliva samples every 2–3 hours over a continuous 64-hour window spanning the day before, the day of, and the day after the fast, measuring cortisol and DHEA levels throughout to map each hormone's full daily rhythm.

What the Researchers Found

Cortisol rhythm changes

• The most consistent finding was a timing shift: cortisol's daily peak arrived nearly 40–50 minutes earlier on the fasting day compared to the surrounding days.
• Amplitude — the size of the swing between the daily high and low — trended higher on the fasting day but did not reach statistical significance for that specific comparison.

DHEA rhythm changes

• The MESOR of the DHEA rhythm (the rhythm-adjusted 24-hour mean level) differed significantly across the three days, p = 0.0381.
• DHEA's overall daily average shifted on the fasting day, though the study reported this primarily as a rhythm-level change rather than a simple before/after comparison.

Chronotype-specific effects

• Morning chronotypes showed the clearest differences in cortisol acrophase (peak timing).
• Evening chronotypes showed the clearest differences in cortisol amplitude (swing size).
• This suggests a single fasting day does not affect everyone's stress hormone rhythm the same way — chronotype meaningfully shaped the response.

What Did Not Change

• The study did not report a significant overall change in DHEA amplitude.
• No adverse safety events were reported from the single-day fasting protocol.

What the Researchers Concluded

The authors concluded that a single day of fasting is enough to measurably shift the daily rhythm of both cortisol and DHEA in adults with obesity, and that these shifts are not uniform — they depend on a person's chronotype, meaning morning-oriented and evening-oriented individuals respond differently to the same fasting day.

What This Means If You Fast

• A single fasting day nudges your stress hormone clock, not just its level. The most notable change here was cortisol's peak arriving earlier in the day, not simply going up or down — worth knowing if you track energy or mood around your fasting days.
• Your chronotype may matter more than generic fasting advice accounts for. If you're a night owl, you may notice more of a change in the size of your cortisol swing; early risers may notice more of a shift in timing. See Fasting and Cortisol: How Stress Hormones Affect Women for more on managing this.
• DHEA shifting alongside cortisol is a reminder that fasting is a whole-system stress response, not an isolated fat-burning switch — supporting the general principle in The Hormonal Hierarchy that cortisol-adjacent hormones move together.
• This was studied in adults with obesity on a hospital-controlled diet, not free-living intermittent fasters, so day-to-day variability for a typical 16:8 or occasional 24-hour faster may look different.
• One fasting day was enough to produce measurable change — you don't need extreme protocols to see a hormonal response, which is consistent with more moderate approaches like those in Alternate Day Fasting for Women: Is It Safe?

Study Limitations

• Small sample size (n=49) limits how confidently these findings generalize to the broader population.
• No untreated control group outside the hospital-controlled diet — all comparisons were within the same participants across three days.
• Participants were specifically adults with obesity; results may not apply the same way to lean or normal-weight individuals.
• The fasting day occurred within a structured 3-week calorie-restriction hospital stay, which is a different context than someone fasting at home around a normal routine.
• Saliva-based hormone measurement, while validated, can be more variable than blood-based assays.
• PubMed's abstract and citation pages were inaccessible at the time of writing (403 network restriction); figures in this article are drawn from the publicly available Frontiers in Nutrition article text and indexed search abstracts rather than a direct PubMed abstract read.

Source

Marciniak, M., Sato, M., Rutkowski, R., Zawada, A., Juchacz, A., Mahadea, D., Grzymisławski, M., Dobrowolska, A., Kawka, E., Korybalska, K., Bręborowicz, A., Witowski, J., & Kanikowska, D. (2023). Effect of the one-day fasting on cortisol and DHEA daily rhythm regarding sex, chronotype, and age among obese adults. Frontiers in Nutrition. PMID: 36814510

Frequently Asked Questions

Does fasting raise cortisol?

This study found that a single 24-hour fast shifted the timing of cortisol's daily peak earlier by 40–50 minutes and produced a non-significant trend toward a bigger daily swing, rather than simply raising overall cortisol levels across the board.

What is DHEA and why does it matter with fasting?

DHEA is a hormone produced by the adrenal glands that serves as a building block for other hormones and is often studied alongside cortisol because the two are produced by the same gland and respond to similar stress signals. This study found DHEA's daily rhythm shifted significantly during a one-day fast.

Does my chronotype affect how fasting impacts my hormones?

Based on this study, yes. Morning chronotypes showed more change in when cortisol peaked during the day, while evening chronotypes showed more change in how big the cortisol swing was — suggesting a one-size-fits-all fasting schedule may not affect everyone's hormones identically.

Is a 24-hour fast safe for hormone health?

This study did not report adverse effects from the single-day water-only fast, but it studied adults with obesity in a hospital-controlled setting. Anyone considering a full-day fast, especially with an existing health condition, should consult a healthcare provider first.

Does this study apply to a normal 16:8 intermittent fasting schedule?

Not directly. This research examined a single full 24-hour fast, not a daily time-restricted eating pattern, so the specific hormone rhythm shifts observed may not translate one-to-one to shorter daily fasting windows.

Related Research and Articles

• Fasting and Cortisol: How Stress Hormones Affect Women
• The Hormonal Hierarchy: Why Cortisol and Insulin Must Come First for Women
• Alternate Day Fasting for Women: Is It Safe?
• Intermittent Fasting and Thyroid Health in Women
• Why Cortisol Control Matters More Than Fasting Length for Women
• Intermittent Fasting and Sleep Quality in Women

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