Cortisol and fasting: what studies show and what it means for a test

Studies of fasting and cortisol don’t agree. Some find that the daily cortisol curve shifts or flattens. Others find lower morning readings afterwards. One review of low-carb diets finds a short-term rise. The studies are small, they use different kinds of fasting, and they sample cortisol at different times of day. None of them shows that fasting causes symptoms through cortisol.

Does fasting increase cortisol?

The evidence is mixed. Here is what the studies we can use found.

• Ramadan in Saudi Arabia. In 23 healthy adults, the daily cortisol pattern was flattened two weeks into Ramadan: evening cortisol was higher than before it, and morning cortisol was lower, though the morning drop in blood readings was not statistically significant (PLoS ONE, 2013). The authors tie this to changed eating and sleeping patterns, since people stayed up until dawn.
• Ramadan in Germany. In 34 healthy adults with morning blood draws, serum cortisol was lower one week after Ramadan than one week before it, and had returned to the starting level a month later (Front Nutr, 2021). It sampled mornings only, so it can’t say what happened in the evening. Its clear drop came a week after Ramadan, not during it, so it isn’t a direct match for the Saudi study.
• A one-day fast. In 49 hospitalized adults with obesity on a calorie-restricted hospital diet, saliva cortisol was sampled over 64 hours. The daily peak came about 48 minutes earlier on the fasting day than the day before. The height of the curve was about 11 percent higher than on the day after, but the paper gives that difference two different P values (0.224 in the text, 0.0224 in its table) (Front Nutr, 2023).
• Time-restricted eating. A review of 14 studies (10 on Ramadan, 4 on other time-restricted eating) found mixed cortisol changes in the four non-Ramadan studies. Skipping dinner went with lower evening cortisol (in one four-day study of 11 people), and skipping breakfast with lower morning cortisol. The authors say there is little research on this and the question needs more study (Nutrients, 2021).

So the honest answer is that fasting can move the pattern around, the direction depends on the study, and nobody can say from these papers what it does to you.

What about keto and low-carb diets?

A meta-analysis of 27 intervention studies in 309 healthy men, which also looked at testosterone, found that low-carbohydrate diets (35 percent or less of energy from carbohydrate, which covers more than strict keto) moderately raised resting cortisol compared with higher-carbohydrate diets during the first three weeks. That result rested on 15 of the studies and 155 men. After three weeks, the effect on resting cortisol was no longer consistent (Nutrition and Health, 2022). That’s about men, over a short time, and about measured levels. It doesn’t tell you how anyone feels or whether any of it matters for health.

Does fasting before a cortisol test matter?

Cortisol follows a daily rhythm, so when the sample is taken matters as much as what you ate. See AM and PM cortisol timing and the body clock.

• Blood cortisol. MedlinePlus doesn’t list a fasting rule for a cortisol blood test. It says stress and exercise can raise cortisol and that you may need to rest first. The seller’s pages for its cortisol tests state no fasting rule, so follow the instructions on your lab order.
• ACTH. MedlinePlus says you may need to fast for an ACTH test, and that samples are often taken early in the morning.
• Saliva. MedlinePlus says not to eat, drink, brush or floss for 30 minutes before a saliva cortisol sample.
• Bundled tests. A glucose or metabolic panel ordered at the same visit may carry its own fasting rule. Follow the lab order for each test.

A result taken during a fast needs the same context as any other: time of day, stress, exercise and medicines. MedlinePlus says a cortisol test alone can’t diagnose the cause of an abnormal level.

Who should check with a prescriber first

NIDDK lists low blood sugar among the symptoms of adrenal insufficiency, along with nausea, vomiting, diarrhea, dizziness and fainting. If you have adrenal insufficiency, or take a corticosteroid, ask your prescriber before you fast. NIDDK also says people with adrenal insufficiency on corticosteroids need immediate medical attention for a severe infection, severe diarrhea, or vomiting that stops them keeping their pills down. Don’t skip or change a steroid dose to fit a fast.

When to see a clinician

Fasting itself isn’t a symptom. See a clinician for long-lasting fatigue, muscle weakness, loss of appetite or weight loss with no clear reason. NIDDK lists these among the most common symptoms of adrenal insufficiency. Persistent symptoms deserve a proper work-up by a clinician. See low cortisol symptoms and cortisol and blood sugar.

Keep reading

• AM and PM cortisol timing

  When the samples are taken and why timing matters.

• Your body clock and cortisol

  The daily rhythm behind every cortisol result.

• Cortisol and blood sugar

  How cortisol and glucose are linked.

• At-home or lab test

  How the ways of testing differ.