What Causes Intermittent Fasting Weight Gain?

Intermittent fasting (IF) is a big trend in the weight loss world. You eat during a set window and fast the rest. Simple, right? But here’s the twist – some people actually gain weight at first. Yeah, it can be frustrating.

Intermittent fasting weight gain in the first week is more common than you’d think. Often, it’s just water shifts or your body adjusting to a new eating rhythm. However, it helps to understand why this happens so you can pick the best intermittent fasting for weight loss and make it work.

How to Tell If Intermittent Fasting Is Working

Here’s the deal: IF isn’t just about dropping pounds fast. Some early signs of success aren’t even scale-related.

Energy Boost

You may feel more energized after just a few days. This is your body switching from carbs to fat for fuel.

What you may notice:

• Fewer energy crashes
• Less reliance on snacks or caffeine
• Steadier focus during the day

That fat-burning mode? It’s a good sign your metabolism is kicking in.

[7] Experts say fasting may improve brain function by helping the brain use ketones, an energy source that is made from fat.

Metabolism Upgrade

IF can improve insulin sensitivity, reduce inflammation, and help your body run more smoothly. Over time[1]:

• Hunger fades between meals
• You stop feeling “hangry” all the time
• Smaller meals feel satisfying

Appetite Control

Hormones such as ghrelin and leptin start to balance out. You feel full sooner and no longer crave sugar as much as before. Still gaining weight while intermittent fasting? Take a closer look at what (and how much) you’re eating during your feeding window [2].

Mental Clarity

Many fasters report clearer thinking and less brain fog. That’s ketones – your brain’s backup fuel – doing their thing.

If you’re not seeing changes on the scale yet, look for:

• Looser jeans
• More definition
• Less bloat

Those are wins too. If you’re dealing with intermittent fasting weight gain in the first week, don’t panic – it may just be water or inflammation, not fat [3].

Read more: Which Beginners’ Fasting Schedule Is Best for You?

How Long Before Intermittent Fasting Starts Burning Fat?

Fasting doesn’t immediately torch fat – your body burns through glucose first. On average, it takes about 12 hours to shift into fat-burning mode[8]. It’s not instant, but it’s worth the wait.

Factors that speed things up:

• A faster metabolism
• Physical activity during your fast
• Choosing the best intermittent fasting for weight loss (such as 16:8 or alternate-day fasting)

Early weight shifts can be misleading – fat loss takes time.

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Why Am I Not Losing Weight on Intermittent Fasting?

If you’re thinking, “intermittent fasting isn’t working”, you’re not alone. A few common reasons for this are:

• Overeating during your eating window
• Too little protein
• Poor food quality
• Stress or lack of sleep [4]
• Your calories are too low, slowing your metabolism

If you’re gaining weight on intermittent fasting, tweak your strategy. Try shorter windows or boost your protein intake.

Welcome to the infamous fasting plateau. It stinks, but it doesn’t mean you’re doing anything wrong.[9].

Why Am I Gaining Weight on Intermittent Fasting?

Gaining weight on intermittent fasting feels unfair, but it’s fixable. You may be:

• Eating more than you realize
• Snacking too much
• Losing muscle from not eating enough protein
• Not moving enough

Can you gain weight from not eating? Absolutely. Severe restriction tells your body to hold on to fat.

If you’ve hit an intermittent fasting weight loss plateau, switch things up:

• Adjust your eating schedule
• Watch your calories
• Add strength training

What Mistakes Lead to Weight Gain During Intermittent Fasting?

One mistake that leads to gaining weight while intermittent fasting is eating high-calorie foods during the feeding window. Some others include:

• Too little protein, which can lead to muscle loss and lower metabolism
• Fasting without staying active won’t burn as much fat
• Skipping meals without tracking calories

And here’s the weird part: sometimes, not eating enough actually causes your body to hold on to fat. Severe calorie restriction can cause your body to conserve energy by slowing down your metabolism, which can lead to fat gain over time. If you’re gaining weight while intermittent fasting, reevaluate your food quality and fasting schedule.

Why You Might Gain Weight

Can you gain weight from not eating? It’s weird but true – if your body thinks food’s scarce, it’ll go into survival mode and slam the brakes on your metabolism. This slowdown can cause weight gain over time.

