Regaining weight after a fast can be frustrating, but it’s normal. We’re evolved to oscillate between fasting and feeding—to lose weight and gain it back again.
When we fast, we trigger metabolic machinery that gets us burning fat and making ketones. We upregulate key factors involved in DNA repair, immune health, and a cellular recycling program called autophagy.
When we feed, we rebuild. We synthesize muscle, rebuild cell parts, and generally grow.
Refeeding will trigger weight gain after a fast. This is inevitable. The human body burns energy at rest through thermoregulation, respiration, and other processes.
It keeps burning energy during a fast, leading to a loss of mass. Some of that mass comes back along with the refeed, and we WANT this. To the degree that fasting appears to benefit organisms, the refeed process appears to be as important as the time spent fasting.
Fasting also increases water loss, so a lot of fasting-driven weight loss is actually just that—water loss.
My main point is this: When you break your fast, you’ll regain some weight—water or otherwise—and that’s a good thing. That being said, there are certainly ways to minimize fat gain in the process. That’s one big reason folks fast in the first place: body recomposition.
I’ll share some practical tips soon. First, let’s establish why weight loss is stimulated during a fast.
Why Do You Lose Weight During a Fast?
To understand why you gain weight after a fast, we need to explore why you lose weight during a fast.
The first reason is intuitive. During a fast—especially an extended fast—you’re in a state of negative energy balance.
In other words, your metabolic rate exceeds your energy intake. Less energy (calories) is coming in than is going out.
Your body must get that energy from somewhere, so it pulls it from stored glucose (glycogen), muscle, and body fat. All of these stored energy sources have mass.
But in most cases, rapid weight loss during a fast isn’t due to fat loss or muscle loss. Rather, it’s due to water loss.
There are two interrelated reasons why fasting accelerates water loss:
- Fasting lowers insulin
- Fasting depletes glycogen
Let’s take these one at a time.
#1: Fasting lowers insulin
The most important roles of insulin revolve around growth and energy partitioning, but this crucial hormone also affects fluid balance. When insulin is low, your kidneys excrete more fluids and electrolytes through urine.
Relevant here: nothing keeps insulin low like eating nothing. Insulin also stays low on a low-carb diet, which is why Keto folks have increased fluid and electrolyte needs.
#2: Fasting depletes glycogen
The average human carries 400 to 500 grams of stored glucose in muscle and liver tissue. This stored sugar, called glycogen, is there in case of periods of deprivation.
When you fast, your body taps into this stored energy. This process, called glycogenolysis, releases a deluge of water, which you subsequently pee out.
Why You Regain Weight After Fasting
If you fast for 12, 14, or 16 hours, your weight rebound probably won’t be super substantial. These intermittent fasts aren’t long enough to cause significant water loss. (Though if you consistently fast for 16-18 hours, it may add up).
Longer fasts of 24 hours or more will provoke greater water loss and therefore greater weight rebound. In this article, I’m talking mostly about fasts of this length.
When you break a fast, your body restocks your liver and muscles with glycogen. Since glycogen is mostly water weight, that can easily add a few quick pounds of water.
This weight rebound is normal and healthy, but there’s at least one trick to minimize it. I’ll talk about it soon.
You also regain non-water weight after a fast. The food has to go somewhere, after all.
So how can you prevent your refeed from becoming body fat?
How To Minimize Fat Gain After Fasting
To minimize fat gain after a fast, you’ll want to minimize these three “i” words:
First, insulin. When you fast, insulin levels plummet to near-unmeasurable levels. This allows you to break off stored body fat (via lipolysis) and burn that fat for energy.
How do you keep insulin low after you fast? Simple: keep carbs low. I hate this cliche, but keeping carbs low helps your body remain a “fat-burning machine”. That’s the “bro-science” answer, anyway.
The more accurate explanation is that consistent blood glucose and insulin levels seem to make it easier to not overeat. Yes, the calories in/out store is more complex than most give it credit, but at the end of the day, calorie intake matters.
You also want to limit inflammation. Some inflammation is normal after you refeed, but too much is harmful, especially on the gut. And gut inflammation creates conditions conducive to weight gain, mainly through mechanisms which up-regulate appetite, causing us to overeat (again, intake).
Fasting suppresses inflammation. Specifically, it suppresses the NLRP3 inflammasome—an inflammatory complex that, when overactivated, is linked to chronic diseases like Alzheimer’s, diabetes, and heart disease.
Feeding triggers inflammation. In one study, healthy adults showed significantly higher levels of NLRP3 inflammasome activation three hours after breaking a 24 hour fast.
To be clear, you don’t want to shut inflammation off completely, but you also don’t want it getting out of control. Unnecessary inflammation is linked to a plethora of health problems, including obesity.
