If you want to know how much fat you can gain in a single day of bingeing, then you want to read this article.

Key Takeaways

  1. Eating significantly more calories than you need to maintain your weight will always cause some fat gain, no matter what foods you eat or how “fast” your metabolism is.
  2. How much fat you gain, though, will depend on how much you eat, whether those calories come from protein, carbs, or fat, and how your body responds to overeating.
  3. The best way to minimize fat gain while bingeing is to keep your fat intake low and get most of your calories from carbs and protein.

Here’s an involable truth of nutrition science:

If you eat more calories than you burn, you’ll gain weight.

And the more calories you eat, the more weight you’ll gain, and the more of that weight is going to be fat.

That begs the question, though.

Is there a limit to how much fat you can gain in, say, a single day?

Can 24 hours of uninhibited gluttony really lead to several pounds of fat gain, as some claim?

Well, that’s what you’re going to learn in this article.

The truth is that you can gain a large amount of fat from a single day of bingeing, but it’s probably less than you think.

By the end of this article, you’ll know how your body stores fat, how much fat you can really gain in a single day, and how to minimize fat gain if you have your heart set on plundering the kitchen.

Let’s jump right in.

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How Much Fat Can You Gain in a Day?

Here’s the short answer: not as much as you might think.

Now for the long answer.

Let’s look at a study published by researchers at the University of Colorado. They fed 16 people 50% more energy than they needed every day for two weeks (about 1,400 extra calories per day).

After two weeks they gained 3 pounds of fat. That works out to 1.5 pounds of fat per week or 0.2 pounds per day.

This is similar to what researchers found in another study where people ate 40% more calories than their maintenance needs for 8 weeks. In this case, they gained 9 pounds of fat. That works out to 1.1 pounds per week or 0.16 pounds per day.

These people were in a 1,200 to 1,500 calorie surplus every day. In the end, they only gained about a fifth of a pound of fat per day.

So, why does the scale always seem to jump up by five to ten pounds after you go a little off the rails?

The answer more or less boils down to three things: sodium, carbohydrate, and water.

When most people eat a lot of food, they inevitably eat a lot more salt and carbohydrate than normal.

Sodium brings water into cells, which is why eating large amounts of it can increase your total body water stores. Most people would call this “bloating” or “water retention” and you’ve probably noticed it after eating a large, salty “cheat meal,” like pizza, burritos, or fries.

A single high-sodium meal could add several pounds of water. This might stay reflected on the scale for several days before your body excretes the extra sodium and water.

Carbohydrate can have a similar effect.

Carbohydrate is mostly stored in the muscles and liver as glycogen. Every gram of glycogen is stored with 3 to 4 grams of water, which means that if you consume 400 grams of carbs, that could bring along 1200 to 1600 grams (~3 to 4 pounds) of water.

Taken together, the increase in sodium, carbohydrate, and water storage could bump your weight up by five to ten pounds overnight.

Obviously, that will come back down, but it can make you think you’ve gained more fat than you really have.

The same thing happened in both of the studies we just covered. After 2 weeks of overeating the subjects gained 7 pounds of scale weight, and after 8 weeks of overeating they gained 17 pounds. In both cases, though, only about half of that was body fat.

The total amount of “bulk” in your stool can also increase your body weight by a pound or two, but that tends to correct itself after a few days.

The bottom line is that if you eat 1,000 to 1,500 calories more than your maintenance needs in a single day, you may only gain around a fifth of a pound of body fat, even if your scale weight tells a different story.

If you’d like to learn how to use a calorie surplus to build muscle instead of gaining fat, then check out this article:

How to Gain Weight (and Muscle) as Fast as Possible

What If You Eat A Lot More than Normal?

The studies we’ve looked at so far involved people “bingeing” on 1,000 to 1,500 more calories than they needed per day.

But what if you’re the kind of person who likes to really “turn it loose” when you go off the rails?

You know, instead of eating a large bowl of oatmeal and a bar of chocolate (~1,000 calories), you decide to demolish an entire pizza, a milkshake, and a 16-ounce pack of Twizzlers (6,000 calories).

What kind of fallout can you expect?

It’s impossible to say exactly how much more you’ll gain for reasons we’ll get to in a moment, but let’s take a shot at it.

Let’s say you need 3,000 calories to maintain your weight every day, and you got 2,000 calories from your regular meals in addition to the binge.

That puts your daily surplus at 5,000 calories per day.

In the studies we discussed earlier, people gained around 0.2 pounds of fat per 1,000 calories they ate above their maintenance needs. If you ate five times that amount, you could expect to gain about a pound of fat.

