Why Am I Gaining Weight While Intermittent Fasting? 5 Reasons

Why Am I Gaining Weight While Intermittent Fasting?

In the first few days, gaining weight on intermittent fasting is almost always water, not fat: refilling your glycogen after a fast pulls 3–4 grams of water in with every gram of carbohydrate. If the gain persists for more than two or three weeks, the cause is different — the eating window has quietly grown, or the meals inside it have.

You complete your fast, break it with a meal, and within 24–48 hours the scale shows you're heavier than when you started. If this has happened to you, you're not alone — and you're not doing something wrong.

Rapid weight gain after a fast is one of the most predictable and well-documented phenomena in fasting. It was documented in meticulous personal detail over a century ago, and modern physiology explains exactly why it happens.

Historical Context: What Upton Sinclair Observed

In his 1911 book The Fasting Cure, journalist and social reformer Upton Sinclair described his experience breaking a twelve-day fast by transitioning to a milk diet. The result startled him: he gained 4.5 pounds in a single day and 32 pounds total over 24 days of refeeding.

Rather than panicking, Sinclair studied the pattern and reached a clear-eyed conclusion:

"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."

Sinclair noticed something important: not everyone regained weight at the same rate. Overweight fasters regained far less than underweight or depleted fasters — because the body was not simply restoring fat. It was restoring what it needed.

His second major twelve-day fast confirmed the pattern. He lost nine pounds in eight days, then spent a week eating oranges and figs and regained most of it quickly. He wasn't troubled by this because he understood the mechanics.

Why the Weight Returns So Fast: The Modern Explanation

1. Glycogen Replenishment

During a fast, your body depletes its glycogen reserves — the stored form of glucose held in your liver (roughly 100–120 grams) and muscles (roughly 300–500 grams depending on body size).

Here is the key fact: for every gram of glycogen your body stores, it binds approximately 3–4 grams of water alongside it.

When you resume eating — especially anything containing carbohydrates or protein — your body immediately begins restoring glycogen. If you replenish 400 grams of glycogen, you store roughly 1.2–1.6 kilograms of water at the same time. This can appear on the scale as 1.5–2 kg within 24–48 hours. It is entirely water, not fat.

2. Water Retention from Rising Insulin

Insulin — which rises when you eat after a fast — signals the kidneys to retain more sodium. Sodium retention pulls water into your cells and tissues. After extended fasting where insulin was very low, this effect is amplified when eating resumes.

Again: this is water, not fat.

3. Return of Gut Contents

A fast empties your digestive tract. When you eat again, food mass, digestive fluids, enzymes, and intestinal contents all occupy space and add weight. After several days of fasting, your digestive system is essentially empty. Returning food adds 0.5–2 kilograms of mass to your gut alone, depending on meal size.

4. Inflammation Response to Refeeding

Reintroducing food after extended fasting causes a mild, transient inflammatory response in the gut lining. This involves local fluid shifts that can show up as puffiness or temporary water retention around the abdomen.

How Much Is Actually Fat?

After a 24–72 hour fast, you may have burned 150–400 grams of actual body fat (roughly 1,350–3,600 calories of fat, depending on your size and activity level). The glycogen and water repletion that follows can easily exceed this on the scale within a day.

This does not mean the fast failed. It means you're measuring the wrong thing in the wrong timeframe.

Weight on a scale reflects water, food mass, and body tissue all mixed together. Fat loss is a weeks-to-months process. Post-fast scale weight in the first 48 hours is almost entirely glycogen and water dynamics.

Regained weight vs. regained fat — what the scale is actually showing

The Ideal Weight Principle: Sinclair's Most Useful Observation

Sinclair's observation that overfat people do not regain their excess weight after fasting — while depleted people restore weight quickly — maps well onto modern understanding.

Your liver preferentially refills glycogen from protein and carbohydrates consumed during refeeding. But your adipose tissue (body fat) only expands when you're in sustained calorie surplus. Breaking a fast with a reasonable meal does not create the sustained surplus needed to rebuild fat stores.

What the scale shows in the first 48 hours is almost never fat. What the scale shows over four to eight weeks is a meaningful signal.

What Sinclair Got Right — and the Science Behind It

Sinclair's observation that "the body comes to its ideal weight" after fasting aligns with what researchers now call body weight set point theory and metabolic reset. Extended fasting reduces the body's defended weight range — particularly in people who are overweight — making it easier to maintain a lower weight afterward.

