Intermittent fasting and other long gaps between meals empty liver glycogen (stored sugar). Free fatty acids rise. Active thyroid hormone T3 falls. After glycogen is gone, the body burns protein. Thymus first, then muscle and liver.[1]

After the liver's glycogen is depleted, fasting destroys the tissues, starting with the thymus, then the muscles and liver.

— Ray Peat[2]

Glycogen window

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A healthy liver holds enough glycogen for a limited gap. Past that, muscle protein becomes sugar. T3 falls and slows burn rate. That slows self-digestion. It is metabolic depression.

In one day of fasting, the body stores several ounces of glucose in the form of glycogen. If you're healthy, you can go up to 24 hours before you are short on glycogen. As the body senses stress, lowering the T3 slows the rate of fuel use. When your glycogen is gone, you have to start using your tissue to make glucose out of your muscles. If your thyroid didn't slow down during this prolonged stress, you would eat your whole body up in just a few days. But since the depressive process turns off metabolism, it slows the rate of degeneration.

— Ray Peat[3]

A healthy person can go 12 to 18 hours. High body PUFA turns a sugar dip into circulating polyunsaturated free fatty acids that damage tissue.[4]

A healthy liver can store glycogen, so a healthy person can go 12 to 18 hours without eating. But if you have too much PUFA in your body, as soon as blood sugar dips, you pull PUFA into circulation and start damaging tissues.

— Ray Peat[5]

Coffee, aspirin, and solid thyroid function make a meal gap less costly.[4]

Largest human trial: muscle loss, not fat-specific loss

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The TREAT trial (UCSF, ~116 participants, one of the largest randomized human trials of time-restricted eating to date) found 16:8 time-restricted eating produced no significant weight loss advantage over three meals a day (-0.94 kg TRE vs -0.68 kg control, between-group difference not significant), while the fasting group showed a significantly larger drop in appendicular lean mass index than controls (-0.16 kg/m2, 95% CI -0.27 to -0.05). The trial's lead investigator, a cardiologist who had personally practiced time-restricted eating for years, stopped the practice after seeing the results.[6]

Cochrane review: no meaningful advantage over regular dieting

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A 2026 Cochrane systematic review pooled 22 randomized controlled trials (1,995 participants) comparing intermittent fasting against regular dietary advice for adults with overweight or obesity. Compared to regular dietary advice, intermittent fasting showed little to no difference in weight loss (mean difference -0.33%, 95% CI -0.92 to 0.26), in reaching a 5% body weight reduction, or in quality of life. None of the differences reached clinical significance.[7] This lines up with the TREAT trial result above: intermittent fasting is not a superior weight-loss method, and it carries the added tissue-loss cost described elsewhere on this page.

Night and diabetes

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Every night blood sugar falls. Free fatty acids rise. Fat burning wastes oxygen. Lactate rises. Protein can become sugar. Mitochondria take damage. Bone mineral appears in morning urine. Longer voluntary fasts deepen that night pattern.[8]

Blood sugar normally falls during the hours of darkness. This is compensated by a rise in free fatty acids in the bloodstream. That is the same pattern that happens when you are under stress. When glycogen is depleted, you mobilize free fatty acids to oxidize for energy. This requires more oxygen and is inefficient. … The longer you go without eating, the more you rely on inefficient fat oxidation.

— Ray Peat[9]

Fasting lowers blood sugar. That drop is stress. Lasting diabetes repair means lower polyunsaturated fat, butter or coconut for cooking, and steady carbohydrate so sugar can support pancreas repair.[10]

It will make your blood sugar go down, but anything that you can call fasting is going to be putting you under stress. The long-range thing that corrects chronic diabetes is to get your polyunsaturated fats (PUFA) lower, to completely eliminate them from your diet as far as possible.

— Ray Peat[11]

Ketogenic fat-burning states lock free-fatty-acid block of full sugar oxidation to CO₂. See Randle cycle and Diabetes.

Eat on a regular schedule. Keep glycogen and blood sugar steady. If a long gap is forced, take a little carbohydrate and minerals (sodium, calcium, magnesium, potassium).[12] Lower body PUFA. Night dips cost less.[4] Judge by warmth, pulse, sleep, mood, and thyroid signs.

See also

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References

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  1. ↑ Ray Peat, Ray Peat email Q&A - Fasting, raypeatemails.com.
  2. ↑ Fasting, Ray Peat email exchange
  3. ↑ JodelleFit: Cortisol Low Testosterone
  4. ↑ 4.0 4.1 4.2 Ray Peat, "One Radio Network: Health of the Human Body", One Radio Network
  5. ↑ One Radio Network: Health of the Human Body
  6. ↑ Lowe DA, Wu N, Rohdin-Bibby L, et al. "Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial," JAMA Intern Med. 2020;180(11):1491-9.
  7. ↑ Garegnani LI, Oltra G, Ivaldi D, et al. "Intermittent fasting for adults with overweight or obesity," Cochrane Database of Systematic Reviews. 2026;2:CD015610.
  8. ↑ Ray Peat, "Jodellefit: Stress and Your Health", Jodellefit
  9. ↑ JodelleFit: Stress and Your Health
  10. ↑ Ray Peat, "Ask the Herb Doctor: September 2021", KMUD
  11. ↑ Ask the Herb Doctor: September 2021 KMUD
  12. ↑ Ray Peat, "Jodellefit: Cortisol, Low Testosterone", Jodellefit