Religious fasting can affect hydration status, but the degree depends on the type of fast, its duration, environmental conditions, and individual factors such as age, health, and activity level[1]. Longer fasts that explicitly forbid drinking (for example, Ramadan daytime fasting) pose a measurable risk of dehydration for some people, while short or water-permitting fasts have little or no harmful effect on hydration for healthy adults[1][3].
Essential context and mechanisms
– What “dehydration” means: dehydration occurs when fluid losses exceed fluid intake, producing a net deficit in body water that can impair physiological function[1]. Mild dehydration may cause thirst, dry mouth, reduced urine output, and lightheadedness; more severe deficits cause marked weakness, cognitive changes, low blood pressure, and kidney stress[1].
– How religious fasts differ: some traditions permit water during fasting windows (for example certain intermittent fasting practices), while others prohibit all oral intake during the fasting period (for example the daily daylight fast in Ramadan).[1][3]
– Time without fluid matters: the longer the uninterrupted period without drinking, the greater the chance that normal losses (urine, sweat, respiration) will produce a net fluid deficit[1]. Studies and reviews note that daytime-only fasts that extend many hours (especially in hot climates or during long summer days) increase the likelihood of decreased fluid intake and elevated markers of hemoconcentration or reduced urine output[3].
– Environmental and behavioral modifiers: heat, physical exertion, and high altitude increase fluid losses and raise dehydration risk during a fast[1]. Likewise, inadequate rehydration during non-fasting hours (for example, consuming mainly diuretics like caffeine or not drinking enough after sunset) compounds the problem[3].
– Individual vulnerability: older adults, young children, pregnant or breastfeeding people, and those with chronic illnesses (kidney disease, uncontrolled diabetes, heart failure) are at higher risk of harmful dehydration during fasting and often require individualized medical advice before fasting[3].
What the evidence shows
– Short-term intermittent fasting that allows water generally does not produce clinically significant dehydration in healthy adults and is often used in research and weight-management programs[1]. One controlled study of alternate-day modified fasting and time-restricted feeding permitted plain water during fasting and reported metabolic changes but water access prevented overt dehydration in participants[1].
– Fasts that forbid liquids for prolonged daytime hours (for example Ramadan) are associated in some studies with transient changes consistent with mild dehydration (reduced urine volume, increased urine concentration, small rises in hematocrit or serum osmolality in some settings), particularly when days are long and temperatures are high[3]. Medical reviews emphasize preventive measures such as adequate hydration during non-fasting hours and avoidance of excessive activity in heat[3].
– Clinical guidance for populations with medical conditions (for example Parkinson disease and other chronic illnesses) advises pre-fast risk assessment and tailored strategies; for hydration specifically, this includes ensuring sufficient fluid intake between sunset and dawn, adjusting medications when needed, and possibly avoiding fasting for high-risk individuals[3].
Practical, evidence-informed advice
– If fasting allows water: drink gradually across non-fasting hours rather than a large single volume; include electrolyte-containing beverages if sweating heavily; avoid excessive caffeine and alcohol that promote fluid loss[1][3].
– If fasting forbids liquids during the day: maximize fluid and salt intake during allowable hours, eat water-rich foods (fruits, soups) before the fast begins, plan lower-intensity activity during fasting hours, and seek shaded or cool environments to reduce sweating[3].
– Watch for warning signs: confusion, fainting, very low urine output, rapid heart rate, very dry mucous membranes, or dizziness are red flags that need prompt medical attention[1][3].
– High-risk groups: children, older adults, pregnant or breastfeeding people, and those with significant chronic diseases should consult healthcare providers and religious authorities before undertaking fasts that restrict fluids; exemptions or modifications are often permissible within many faith traditions[3].
Sources
https://pmc.ncbi.nlm.nih.gov/articles/PMC12731896/
https://www.frontiersin.org/journals/neurology/articles/10.3389/fneur.2025.1720571/full
