--- canonical_name: Dry Fasting alternate_names: Absolute Fasting, Water-Free Fasting, Hard Dry Fasting, Soft Dry Fasting, Intermittent Dry Fasting, Dawn-to-Dusk Dry Fasting, Diurnal Dry Fasting canonical_topic: Dry Fasting for Health & Longevity short_topic_lc: dry_fasting creation_date: 2026-1002-0808 creator_ai_fullname: Opus 5.5 ep_keywords: Fasting, Intermittent Fasting, Religious Fasting, Ramadan Fasting, Time-Restricted Eating --- # Dry Fasting for Health & Longevity
Evidence Review created on 10/02/2026 using [AI4L](https://github.com/forever-healthy/AI4L) / Opus 5.5 **Also known as:** Absolute Fasting, Water-Free Fasting, Hard Dry Fasting, Soft Dry Fasting, Intermittent Dry Fasting, Dawn-to-Dusk Dry Fasting, Diurnal Dry Fasting ## Motivation Dry fasting means going without both food and all fluids, including water, for a set period. It ranges from daily sunrise-to-sunset fasts to rarer fasts lasting several days. It attracts interest because it layers deliberate water restriction on top of the usual shift from burning food to burning stored fat, which proponents claim adds effects beyond ordinary fasting. Most human data come from religious practice. Hundreds of millions of Muslims fast from dawn to sunset during the month of Ramadan each year, and members of the Bahá'í Faith keep a similar nineteen-day fast every March. These large yearly fasts have prompted studies of body weight, heart and metabolic health, and kidney safety, while multi-day dry fasting has been examined only in very small supervised groups. This review examines the evidence on the benefits and risks of both daytime and multi-day dry fasting, how the practice compares with fasting that allows water, and what the findings mean for health-focused adults pursuing a longer healthy life. **[Benefits](#expected-benefits) - [Risks](#potential-risks--side-effects) - [Protocol](#therapeutic-protocol) - [Conclusion](#conclusion)** ## Recommended Reading This section lists overview-level primary research and magazine content that discuss dry fasting or its therapeutic category in depth. - [Hypoglycemia compensation mechanisms in dry fasting.](https://pubmed.ncbi.nlm.nih.gov/40585323/) - Papagiannopoulos-Vatopaidinos et al., 2025 Tracks how the body averts hypoglycemia (low blood sugar) and how insulin, thyroid and cholesterol shift during a supervised five-day dry fast. The lead author runs a fasting-therapy practice (financial interest). - [Effects of Daytime Dry Fasting on Hydration, Glucose Metabolism and Circadian Phase: A Prospective Exploratory Cohort Study in Bahá'í Volunteers.](https://pubmed.ncbi.nlm.nih.gov/34395487/) - Koppold-Liebscher et al., 2021 Charité Berlin cohort of 34 healthy adults tracking hydration, body composition and body-clock timing during the 19-day Bahá'í fast. Several authors work in a hospital department that offers therapeutic fasting. - [Intermittent fasting from dawn to sunset for four consecutive weeks induces anticancer serum proteome response and improves metabolic syndrome.](https://pubmed.ncbi.nlm.nih.gov/33110154/) - Mindikoglu et al., 2020 Pilot study of 14 adults with metabolic syndrome (clustered excess waist, blood pressure, glucose and blood fats) who dry-fasted over 14 hours daily, reporting shifts in blood proteins linked to cancer and metabolism. - [Benefits of Intermittent Fasting](https://www.lifeextension.com/magazine/2021/2/benefits-of-intermittent-fasting) - Susan Palmer Life Extension Magazine overview of fasting biology, including autophagy (cellular recycling of damaged components), with a sidebar summarizing a four-week dawn-to-dusk fast without food or drink and its weight and blood-pressure changes. Only four items are listed because little overview-level content discusses dry fasting in depth; most fasting content addresses water-permitted fasting, and the remaining candidates were duplicates from the same research groups. No dry-fasting content was found from Rhonda Patrick, Peter Attia, Andrew Huberman, Chris Kresser or Lifespan.io; their fasting material addresses water-permitted fasting and time-restricted eating (confining all food to a fixed daily window), and their site searches returned no dedicated dry-fasting article, episode or segment. ## Grokipedia [Dry Fasting](https://grokipedia.com/page/Dry_Fasting) AI-generated encyclopedia overview defining soft, hard, intermittent and prolonged dry fasting, with history and summarized research; useful for terminology rather than for grading evidence. ## Examine [Ramadan Fasting](https://examine.com/diets/ramadan-fasting/) Examine's dedicated page for dawn-to-dusk dry fasting, defining it as abstaining from food and water from sunrise to sunset; its study summaries are for members only. Examine has no page on multi-day dry fasting. ## ConsumerLab No ConsumerLab article on dry fasting exists. ConsumerLab tests supplements and consumer products, and dry fasting involves no product. ## Systematic Reviews The following systematic reviews and meta-analyses cover the effects and principal risks of dawn-to-dusk dry fasting, all studied in Ramadan observers. - [Cardiometabolic and obesity risk outcomes of dawn-to-dusk, dry intermittent fasting: Insights from an umbrella review.](https://pubmed.ncbi.nlm.nih.gov/40081802/) - Faris et al., 2025 Pools twenty meta-analyses of Ramadan dry fasting, finding small falls in weight, waist, body fat, diastolic pressure, cholesterol and triglycerides. - [Effect of Religious Fasting in Ramadan on Blood Pressure: Results From LORANS (London Ramadan Study) and a Meta-Analysis.](https://pubmed.ncbi.nlm.nih.gov/34619991/) - Al-Jafar et al., 2021 Thirty-three studies with 3,213 participants show modest blood-pressure falls in healthy, hypertensive and diabetic fasters, but not in kidney disease. - [Effect of diurnal fasting on sleep during Ramadan: a systematic review and meta-analysis.](https://pubmed.ncbi.nlm.nih.gov/31832984/) - Faris et al., 2020 Quantifies a principal cost: across 24 studies, nightly sleep shortened by almost an hour, with slightly more daytime sleepiness. - [Ramadan Fasting and Kidney Stones: A Systematic Review.](https://pubmed.ncbi.nlm.nih.gov/32981032/) - Amjadi et al., 2020 Addresses a key dehydration concern; five heterogeneous, low-quality studies mostly showed no difference in stone formation between fasting and non-fasting periods. - [Impact of Ramadan