Most people who water-fast for five days lose somewhere between 2% and 10% of their starting body weight, which for an average adult works out to roughly 4 to 20 pounds on the scale.1PubMed Central. Efficacy and safety of prolonged water fasting: a narrative review of human trials That range is wide because the number depends heavily on how much you weigh going in, how much glycogen and water your body is holding, and your individual metabolism. The more useful question, though, is what that weight actually consists of, because the answer is not nearly as satisfying as the scale might suggest.
What the Scale Is Actually Measuring
The first couple of pounds you lose during a fast are not fat. They are glycogen and the water that tags along with it. Your liver and muscles store a few hundred grams of glycogen as a quick-access fuel reserve, and every gram of glycogen binds roughly three grams of water. Once you stop eating, your body burns through those glycogen stores within roughly 12 to 24 hours, releasing all that bound water in the process.2PubMed Central. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting This is why people often see a dramatic drop on the scale in the first day or two of a fast. It feels exciting, but it reverses almost completely the moment you eat carbohydrates again and your body restocks those glycogen shelves.
On top of glycogen-bound water, fasting triggers a separate process of sodium and water loss through the kidneys. When your body shifts to burning fat and producing ketones, the organic acids generated during that process pull sodium out in your urine, and water follows the sodium.3JCI Insight. The mechanism of the natriuresis of fasting This diuretic effect adds to the early scale drop and also partly explains why people feel lightheaded or weak during the first days of a fast. Once you start eating again, sodium excretion drops quickly and the body reabsorbs fluid, which is why some of that “lost” weight reappears within 48 hours of breaking the fast.
How Much of That Weight Is Actually Fat
This is where the numbers get sobering. A prospective trial that tracked healthy men through a 10-day fast using detailed body-composition measurements found that fat loss accounted for only about 40% of total weight lost. The other 60% was categorized as lean soft tissue loss, which breaks down further into extracellular water loss (about 44% of that lean tissue figure), early glycogen depletion with its associated water (about 14%), and actual metabolically active tissue like muscle, organs, and other structures (about 42% of the lean tissue loss, or roughly a quarter of total weight lost).4PubMed Central. Is muscle and protein loss relevant in long‐term fasting in healthy men? A prospective trial on physiological adaptations
A separate study looking at seven days of fasting found similar patterns. After one week, participants lost about 4.6 kg of lean mass (an 8% decrease) and about 1.4 kg of fat mass (an 8.4% decrease).5Nature Communications. Effects of seven days’ fasting on physical performance and metabolic adaptation during exercise in humans In absolute terms, the lean mass loss outpaced the fat loss by more than three to one. That does not mean you lost three times as much muscle as fat, because “lean mass” on a body scanner includes all the water and glycogen shifts described above. But it does mean the headline number on your scale is doing a lot of work that has nothing to do with the fat tissue you probably want to get rid of.
To put a rough estimate on actual fat loss: your body can oxidize somewhere around half a pound to three-quarters of a pound of fat per day during a water fast, depending on your size and metabolic rate. Over five days, that translates to perhaps 2.5 to 4 pounds of true fat loss for most people, even if the scale shows 8 to 15 pounds gone.
What Happens to Your Metabolic Rate
One of the persistent myths about extended fasting is that it immediately tanks your metabolism. The early data tells a different story. A study that measured resting energy expenditure during a short starvation period found it actually increased during the first few days, rising from about 4.0 kJ/min on day one to about 4.5 kJ/min on day three. That bump correlated with a surge in norepinephrine, which roughly doubled over the same period.6The American Journal of Clinical Nutrition. Resting energy expenditure in short-term starvation is increased and correlated with an increase in serum norepinephrine This makes some evolutionary sense: when food disappears, the body initially ramps up alertness and energy mobilization to help you go find more.
But this boost does not last. A study tracking volunteers through a 21-day fast found that resting energy expenditure showed no significant drop by day three, but had fallen by about 7.5% by day nine and continued declining, reaching roughly 20% below baseline by day twenty.7Scientific Reports. Analysis of physiological and biochemical changes and metabolic shifts during 21-Day fasting hypometabolism For a five-day fast, you are likely catching the tail end of the initial metabolic bump and entering the beginning of the downward slide. Your metabolism probably has not cratered yet, but it is starting to slow. One volunteer in that longer study experienced a maximum metabolic drop of nearly 54%, though that was an outlier in a much longer fast.
A five-day study that measured basal energy expenditure specifically at the five-day mark confirmed a decrease, along with reductions in body mass, waist circumference, glucose, insulin, and leptin.8PubMed Central. Physiological and psychological responses to five-day fasting So the metabolic picture for a five-day fast is mixed: you probably get a brief bump early on, then things start drifting downward as your body conserves energy.
