Intermittent fasting is a pretty common weight loss strategy, but sometimes, it seems to have the opposite effect. Can intermittent fasting actually cause weight gain? 

It’s certainly possible to follow an intermittent fasting schedule and find yourself gaining weight … which can be distressing, to say the least! What gives?! 

Here’s the challenge: there are some potential intermittent fasting side effects that can throw you off. Plus, your body may need to get used to intermittent fasting before you see results. 

It’s like breaking in new shoes: they may not be comfy at first, but once they’ve adjusted to your feet, boom! No more growing pains. 

So, if you’ve been wondering, “Why am I gaining weight while on an intermittent fasting schedule?!” let’s explore what can get off track and identify how to fix it. 

Is it possible to gain weight while intermittent fasting?

If you’re human, it’s always possible to gain weight — our bodies have that ability built in (and for a good reason). But it can feel surprising that we can gain weight while intermittent fasting.

We’re putting in the effort, right? So, when the weight not only doesn’t come off but actually goes on, it feels completely backward. How can we be working this hard and still be moving in the wrong direction?!?!

It’s easy to jump from there to the idea that it’s intermittent fasting itself that causes weight gain.

But it’s not fasting’s fault. 

Unsurprisingly, it’s the usual suspects that lead to gaining weight on intermittent fasting. Let’s unpack a few of those to see if you can find the sneaky little culprit that’s getting in your way. 

8 reasons why you’re gaining weight on intermittent fasting

If you’ve gained weight on intermittent fasting, chances are one (or more) of these eight reasons is to blame.

1. You’re eating too much

Let’s be real, fasting can make you pretty hungry, and sometimes you might satisfy that hunger a leetle bit too well. You might not realize how calorie-dense some of your go-to daily foods are. 

Both of these can cause you to eat more calories than you burn in a day. Regardless of whether you’re following an intermittent fasting schedule or not, that will cause weight gain. 

Remember that this is a lifestyle, not a diet. So, consider how to make wise choices and eat mindfully within your eating window.

2. Your daily calories are too low

You’ll often hear that not eating enough calories can cause weight gain. Which is sort of true … but also completely false. 

You can’t eat too little — aka less than you burn — and not lose weight. It’s biologically impossible. What you can do is cut your calories so low that it kicks you into four unhelpful potential situations:

1. Rebound eating. Your calories are so low that, a few days in, you’re ravenous, your cravings are giant, insatiable monsters, and it’s open season on every high-fat, high-sugar food you’re desperate for. You can’t help but overeat.
2. Low energy. If your calories are super low, you’re probably just about covering your energy needs to get through the day. There’s no extra in the tank for being active. It seems counterintuitive, but eating enough to move your body regularly is key to eating less than you burn.
3. Consistency challenges. It’s tough to stick to lowered calories over the long term, but it’s what you do over the long term that leads to results. 
4. Metabolic adaptations. If calories stay too low for too long, your body adapts to needing fewer calories to fuel it,[1] which can make gaining weight much easier to do. 

3. You’re not eating enough protein

Chances are you’ve heard a million times that protein is important for losing weight.[2] Don’t tune this out — it’s as critical as it’s cracked up to be.

In real terms, not eating enough protein means: 

• Not feeling full throughout the day. You have killer cravings plus off-the-chart hunger and can’t help but grab unwise snacks because all you can think about is food.  
• Losing muscle when you’re losing weight. This makes you less able to keep that weight off — and more likely to gain more — because a more-muscled body burns more calories per activity than a less-muscled one. 

4. You’re eating the wrong foods

Intermittent fasting isn’t magic. It’s simply a way of structuring your food intake that takes advantage of a couple of helpful biological facts. 

1. Ketosis

Going without eating for a time (12+ hours) can stimulate ketosis and allow you to burn your fat stores for energy.[3] 

2. Eating less

By having a set period where you don’t eat, you can: 

• potentially curb some unhelpful eating habits, like snacking through the evening when you’re not hungry and/or
• eat less without having to track or monitor your food.