fasting on kidney function and related outcomes in chronic kidney disease and kidney transplant recipients: a systematic review and meta-analysis.](https://pubmed.ncbi.nlm.nih.gov/39572100/) - Bello et al., 2024 Thirty-two observational studies (2,592 patients) found no consistent change in kidney function in kidney disease or transplantation; evidence quality was low. No systematic review or meta-analysis covers multi-day dry fasting. ## Mechanism of Action Dry fasting combines two separate stresses: energy deprivation and water deprivation. - **Energy deprivation:** Without food, insulin falls and the body shifts from stored liver sugar (glycogen) to fat breakdown and ketones (fat-derived fuel), which lowers calorie intake and body fat over repeated days ([Mähler et al., 2021](https://pubmed.ncbi.nlm.nih.gov/35011024/)). - **Water deprivation:** Ongoing losses through urine, breath and skin concentrate body fluids and shrink blood volume. This releases vasopressin (antidiuretic hormone, which makes the kidneys retain water), activates the renin–angiotensin–aldosterone system (RAAS, a hormone cascade that conserves salt and water) and raises cortisol. Across a five-day dry fast these hormones rose sharply and weight fell about 7 kg, mostly as water ([Papagiannopoulos-Vatopaidinos et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31958788/)). - **Metabolic water (competing views):** Burning fat yields roughly 1.1 g of water per gram. Proponents argue this forces greater fat use and stronger autophagy than water fasting; critics note this water covers only a small share of daily losses and that most rapid weight loss is fluid, not fat. - **Daytime-only fasting:** In dawn-to-dusk regimens, night-time drinking restores hydration; blood concentration stayed within the normal range in Bahá'í fasters, while meal timing shifted body-clock markers ([Koppold-Liebscher et al., 2021](https://pubmed.ncbi.nlm.nih.gov/34395487/)). - **Pharmacology:** Dry fasting is a behavioral practice, not a compound, so half-life, selectivity, tissue distribution and drug-metabolizing enzymes do not apply. ## Historical Context & Evolution Abstaining from both food and drink began as religious discipline, not as a health protocol. Islam's Ramadan fast (dawn to sunset for a lunar month, observed since the 7th century), the Bahá'í nineteen-day fast each March, Judaism's 25-hour Yom Kippur fast and ascetic monastic traditions all exclude water. In the late Soviet and post-Soviet period, Russian practitioners such as Sergei Filonov and Leonid Shchennikov promoted multi-day "dry therapeutic fasting" as a faster, stronger alternative to water fasting, drawing mainly on clinical anecdote. Online fasting communities spread these ideas in the 2010s, marketing dry fasting as a longevity tool. Scientific attention came mostly from Ramadan research. From the 1980s, hundreds of before-and-after studies measured weight, blood fats, blood pressure and kidney function, first to judge safety in chronic disease; they generally found small, transient falls in weight and blood pressure and few serious problems in healthy adults ([Faris et al., 2025](https://pubmed.ncbi.nlm.nih.gov/40081802/)). Since 2018, Charité Berlin studies of Bahá'í fasters, Baylor College of Medicine protein-profiling work and supervised five-day dry fasts at a Mount Athos monastery hospital have reframed the question around health. An international expert consensus on fasting terminology defined fasting as abstinence from some or all foods, or foods and beverages, a definition that encompasses dry fasting ([Koppold et al., 2024](https://pubmed.ncbi.nlm.nih.gov/39059384/)). Opinion has shifted from treating daytime dry fasting mainly as a patient-safety issue toward studying it as a possible health intervention, while the safety of multi-day dry fasting remains largely unmeasured. ## Expected Benefits ### High 🟩 🟩 🟩 No benefit reaches High: the human evidence consists of uncontrolled before-and-after cohorts of religious fasters and single small controlled trials, not replicated randomized trials. ### Medium 🟩 🟩 #### Body Weight and Body Fat Reduction Dawn-to-dusk dry fasting lowers body weight and body fat over a fasting month, mainly through lower calorie intake. A meta-analysis of 85 Ramadan studies found a small loss that grew with longer daily fasts ([Jahrami et al., 2020](https://pubmed.ncbi.nlm.nih.gov/32157368/)); another of 70 studies found fat loss mainly in people with overweight ([Fernando et al., 2019](https://pubmed.ncbi.nlm.nih.gov/30813495/)). A small randomized controlled trial (RCT, participants assigned by chance) of Bahá'í fasting confirmed lower intake and weight ([Peters et al., 2024](https://pubmed.ncbi.nlm.nih.gov/39203800/)). Most weight returns within 2–5 weeks. **Magnitude:** −1.02 kg (95% confidence interval [CI, the range likely to contain the true value] −1.16 to −0.88) over Ramadan; body fat −1.46 percentage points (CI −2.57 to −0.35) in people with overweight; −1.92 kg over 19 days in the Bahá'í trial. #### Blood Pressure Reduction Blood pressure falls modestly during a dawn-to-dusk fasting month, and the fall persisted after adjustment for changes in weight, body water and fat mass. A meta-analysis of 33 studies (3,213 participants) combined with the London Ramadan Study found reductions in healthy people and in those with hypertension or diabetes, but not in chronic kidney disease ([Al-Jafar et al., 2021](https://pubmed.ncbi.nlm.nih.gov/34619991/)). Data are observational before-and-after comparisons; no randomized comparison with water-permitted fasting exists, and durability after the fasting month is unknown. **Magnitude:** Systolic −3.19 mmHg (CI −4.43 to −1.96) and diastolic −2.26 mmHg (CI −3.19 to −1.34) in the pooled analysis. #### Fasting Glucose and Metabolic Syndrome Severity Fasting blood glucose and a composite metabolic syndrome severity score (combining waist, glucose, triglycerides, HDL cholesterol [high-density lipoprotein, the "good" cholesterol] and blood pressure) improve modestly during Ramadan across ethnic groups and both sexes ([Jahrami et al., 2022](https://pubmed.ncbi.nlm.nih.gov/36333203/)). An umbrella review confirmed lower fasting glucose ([Faris et al., 2025](https://pubmed.ncbi.nlm.nih.gov/40081802/)). In adults without diabetes, a small RCT using continuous glucose monitoring found no