How Your Body Protects Its Own Protein
One of the body’s more impressive adaptations to fasting is a gradual reduction in protein breakdown. After about a week of fasting, whole-body protein breakdown rates in obese subjects dropped from roughly 2.0 g/kg per day to about 1.5 g/kg per day, a meaningful decrease that helps spare muscle and organ tissue as the fast continues.9PubMed. Whole body protein breakdown rates and hormonal adaptation in fasted obese subjects This protein-sparing effect is driven partly by the rise in ketone bodies, which the brain can use as fuel instead of glucose, reducing the need for the body to break down amino acids to manufacture new glucose.
By day four or five of a fast, more than 95% of people show detectable ketones in their urine, signaling that the metabolic switch to fat-derived fuel is well underway.10PubMed Central. Long-Term Fasting-Induced Ketosis in 1610 Subjects: Metabolic Regulation and Safety There is individual variation here: some people produce only trace amounts of ketones even after several days, while others ramp up rapidly. How quickly you shift into ketosis affects both how you feel during the fast and how effectively your body spares protein.
Hunger, Hormones, and How You Will Feel
The hunger question is what most people dread, and the answer is counterintuitive. Ghrelin, the hormone most associated with appetite, does not simply climb higher and higher as the fast goes on. Research shows that ghrelin maintains its normal daily rhythm even during extended fasting, spiking around the times you would normally eat and then falling back on its own.11European Journal of Endocrinology. Spontaneous 24-h ghrelin secretion pattern in fasting subjects: maintenance of a meal-related pattern Average 24-hour ghrelin levels actually declined slightly over the fasting period rather than building relentlessly.12The Journal of Clinical Endocrinology & Metabolism. Fasting Unmasks a Strong Inverse Association between Ghrelin and Cortisol in Serum: Studies in Obese and Normal-Weight Subjects Most people who have done extended fasts report that days two and three are the hardest for hunger, and by day four or five, it genuinely eases.
Growth hormone tells a more dramatic story. A five-day fast roughly tripled the 24-hour integrated growth hormone concentration and doubled the maximum pulse amplitude compared to fed conditions.13PubMed Central. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man This surge is thought to help preserve lean tissue and mobilize fat stores. Interestingly, the growth hormone increase does not appear to correlate with weight loss itself, suggesting it is a direct response to the fasting state rather than a byproduct of losing weight.14PubMed Central. Weight loss-independent changes in human growth hormone during water-only fasting: a secondary evaluation of a randomized controlled trial
Mood is the other surprise. While you might expect five days without food to leave people irritable and foggy, the five-day fasting study found that participants experienced improved mood overall, with increased vigor and reduced tension.8PubMed Central. Physiological and psychological responses to five-day fasting People with higher emotional well-being going into the fast also tended to lose more fat. This does not mean fasting is universally pleasant, but the stereotype of five days of misery does not hold up for everyone.
Does Starting Weight Change the Picture
Your body composition going into the fast has a meaningful effect on how things play out. A person carrying more body fat has a larger energy reserve to draw on and can sustain a fast with less metabolic stress. Paradoxically, lean people actually lose weight faster during a fast, measured as a percentage of body weight, because their bodies have fewer fat stores and must tap into protein-containing tissues sooner.15PubMed Central. Weight loss will be much faster in lean than in obese hunger strikers This means a lean person doing a five-day fast is at greater risk of losing proportionally more muscle and organ tissue relative to fat, making the fast both less effective for fat loss and potentially more dangerous.
For someone who is already at a healthy weight, a five-day fast is a more aggressive intervention than it is for someone with significant fat reserves. The protein-sparing mechanisms described earlier work better when there is ample fat to burn. If you are already lean and considering a five-day fast, the risk-to-benefit ratio shifts in an unfavorable direction.
What Happens When You Start Eating Again
The refeeding phase is where most people underestimate what is coming. First, the practical reality: as soon as you eat carbohydrates, your body restocks glycogen and pulls water back into your muscles and liver. The sodium your kidneys were flushing gets retained again. Most people regain 3 to 5 pounds within the first two or three days of eating, and this is entirely normal water and glycogen, not fat. The “true” weight loss, meaning fat that stays gone, is the much smaller number discussed earlier.
Beyond the immediate water bounce, animal research shows that the body develops adaptations during weight loss that make it exceptionally efficient at regaining fat. Dietary fat oxidation becomes suppressed, and the body preferentially channels incoming calories toward fat storage.16PubMed. Weight regain after sustained weight reduction is accompanied by suppressed oxidation of dietary fat and adipocyte hyperplasia This is not unique to fasting; it happens after any substantial weight loss. But the speed and severity of a five-day fast can amplify the rebound effect if you return to unrestricted eating immediately afterward.
