Single-Day Weight & Fat Gain Per 20 Different Studies

Everyone knows that if you eat more calories than you burn, you gain weight. And the more calories you eat, the more weight you gain.

That begs the question, though: Can you gain weight in a day?

And if so, how much weight can you gain in a day?

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

In other words, if you really “let yourself go,” how much fallout can you expect?

Get evidence-based answers to these questions and more in this article. 

Table of Contents

• Can You Gain Weight in a Day?
• How Much Weight Can You Gain in a Day?
• Not All Weight Gain Is Fat Gain
• Carbohydrate Intake and Body Weight
• What If You Eat A Lot More than Normal?
• How Many Calories to Gain a Pound of Fat?
• How Much Weight Can You Gain in a Day: FAQs
• FAQ #1: Can you gain 5 pounds in a day?
• FAQ #2: Why did I gain 4 lbs overnight?
• FAQ #3: Why am I gaining weight on 800 calories a day?

Can You Gain Weight in a Day?

Yes, you can gain weight in a day, but it’s usually not because you’re gaining significant amounts of body fat (more on this soon). To gain body fat, you must consistently overeat.

In other words, a single day of overeating may cause a slight uptick on the scale, but it won’t translate to noticeable fat gain unless overeating becomes a recurring pattern.

How Much Weight Can You Gain in a Day?

Many people wonder how much weight can you gain in a day.

And the answer is: not as much as you might think.

For evidence of this, we can look to a study conducted by scientists at the University of Colorado. 

To simulate a bout of overeating, the researchers fed 16 men 50% more calories than they needed to maintain their weight every day (about 1,400 extra calories per day). 

After two weeks, they gained 3 pounds of fat, which is 1.5 pounds of fat per week or 0.2 pounds per day.

This study wasn’t a perfect representation of how people usually binge (overeating during the holidays, for example). Most of us might overeat for a day or two but not several weeks, as in this study, but we can still use the data to ballpark how much fat we might gain when overeating for a day or two.

In this case, they gained ⅕ of a pound of fat when they maintained a 1,400 calorie surplus.

Another longer study conducted by scientists at the Pennington Biomedical Research Center bolster these results.

In this study, the researchers had 29 overweight men eat 40% above their TDEE (40% more calories than they needed to maintain their weight every day), which works out to a calorie surplus of 1,200-to-1,500 calories per day.

After 8 weeks, they’d gained 9 pounds of fat, or about 1.1 pounds per week or 0.16 pounds per day.

Despite snarfing high-calorie foods on the daily, once again they only gained about ⅕ of a pound of fat per day.

Finally, a study conducted by scientists at Loughborough University looked at the effects of overeating high-fat foods for a single day.

The researchers had 15 healthy, normal weight, physically active (exercising at least 30 minutes three days per week) men and women eat 78% above their TDEE. This worked out to 6,000 calories per day compared to their normal calorie intake of 3,350 calories per day.

Their diet was designed to be extremely high fat, providing 68% of total calories from fat. 

The researchers didn’t measure the participants body fat percentage, but they did record their weight before and after the all-day binge.

The result? 

On average, the participants gained 1.76 pounds. 

That’s considerably more than the other two studies, but much of the weight they gained wasn’t body fat. And even if all of the weight these people gained were fat, two pounds of weight gain isn’t catastrophic considering how much these people ate. 

Not All Weight Gain Is Fat Gain

At this point you may be wondering why the results from these studies don’t line up with your experiences.

“Sure,” you might think, “studies show people don’t gain that much fat when they overeat, but why do I always gain 5-to-10 pounds after the holidays?”

The answer more or less boils down to four things: your sodium, carbohydrate, and water intake, and the weight of your stool.

Consuming large amounts of sodium, carbohydrate, and water causes a disproportionate increase in body weight despite not significantly increasing body fat.

And when most people eat a lot of food, they inevitably eat a lot more sodium and carbohydrate than normal. 

Sodium and carbohydrate don’t cause much weight gain on their own. Instead, they increase your body weight by increasing your whole-body water stores. 

Sodium brings water into cells, which is why eating large amounts 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 increase your whole-body water stores enough to add several pounds to your body weight. This might increase your scale weight for several days before your body disposes of the excess sodium and water.

Carbohydrate Intake and Body Weight

Carbohydrate can have a similar effect on your body weight as sodium.