change in 24-hour average glucose ([Peters et al., 2024](https://pubmed.ncbi.nlm.nih.gov/39203800/)). **Magnitude:** Metabolic syndrome severity score −0.22 (CI −0.24 to −0.01); fasting glucose 4.98 to 4.77 mmol/L; pooled fasting-glucose standardized mean difference (SMD, effect expressed in standard-deviation units) −0.23 (CI −0.33 to −0.13) across 16,106 people. ### Low 🟩 #### Blood Lipid Changes ⚠️ Conflicted Meta-analyses disagree: one of 91 studies found lower LDL cholesterol (low-density lipoprotein, the "bad" cholesterol) and higher HDL ([Jahrami et al., 2021](https://pubmed.ncbi.nlm.nih.gov/34167865/)); another of 33 studies found LDL rose slightly ([Mirmiran et al., 2019](https://pubmed.ncbi.nlm.nih.gov/31377182/)). Diet, sex and baseline levels likely explain this. Net: lipid effects are small and inconsistent. **Magnitude:** LDL SMD −0.115 (CI −0.197 to −0.034) in one analysis versus +1.81 mg/dL (CI 0.55 to 3.07) in healthy adults in the other. #### Liver Enzyme Improvement A meta-analysis of 20 studies (601 healthy adults) found small falls in the liver enzymes AST and GGT (blood markers of liver-cell stress) but no significant change in ALT (alanine aminotransferase, the enzyme most tied to fatty liver) ([Faris et al., 2021](https://pubmed.ncbi.nlm.nih.gov/34273453/)). Uncontrolled designs limit inference. **Magnitude:** AST SMD −0.26 (CI −0.38 to −0.13); GGT SMD −0.53 (CI −0.84 to −0.22). #### Reduced Gum Inflammation In an experimental gingivitis model (gum inflammation induced by pausing oral hygiene for 9 days), 23 Bahá'í dry fasters showed a smaller rise in gum bleeding than regular eaters ([Pappe et al., 2025](https://pubmed.ncbi.nlm.nih.gov/40059409/)). The dry-fasting arm was not randomized, introducing bias. **Magnitude:** Bleeding on probing −9.48 percentage points (CI −17.18 to −1.79) versus the regular-diet control. #### Improved Well-Being and Mindfulness Surveys of 146 Bahá'í fasters, with interviews, reported higher mindfulness and well-being and lower stress, anxiety and fatigue during fasting, with mindfulness still elevated three months later ([Ring et al., 2022](https://pubmed.ncbi.nlm.nih.gov/35268012/)). The religious setting and lack of a control group limit generalization. **Magnitude:** Mindfulness score (Mindful Attention Awareness Scale, 1–6) rose from 4.23 before fasting to 4.57 in the third week and was 4.39 three months later. ### Speculative 🟨 #### Anti-Tumor and Anti-Atherosclerotic Protein Profiles Small cohorts showed more tumor-suppressor proteins and less apolipoprotein B (an artery-clogging protein) ([Mindikoglu et al., 2022](https://pubmed.ncbi.nlm.nih.gov/36506940/)). Dry-fasted serum slowed liver-cancer cells in vitro (lab culture) ([Mindikoglu et al., 2025](https://pubmed.ncbi.nlm.nih.gov/40551123/)); no clinical outcomes exist. #### Stronger Autophagy Than Water Fasting Proponents claim water restriction intensifies autophagy and fat use through metabolic water production. The basis is mechanistic reasoning and anecdote; no human study has compared autophagy between dry and water fasting. #### Anti-Inflammatory Shifts ⚠️ Conflicted Immune signaling proteins (interleukin-1β, interleukin-6) fell in a 50-person Ramadan cohort ([Faris et al., 2012](https://pubmed.ncbi.nlm.nih.gov/23244540/)); C-reactive protein (CRP, an inflammation marker) rose in ten five-day dry fasters ([Papagiannopoulos-Vatopaidinos et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31958788/)). Net: fast length decides. #### Circadian Rhythm Shift In an uncontrolled cohort of 16 Bahá'í fasters, body-clock timing, estimated from immune-cell gene activity, advanced about one hour ([Koppold-Liebscher et al., 2021](https://pubmed.ncbi.nlm.nih.gov/34395487/)). It reverted within three weeks; health relevance is unknown. ## Benefit-Modifying Factors - **Genetic polymorphisms:** No gene variant has been studied as a modifier of dry-fasting benefits; variants in vasopressin signaling or thirst regulation are theoretical candidates only. - **Baseline adiposity:** Higher starting body mass index (BMI, weight relative to height) predicts larger weight loss, and body-fat reduction occurs mainly in people with overweight or obesity ([Fernando et al., 2019](https://pubmed.ncbi.nlm.nih.gov/30813495/)). - **Baseline lipids and glucose:** Higher baseline triglycerides predict larger triglyceride falls ([Mirmiran et al., 2019](https://pubmed.ncbi.nlm.nih.gov/31377182/)); people with metabolic syndrome show the clearest glucose and blood-pressure shifts. - **Sex:** Across 29 studies, men and women differed in dietary, body-composition and blood outcomes during Ramadan ([Abdelrahim et al., 2023](https://pubmed.ncbi.nlm.nih.gov/37141819/)); male sex moderated LDL change, and women's religious fasts pause during menstruation. - **Pre-existing health conditions:** Blood pressure fell in hypertension and diabetes but not in chronic kidney disease ([Al-Jafar et al., 2021](https://pubmed.ncbi.nlm.nih.gov/34619991/)); with metabolic syndrome and fatty liver, dry-fasted serum slowed liver-cancer cells, unlike serum from healthy fasters ([Mindikoglu et al., 2025](https://pubmed.ncbi.nlm.nih.gov/40551123/)). - **Age:** Older age modified HDL changes ([Jahrami et al., 2021](https://pubmed.ncbi.nlm.nih.gov/34167865/)); older adults' blunted thirst adds dehydration cost, which may shrink the net benefit. - **Daily fast length and climate:** Longer daily fasting time predicts more weight loss ([Jahrami et al., 2020](https://pubmed.ncbi.nlm.nih.gov/32157368/)); long, hot summer days intensify both benefits and dehydration. - **Eating-window behavior:** Large, sugary night meals and reduced daytime activity blunt weight and lipid benefits. ## Potential Risks & Side Effects ### High 🟥 🟥 🟥 #### Hypoglycemia With Glucose-Lowering Medication Removing daytime food while insulin or sulfonylureas (drugs that force insulin release, e.g., gliclazide) keep acting causes hypoglycemia (dangerously low blood sugar). In the population-based EPIDIAR survey of 12,243 people with diabetes in 13 countries, severe hypoglycemia was several-fold more frequent during Ramadan, especially when doses were changed ([Salti et al., 2004](https://pubmed.ncbi.nlm.nih.gov/15451892/)). A meta-analysis of drug studies during Ramadan found DPP-4 inhibitors (drugs that prolong insulin-releasing gut hormones) caused fewer episodes