Why Do Some People Gain Weight When Starting Intermittent Fasting?

You start intermittent fasting with high expectations, follow the rules, and then step on the scale to find it has actually gone up. It's one of the most discouraging experiences for new fasters — and one of the most misunderstood. The good news is that in most cases, this is completely explainable, temporary, and often has nothing to do with actual fat gain.

The Short Answer

Weight gain at the start of intermittent fasting is almost never real fat gain. It is almost always water retention, glycogen fluctuation, or the direct result of eating the wrong foods — especially sugar and refined carbohydrates — during the eating window. Understanding which of these is happening is the first step to fixing it.

Reason 1: Your Body Is Still Running on Sugar

The single most common reason people struggle with weight gain at the start of fasting is that they haven't changed what they eat — only when they eat. If your eating window is still full of bread, pasta, rice, sauces, and sugary drinks, your insulin levels stay elevated every time you eat. High insulin tells your body to store fat, not burn it. No matter how long your fasting window is, if your eating window is full of carbohydrates, the fat-burning signal never arrives.

Intermittent fasting works by lowering insulin over time, allowing the body to access stored fat for energy. If you're eating foods that spike insulin every meal, you're working against the process. The fix is to clean up the food before worrying about the clock. Prioritise protein, healthy fats, and vegetables — and remove sugar, grains, and processed foods from the eating window.

Reason 2: Water Weight and Glycogen Fluctuation

When you first reduce your eating window, your body's glycogen stores — glucose stored in the liver and muscles — begin to deplete. Each gram of glycogen is stored with roughly three grams of water. When glycogen drops, water is released. But the reverse is also true: if you eat a high-carbohydrate meal after a fast, your body rapidly refills glycogen stores and pulls that water back in. This can mean a 1–3 kg swing on the scale within 24–48 hours, with no fat involved whatsoever.

In the first week or two of fasting, the scale can move in both directions depending entirely on what you ate the day before. This isn't meaningful for fat loss tracking. Body measurements and how your clothes fit are far more reliable signals early on.

Reason 3: Eating More Than You Realise During the Eating Window

Some people unconsciously eat more when they compress their eating window because the perception of "I've been fasting all day, I deserve a big meal" leads to overconsumption. If you were previously eating three moderate meals and you now eat one or two very large meals loaded with carbohydrates and ultra-processed food, your total calorie intake can actually increase despite fasting.

Fasting is not a licence to eat anything in unlimited quantities during the eating window. The quality and quantity of food still matters. Focus on meals that are built around fat and protein — these are naturally satiating and much harder to overeat than high-carbohydrate meals.

Reason 4: Cortisol-Driven Water Retention

Fasting is a mild physiological stress — and in the early days, especially if the approach is too aggressive, the body may respond with a temporary cortisol increase. Cortisol causes the body to retain water and sodium. For women in particular, this can add 1–2 kg of apparent weight within a few days of starting, with nothing to do with fat.

This is why beginners are advised to build up the fasting window gradually — from 12 hours to 14 to 16 — rather than jumping straight to 20 hours or one meal a day. A gradual approach reduces cortisol stress, allows the body to adapt, and produces steadier results.

Reason 5: The First 10 Days Are the Hardest

The early days of fasting are a transition period. The body has been dependent on a constant supply of glucose from regular meals, and it takes time to shift to fat-burning as the primary energy source. During this adjustment period — roughly the first 7–10 days — hunger, cravings, and erratic energy levels are normal. Some people respond to this discomfort by overeating when their eating window opens, which can temporarily push the scale up.

After the first 10 days, hunger tends to settle significantly. Once the body becomes fat-adapted — meaning it can draw on stored fat efficiently for fuel — the fasting window starts to feel natural and the scale begins moving in the right direction.

What the First 3 Kilos Actually Look Like

The first 3 kilograms of weight lost during intermittent fasting are often a mix of actual fat and stored water. When glycogen depletes properly, you can lose 3–4 kg of water weight in the first week alone. This is one reason results in the first week can look dramatic — but it also means early fluctuations, both up and down, are largely water, not fat.

Real fat loss begins once the body has cleared the glycogen stores and insulin levels have stabilised. This typically starts to show consistently from weeks 2–4 onward, depending on food quality and fasting duration.

How to Stop the Scale Going Up

If you're seeing unexpected weight gain at the start of intermittent fasting, work through this checklist:

• Check your food quality first. Are there still sugars, grains, sauces, or processed foods in your eating window? These are the most likely culprit.
• Avoid overeating when the window opens. Start your meal slowly — a small salad or vegetables — then eat your main food. Don't rush.
• Don't jump straight to extreme fasting. Start at 12–14 hours and build up over a few weeks. Aggressive starts often trigger cortisol and water retention.
• Track your measurements, not just the scale. Waist, hips, and how clothes fit are better short-term indicators than the scale.
• Give it at least 2–3 weeks before drawing conclusions. The body needs time to shift from sugar-burning to fat-burning. Early fluctuations are noise.

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Frequently Asked Questions

Is it normal for the scale to go up in the first week of fasting? Yes, very common. Glycogen and water fluctuations can cause 1–3 kg swings in either direction in the first week, regardless of actual fat gain or loss.

How long until intermittent fasting starts reducing weight? Most people see consistent downward movement from week 2–4 onward, once the body adapts and food quality is addressed. The first week is often dominated by water fluctuations.

Can eating too little during the eating window cause weight gain? Indirectly — if you eat too little, cortisol rises, which can cause water retention and signal the body to conserve fat. Eating enough fat and protein during the window matters.

Should I change my eating window if I'm gaining weight? Not necessarily — check your food quality first. The timing of fasting matters less than what you eat during the eating window.

Does everyone gain weight at the start of intermittent fasting? No. Some people lose weight in the first week, particularly those switching away from a high-carbohydrate diet. Those who stay on high-carb foods are more likely to see initial fluctuations.

Related Articles

• What happens to your body during intermittent fasting?
• What are the first 3 kilos of intermittent fasting weight loss?
• Why did I stop losing weight on intermittent fasting?

This article is for informational purposes only and is not medical advice.