Dry fasting: the trend with almost no evidence — and why that's the real story

You've probably seen one. Someone on your feed, day three, no food and not a single sip of water, saying they've never felt so clear-headed. Dry fasting gets pitched as ordinary fasting with the dial turned up: burn fat faster, switch your cells' clean-up crew into overdrive, flush out whatever needs flushing.

It sounds like a hack. So I went looking for the studies behind it.

What I found is more interesting than "it works" or "it's dangerous". The honest answer is that almost nobody has properly studied it — and that gap is the real story.

First, what dry fasting actually is

Dry fasting means no food and no fluids — water included. But the phrase covers two very different things.

The first is dawn-to-dusk dry fasting: nothing to eat or drink while the sun is up, then eating and drinking normally at night. This is what millions of people do every year during Ramadan, for reasons of faith rather than diet — and it means we actually have a fair amount of data on it.

The second is multi-day dry fasting: one, three, five days or more without food or water. This is the version that tends to go viral.

It's worth knowing that fasting research is young enough that experts only agreed on a shared vocabulary for it in 2024 (Koppold et al., Cell Metabolism, 2024). When the basic words weren't settled until two years ago, you can guess how settled the answers are.

What the evidence shows

Multi-day dry fasting. The most detailed human data come from two studies by the same research group. In each one, ten volunteers did a five-day dry fast. There was no comparison group (Papagiannopoulos-Vatopaidinos et al., Complementary Medicine Research, 2020; Metabolism Open, 2025).

Here's what they measured. People lost about 7 kg in five days. Their blood became more concentrated — haematocrit up 11%, albumin up 18% — and the hormones your body uses to hold on to water shot up: renin by 315%, the antidiuretic hormone by 60%. Think of it as the body pulling every lever it has to stop water leaving.

Cortisol, a stress hormone, rose by 495% — roughly six times its starting level. Uric acid doubled. In the second study, LDL cholesterol went up and thyroid hormones went down.

The authors read their results as showing short-term anti-inflammatory and protective effects. It's worth knowing that one of the most standard inflammation markers they measured, CRP, went up by 167%. Both things are in the same paper; you can read it and judge for yourself.

Dawn-to-dusk dry fasting. Here the picture is calmer. In a small study of 13 people with a higher body weight who fasted from dawn to dusk for 30 days, several inflammatory molecules in the blood went down (Al Lami et al., Metabolism Open, 2024). It was observational, with no comparison group — a promising signal, not a conclusion.

For people with kidney disease or a kidney transplant, a review of 32 studies covering 2,592 people found that the effects of Ramadan fasting on kidney function were inconsistent, and the authors urged caution because the underlying studies were limited (Bello et al., BMJ Open, 2024). For healthy people, a review of fluid restriction during Ramadan found that daytime dehydration, headaches, irritability and tiredness are common, with no direct harm proven (Leiper et al., European Journal of Clinical Nutrition, 2003). That review is more than twenty years old — which tells you how rarely this question gets revisited.

Why scientists can't give you a straight answer

It's tempting to think doctors disagree about dry fasting because some are open-minded and others are stuck in their ways. The real reason is less dramatic: the studies haven't been done.

Ten people per study, with no comparison group, means we can't tell which changes come from dry fasting and which would happen with any five days of not eating. Almost all of the multi-day data come from one research group, and nobody has independently repeated the work.

There's also a hard ethical wall. Deliberately keeping volunteers from drinking water for days is very difficult to justify in a trial. That means the strongest kind of evidence for long dry fasts may never exist.

And one more thing. I searched the medical literature for published cases of people harmed by dry fasting and didn't find any. That sounds reassuring, but it isn't: no recorded cases doesn't mean "safe", it means nobody has been collecting the data.

What you'll hear online

"It burns fat faster than a water fast." No study has compared fat loss on a dry fast with fat loss on a water fast. The claim has simply never been tested.

"It puts autophagy into overdrive." Autophagy — your cells breaking down and recycling their own worn-out parts — is real. But it's very hard to measure inside a living person. A 2025 study that tracked autophagy activity in people's blood cells over six months of time-restricted eating opens by noting that this effect "has never been shown in humans" — and even then, it found only a hint (Bensalem et al., Journal of Physiology, 2025). Nobody has measured it during a dry fast at all.

"Your body makes its own water." It's true that burning fuel produces a little water. But in the studies above, people's bodies were still clearly short of water.

"It detoxes you." Your liver and kidneys do that every day, whether you eat or not, and there are no controlled trials showing that detox diets do anything extra (Klein & Kiat, Journal of Human Nutrition and Dietetics, 2015).