than sulfonylureas ([Shiju et al., 2022](https://pubmed.ncbi.nlm.nih.gov/35242880/)). Risk is minimal without these drugs. **Magnitude:** Severe hypoglycemia 0.14 vs 0.03 episodes/month (type 1 diabetes) and 0.03 vs 0.004 (type 2 diabetes); odds ratio (OR, relative odds) 0.38 (CI 0.26–0.55) for DPP-4 inhibitors versus sulfonylureas. ### Medium 🟥 🟥 #### Headache and Migraine Aggravation Fasting-day headaches are common and mostly tension-type, driven by caffeine withdrawal, dehydration, low blood sugar and sleep loss. Among hospital staff, first-day headaches were far more frequent in fasters than non-fasters ([Awada & al Jumah, 1999](https://pubmed.ncbi.nlm.nih.gov/11279933/)). In 293 people with migraine, attack days and severity rose during Ramadan ([Al-Hashel et al., 2021](https://pubmed.ncbi.nlm.nih.gov/34464831/)). A 14-country survey identified pre-existing headache disorders as the strongest predictor ([Ceren Akgor et al., 2026](https://pubmed.ncbi.nlm.nih.gov/42428755/)). **Magnitude:** First-day headache in 41% of fasters vs 8% of non-fasters; migraine days 10.4 vs 6.9 per month. #### Hypohydration Fatigue and Reduced Alertness Water deprivation causes hypohydration (a body-water deficit) that concentrates body fluids and reduces blood volume. In a randomized crossover trial, 24 hours without water cost 2.6% of body weight; test performance held, but participants felt more tired, less alert and needed more effort ([Szinnai et al., 2005](https://pubmed.ncbi.nlm.nih.gov/15845879/)). In daytime-only fasting, hydration largely normalizes overnight ([Koppold-Liebscher et al., 2021](https://pubmed.ncbi.nlm.nih.gov/34395487/)); multi-day fasts produce progressive hemoconcentration (thickening of blood as plasma water falls). Beyond two to three days, losses near 1.4 kg daily make high blood sodium, fainting and kidney injury plausible. **Magnitude:** Tiredness +1.0 and alertness −0.9 points on a 5-point scale after 24 hours; hematocrit (share of blood volume made of red cells) +11% after five days of dry fasting ([Papagiannopoulos-Vatopaidinos et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31958788/)). #### Shortened Sleep Night-time meals and a pre-dawn meal compress sleep. A meta-analysis of 24 studies (646 participants) found shorter total sleep time and slightly higher daytime sleepiness on the Epworth Sleepiness Scale (a validated sleepiness questionnaire) ([Faris et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31832984/)). The cause is the eating schedule rather than water restriction itself. **Magnitude:** Total sleep time 7.2 to 6.4 hours per night; Epworth score 6.1 to 7.0. ### Low 🟥 #### Transient Kidney Strain ⚠️ Conflicted Stepwise water restriction ending in 16 hours without water raised kidney-stress biomarkers in a randomized crossover trial ([Domeier et al., 2026](https://pubmed.ncbi.nlm.nih.gov/41758613/)), yet fasting emergency patients had fewer acute kidney injury (AKI, sudden loss of kidney function) cases ([AlAbdan et al., 2022](https://pubmed.ncbi.nlm.nih.gov/35125093/)). Net: transient strain, no evident lasting harm. **Magnitude:** AKI adjusted OR 0.65 (CI 0.44–0.98) in fasting versus non-fasting matched patients. #### Kidney Stone Formation Concentrated urine favors stone formation. Yet a systematic review found most studies showed no difference between fasting and non-fasting periods, though its five studies were heterogeneous and of low quality ([Amjadi et al., 2020](https://pubmed.ncbi.nlm.nih.gov/32981032/)). **Magnitude:** Not quantified in available studies. The review could not pool its five heterogeneous studies into a single estimate. #### Peptic Ulcer Complications Hospital series report more peptic ulcer (stomach-lining sore) perforations and bleeding during Ramadan, plausibly through altered gastric acid rhythms. A 10-year series found perforation admissions rose during Ramadan ([Tokocin et al., 2026](https://pubmed.ncbi.nlm.nih.gov/42571034/)), echoing earlier reports ([Dönderici et al., 1994](https://pubmed.ncbi.nlm.nih.gov/7939395/)). Individual fasting status was not recorded, so evidence is indirect. **Magnitude:** Perforation admissions 10.5 vs 6.1 per month; incidence rate ratio (IRR, relative rate) 1.72 (CI 1.24–2.39) during Ramadan versus other periods. #### Hyperglycemia and Ketoacidosis in Diabetes Fasters with diabetes risk hyperglycemia (high blood sugar) and diabetic ketoacidosis (DKA, dangerous acid build-up from insulin shortage), especially when insulin is cut. A three-year hospital review found DKA presentations clustered in Ramadan, though cases were milder ([Alsufyani et al., 2026](https://pubmed.ncbi.nlm.nih.gov/42741250/)). The single-center retrospective design limits inference. **Magnitude:** DKA presentations during Ramadan 1.75 times those of the preceding month and 1.51 times those of the following month. #### Uric Acid Elevation During a supervised five-day dry fast in ten volunteers, serum uric acid roughly doubled as the kidneys conserved water and fat breakdown increased ([Papagiannopoulos-Vatopaidinos et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31958788/)). This raises gout and uric-acid stone concerns; daytime-only fasting was not assessed. **Magnitude:** Uric acid +103% (±19%) by day five. #### Cerebral Venous Thrombosis in Predisposed People A regional series found more cerebral venous thrombosis (clots in brain veins) during Ramadan than in other months, plausibly worse in estrogen contraceptive users ([Saadatnia et al., 2009](https://pubmed.ncbi.nlm.nih.gov/19723447/)). Arterial stroke and heart-attack rates were unchanged in a meta-analysis ([Turin et al., 2016](https://pubmed.ncbi.nlm.nih.gov/27186152/)). **Magnitude:** 5.5 vs 1.95 cases per month in fasting versus non-fasting months. #### Stress-Hormone and Lipid Surge in Multi-Day Fasts Across five dry-fasting days in ten volunteers, cortisol, renin (a kidney hormone) and CRP rose steeply ([Papagiannopoulos-Vatopaidinos et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31958788/)), while LDL cholesterol rose and thyroid hormones fell ([Papagiannopoulos-Vatopaidinos et al., 2025](https://pubmed.ncbi.nlm.nih.gov/40585323/)). Findings are uncontrolled and apply to multi-day fasting. **Magnitude:** Cortisol +495% and CRP +167% by day five. #### Fat-Free Mass Loss ⚠️ Conflicted One meta-analysis found fat-free mass (muscle, organs