4 ways to minimize weight gain after a fast

It’s no mystery why fasting promotes weight loss. When folks eat less frequently, they usually consume fewer calories overall — fewer calories in, more pounds lost. 

But many people are surprised and frustrated when they regain weight after a fast, especially after longer fasts that shave off several pounds. This article will explain this phenomenon and give strategies for minimizing post-fasting weight gain.

For some important context up front: Weight fluctuation is inevitable — and normal — as humans rotate between fasting and feeding. We burn energy and lose mass during a fast. We gain some or all of it back when we eat. You can see this on a smaller scale in your day-to-day — you’ll weigh more in the evening (after eating, drinking, etc.) than in the morning. When you fast for longer periods of time than just overnight, you’ll see larger fluctuations.

Much of this seesaw is water weight fluctuation, due to the body naturally retaining and excreting water due to factors like sodium, fluid, and food intake. These fluctuations, even during a fast, are normal and healthy. But what many folks seek is sustainable fat loss from fasting, not just those temporary fluctuations. The first three strategies I share in this article aim to support this goal, while the fourth aims to reduce water weight fluctuations. 

Here are the four tips we’ll talk through:

• Don’t overeat
• Limit carbs
• Eat anti-inflammatory foods
• Get enough fluids and electrolytes

To begin, let’s first dig deeper into why fasting works for weight loss.

Why Do You Lose Weight During a Fast?

You lose weight during a fast for two reasons — consuming fewer calories (which leads to long-term weight loss), and losing water weight (temporary weight loss). Let’s explore each reason. As you’ll see, they overlap. 

#1: Consuming fewer calories

To state what might be obvious, your body burns more calories than it consumes while fasting. Fewer calories in + the same calories out = weight lost. 

It makes sense. When you eat more food than you need, you store the excess in your muscles (as easily accessible sugar, called glycogen) and fat cells (as fat). When you skip meals and don’t eat enough to support your body’s day-to-day processes, your body reaches for those energy stores in muscle and fat to keep it going.

Each of these stored calorie sources has weight to it. When you’re fasting long enough for your body to need to draw from these energy sources, you burn this energy and lose the associated weight (particularly losing fat and its associated weight in longer fasts). 

But what about weight shifts of several pounds in the early stages of fasting? That’s typically not fat loss, but water fluctuations. 

#2: Losing water weight

Water fluctuations during a fast typically happen for two main reasons. One, folks tend to consume less water on a fast, since food can contain over 50% water molecules. Two, fasting also makes your body excrete more fluidOpens in a new tab. Less water coming in, more going out — that’s a recipe for losing water weight. 

Let’s explore the water going out part. We already mentioned glycogen, which is how our bodies store sugar/carbohydrates for later use. The average human carries ~400 to 500 grams of glycogen in muscle and liver cells, and each gram of glycogen is bound to ~3 grams of water. When you fast, your body breaks down glycogen to access its quick energy, releasing the bound waterOpens in a new tab which you subsequently excrete. 

Now, I also need to mention insulin and its role in water retention. Insulin is best known as a growth and blood sugar regulation hormoneOpens in a new tab. When you consume food, insulin levels rise to store that energy. When you fast, insulin levels fall, and you burn more body fat. Relevant here: low insulin levels also tell your kidneys to excrete more fluid. 

To sum up, while long or repeated fasting can lead to weight (fat) loss, in the initial stages you also lose water weight from glycogen and because you’re excreting more fluids. Now it makes more sense why some of the weight comes back. 

Why You May Regain Weight After Fasting

The main reason folks regain weight after breaking a fast is that water weight returns. 

With food back on the menu, insulin levels rise to help store the energy you’re consuming, and glycogen reforms in organs like your muscles and liver (a good thing to provide quick energy when you need it, especially during exercise). Both moves increase fluid retention.

Fasts of a day or more can provoke significant water loss and subsequent weight rebound. I’ll share a 2022 studyOpens in a new tab to illustrate. On the sixth day of a 10-day fast, 13 healthy men lost an average of 11.4% of their total body water. For a 200-pound man, that’s about 14 pounds of water weight. And guess what? That lost water weight all came back by day five after breaking the fast. 

Body fat showed a different trend, though. The men’s average body fat percentage declined during the fast and continued to decline when they resumed normal eating. In other words, they lost several pounds of body fat during the fast and then kept losing more. It appeared the fasting intervention flipped a “metabolic switch” that prompted their bodies to continue burning more fat in general. 

Still, most of us aren’t doing 10-day fasts (and shouldn’t, without support from a healthcare provider). But let’s say you want to minimize water regain after shorter periods of fasting. Or more importantly if your goal is weight loss, you want to lose fat and keep it off. Keep reading. 

