The Weight You Gain In One Day Or One Week Isn’t Fat

Let me ask you a question.

Did you gain a bunch of weight within a short period of time?

Specifically, did you gain a few pounds (e.g. 1lb, 2lbs, 3lbs, 4lbs, 5lbs, or even as much 10lbs) either overnight, over the span of a few days, or over the span of a week?

And, if so, do you think this weight gain is fat?

Cool, because I’m here to tell you it’s definitely NOT fat.

Here’s Why You Think You Gained Fat

Let me guess what happened.

You had a bad day of eating, or a bad few days, or even a bad week where you either ate “bad foods” you weren’t supposed to eat, ate more calories than you were supposed to eat, or a combination of both.

Or maybe you even allowed yourself to relax a bit with your diet for some short period of time because it was a holiday, birthday, vacation, or something like that.

Whichever it may be, the end result is the same…

You’ve now stepped on the scale to weigh yourself and the scale shows that you’ve gained weight… and you assume this weight is body fat.

This is exactly what most people think, so please allow me to explain why it’s not actually true.

You Gained Weight, But It’s NOT Fat

Yes, you DID gain weight.

I’m not disputing the weight gain itself. Assuming you didn’t just recently break your scale or something, then your scale isn’t wrong. You’ve legitimately gained weight.

In fact, if you look in the mirror, you may also see a difference. You don’t look as lean/defined as you did before this weight gain happened. This is also real. You’re not imagining it (at least, not entirely).

And if you take measurements regularly, there’s a really good chance that certain measurements (e.g. stomach) have also increased. Your clothes may even fit tighter, too.

Yet again, this is all real

But here’s the thing.

This is all proof of the fact that you gained weight during this time. But “weight” doesn’t automatically mean “fat.”

Weight vs Fat

You see, “weight” can be all kinds of different things besides body fat. For example:

  1. Muscle
  2. Poop
  3. Water
  4. Glycogen
  5. Stomach content (i.e. food waiting to be digested)

And do you see #3, #4, and #5 specifically? Funny thing about them.

These are 3 temporary factors that are constantly causing your body weight to fluctuate from one day to the next or one week to the next. Hell, even one hour to the next.

And do you know what else is funny about them?

When you eat differently than you typically do or eat more than you typically do – for example, more carbs than usual, more sodium than usual, more alcohol than usual, more overall volume of food than usual… just like you did in the scenario we’re talking about here – these three forms of “weight” will make the number on the scale increase, and make certain measurements go up, and make your clothes fit tighter, and even make you look worse/fatter when you look in the mirror.

Sound familiar?

Yup, it’s the very same things you’re experiencing right now and assumed meant you gained a bunch of fat.

Only… you didn’t gain a bunch of fat. You gained a bunch of weight, and when this happens over a relatively short period of time (e.g. a day, or a few days, or a week), you can bet that most (if not all) of that weight is a result of these other meaningless factors, and NOT fat.

How can I be so sure, you ask?

Because of math.

Here, I’ll prove it.

Proof That You Didn’t Gain Fat 

There are about 3,500 calories stored in a single pound of body fat. (source)

That means, for every additional 3,500 calories you eat above your maintenance level, you will gain 1 pound of body fat.

(Note: In reality, it will actually take more than 3,500 calories over maintenance to gain 1lb of fat, because overeating causes the Thermic Effect Of Food (TEF) and Non-Exercise Activity Thermogenesis (NEAT) to increase, which means you’ll automatically burn off some of the extra calories consumed. But let’s ignore this fact for now and pretend that exactly 3,500 calories over maintenance = exactly 1lb of fat gained. Cool? Cool. Additional details here: How Many Calories In A Pound Of Fat)

Now let’s think about that for a second.

Example #1

Let’s pretend we have some example person with a maintenance level of 2,500 calories who overate yesterday and woke up today weighing 1-3lbs more than they did the day before… and they think it’s all fat

In order for that to be true, they would have had to consume 6,000 calories (2,500 maintenance + additional 3,500 calories) in that 24 hour period to gain 1lb of fat the next day, 9,500 calories to gain 2lbs of fat, and 13,000 calories to gain 3lbs of fat.