For multi-day dry fasting, the answer to question two is: about ten people, for five days.

What would actually settle it

There's a study that could be done safely and would answer a lot: take people who want to try dawn-to-dusk fasting and randomly split them into two groups — one with water at night only, one allowed water through the day — with the same eating window and the same calories, and keep a close eye on kidney function, salts in the blood and blood pressure. If you work in research, it's a genuinely open question worth funding.

What you can do

If you're curious about fasting, the version with real data behind it is the gentler one: dawn to dusk, or simply a longer overnight gap between dinner and breakfast. Multi-day dry fasting is unmapped territory, and you'd be one of very few people to have walked it.

If you take any regular medication — for blood pressure, diabetes, your mood (lithium in particular — even if you've stopped it; Stone, Journal of the American Board of Family Practice, 1999) or your kidneys — or you're pregnant, talk to your doctor before cutting fluids. Your doctor will know what applies to you.

The most honest thing science can say about dry fasting right now is "we don't know yet". That's not a dead end. It's an invitation to find out properly.

Curiosity first.

— Dr. Brugal

Voices in the debate

If you'd like to hear the case for dry fasting from people who practise, champion or study it, these are names you'll come across. Their views often go beyond what published human studies have shown so far — which is exactly why it's worth hearing them and reading the studies side by side.

• Sergey Filonov — a Russian practitioner who runs dry-fasting programmes and has written about them in 20 Questions & Answers About Dry Fasting (Siberika). Book
• Ioannis Papagiannopoulos-Vatopaidinos — lead author of the two five-day dry-fasting studies above, who sees potential for dry fasting as a therapy. Study (2020)
• Dr Ayse Mindikoglu — researcher at Baylor College of Medicine who studies dawn-to-dusk dry fasting and its effects on blood proteins, and who has said more studies are needed. Baylor College of Medicine

For the wider debate on fasting, including the doctors who champion its broader benefits, see our series guide: Why scientists still can't agree on fasting (coming soon).

References

All studies retrieved via PubMed.

1. Koppold DA, Breinlinger C, Hanslian E, et al. International consensus on fasting terminology. Cell Metabolism. 2024. Expert consensus statement. doi:10.1016/j.cmet.2024.06.013 (PMID 39059384)
2. Papagiannopoulos-Vatopaidinos IE, Papagiannopoulou M, Sideris V. Dry fasting physiology: responses to hypovolemia and hypertonicity. Complementary Medicine Research. 2020. Uncontrolled study, 10 people, 5-day dry fast. doi:10.1159/000505201 (PMID 31958788)
3. Papagiannopoulos-Vatopaidinos IE, Papagiannopoulou M, Dotsika E, et al. Hypoglycemia compensation mechanisms in dry fasting. Metabolism Open. 2025. Uncontrolled study, 10 people, 5-day dry fast. doi:10.1016/j.metop.2025.100363 (PMID 40585323)
4. Al Lami Z, Kurtca M, Atique MU, et al. Dawn-to-dusk dry fasting decreases circulating inflammatory cytokines in subjects with increased body mass index. Metabolism Open. 2024. Observational study, 13 people, 30 days. doi:10.1016/j.metop.2024.100274 (PMID 38455231)
5. Bello AK, Lloyd A, Osman MA, et al. Impact of Ramadan fasting on kidney function and related outcomes in chronic kidney disease and kidney transplant recipients: a systematic review and meta-analysis. BMJ Open. 2024;14(11):e085329. Meta-analysis of 32 observational studies, 2,592 people. doi:10.1136/bmjopen-2024-085329 (PMID 39572100)
6. Leiper JB, Molla AM, Molla AM. Effects on health of fluid restriction during fasting in Ramadan. European Journal of Clinical Nutrition. 2003. Review. doi:10.1038/sj.ejcn.1601899 (PMID 14681711)
7. Bensalem J, Teong XT, Hattersley KJ, et al. Intermittent time-restricted eating may increase autophagic flux in humans: an exploratory analysis. Journal of Physiology. 2025. Exploratory analysis of a randomised trial, humans. doi:10.1113/JP287938 (PMID 40345145)
8. Klein AV, Kiat H. Detox diets for toxin elimination and weight management: a critical review of the evidence. Journal of Human Nutrition and Dietetics. 2015;28(6):675-86. Review. doi:10.1111/jhn.12286 (PMID 25522674)
9. Stone KA. Lithium-induced nephrogenic diabetes insipidus. Journal of the American Board of Family Practice. 1999;12(1):43-7. Case report and review. doi:10.3122/15572625-12-1-43 (PMID 10050642)

Awareness content — not medical advice. Always talk to your doctor or pharmacist before making changes.