and water) fell alongside fat ([Fernando et al., 2019](https://pubmed.ncbi.nlm.nih.gov/30813495/)); a later one of 54 studies found no change ([Wulandari et al., 2025](https://pubmed.ncbi.nlm.nih.gov/40221545/)). Net: any lean-mass loss is small and partly fluid. **Magnitude:** Fat-free mass loss about 30% smaller than fat-mass loss in the first analysis; no significant change in the second. #### Reduced Exercise Performance ⚠️ Conflicted One meta-analysis found lower maximal aerobic capacity during Ramadan ([Correia et al., 2020](https://pubmed.ncbi.nlm.nih.gov/32408718/)); another found aerobic capacity and strength preserved but sprint power reduced, notably in morning tests ([Abaïdia et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31960369/)). Net: sprint and all-out output suffer most, while strength holds. **Magnitude:** Maximal oxygen uptake SMD −2.20 during Ramadan fasting. #### Accelerated Aging From Chronic Under-Hydration In a 25-year cohort of 15,752 adults, serum sodium above 142 mmol/L, a marker of habitual under-hydration, predicted chronic disease and older biological age ([Dmitrieva et al., 2023](https://pubmed.ncbi.nlm.nih.gov/36599719/)). Whether periodic dry fasting raises average sodium is unknown. **Magnitude:** Hazard ratio (HR, relative rate over time) 1.39 (CI 1.18–1.63) for chronic disease. ### Speculative 🟨 ## Risk-Modifying Factors - **Genetic polymorphisms:** Sickle cell trait (one copy of the sickle hemoglobin gene) and Factor V Leiden (an inherited clotting-factor variant) plausibly raise dehydration-related sickling and clot risk; untested in dry fasting. - **Baseline biomarkers:** Reduced kidney function, serum sodium above 142 mmol/L, high uric acid or elevated HbA1c (3-month average blood sugar) mark higher kidney, gout and blood-sugar risk. - **Sex:** Women showed slower reaction times after water deprivation ([Szinnai et al., 2005](https://pubmed.ncbi.nlm.nih.gov/15845879/)); estrogen contraceptive users face higher brain-vein clot risk while fasting ([AlSheef et al., 2020](https://pubmed.ncbi.nlm.nih.gov/33026046/)). - **Pre-existing health conditions:** Diabetes on insulin or sulfonylureas, migraine, kidney stones, gout, peptic ulcer, kidney disease, heart failure and epilepsy raise risk; poor seizure control predicted seizures during Ramadan ([Ibrahim et al., 2026](https://pubmed.ncbi.nlm.nih.gov/41707432/)). - **Age:** Older men were less thirsty and rehydrated incompletely after 24 hours without water ([Phillips et al., 1984](https://pubmed.ncbi.nlm.nih.gov/6472364/)), raising dehydration, fall and kidney risk. - **Environment and workload:** Heat, altitude, physical labor and long summer fasts multiply fluid losses and heat-illness risk. ## Key Interactions & Contraindications - **Insulin and sulfonylureas (glimepiride, gliclazide, glyburide):** Major caution — severe hypoglycemia during the fast. Mitigation: dose reduction and shifting doses to the evening meal with the prescriber; glucose checks at mid-day and before sunset. - **SGLT2 inhibitors (empagliflozin, dapagliflozin; drugs that make the kidneys excrete glucose):** Caution — added fluid loss, low blood pressure and ketoacidosis (acid build-up from ketones). A Ramadan meta-analysis found no major events ([Gad et al., 2022](https://pubmed.ncbi.nlm.nih.gov/34953158/)). Mitigation: evening-meal dosing. - **Diuretics (drugs that increase urine output; furosemide, hydrochlorothiazide, spironolactone):** Caution — amplify dehydration, sodium and potassium disturbances, dizziness and kidney strain. Mitigation: move dosing to the evening; check electrolytes and kidney function. - **ACE inhibitors and ARBs (blood-pressure drugs that block the angiotensin hormone system; lisinopril, ramipril, losartan):** Monitor — with low blood volume they can precipitate AKI and low blood pressure. Mitigation: evening dosing, adequate overnight fluids, creatinine check in week three. - **Lithium:** Major caution — dehydration raises blood lithium toward toxicity (tremor, confusion). Mitigation: serum lithium monitoring; many clinicians regard dry fasting as unsuitable during lithium therapy. - **NSAIDs (nonsteroidal anti-inflammatory drugs), over-the-counter (ibuprofen, naproxen, high-dose aspirin):** Caution — they reduce kidney blood flow, compounding AKI risk during dehydration. Mitigation: avoidance during fasting periods; acetaminophen after sunset for fasting headaches. - **Warfarin:** Monitor — INR (a clotting-time measure) was stable during Ramadan but values above the target range, which raise bleeding risk, were more likely afterward ([Rabea et al., 2022](https://pubmed.ncbi.nlm.nih.gov/35179616/)). Mitigation: check INR after the fasting month. - **Levothyroxine:** Monitor — shifted dosing relative to meals raised TSH (thyroid-stimulating hormone) after Ramadan, signaling under-treated hypothyroidism with fatigue and weight gain ([Belal et al., 2024](https://pubmed.ncbi.nlm.nih.gov/37733226/)). Mitigation: consistent timing and a TSH recheck about six weeks later. - **Estrogen-containing oral contraceptives (ethinylestradiol combinations):** Caution — dehydration plus estrogen raises brain-vein clot risk. Mitigation: overnight hydration targets; non-estrogen contraception where other clot risk factors exist. - **Multiple-daily-dose drugs (some antibiotics, e.g., amoxicillin, and antiepileptics, e.g., levetiracetam):** Caution — long dosing gaps may reduce effect or trigger seizures. Mitigation: switch to once- or twice-daily formulations with the prescriber. - **Caffeine (coffee, energy drinks, caffeine supplements):** Monitor — mild diuretic and cause of withdrawal headache on fasting days. Mitigation: tapering over one to two weeks beforehand; intake only at the pre-dawn meal. - **Glucose-lowering supplements (berberine, cinnamon extract, chromium):** Caution — additive effect with fasting on blood sugar, raising hypoglycemia risk, especially alongside medication. Mitigation: pausing them or evening-meal dosing. - **Creatine and high-dose vitamin C:** Monitor — creatine raises serum creatinine and water demand; vitamin C above 1 g daily increases urinary oxalate, adding stone risk with concentrated urine. Mitigation: intake only in the eating window, with fluids. - **Herbal diuretics (dandelion, horsetail, high-dose green tea extract):** Caution — additive fluid loss and electrolyte