There is also a genuine medical risk with refeeding after an extended fast. Refeeding syndrome occurs when the sudden reintroduction of calories causes dangerous shifts in electrolytes, particularly phosphorus, potassium, and magnesium. Symptoms typically appear within two to five days of refeeding and can range from mild to life-threatening depending on the degree of prior malnutrition.17PubMed Central. The Refeeding Syndrome: a neglected but potentially serious condition for inpatients. A narrative review Clinical guidelines classify refeeding syndrome by the degree of electrolyte drop, with a decrease of more than 30% in phosphorus, potassium, or magnesium considered severe.18PubMed. ASPEN Consensus Recommendations for Refeeding Syndrome For a healthy person doing a one-off five-day fast, severe refeeding syndrome is unlikely but not impossible, particularly if you break the fast with a large, carbohydrate-heavy meal. Starting with small, easily digestible portions and gradually increasing intake over a day or two is standard advice for good reason.
Fasting Versus Just Eating Less
If the goal is lasting weight loss, the uncomfortable truth is that a five-day fast does not appear to outperform sustained calorie restriction over the long term. A systematic review and meta-analysis comparing fasting-based strategies to continuous calorie restriction in obese adults found that while fasting produced some short-term metabolic benefits, it did not show superior long-term outcomes for weight management.19PubMed Central. Is Fasting Superior to Continuous Caloric Restriction for Weight Loss and Metabolic Outcomes in Obese Adults? A Systematic Review and Meta-Analysis of Randomized Clinical Trials Both approaches are effective, but the idea that fasting gives you a metabolic edge that calorie restriction cannot match is not supported by the current evidence.
This matters because a five-day water fast is a genuinely difficult and potentially risky undertaking. If the weight-loss results at six months or a year are similar to what you would get from a sustained moderate calorie deficit, the practical question becomes whether the fast offers other benefits that justify the difficulty and the risks.
Effects That Have Nothing to Do With the Scale
Some of the most interesting research on prolonged fasting has nothing to do with weight. At the cellular level, fasting appears to activate autophagy, a housekeeping process in which cells break down and recycle damaged components. One study found that fasting increased expression of the autophagy marker LC3B-II by roughly 30% in human skeletal muscle, suggesting the process ramps up during a fast, though interpreting the full picture of autophagy activity in living humans remains technically challenging.20PLoS ONE. Fasting Increases Human Skeletal Muscle Net Phenylalanine Release and This Is Associated with Decreased mTOR Signaling
The immune system is another area drawing attention. Mouse studies and early human data suggest that prolonged fasting can reduce circulating levels of IGF-1 and trigger changes in hematopoietic stem cells that promote regeneration of the immune system.21Cell Stem Cell. Prolonged Fasting Reduces IGF-1/PKA to Promote Hematopoietic-Stem-Cell-Based Regeneration and Reverse Immunosuppression Research on fasting-mimicking diets in humans has shown changes in blood cell profiles consistent with immune rejuvenation, including a reversal of the age-related decline in the ratio of lymphoid to myeloid cells.22Cell Metabolism. Fasting-mimicking diet promotes healthspan and reduces signs of aging These are early-stage findings and should not be mistaken for proven clinical benefits, but they represent genuinely interesting biology that goes well beyond simple weight loss.
The Evolutionary Backdrop
The reason your body has such an elaborate set of responses to fasting, from glycogen depletion to ketone production to protein sparing to growth hormone surges, is that starvation has been one of the most persistent selection pressures on the human genome.23Mechanisms of Ageing and Development. Starvation in humans: Evolutionary background and contemporary implications Your ancestors who survived famines were the ones whose bodies could efficiently switch fuel sources, preserve critical tissues, and stay sharp enough to find food while running on empty. The hormonal cascades that kick in during a fast, including suppression of reproductive hormones and thyroid output to conserve energy, are direct reflections of that evolutionary history.24PubMed Central. Neuroendocrine adaptations to starvation
This is both reassuring and cautionary. The human body is remarkably well-equipped to handle a five-day fast, biologically speaking. But those same adaptive mechanisms, the metabolic slowdown, the efficient fat regain on refeeding, the hormonal shifts that redirect energy away from non-survival functions, are also working against you if your only goal is permanent fat loss. Your body interprets a five-day fast as a survival event and responds accordingly, prioritizing recovery and energy storage once food returns. Understanding this evolutionary context helps explain why the scale drops dramatically during the fast but the lasting results are far more modest.