In the big scheme of things, that really isn’t that much body fat, and you should be able to get rid of it with a week of proper dieting.

If we look at real world examples of extreme hedonism, like Thanksgiving, Christmas, and New Years, then we have even less reason to worry. On average, people only gain around one pound of weight during the holidays.

Talk with people who’ve tried this and you’ll quickly realize that they often don’t gain as much as you’d expect. After a few days of eating normally and allowing their extra water and glycogen stores to decrease, their weight settles back to where it was before the calorie bonanza.

In other words, calorie intake and fat gain don’t go up in lockstep.

Why, might you ask?

Let’s find out.

Why Don’t You Get (Super) Fat When You Binge?

why dont you get fat when you binge

It’s easy to find examples of people who’ve eaten ungodly amounts of food without getting fat.

Case in point, google “10,000 calorie challenge.” You’ll find countless examples of people who’ve eaten enough food to put a plowhorse into a coma, yet after a few days they’re back to their pre-gorge weight.

In both of the studies we looked at earlier, the people should have gained about 0.75 pounds per day. Instead, they only gained about a fifth of a pound per day.

Are these people defying the principles of energy balance? Are they just blessed with good genes?

Not exactly.

Overeating will always cause some level of fat gain, but several changes take place in your body that reduce the number of calories that are stored as fat.

Namely . . .  

  1. You burn more calories to digest more food.
  2. You store some macronutrients as fat more easily than others.
  3. You (may) move more when you eat more.
  4. You don’t absorb every calorie you eat.

Let’s look at each in turn.

You burn more calories to digest more food.

When you eat a meal, your body has to expend energy to digest and process the food. This is known as the thermic effect of food (TEF).

Research shows that it accounts for approximately 10% of total daily energy expenditure. That is, about one in ten calories is spent simply digesting your meals.

In this way, your metabolism does increase when you eat more food.

How much your metabolism increases or decreases depends on the size and composition of the meal.

Smaller meals require less energy to digest, so they cause a smaller increase in energy expenditure.

Large meals, on the other hand, require far more energy to digest, so they cause a much greater rise in energy expenditure.

Some macronutrients also raise TEF more than others.

Protein has the highest TEF of around 30%.

Carbohydrate has a TEF of 5 to 10%.

Fat has a TEF of 0 to 3%.

What this means is that meals higher in protein and carbohydrate are going to cause a greater increase in TEF than meals high in fat.

Thus, let’s say you overeat 1,000 calories. Right off the bat, you’re losing about 100 of those calories to the thermic effect of food. If the meal is high in carbs and protein, it might be closer to 150 calories.

You store more or less of some macronutrients than others.

How many calories you store as fat also depends on what percentage of those calories are coming from protein, carbohydrate, and fat.

This is because not all macronutrients are processed the same way by the body.

Protein is used almost entirely for repairing, building, and regenerating cells, hormones, and other molecules in the body, and the excess is burned off as fuel in the liver. It’s almost never directly converted into body fat.

Carbohydrate is mostly stored in muscles as glycogen. Despite what many people believe, carbs are rarely converted into body fat.

Fat is chemically very similar to the kind of molecules stored in body fat cells, and thus your body prefers to store dietary fat as body fat and burn carbs instead.

Eating too many carbs can still make you gain fat, though, but not in the same way as overeating fat.

Chemically speaking, carbs are very different from the kind of molecules stored as body fat (triglycerides), and they have to undergo an energy-intensive process known as de novo lipogenesis (DNL) to be converted into body fat.

The thing is, DNL only ramps up enough to make a difference in your body fat levels after several days of gorging yourself on carbs. We’re talking 700 to 1,000+ grams per day for most people.

Even then, about 15 to 25% of the energy in carbohydrate is lost during the process of converting it into fat, so 100 calories’ worth of carbs might only turn into 75 or 85 calories’ worth of body fat.

Overeating carbs can increase fat storage in another way, though. As your body burns more and more carbs for energy, it burns proportionally less of the fat you eat, and thus more dietary fat is stored as body fat.

What this means is that the more carbs you eat, the more fat you’ll store and the less you burn throughout the day.

And in case you’re wondering, alcohol is burned immediately because the body has no way to store it, so it’s never directly stored as fat. It does, however, shut down fat burning in a similar manner to carbs, which means that while alcohol is in your system you’re going to store almost all of the fat you eat.

You (may) move more when you eat more.

will one day of overeating ruin my diet

There’s some truth to the idea that some people have naturally “fast” metabolisms, and can eat more than others without gaining as much weight.