disturbance. Mitigation: avoidance during dry-fasting periods. - **Heat, sauna, alcohol and endurance exercise:** Caution — each adds fluid loss, with heat illness and fainting as consequences. Mitigation: no sauna or prolonged exertion during the fast; limited alcohol in the eating window. **Populations who should avoid Dry Fasting:** - Pregnant or breastfeeding women; placental weight was lower with Ramadan fasting ([Glazier et al., 2018](https://pubmed.ncbi.nlm.nih.gov/30359228/)) - Type 1 diabetes, or type 2 diabetes on insulin or sulfonylureas with HbA1c above 9% or severe hypoglycemia within the past 3 months - Chronic kidney disease stage 3b–5 (eGFR, estimated glomerular filtration rate, below 45 mL/min/1.73 m²), kidney transplant recipients without specialist oversight, or two or more prior kidney stones - Heart failure NYHA class III–IV (New York Heart Association class: symptoms with minimal activity or at rest) or a heart attack within the past 3 months - Prior cerebral venous thrombosis, known thrombophilia (inherited clotting tendency) or sickle cell disease - Gout with a flare in the past 12 months or serum uric acid above 8 mg/dL - Adults aged 75 or older, or frail older adults - Current or past eating disorder, or BMI below 18.5 kg/m² - Lithium therapy, or epilepsy with a seizure in the past 3 months - Anyone attempting dry fasts longer than 24 hours without medical supervision ## Risk Mitigation Strategies - **Overnight hydration target:** Spreading 2–3 L of fluid between sunset and dawn blunted fasting rises in creatinine and urea in a randomized trial ([Tarabeih et al., 2023](https://pubmed.ncbi.nlm.nih.gov/36549977/)); mitigates hypohydration and kidney strain. - **Pre-dawn meal with water and salt:** A pre-dawn meal with about 500 mL water, protein and moderate salt mitigates daytime hypohydration, headache and fatigue. - **Caffeine taper:** Gradual caffeine reduction over one to two weeks beforehand, with any intake limited to the pre-dawn meal, prevents withdrawal headache. - **Medication review before starting:** Prescriber adjustment of glucose-lowering, diuretic, lithium and blood-pressure drug doses and timing two to four weeks beforehand prevents hypoglycemia, AKI and drug toxicity. - **Glucose checks for medication users:** Checks at mid-day and before sunset, with the fast broken below 70 mg/dL (3.9 mmol/L) or above 300 mg/dL (16.7 mmol/L), prevent severe hypoglycemia and hyperglycemia (high blood sugar). - **Stop rules:** Breaking the fast for dizziness on standing, confusion, chest pain, no urination for over 8 hours, or loss above 3% of body weight within one day prevents severe dehydration and kidney injury. - **Heat and exertion limits:** Avoiding outdoor heat, sauna and intense training in the last hours of the fast, and training one to two hours after the evening meal, prevents heat illness, fainting and performance loss. - **Protein and resistance training:** Protein intake of at least 1.2 g/kg/day within the eating window plus twice-weekly resistance training limits fat-free mass loss. - **Sleep protection:** A fixed sleep window plus a 20–30-minute afternoon nap when the pre-dawn meal shortens nights mitigates sleep loss and daytime sleepiness. - **Supervision for multi-day fasts:** Limiting unsupervised dry fasts to 24 hours, with longer fasts only under daily clinician checks of weight, sodium and creatinine, prevents hypernatremia (high blood sodium) and AKI. - **Stone prevention:** For stone-prone people, enough overnight fluid to produce pale morning urine, plus less salt and animal protein at night, mitigates kidney stone formation. ## Therapeutic Protocol - **Dawn-to-dusk intermittent dry fasting (Ramadan model):** No food or fluid from dawn to sunset, 12–18 hours depending on latitude and season, for 29–30 consecutive days, with pre-dawn and post-sunset meals. The most studied form. - **Sunrise-to-sunset 19-day fast (Bahá'í model):** Abstinence from sunrise to sunset for 19 days each March; studied by the Charité Berlin group of Andreas Michalsen and Daniela Koppold as a secular template. - **Short daily dry window:** Some practitioners keep a 12–16-hour dry window overlapping sleep (e.g., 20:00–10:00), combining time-restricted eating with overnight fluid restriction; no dedicated trials exist. - **Soft versus hard dry fasting:** "Soft" permits bathing and tooth-brushing without swallowing; "hard" forbids all water contact. Hard forms come from Russian practitioners Sergei Filonov and Leonid Shchennikov; no evidence shows different outcomes. - **Prolonged dry fasting:** 24–36-hour fasts (as on Yom Kippur) are common; 3–5-day fasts, popularized by Filonov, have been studied only as a supervised five-day protocol at a Mount Athos monastery hospital. - **Competing approach, water-permitted fasting:** Time-restricted eating, water-only fasting (used by Peter Attia) or Valter Longo's fasting-mimicking diet (a five-day low-calorie plant-based regimen) provide the fasting stimulus without fluid restriction; no trial shows dry fasting superior. - **Eating-window structure:** Practitioners typically break the fast with water and a light starter, then a full meal, spreading 2–3 L of fluid overnight alongside protein, vegetables, fruit and moderate salt. - **Best time of day:** Daytime fasting matches religious practice; lower evening energy expenditure after meals suggests an earlier eating window may suit secular practitioners ([Mähler et al., 2021](https://pubmed.ncbi.nlm.nih.gov/35011024/)). - **Half-life and dose splitting:** Not applicable to a behavioral practice; there is no compound half-life or single-versus-split dosing. The "dose" is fast length and frequency. - **Genetic polymorphisms:** No variant has been studied for protocol choice; carriers of sickle cell trait or Factor V Leiden may favor shorter, daytime-only fasts with strict overnight hydration. - **Sex differences:** Reaction-time slowing after water deprivation appeared in women but not men ([Szinnai et al., 2005](https://pubmed.ncbi.nlm.nih.gov/15845879/)); estrogen contraceptive users and menstruating women may favor shorter or interrupted protocols. - **Age:** Adults over 65 feel less thirst and rehydrate incompletely ([Phillips et al., 1984](https://pubmed.ncbi.nlm.nih.gov/6472364/)), favoring shorter dry windows, explicit overnight fluid targets and no multi-day