But here’s the thing. Both of these rely on you eating foods that satisfy your appetite, give you energy, and keep your cravings in check. 

If you’re choosing foods that trigger cravings, are easy to overeat, and leave you hungry and lethargic, it’s going to be much harder to eat the amount of food you need to lose weight.

If you need help improving your food choices, take our Simple quiz and get hooked up with our smart, supportive app. When you track your food in the app, we make suggestions on how to healthify your meals! 

5. You’re drinking too much caffeine

If you’re a coffee fan, one of the nicest things about intermittent fasting is the first coffee of the day. So what if you gotta wait for breakfast? Who cares? You have COFFEE! 

Coffee brings energy to the party, which is helpful in the short term, but it’s not a source of real energy. It’s more of a temporary holdover. Real energy — the kind that powers you all day to feed the kids, walk the dog, argue with your boss, etc. — comes from food.

Sometimes, we use coffee as a crutch when trying to lose weight, but this strategy backfires easily. Overdoing caffeine can interfere with your sleep quality and jack up your stress hormones,[4] making eating too much an incredibly easy next step. 

6. You’re not active 

Exercise not only burns calories but also allows your body to better use the food you eat. For instance, active people build more muscle and store less fat from what they eat than sedentary people. 

Think of being active as a multiplier of success — it makes your health-promoting food choices work harder for your body and well-being.

7. You’re eating too many carbs

Eating too many carbohydrate-based foods like bread and pasta can increase your salt intake overall. That, in turn, can influence fluid retention, and when you’re retaining fluid, your weight increases. 

8. Non-diet-related weight gain

Finally, let’s call out the non-diet-related factors that can impact the number on the scale. 

Hormonal changes, fluid intake, muscle-glycogen retention after exercising, muscle gain — all these can make you heavier on the scale. Yet none of them necessarily mean you’ve gained fat (which tends to be the kind of weight we’re trying to lose).

To understand more fully how this might be influencing your results, read more about the difference between fat loss vs. weight loss here.

10 tips for success when intermittent fasting to lose weight

Let’s flip from looking at the problems to finding solutions. Here are 10 tips to help you succeed with intermittent fasting for weight loss.

1. Eat fewer calories than you burn … but not so few that you end up rebounding into overeating. Try our calorie calculator to get a feel for what your “just right” is. 
2. Eat more nutrient-dense whole foods. Find the ones that energize you, give you plenty of vitamins and minerals — and that you like. Things like fruits, vegetables, and whole grains (including quinoa) are a great choice. You’re gonna have to eat for the rest of your life, so you might as well enjoy the foods you’re choosing!
3. Eat plenty of protein-rich foods. Prioritize lean sources, like chicken, fish, eggs, and tofu, and find a few trusty recipes that are easy to make and tasty to eat. 
4. Eat healthy fats. Ai for about a thumb’s worth with every meal. Examples include olive oil, unsalted nuts, avocado, and margarine-based spreads. You’ll feel more satisfied, you’ll have more energy, and things will taste better.
5. Eat fiber-rich foods. Get your 5-a-day in by eating more fruits and vegetables.[5] Your gut will thank you for it. And you’ll feel fuller and more satisfied with fewer calories.

Dive into how to crush your protein, healthy fat, and fiber needs (also featured: whole grains, calcium-rich foods, and fruits!) with our guide on what to eat during intermittent fasting.

