How Fasting Brings You to Your Ideal Body Weight (Not Just Thin)
Extended fasting appears to normalise body composition rather than simply reduce weight. People who are significantly overweight tend to lose fat without losing proportional muscle.
The Direct Answer
Extended fasting appears to normalise body composition rather than simply reduce weight. People who are significantly overweight tend to lose fat without losing proportional muscle. People who are underweight sometimes gain healthy weight after fasting. The mechanism involves the body selectively consuming damaged and excess tissue during a fast while preserving functional lean mass.
Sinclair's Observation in 1911
In The Fasting Cure (Mitchell Kennerley, 1911), Upton Sinclair wrote:
"After a complete fast the body will come to its ideal weight. People who are very stout will not regain their weight; while people who are under weight may gain a pound or more a day for a month."
He based this on his own two 12-day fasts and the written accounts of over 100 readers who reported their results. The pattern he observed was consistent enough to become one of his central arguments: fasting is not about becoming thin, but about allowing the body to find its natural balance.
One of his case summaries: a man weighing 220 pounds with severe dropsy (fluid retention) and legs described as "like sacks of water" fasted for seven days on water alone, then followed a light diet for four weeks, and returned to farm work — chopping wood and pitching hay — at a weight described as robust and appropriate for his frame. Another case involved a woman who could barely walk when she began, fasted 10 days and then 8 more with a milk recovery diet, and was later described as having "superabundant and radiant health."
These are anecdotal cases from 1911 and should be understood as historical accounts, not clinical evidence. But the consistency of the pattern across hundreds of reported cases made Sinclair confident enough to state it as a general principle.
The Theory Behind It: What the Body Burns First
Sinclair's explanation was that the body, given complete digestive rest, goes to work consuming what it least needs: "morbid tissue" — waste accumulations, inflammatory deposits, excess fat — before it touches healthy muscle or organ tissue.
He wrote: "The body will not fast unless it has a supply of stored material to draw upon. When it is forced to live upon itself, it chooses the abnormal growths and diseased tissues first."
Modern science gives this process a name: autophagy. During extended fasting, cells begin breaking down and recycling damaged proteins, dysfunctional organelles, and excess lipid deposits. This process is particularly active in adipose (fat) tissue and in cells that have accumulated metabolic damage.
The practical outcome: fasting tends to reduce fat mass selectively while preserving lean mass (muscle, bone, organ tissue). This is why people who fast don't simply become "thin versions of themselves" — they often describe a change in body composition that a scale number alone doesn't capture.
Why Very Overweight People Often Lose Fat Without Becoming Frail
Sinclair noticed that very overweight individuals seemed to tolerate extended fasting better than expected — and to achieve results that were proportionate rather than extreme. This makes physiological sense: the body draws on fat stores for fuel, and when there is a large surplus of stored fat, the body can sustain a fast for longer before any protein catabolism (muscle breakdown) begins.
Modern studies on alternate day fasting and time-restricted eating support this. In overweight populations, fasting protocols consistently reduce fat mass while lean mass changes are minimal — typically within measurement error. The body, it seems, does have a preference for burning what it has in excess.
Why Underweight People Might Gain After Fasting
This is the more surprising half of Sinclair's observation. He described several cases of people who were thin or underweight who regained weight rapidly and healthily after fasting — including his own post-fast recovery, where he gained 32 pounds in 24 days on a milk diet.
The explanation is less about the fast itself and more about the post-fast period. After an extended fast, the gut and digestive system are essentially reset: inflammation reduced, absorption capacity restored. When eating resumes with high-quality, easily digestible food, the body can extract nutrition from meals it may have previously struggled to process efficiently.
This doesn't make fasting a weight-gain tool for underweight people — it means the combination of digestive rest and quality refeeding can allow the body to absorb nutrition better than before. Underweight people should not attempt fasting without medical guidance.
The Post-Fast Diet Makes All the Difference
Sinclair was emphatic on one point: what you eat after a fast determines whether the results hold. Return to sugar, white bread, pasta, and packaged foods and the problems in the gut resume. The weight returns. The inflammation rebuilds.
He recommended a gradual return to food beginning with fruit juice, then warm milk in small amounts, then raw fruits and nuts — avoiding starch and sugar, which he described as creating a "yeast-pot" of fermentation in the intestine.
Modern fasting practice aligns with this principle: the eating window matters as much as the fasting window. Clean protein, quality fats, vegetables, and fermented foods during the eating period preserve the benefits of the fast. Sugar and processed carbohydrates undermine them. See what to eat during intermittent fasting for a practical breakdown.
Connection to Modern Body Composition Research
Contemporary studies on intermittent fasting consistently show:
- Fat mass decreases significantly in overweight and obese populations
- Lean mass is largely preserved — often to a greater degree than with calorie restriction alone
- Metabolic rate is generally maintained (unlike prolonged severe calorie restriction, which can reduce resting energy expenditure)
- Visceral fat (the metabolically harmful fat around organs) tends to reduce markedly, often proportionally faster than subcutaneous fat
Sinclair's intuition that the body moves toward an "ideal" composition during fasting is not far from what body composition science now documents: selective fat loss with preservation of the tissues that matter most.
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Frequently Asked Questions
Does fasting cause muscle loss?
Short-duration fasting (16–24 hours) has minimal effect on muscle mass in most people. Extended fasting can cause some protein catabolism, but the body prioritises fat tissue and damaged cells first. Studies on alternate day fasting and time-restricted eating consistently show lean mass is preserved when protein intake is adequate during the eating period. See does intermittent fasting destroy muscle? for the full picture.
How long do you need to fast to reach your ideal body weight?
Sinclair was referring primarily to extended or prolonged fasting (multi-day fasts) when he made this observation. For most people practicing intermittent fasting (16:8, 5:2), the process is gradual over weeks and months rather than occurring in a single prolonged fast. Consistency over time matters far more than the length of any individual fast.
Can fasting help underweight people gain weight?
Sinclair reported cases where underweight people gained weight after fasting, once the digestive system had rested and absorption improved. Modern science doesn't specifically study this population in IF contexts. People who are significantly underweight should not attempt fasting without medical supervision.
Why does belly fat seem to be the last to go?
Visceral fat (belly fat) is strongly correlated with insulin levels. When insulin stays elevated from carbohydrate-heavy eating, the body stores fat preferentially around the abdomen. As fasting consistently brings insulin down, visceral fat reduces — but it's typically the last depot to clear. See why belly fat is the last to go on intermittent fasting for more.
How do I know if I'm losing fat or muscle?
The best indicators are non-scale: how clothes fit, energy and strength levels, and how you look in photos over time. A tape measure around the waist often tells you more than a scale. If strength and energy are maintained while your waist measurement decreases, you're almost certainly losing fat rather than lean tissue.
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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.