The reason, though, has more to do with how their activity levels change when they overeat versus some innate fat burning ability.

Research shows that some people will spontaneously and subconsciously move more throughout the day when they overeat, and that this can add up to about a 700-calorie increase in energy expenditure.

That is, if people are fed 1,000 calories more per day than they need, some will burn off about three quarters of that just by fidgeting, tapping their legs, and moving around more throughout the day.

These activities are known as non-exercise activity thermogenesis (NEAT).

One thing you should know about NEAT, though, is that it varies a great deal from person to person.

When people eat more calories than they need, some might actually burn fewer calories and others might burn almost 1,000 calories more per day. Women also tend to have a smaller rise in NEAT than men when overeating.

That’s why I say you may move more when you eat more—not everyone responds the same way.

That said, the average increase in energy expenditure works out to about 300 calories per day, which is enough to make a significant dent in your binge.

You don’t absorb every calorie you eat.

As you may have noticed at one point or another, not every calorie you eat is entirely digested.

Some foods, particularly ones high in fiber like grains, nuts, seeds, and vegetables often make their way through the digestive system without being completely broken down.

What’s more, these high-fiber foods can also interfere with the absorption of fat, meaning more gets excreted in your poop.

So, does this mean that many of those extra cheat day calories just go in one end and out the other as long as you eat plenty of fiber?

Nope.

The body is very good at extracting calories out of the foods you eat, and the vast majority of the calories you eat are absorbed.

If you consume a titanic amount of food with a large amount of fiber, then you may excrete a few more calories of fat in your poo, but don’t count on it making any difference in your physique.

What if You Binge on Carbs and Protein?

Now, you may be thinking, “Can’t I just overeat carbs and keep my fat intake at zero, and gain no body fat?”

Researchers tested this idea in one of the studies we looked at earlier.

They not only fed 16 people 50% more calories than they needed for two weeks, but they split the groups in half and gave each group different diets.

One group got their extra calories from fat and the other group got them from carbohydrate. After two weeks, the group following the high-fat diet gained 2.7 pounds of body fat, and the group following the high-carb diet gained 2.4 pounds of body fat.

And if you only look at the obese people in the study, both kinds of diets caused the same amount of fat storage.

In other words, it doesn’t seem to matter–you’ll gain the same amount of fat from carbs and dietary fat.

That said, this study was over several weeks, and the people in the high-carb group were still eating about 30% of their calories from fat, which means virtually all of that would be stored as body fat throughout the study.

What happens if you keep your fat intake low and binge on carbs for a day or two?

Can you expect much, if any fat gain?

Probably not.

Researchers at the University of Lausanne tested this idea by giving one group of people a normal “balanced” diet that included 300 grams of carbs, 90 grams of protein, and 90 grams of fat. The other group got a whopping 800 grams of carbs and the same amount of protein and fat. 

In this case, the researchers didn’t just measure total body fat gain. They took things one step further and measured how much of the extra carbohydrate was converted into body fat.

What did they find?

After four days of eating like this, only about 13% of the extra carbs were being converted into fat in the high-carb group.

The researchers didn’t calculate exactly how much body fat gain this would translate into, but it’s likely less than a quarter pound of body fat per day.

So, if you’re only overeating for a day, you can significantly minimize your chances of gaining body fat by keeping your fat intake low and primarily overeating carbohydrate and protein.

Not only that, but periodically increasing your carbohydrate intake can have several benefits for your performance, body composition, and mood. Check out this article to learn more:

The Science of Carb Cycling: How It Works and How to Do It Right

The Bottom Line on How Much Fat You Can Gain in a Day

Bingeing will always cause some level of fat gain.

How much, though, depends on what you eat, how much you eat, and how your body responds to overeating.

In the final analysis, here’s what we can say:

  1. It’s impossible to tell exactly how much fat you’ll gain when overeating, but a good rule of thumb might be a fifth of a pound of body fat per 1,000 calories above your maintenance needs.
  2. In a worst case scenario, where you eat thousands of extra calories along with a large amount of fat, you can expect to gain maybe a pound or two of fat in a single day.
  3. If you keep your fat intake low to moderate, and mostly overeat high-carb and high-protein foods, then you can expect to gain closer to a quarter pound of fat at most.

The bottom line is that you don’t get fat from a single day of overeating. You get fat from continuously eating far more calories than your body needs.

The best way to avoid that, then, is to eat plenty of whole, nutritious, minimally processed foods that you enjoy.

Check out this article to learn how:

How to Get the Body You Want With Flexible Dieting

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