6. Drink lots of water. When you’re fasting, you need to drink more to stay hydrated. Bonus points: water actually has a positive impact on losing weight.[6] If plain water bores you silly, check out this guide on what you can drink while fasting for ways to zhuzh up your hydration.
7. Get moving. It doesn’t have to be formal exercise — you never need to set foot in a gym if that’s not your bag. Move your body in a way that makes you feel good. Our guide to intermittent fasting and working out will give you some ideas.
8. Learn to process and release stress. Stress is a key trigger for eating — just as strong (sometimes stronger) in many cases as hunger.[7] Having ways to soothe stress that don’t involve food is a massive piece of the weight loss puzzle. 
9. Sleep well. When you’re underslept, your appetite is harder to control (thanks, hormones!).[8] If you often miss getting your 7–9 hours (or however long makes you feel rested), improving your sleep routine is well worth putting some energy into. 
10. Get some backup. Talk to your doctor, work with a registered dietitian, and get a good buddy on your side. Everything worth doing (especially hard things) is easier, more fun, and less scary with supportive company.

Support is on hand via our Simple quiz. Tell us a little about yourself and what you want to achieve, and we’ll get you on the right path!

When do you start losing weight on intermittent fasting?

Weight loss on intermittent fasting varies from person to person and will depend on lots of factors, like:

• calorie intake
• food choices
• activity levels
• body weight

Losing weight is a process that takes time. You may get some quick wins, and a few pounds may drop off right away, or it may take your body a little longer to show progress. 

Give it 2–4 weeks. If nothing has changed by then — no weight loss, no inch loss, no noticeable changes in photos or how your clothes fit — check in with your actions. Look at the list above, read this article on what to do if intermittent fasting is not working, and see where you can adjust. 

We know having patience isn’t always easy! Hang in there and stay consistent with what you need to do. The results will come. 

How much weight can you lose while fasting?

One study shows it’s possible to lose up to 10 lbs over three months with fasting,[9] but results will vary from person to person. Just like the time it takes to lose weight, the amount of weight you can lose will depend on many things, like:

• your starting weight
• your body composition
• your age and sex
• any medications you’re on
• how much energy you’re ready, willing, and able to put into the process
• how active you can be

To keep it safe and sustainable, shoot for 1–2 lbs a week.[10] Adjust your actions as needed based on your results. And always listen to your body and look after your overall physical and mental health as you go.

What is the best intermittent fasting schedule for weight loss?

Losing weight is undoubtedly one of the benefits of intermittent fasting.[11] If that’s your goal, which schedule gives the best results?

The short answer:

The best schedule is the one that you can do sustainably.

What you can do over a lengthy period of time will yield the best, most long-lasting results. For instance, if you try 18:6 but find it so restrictive you quit three weeks in, then it’s not the best schedule for you. 

With that said, let’s look at your options. 

Simple-approved fasting schedules

Time-restricted eating (TRE) is the intermittent fasting schedule we recommend here at Simple. Within this group, you’ll find: 

• intermittent fasting 12:12
• intermittent fasting 14:10
• intermittent fasting 16:8 
• intermittent fasting 18:6 

TRE also includes intermittent fasting on a 20:4 split, but we suggest you skip that option. The short eating window makes it tough to get all the nutrients you need. Plus, we don’t recommend fasts lasting 18+ hours since there isn’t really any evidence to suggest that they provide any extra benefits.

If we had to choose which of these was the “best” for weight loss, we’d say studies show that 16:8 and 18:6 can both yield promising results.[9] 

Where 12:12 and 14:10 shine is helping you safely practice either 16:8 or 18:6. The longer fasts are pretty hard to jump straight into, so practicing shorter fasts till you’re comfortable and feeling proficient is a good way to go! And remember: it’s always a good idea to consult your healthcare team if you’re changing up your eating habits, but it’s especially important if you’re planning to fast for 18+ hours.  

Other fasting schedules 

Science tells us that both the 5:2 diet and alternate-day fasting (ADF) can be beneficial for weight loss. Our stance on these is this: 

Try 5:2 and ADF only if your doctor gives them the stamp of approval.

Then there are schedules like water fasting, OMAD (one meal a day), and Eat Stop Eat. None of these options give superior results compared to TRE schedules like 18:6 or 16:8, but they do increase the risk to your health. 

Always check with your doctor if you plan on trying fasts longer than 18 hours.