4 Ways To Minimize Weight Gain After Fasting

Want to prevent weight regain after fasting? Three tips to support that goal and long-term fat loss are: Don’t overeat, limit carbs, and consume anti-inflammatory foods. And a fourth tip to help stabilize water weight fluctuations is to hydrate properly. Let’s explore the strategies and science behind these. 

#1: Don’t overeat

To lose fat long-term on a fasting program, you’ll want to overall consume slightly fewer calories than your body burns on a day-to-day basis.

This may seem obvious, but understand it’s possible to fast and still consume more calories than you expend. For example, let’s say your body needs ~2500 calories a day to function (you can estimate your daily energy burn with a calculator like this oneOpens in a new tab). If you fast for 16 hours of the day, you’ll likely still gain weight if you eat over 2500 calories during the 8 hours you allow yourself to eat. You’re consuming more calories than you’re burning. Keep in mind, though, that most folks naturally eat less on fasting protocols, so calorie counting isn’t usually necessary.

While you’re less likely to overdo calories on longer fasts, the opposite can also be the case: undernutrition. I see this frequently in lean biohacker types who lose significant muscle on aggressive fasting protocols. A 2023 review found that during water-only fasts of 5–20 days, ⅔ of the weight loss was muscleOpens in a new tab, while just ⅓ was fat. To reduce this risk and make weight loss more sustainable, you can consider shorter fasts and prioritize protein. Read more about tips for fasting hereOpens in a new tab. 

An intuitive approach to prevent over- or under-eating outside of your fasting window is to eat to fullness. This’ll do the job for most folks.

If you continue to struggle with overeating, though, working with a trusted health practitioner can be impactful, as can getting more granular by tracking your food intake. Whether you take the intuitive approach or track data closely, the important part is to listen to your body, adjust accordingly, and remember that weight loss doesn’t all need to happen in one go. A sustainable approach to fat loss may take at least a couple months.

#2: Limit carbs

Limiting carbs prevents weight gain after a fast in several ways. A low-carb diet can:

1. Reduce overall calorie intakeOpens in a new tab by increasing feelings of fullness (protein and fat are more filling nutrients than carbs.
2. Reduce fat storage by limiting the insulin response. 
3. Reduce water weight fluctuations by limiting the insulin response (insulin will stay lower and more stable in fasting and feeding periods).

You can stay very-low carb by eating foods like eggs, lean protein, and cooked greens. You can also tinker with a low-ish carb approach (50–150 grams of carbs daily) as detailed in my book Wired To Eat. Start low and gradually increase your carbs if this approach doesn’t feel right for your body.  

#3: Eat anti-inflammatory foods

Eating foods that limit inflammation may also reduce fat and water regain after a fast.

Inflammation is an immune response. It helps protect us from germs, heal wounds, and even process food. The problem occurs when inflammation becomes chronic. Chronic inflammation is a broken immune response no longer protecting or healing us (excess inflammatory cells cause damage!).

Chronic inflammation is linked to many modern diseases, including obesity. The connectionOpens in a new tab between the two isn’t entirely clear, but folks with chronic inflammation tend to have excess body fat and vice versa. Researchers believe that inflammation may drive problems with insulin, leading to increased fat storage. 

Inflammation can also trigger edemaOpens in a new tab — excess fluid accumulation in your body’s tissues. Ever feel like the Michelin man with a bad cold or flu? That’s edema. This effect may also occur when folks eat an inflammatory, modern diet; processed, high-sugar diets are linked to chronic inflammationOpens in a new tab. 

Yet any type of eating will trigger some inflammation. When we eat, we don’t just consume nutrients but also bacteria, so immune cells arrive to protect usOpens in a new tab. Post-feeding inflammation also helps us process glucose from eating carbs. This low inflammation after eating is not a bad thing! It’s the chronic inflammation and excess fluid accumulation that folks want to watch out for.

So, fasting suppresses inflammation, and feeding increases itOpens in a new tab. But we can minimize excess inflammation by eating healthy foods after a fast. Besides avoiding ultra-processed, sugary, inflammatory foods like cookies and Cheetos, here are three tips for a low-inflammation diet. 

1. Avoid trigger foods. Don’t break your fast with foods that you know cause you digestive woes like stomach pain, bloating, or gas — signs of gut inflammation. Common problem foods are eggs, soy, nuts, dairy, grains, and alcohol, but everyone’s body is different and you may be able to tolerate foods that others can’t. Cooking foods (such as steaming broccoli vs eating it raw) can also help digestion. 
2. Limit saturated fat. High intakes of saturated fat can increase lipopolysaccharideOpens in a new tab, an inflammatory marker in the gut. Saturated fats are common in processed foods, so eating healthy and prioritizing low-saturated fat options like olive oil and avocados can help support this goal.
3. Eat lean protein. Lean protein is also low in saturated fat. Plus, prioritizing protein after a fast can help you rebuild any muscle and organ tissue broken down during the fast, and stay full longer so you’re less tempted by sugary foods.

Lastly, let’s talk hydration.