Now I don’t care how “bad” of a day you had or how much you “messed up” your diet or whatever else may have happened. I can promise you that you didn’t eat anything close to these kinds of amounts (most people couldn’t eat this much if they purposely tried to).

Which means you didn’t gain anything close to this much actual body fat, either.

In my years of coaching people, when someone ends up eating more than they were intending to, it’s typically by just a few hundred extra calories. That’s nothing!

And the realistic worst case scenario? Maybe it was as much as 1,000 calories.

Even THAT isn’t enough to cause a single pound of fat to be gained.

Think about that for a second.

Eating 1,000 calories more than your typical daily maintenance calorie intake is only going to cause a teeny tiny fraction of a pound of fat to be gained AT MOST, along with a bunch of meaningless temporary weight (i.e. water, glycogen, stomach content)… all of which will be lost as soon as your diet returns to normal.

Example #2

Here’s another example, this time from someone in my 1-on-1 coaching program who recently asked me about this exact scenario.

His goal at the moment is fat loss, so I have him in a moderate caloric deficit successfully losing fat for the last few weeks.

However, he recently returned home from a short trip (a few days), and when he weighed himself, he saw that he gained 5 pounds from what he weighed before the trip.

He knew he overate during this trip, but is it possible he overate enough to gain 5 pounds of fat in the span of a few days?

The answer is hell no.

Why? Because in order for that to have happened, he would have had to…

  1. Overeat enough calories during these few days to completely wipe out the caloric deficit he intended to be in, thus putting him back at his maintenance level…
  2. AND THEN, on top of the overeating that takes him from a deficit back up to maintenance, he’d need to overeat an additional 17,500 calories more than this maintenance amount during these few days as well. 

Please do me a favor and read that again.

These are the required conditions for this person to gain 5lbs of actual body fat during the few days he spent on his trip.

Do you think that happened? Do you think he overate this much over the span of 3-4 days? 

The answer is hell no.

Which is why I told him not to worry, because the 5lbs he gained is mostly (if not entirely) just temporary weight gain that he’ll lose soon after his diet returns to normal.

Cut to a few days later, and he gave me an update…

Also Jay, you were absolutely right about my weight gain over the work trip. Back down to 170lbs after I freaked out and thought I gained 5 pounds. The human body is one crazy ass system of systems.

It sure is. 🙂 

You Have 4 Options To Choose From

Your goal is to stick to your diet as consistently as you realistically can.

But even if you do an amazing job of that, you’re still going to eventually have a day, or a few days, or maybe even a whole week where you end up eating differently than you were intending to or eating more than you were intending to.

Don’t worry, it happens to everyone.

But when this inevitably happens, you have 4 options to choose from in terms of what you do next…

1) You can incorrectly assume that you’ve gained a bunch of fat and then feel unhappy, mad, guilty, depressed, frustrated, etc. etc. etc. because of it. Which would be very wrong.

2) You can incorrectly assume that you’ve gained a bunch of fat and then unnecessarily force yourself to do a ton of cardio or eat a lot less during the the next day(s) to “make up for it.” Which would be very wrong.

3) You can incorrectly assume you’ve gained a bunch of fat and let this thought turn into the “screw it” mentality, where you tell yourself you’ve already messed things up so badly that it doesn’t even matter anymore, so you might as well keep messing things up and continue to overeat because this day/week is already a lost cause. Which would be very wrong, and also the scenario that actually leads to body fat being gained.


4) You can correctly realize the weight gain you’re seeing in a short period of time like this is mostly (if not entirely) just meaningless temporary weight (NOT fat) that you’ll lose soon after your diet returns to normal. And that as long as you get right back on track with your diet, you’ll be perfectly fine and probably still end up losing fat this week.

I vote for option 4.

What’s Next?

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