How Much Weight Do You Lose During a Fast? Breaking Down the Numbers

One of the most common questions people ask before starting a fast is simple: how much weight will I actually lose? The answer is more nuanced than most people expect — and the accounts from Upton Sinclair's 1911 book The Fasting Cure offer a surprisingly honest picture of what happens to body weight during extended fasting, one that modern science has largely confirmed more than a century later.

The Short Answer

In the first 2–4 days of a fast, weight loss is rapid — often 1–2 kg per day — but most of this is water and glycogen, not fat. After day 4–5, the rate slows to roughly 200–400g per day of genuine tissue loss. The total weight lost across a 6–10 day fast averages 4–7 kg, with significant individual variation based on starting weight, hydration, and metabolic state.

What Sinclair's Own Fasts Revealed

Upton Sinclair documented his personal fasting experiences in remarkable detail. During his first 12-day fast, he lost 15 pounds (approximately 6.8 kg) in the first four days alone — a rate he later acknowledged indicated his tissue was in a "very poor state" from years of suboptimal eating and chronic illness.

After that initial rush, he lost only about 2 more pounds over the remaining 8 days of the same fast. Total: roughly 17 pounds in 12 days, but with a wildly uneven distribution between early and later stages.

His second 12-day fast told a different story: having cleaned up his diet and fasted before, he lost only 9 pounds over 8 days — a much lower rate, consistent with a healthier starting state and depleted glycogen stores that were already used to periodic restriction.

These two accounts, written in 1911 without the benefit of modern metabolic science, capture something researchers now understand clearly: the rate of weight loss during fasting is not constant, and the dramatic early loss is predominantly water.

Breaking Down the Numbers

Days 1–4: The Water Phase

Your body stores glucose as glycogen in muscles and the liver — approximately 400–500g of glycogen, which is bound to roughly three to four times its weight in water. When you fast, glycogen depletes within the first 24–48 hours, releasing this water and creating dramatic scale weight reduction.

This is why people regularly report losing 3–5 kg in the first few days of a fast or a low-carbohydrate diet. Sinclair's 15-pound drop in four days fits exactly this pattern — amplified by a large initial glycogen store and the poor tissue hydration that comes with years of inflammatory eating.

Days 5 Onwards: Genuine Fat Loss

After glycogen is depleted and water loss slows, the body shifts primarily to burning stored fat. At this stage, the weight loss rate drops considerably — to roughly 200–400g per day for most adults fasting at rest.

This is not disappointing; it reflects genuine fat metabolism. A pound of fat contains approximately 3,500 kcal. An average adult burning 1,500–2,000 kcal per day while fasting loses approximately 200–400g of actual fat daily — precisely what Sinclair's case records show in the later stages of fasts, and what modern indirect calorimetry studies confirm.

What About Muscle Loss?

Sinclair was not aware of muscle protein breakdown as a mechanism, but this is a genuine concern during extended fasting. Modern research suggests the body preferentially breaks down fat before protein — particularly in the early weeks of fasting — but prolonged fasts (beyond 5–7 days) do begin to draw on lean tissue as a fuel source.

For daily intermittent fasting (16:8 or OMAD), muscle loss is not a significant concern in people with adequate protein intake. For extended multi-day fasts, the risk increases with duration.

The Body Burns Damaged Tissue First

One of Sinclair's more striking observations — and one that modern autophagy research has partially validated — is that the body selectively metabolises abnormal and diseased tissue before healthy tissue during fasting. He described tumours, waste matter, and "morbid tissue" being consumed in preference to functional tissue.

Modern research on autophagy (the cellular self-cleaning process activated during fasting) confirms a version of this: damaged cells, misfolded proteins, and dysfunctional cellular components are preferentially broken down and recycled during the fasting state. While this does not translate to dramatic tumour dissolution in humans, it does suggest the body has a sophisticated prioritisation system for what it breaks down first.

The 277 Cases: What Real Fasters Lost

Sinclair collected accounts from 109 people reporting on 277 fasting episodes. The average fast length across these cases was 6 days. While he did not publish precise weight loss tables, the cases describe typical losses of 6–12 pounds (3–5 kg) across a 5–7 day fast — consistent with modern estimates combining initial water loss with subsequent fat mobilisation.

Importantly, half the cases where permanent results were not achieved were attributed to breaking the fast incorrectly and returning to poor eating habits — not to any failure of fasting itself.

The Rebound: What Happens After You Break the Fast

Here is the part that surprises most people. Weight returns rapidly after breaking a fast — and according to Sinclair, this is entirely normal and expected.

After his first 12-day fast, Sinclair began a milk diet and gained 4.5 pounds on the very first day. Over 24 days on the milk diet, he regained 32 pounds — far more than he had lost during the fast itself.

This is not failure. When you fast, your glycogen stores are depleted and your body holds far less water. The moment you begin eating — even very lightly — glycogen and water rush back in simultaneously. A 3–5 kg gain in the first 48 hours of eating after a multi-day fast is normal and does not represent fat regain.

Modern research on post-fast refeeding confirms this same pattern: rapid weight restoration in the first days reflects glycogen and fluid repletion, not fat accumulation. The genuine fat loss achieved during the fast is preserved.

What This Means for Daily Intermittent Fasting

The dramatic weight fluctuations that characterise extended multi-day fasts are not relevant to daily intermittent fasting (16:8 or OMAD). With a daily fasting window:

• Glycogen is partially depleted each night but replenished during the eating window
• Water weight fluctuates by 0.5–2 kg day to day (normal)
• Fat loss is gradual and consistent — typically 0.5–1 kg per week when combined with appropriate food choices

The scale will move more slowly with daily intermittent fasting than with a 5-day extended fast, but the fat loss is real, sustainable, and far easier to maintain long-term.

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Frequently Asked Questions

How much weight do you lose in a 3-day fast?

Most people lose between 2–4 kg in a 3-day fast, but the majority of this is water weight from glycogen depletion. True fat loss over 3 days is roughly 400–600g at most.

Why did I lose so much weight on the first day of fasting?

The rapid weight loss in the first 24–48 hours of a fast is almost entirely water and glycogen. When your body exhausts its glucose stores, it releases the water bound to those stores — typically 2–3 kg very quickly. This is not fat loss.

Will I regain all the weight I lost after a fast?

Some weight returns quickly after a fast because glycogen stores refill and water returns with them. This is normal and does not represent fat regain. If you eat sensibly after breaking a fast, you will retain the genuine fat loss achieved during fasting.

How much fat do you actually burn during a fast?

A healthy adult burns approximately 200–400g of fat per day during fasting, after the initial 2–4 day glycogen depletion phase. Over a 7-day fast, total fat loss is roughly 1.5–2.5 kg. The rest of the scale change is water.

Why do very overweight people seem to lose more during a fast?

Heavier individuals have larger glycogen stores, more water bound to tissue, and greater daily energy requirements — all of which translate to larger initial weight losses during fasting. Sinclair noted this pattern repeatedly in his cases, and modern research confirms it.

Related Articles

• What happens in the first 3 kilos of intermittent fasting weight loss
• How much weight can you lose with intermittent fasting?
• Extended fasting (5+ days): what to expect and how to prepare

This article draws on historical research from 1911 and is for informational purposes only — not medical advice. Always consult a qualified healthcare professional before making any dietary changes.

Sinclair, U. (1911). The Fasting Cure. Mitchell Kennerley.