protocols. - **Baseline biomarkers:** Serum sodium above 142 mmol/L, eGFR below 60, uric acid above 7 mg/dL or HbA1c above 7% favor shorter windows and closer monitoring; higher BMI predicts larger weight loss. - **Pre-existing health conditions:** Migraine, kidney stones, gout, medication-treated diabetes, heart failure and epilepsy shift the balance toward shorter or no dry fasting; metabolic syndrome shows the largest cardiometabolic responses. ## Discontinuation & Cycling - **Short-term, periodic practice:** Dry fasting is used in bounded periods (a month per year, occasional 24-hour fasts), not continuously; the agents here are food and water, withdrawn together and restored together. - **Withdrawal effects:** None known; weight and body-composition changes drift back within two to five weeks after a fasting month ([Fernando et al., 2019](https://pubmed.ncbi.nlm.nih.gov/30813495/)). - **Ending a fast (tapering):** Fasts usually end with water sipped over 30–60 minutes, then a light meal; after multi-day dry fasts, fluids come first and food returns over one to two days to avoid refeeding syndrome (dangerous electrolyte shifts). - **Cycling:** Religious models cycle naturally (an annual month); practitioners repeat 24-hour dry fasts weekly or monthly. No data show that cycling preserves or enhances effects. - **When to stop:** Recurrent kidney stones, rising creatinine, gout flares or severe headaches during fasts indicate the practice is not tolerated. ## Sourcing and Quality - **Not a product:** Dry fasting involves no substance to buy, so purity, formulation and third-party testing of the intervention itself do not apply. - **Retreats and coaching:** Commercial dry-fasting retreats and online programs, often promoting multi-day "hard" fasts, have a financial interest in adoption; markers of quality are physician oversight, laboratory monitoring and clear stop criteria. - **Electrolyte products for the eating window:** Third-party tested products (NSF Certified for Sport or Informed Sport seals, e.g., Thorne or Klean Athlete) without stimulants limit contamination risk; they belong only in non-fasting hours. - **Monitoring tools:** A validated upper-arm blood pressure monitor, a body-weight scale and, for medication users, a glucometer or CGM (continuous glucose monitor, a skin sensor reading glucose every few minutes). ## Practical Considerations - **Time to effect:** Weight and waist changes appear by the second to third week of a fasting month and peak shortly after it; most revert within weeks ([Wulandari et al., 2025](https://pubmed.ncbi.nlm.nih.gov/40221545/)). - **Common pitfalls:** Sugary, oversized night meals, skipping the pre-dawn meal, heavy caffeine use, training in heat, and mistaking rapid water-weight loss in multi-day dry fasts for fat loss. - **Regulatory status:** Unregulated lifestyle practice. Religious traditions exempt the ill, pregnant, elderly and travelers; no medical body endorses multi-day dry fasting. - **Cost and research incentives:** Free. No company can patent it and insurers or health systems gain little from studying it, a structural bias that leaves the evidence small, observational and largely funded by universities. - **Climate and latitude:** Daily fast length ranges from about 11 to over 18 hours with season and latitude; longer, hotter days raise dehydration risk. - **Occupational fit:** Fluid restriction late in the fast conflicts with jobs requiring vigilance, such as driving, surgery or operating machinery. ## Interaction with Foundational Habits - **Sleep:** Blunting, indirect. Night-time meals and pre-dawn eating shorten sleep by almost an hour ([Faris et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31832984/)); caffeine and heavy late meals worsen it. A last meal two to three hours before bed, a fixed sleep window and short naps limit the loss. - **Nutrition:** Potentiating when the eating window is high quality. Fluid-rich foods (fruit, soups, yogurt), adequate protein and moderate salt support hydration; fried, sugary night meals erode weight and lipid gains. Fewer high-oxalate foods (spinach, nuts) lower stone risk. - **Exercise:** Blunting for high-intensity work. Sprint power falls during fasting, notably in morning tests ([Abaïdia et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31960369/)), while strength is largely preserved. Training after the evening meal, or just before it with immediate rehydration, plus continued resistance work protects performance and lean mass. - **Stress management:** Blunting in multi-day fasts, potentiating in daytime fasts. Cortisol rises steeply in multi-day dry fasting ([Papagiannopoulos-Vatopaidinos et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31958788/)), yet Bahá'í fasters report lower stress and higher mindfulness ([Ring et al., 2022](https://pubmed.ncbi.nlm.nih.gov/35268012/)); meditation, prayer and adequate sleep likely shape the response. ## Monitoring Protocol & Defining Success **Baseline testing:** Before a first fasting month or any dry fast longer than 24 hours, a baseline panel covers kidney function, electrolytes, uric acid, glucose markers, lipids and inflammation, together with seated and standing blood pressure, body weight and, ideally, body composition. People taking glucose-lowering, diuretic, blood-pressure, thyroid, anticoagulant (blood-thinning) or lithium therapy add a medication review with the prescriber. **Ongoing monitoring:** Cadence for dawn-to-dusk fasting: weight and blood pressure weekly, laboratory tests in week three of the fasting month and again two to four weeks after it ends, then annually before each fasting season. Glucose-lowering medication users check glucose daily at mid-day and before sunset. Multi-day dry fasts require daily weight, sodium and creatinine checks under clinical supervision. Success means stable kidney markers alongside improved weight, waist and blood pressure. | Biomarker | Optimal Functional Range | Why Measure It? | Context/Notes | |-----------|--------------------------|-----------------|---------------| | Serum sodium | 137–142 mmol/L | Hydration status | Conventional 135–145 mmol/L; above 142 linked to faster biological aging; draw late in the fast to capture the peak | | Creatinine & eGFR | eGFR above 90 mL/min/1.73 m² | Kidney strain | eGFR = estimated glomerular filtration rate; conventional cutoff 