Your body stores carbohydrate in the muscles and liver in the form of glycogen. It also stores every gram of glycogen with 3-to-4 grams of water, which means if you consume 400 grams of carbs, that could bring along 1200-to-1600 grams (~3-to-4 pounds) of water into your muscles and liver.

Taken together, the increase in sodium, carbohydrate, and water storage could bump your weight up by 5-to-10 pounds or more overnight. This can also give you a bloated, puffy appearance (which also goes away). 

The good news is your body will excrete most of this extra sodium and water, and your carbohydrate stores will gradually return to normal, but until this occurs you may think you’ve gained several pounds of fat if you go by your scale weight alone. 

Finally, another reason your body weight will skyrocket after a day or two of overeating is an increase in your stool weight. 

Until your body has digested and excreted all of the extra food mass from your feasting, you’ll likely be carrying around several additional pounds of food in your digestive tract, which further bumps up your body weight. This tends to go away after dropping the kids off at the pool a few times.

You see these effects in studies, too.

In the first study you learned about a moment ago, the participants gained 7 pounds of scale weight after 2 weeks of overeating, and in the second study, they gained 17 pounds after 8 weeks of overeating. In both cases, only about half of this increase in body weight was actual body fat (the rest being water, carbs, and stool).

The bottom line is if you eat 1,000-to-1,500 calories more than you need to maintain your weight in a single day, you’ll probably only gain ⅕-to-¼ of a pound of fat, even if your scale weight tells a different story.

Let’s say you really throw caution to the wind and eat 2,000-to-3,000 calories more than you need to maintain your weight (fairly common on Thanksgiving).

The damage? 

Maybe half a pound of fat gain.

The one big caveat here is that this assumes you’re only overeating for a day or two. It’s continuous overeating that leads to weight gain, obesity, diabetes, and the many other health complications associated with overeating.

What If You Eat A Lot More than Normal?

Most of 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’ll the consequences be?

It’s impossible to say exactly how much you’ll gain for reasons you’ll learn in a moment, but let’s try to puzzle this out with napkin math.

Let’s say you need 3,000 calories to maintain your weight every day.

On the day of your binge, you eat 2,000 calories from your normal meals, and 6,000 calories from your pizza, milkshake, and Twizzlers.

2,000 + 6,000 = 8,000 total calories eaten.

8,000 calories eaten – 3,000 calories burned = a 5,000 calorie surplus.

In the studies you learned about earlier in this article, 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—5,000 calories more than you need to maintain your weight—you could expect to gain about a pound of fat.

You can get rid of that with about a week of proper dieting.

(And if you’d like specific advice about how many calories, how much of each macronutrient, and which foods you should eat to lose fat quickly, take the Legion Diet Quiz.)

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 body weight during the holidays. 

Talk with people who’ve partaken in binges like this, and you’ll quickly realize they often don’t gain as much fat as you’d expect. After a few days of eating normally and allowing their extra water, sodium, 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. 

How Many Calories to Gain a Pound of Fat?

Given that a pound of body fat contains approximately 3,500 calories worth of energy, many people believe that to gain a pound of fat, you must eat 3,500 calories.

However, this idea oversimplifies how weight gain works. Factors like your basal metabolic rate (BMR), body composition, activity level, and initial body fat level all play a role.

Another contributing factor is the source of your calories.

For instance, your body tends to store calories from fat more easily as body fat compared to calories from proteins or carbs. So, eating 3,500 calories from fat is more likely to result in a pound of fat gain than the same number of calories from protein.

Thus, the “3,500-calorie rule” is a general guide, but you shouldn’t apply it to every person in every situation. The actual calories needed to gain a pound of fat can differ from person to person.

How Much Weight Can You Gain in a Day: FAQs

FAQ #1: Can you gain 5 pounds in a day?

You could gain 5 pounds in a day, but you probably couldn’t gain 5 pounds of fat in a day. Most sudden weight increases come from water retention, increased sodium and carbohydrate intake, and the physical weight of the food you’ve eaten, not from fat.

FAQ #2: Why did I gain 4 lbs overnight?

Gaining 4 lbs overnight is likely due to water retention from eating high-sodium and high-carb foods, not actual fat gain. To learn more about water retention, check out this article:

What Causes Water Retention and How to Get Rid Of It

FAQ #3: Why am I gaining weight on 800 calories a day?

Gaining weight on 800 calories a day is almost impossible for adults. You’re probably facing another issue. Check out this article to learn some potential solutions:

Stopped Losing Weight? Here’s Why (and How to Fix It)