His milk-diet refeeding protocol also intuitively avoided the mistake most people make: loading the gut with hard-to-digest food immediately after a fast. Liquid, slowly-introduced nutrition minimises the inflammatory refeeding response and the digestive shock of sudden large meals.

What This Means Practically

If you're fasting for weight loss, measure results over weeks, not hours.

The post-fast rebound you see in the first 48 hours is water mechanics. The fat loss that happened during your fast is real — but it won't show cleanly on the scale until the water dynamics settle.

How to reduce the scale rebound after fasting:

Break your fast with protein and fat first — eggs, meat, fish, avocado — rather than carbohydrates. Protein and fat replenish the body without triggering a massive glycogen restoration. The scale rebound is noticeably smaller when the first meal is low in starches and sugars.

How long to wait before weighing yourself:

After a 24–72 hour fast, wait at least 3–5 days before using the scale as a meaningful data point. After a longer extended fast (5+ days), wait 7–10 days for a fair reading.

Don't overcorrect by under-eating in the days after a fast. Restricting aggressively right after a fast stresses the body, depletes electrolytes, and makes the experience miserable. A normal, clean eating window restores glycogen at a pace that is physiologically healthy.

Still Gaining After Three Weeks? Then It Is Not Water

Everything above explains the scale going up in the first few days. If you have been fasting daily for three weeks or more and the trend is still upward, glycogen and water have long since settled and something else is happening. In practice it is almost always one of five things, in this order of likelihood.

1. The eating window has crept wider. Most people start at 16:8 and drift. A first coffee with milk at 10am and a last handful of nuts at 10pm is a 12-hour window wearing a 16:8 label. Write down the exact time of your first and last calorie for three days — the number is usually a surprise.

2. The meals inside the window got bigger to compensate. Fasting suppresses appetite for most people, but not all. If you break an 18-hour fast genuinely ravenous and eat past comfortable every time, the daily total can exceed what you ate before you started. Fasting controls when, not how much; it only produces a deficit if the window does not fully repay the skipped meal.

3. You are eating more of the foods that hold water. Salt and refined carbohydrates both increase water retention independently of fat. A week of restaurant food can add 1–2 kg on the scale with no change in body fat at all.

4. You are gaining muscle while losing fat. If you started resistance training around the same time you started fasting — very common — the scale can stay flat or rise while your waist shrinks. This is the only reason on this list that is good news, and the tape measure will tell you within a month.

5. The weigh-ins are not comparable. Weighing after a meal, after a salty dinner, at a different time of day, or on carpet rather than a hard floor all move the number by more than a week of real fat loss. Weigh yourself first thing in the morning, after the toilet, before drinking, on the same hard surface, and only compare weekly averages.

If none of these fit and the gain has continued for two months on a genuinely stable window, that is worth raising with a doctor — thyroid problems and some medications cause weight gain that no eating pattern will fix.

For the Complete Guide

Track every fast automatically — get FastSyncer on the App Store — your first week is free.

Frequently Asked Questions

Why did I gain 2 kg overnight after breaking my fast?

Glycogen replenishment and water retention. Each gram of glycogen stored binds 3–4 grams of water. A meal containing moderate carbohydrates after a fast can trigger 1–2 kg of water weight appearing on the scale overnight. This is normal and temporary.

Is the weight I gain after a fast real fat?

Almost certainly not — at least not in the first 2–5 days. Rapid fat gain requires a sustained calorie surplus maintained over days to weeks. A single meal or even a day of eating after a fast does not create enough surplus to produce meaningful fat gain.

How long until post-fast weight stabilises?

Typically 3–7 days after a short fast (24–72 hours), and 7–14 days after an extended fast (5–10 days). The longer the fast, the more depleted your glycogen stores are, and the longer repletion takes.

Upton Sinclair gained 32 pounds after his twelve-day fast — will I do that too?

No — Sinclair's extreme refeeding used a high-calorie milk diet specifically designed to rebuild a depleted body after twelve days of complete fasting. A daily 16–18 hour intermittent fast produces only partial glycogen depletion. The rebound after a standard overnight fast is measured in grams, not pounds.

Should I avoid carbs after breaking a fast to prevent weight gain?

If minimising the scale rebound matters to you, starting your first meal with protein and fat (not carbohydrates) does produce a smaller spike. But this is an aesthetic consideration. Eating balanced, clean food after a fast is entirely appropriate — the water weight that follows is temporary and not harmful.

Related Articles

• How to break a fast the right way
• What should your first meal be after fasting?
• How much weight can you lose with intermittent fasting?

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.

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