60; creatinine also rises with creatine use and heavy training | | BUN | 10–16 mg/dL | Dehydration signal | BUN = blood urea nitrogen; conventional 7–20 mg/dL; BUN-to-creatinine ratio above 20 suggests low blood volume | | Uric acid | Below 5.5 mg/dL | Gout and stone risk | Conventional up to 7.0 mg/dL (men) and 6.0 mg/dL (women); roughly doubled in a five-day dry fast ([Papagiannopoulos-Vatopaidinos et al., 2020](https://pubmed.ncbi.nlm.nih.gov/31958788/)) | | Potassium | 4.0–4.8 mmol/L | Electrolyte balance | Conventional 3.5–5.0 mmol/L; diuretics and ACE inhibitors (angiotensin-converting enzyme blockers) shift levels | | Fasting glucose | 75–90 mg/dL | Glucose control | Conventional below 100 mg/dL; draw after an overnight fast outside the dry-fasting period | | HbA1c | Below 5.4% | Three-month glucose average | HbA1c = glycated hemoglobin; conventional below 5.7%; pair with fasting glucose | | LDL-C, apoB, TG | LDL-C below 100 mg/dL; apoB below 80 mg/dL; TG below 100 mg/dL | Cardiovascular risk | LDL-C = LDL cholesterol; apoB = apolipoprotein B, a count of artery-clogging particles; TG = triglycerides; conventional LDL-C below 130 mg/dL, apoB below 130 mg/dL, TG below 150 mg/dL; draw well hydrated, 10–12 hours after eating | | hs-CRP | Below 1.0 mg/L | Inflammation | hs-CRP = high-sensitivity C-reactive protein; conventional below 3.0 mg/L; rises in multi-day dry fasts | | Hematocrit | 40–50% (men); 37–45% (women) | Hemoconcentration | Rises with dehydration; relevant to clot risk; best paired with sodium | | Urine specific gravity | 1.005–1.020 on waking | Overnight rehydration adequacy | Above 1.025 on waking suggests insufficient overnight fluid; home dipstick, morning sample | | Blood pressure | Below 120/80 mmHg | Cardiovascular effect and standing drop | Standing drop above 20 mmHg systolic indicates orthostatic hypotension (blood-pressure fall on standing); measure seated then standing | | TSH (levothyroxine users) | 0.5–2.5 mIU/L | Thyroid dose adequacy | TSH = thyroid-stimulating hormone; conventional 0.4–4.5 mIU/L; recheck about six weeks after the fasting month | **Qualitative markers:** - Energy and concentration in the last hours of the fast - Headache frequency and severity - Thirst intensity and urine color on waking - Dizziness on standing - Sleep duration and quality - Mood, irritability and sense of well-being - Exercise tolerance and recovery ## Emerging Research - **Mitophagy (removal of damaged mitochondria, the cell's power plants) under dry versus water-permitted fasting:** [NCT07638696](https://clinicaltrials.gov/study/NCT07638696) – observational, 120 healthy volunteers, not yet recruiting; compares mitophagy genes PINK1 and PRKN and mitochondrial DNA copy number between Ramadan dry fasting and 16:8 eating (a daily 8-hour eating window). Could confirm or refute the stronger-autophagy claim. - **Gut and vaginal microbiome during dry fasting:** [NCT07001358](https://clinicaltrials.gov/study/NCT07001358) – the Rufaida Project, an observational study of up to 350 Muslim women, 50 of whom give stool and vaginal samples before, during and after Ramadan while the rest complete questionnaires only; primary endpoint is microbial composition by gene sequencing. - **Salt loading to limit dehydration:** [NCT05839860](https://clinicaltrials.gov/study/NCT05839860) – completed interventional study of 23 adults testing 6–9 g salt with water before a seven-hour dry fast; primary endpoints are urine output, body weight and urine concentration. - **Randomized comparisons with time-restricted eating:** A pilot randomized trial of Bahá'í fasting versus time-restricted eating found no glycemic harm ([Peters et al., 2024](https://pubmed.ncbi.nlm.nih.gov/39203800/)); larger trials comparing dry and water-permitted windows could show whether water restriction adds anything. - **Prolonged dry fasting physiology:** A five-day study reported rising LDL cholesterol and IGF-1 (insulin-like growth factor 1, a growth-signaling hormone) with falling thyroid hormones ([Papagiannopoulos-Vatopaidinos et al., 2025](https://pubmed.ncbi.nlm.nih.gov/40585323/)); replication could weaken longevity claims. - **Anticancer protein signals:** Dry-fasted serum slowed liver-cancer cell growth in vitro ([Mindikoglu et al., 2025](https://pubmed.ncbi.nlm.nih.gov/40551123/)); human outcome studies could strengthen or overturn this signal. - **Hydration and biological aging:** Higher serum sodium predicted faster aging and chronic disease ([Dmitrieva et al., 2023](https://pubmed.ncbi.nlm.nih.gov/36599719/)); measuring sodium across repeated dry-fasting cycles would test whether the practice adds this risk. ## Conclusion Dry fasting, going without food and all fluids for part of the day or, more rarely, for several days, is understood mainly through daytime religious fasts. For health-focused adults, that evidence points to modest, short-lived drops in body weight, body fat and blood pressure and small improvements in blood sugar, much like other ways of limiting eating to part of the day. Signals on cholesterol, liver markers, inflammation and cellular clean-up are mixed or rest on laboratory findings. Nothing yet shows that removing water adds benefit beyond fasting that allows water. The costs are real but mostly manageable in the daytime form: headaches, tiredness, shorter sleep and weaker all-out exercise performance. Dangerously low blood sugar threatens people taking certain diabetes medicines, and kidney, clotting and gout concerns apply to those already prone to them. Multi-day dry fasts cause large fluid losses and steep stress-hormone rises, yet have been studied in only a handful of volunteers. The evidence rests on before-and-after studies of religious fasters, with few randomized comparisons and no long-term outcome data. Much of the multi-day research comes from a clinician who runs a fasting-therapy practice, and some daytime research from a hospital department offering fasting treatment. Because the practice is free and cannot be patented, little industry money funds research, keeping studies small. On current evidence, daytime dry fasting is a tolerable but unproven variation on intermittent fasting, while prolonged dry fasting remains high-risk and largely unexamined. **[Top](#top) - [Benefits](#expected-benefits) - [Risks](#potential-risks--side-effects) - [Protocol](#therapeutic-protocol)**