It’s also common to hit a weight loss plateau [5]. When this happens, the scale stops moving, even if you’re doing everything “right”. To push through, try these strategies:

• Change your fasting schedule
• Keep an eye on calorie intake
• Remain active with regular movement or exercise

How to Adjust Intermittent Fasting if You’re Gaining Weight

If you’re gaining weight, don’t give up just yet. Here’s what to tweak before you throw in the towel:

• Track your calorie intake even during fasting.
• Reduce your feeding window – for example, from 10 hours to 8
• Incorporate exercise, particularly strength training, to preserve muscle.
• Try a different fasting method. Sometimes, switching helps you break the intermittent fasting weight loss plateau.

What’s the Best Intermittent Fasting for Weight Loss?

It depends on your lifestyle and goals. If you’re not losing weight – or worse, gaining weight – on intermittent fasting, you may be using the wrong schedule. The truth is that IF isn’t one-size-fits-all. The best method depends on your body, lifestyle, and eating habits. 

Here’s a breakdown of some of the most effective approaches to find the best intermittent fasting for weight loss.

16:8 Method – Beginner Friendly

You fast for 16 hours and eat during an 8-hour window.

• Great for: IF beginners and daily consistency.
• Tip: Eat whole, nutrient-dense meals to avoid gaining weight while intermittent fasting.
• Watch out for grazing – some people report intermittent fasting weight gain in the first week of this plan due to snacking or overeating – stick with it and adjust mindfully.

18:6 or 20:4 Method – Breaks Plateaus

These tighter eating windows prompt your body to enter deeper fat-burning modes.

• Great for: Intermediate fasters and people whose fat loss has stalled.
• Perfect: If you’re stuck in an intermittent fasting weight loss plateau.
• Tip: Stay hydrated and include protein to maintain energy and muscle.

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Alternate-Day Fasting (ADF)

• Fast (or eat <500 calories) every other day.
• Good for faster results.
• May improve insulin sensitivity.
• Can reduce inflammation [6]

It’s a powerful reset when intermittent fasting is not working, but it’s not suitable for everyone in the long term.

5:2 Diet – Flexible and Gentle

Eat normally for 5 days and restrict your calories (to around 500-600) on two non-consecutive days.

• Great for: Busy lifestyles and lower stress.
• Benefit: Easier to stick with over time.
• Tip: Choose whole foods on fasting days to stay full.

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One Meal a Day (OMAD)

You eat all your calories in a single daily meal, usually dinner.

• Great for: Simplicity and routine.
• Benefit: Strong appetite control for some.
• Warning: It may cause intermittent fasting weight gain in the first week if you’re not careful.

Why the Right Plan Matters

Choosing the right method can mean the difference between fat loss and frustration.

If you’re:

• Feeling low on energy
• Struggling with cravings
• Not seeing results
• Gaining weight while intermittent fasting

Your body may just be trying to tell you something – listen to it. 

Read more: The List Of The Best And Worst Foods To Break A Fast

When Should You Quit Intermittent Fasting Due to Weight Gain

Are you still gaining weight while intermittent fasting after a few weeks? Or just feeling miserable? It may be time to:

• Try a new method
• Eat more protein
• Cut back on snacking
• Focus on food quality

Remember that not all weight gain is fat – it could be muscle if you’re lifting weights.

Frequently Asked Questions

• Can too much protein cause weight gain?

Yes, it can. While protein supports muscle growth and satiety, consuming more than your body needs can still lead to too many calories, which may result in weight gain while intermittent fasting – particularly if you’re not tracking your intake. Stick to moderate portions and prioritize lean protein sources.

• Does fasting for longer prevent weight gain?

Not always. Fasting longer may boost fat burning, but it can also backfire. Extended fasts may trigger overeating during feeding windows or slow your metabolism over time. If you’re gaining weight on intermittent fasting, more hours of fasting isn’t always the fix – the quality of food and total calories still matter.

• Why do I gain weight after stopping intermittent fasting?

Once you stop fasting, your body may regain weight if you return to frequent eating or higher-calorie meals. This is common, particularly if you don’t replace fasting with another structured plan. Intermittent fasting can lead to weight gain in the first week of the plan, which can be attributed to water, glycogen, or true fat, depending on your diet and activity levels after the fast.

• Do I really have to track every bite I eat?

It sounds annoying, but it’s surprisingly eye-opening. While IF often reduces appetite naturally, tracking calories can be essential – particularly if you’ve hit an intermittent fasting weight loss plateau or are gaining weight from not eating enough. A simple food journal or app helps you stay in a sustainable calorie range without overthinking every bite.

The Bottom Line

Don’t give up just because intermittent fasting isn’t working today. Adjust your plan. Focus on how you feel – not just the scale. Whether you’re gaining weight on intermittent fasting or hitting an intermittent fasting weight loss plateau, your body may just need a little more time, or a new approach. You’ve got this!