The strategies in the next section will help you limit unnecessary inflammation, fat gain, and weight gain generally after a fast.
8 Ways To Minimize Weight Gain After Fasting
The focus of this section is on minimizing inflammation and fat gain after a fast. But there are also a couple of tricks to curb water rebound. I’ll cover those first.
#1: Take sodium
An old study just came across my desk. It was from 1968, which is bound to get a few eye rolls.
But c’mon people, they still did science in the sixties. Heck, Einstein discovered relativity almost sixty years before the sixties, back in 1905.
Back to the text though. In the study, researchers fasted 13 obese patients for 10 days. Seven of the patients consumed water only, while the other six consumed water with about two grams of a salt supplement.
Both groups lost significant weight, mostly due to water loss. The researchers also noted that urinary excretion of sodium (called natriuresis) kicked up BIG TIME during the fast.
After the fast, both groups regained weight upon refeeding. But here’s the interesting part: the salt-supplemented group regained significantly less weight. (Only about 39% of the initial amount lost.)
Regardless, this is another reason to eat salt (about 2.5 teaspoons daily) during a fast. More importantly, getting enough sodium can help prevent the headaches, insomnia, and low energy that many people experience while fasting. These symptoms are often driven by sodium deficiency.
#2: Drink enough fluids
The low-insulin state of fasting has a diuretic effect. You lose more fluids.
If you don't replace these fluids, your weight will decrease, but it will only be water weight. And you may suffer the headaches, fatigue, and muscle cramps of dehydration.
Let’s shift to preventing inflammation now.
#3: Limit carbs
If you want to minimize insulin, inflammation, and fat storage after a fast, you’ll want to minimize carbs.
We already covered the insulin part. Eating carbs stimulates more insulin release than fat or protein. It shuts down fat burning.
Eating too many carbs after a fast may also increase inflammation. In mice, for instance, a high-carb refeed after a 48 hour fast caused significantly more inflammation than a low-carb refeed.
Why did this happen? Because high glucose levels (driven by carbohydrate intake) activate toll-like receptors—basically signaling beacons for inflammation.
Fructose (fruit sugar) also promotes fat storage, but for different reasons. The bottom line is that consuming too much of or the wrong type of carbohydrate tends to promote inflammation, overeating, or fat storage. Minimizing fruit, sugar, and starch is a good way to minimize fat gain after a fast.
#4: Avoid problem foods
If you’re sensitive to certain foods, avoid these foods when you break your fast. This will help minimize gut inflammation.
Common problem foods include eggs, soy, nuts, dairy, all grains including gluten, and alcohol. Raw foods also tend to be problematic because they’re harder to digest. Save the salad for another day.
#5: Limit saturated fat
I’m not against saturated fat. From what I’ve seen, there’s no convincing evidence that dietary saturated fat causes heart disease. Plus, I love me a good ribeye.
But compared to a diet high in monounsaturated fat, a diet high in saturated fat has been shown to increase intestinal absorption of a nasty toxin called lipopolysaccharide (LPS). And with LPS comes inflammation.
Because of this, I think it makes sense to limit saturated fat for your first meal back. Lean towards monounsaturated fats like olive oil instead. (Fun fact: a compound in olive oil called oleanolic acid has been shown to decrease gut permeability caused by LPS).
#6: Eat lean sources of protein
Protein is the most important nutrient to consume after a fast. It’s essential for rebuilding and repairing all the tissues and molecules that have been “broken down” and “reset” while fasting.
Chicken, fish, and leaner cuts of beef are your friends here. If possible, consume a fish (like cod, salmon, or trout) with omega-3 fatty acids. Omega-3s appear to suppress the inflammatory harms of LPS.
#7: Drink green tea
Drinking green tea is a quick way to prevent inflammation while refeeding. Rats fed green tea before (or after) a fast were protected from the gut-damaging effects of LPS.
#8: Don’t gorge yourself
Even if you do everything else right, overeating after a fast can derail you.
When the clock finally hits mealtime, it’s tempting to play the role of the starving jackal who’s just located a fresh buffalo carcass. Believe me, I know. But if you behave like the jackal, the consequences may include:
- Food on your shirt
- Increased inflammation
- Nagging feelings of guilt
- Increased weight gain
- Concerned looks from dining companions
The smart thing to do is plan a normal-sized meal. A plate of chicken or fish with low-carb vegetables soaked in olive oil is a good template to play with.
Combine this with proper hydration (sodium plus fluids) and you’ll be well-positioned to benefit from your fasting feeding cycle.
And don’t worry if you regain some of the weight you lost during a fast. That’s what’s supposed to happen.