How much fat can you gain in one day?

Updated: Jul 26

Put up your hand if you’ve ever been worried about fat gain? Oh… I see… everyone has their hands up… Good thing then that is exactly what we’re going to cover today and forever put your minds at ease!

The thought of fat gain is, for many, a quite debilitating anxiety, restricting them from social events, food experiences, worsening their relationship with food and self and even contributing to the development (or furtherment) of disordered eating / eating disorders.

In this article, we will aim to not only give a full overview of how weight change and fat storage works, but also how much you could feasibly store in one day.

Spoiler alert; we’re going to ask you to sit back, relax, crack open that chocolate bar and munch away while you read because, guess what, it won’t have any f&%king effect on your body weight / fat long (or likely even short) term.

How weight change works

Understanding weight change is our first port of call; outside of changing body composition at a steady weight (described as “body recomposition”), almost all contributions to body fat gain (or indeed explanations surrounding why body fat has been lost) can be explained by changes in body mass / weight.

If we gain weight (especially over time), it is likely that some of that will be fat gain. Acute changes in weight may be down to;

More gradual change in bodyweight is a greater indicator of “actual weight” i.e. increases in lean body mass, fat mass, bone density etc.

As it relates to lean body / muscle mass and fat mass (and in some context dependant cases bone density), these will change based on a very basic principle; energy in vs. energy out. Or, if you’re more familiar with this description; calories in vs. calories out.

If your goal is to lose weight, then you’d need to consume less calories than the number required to maintain your weight.

If your goal is to gain weight then you’d need to consume more calories than required to maintain.

If your aim is to maintain your weight and focus more on body re-composition then you’d need to eat at a calorie intake around your maintenance and focus more so on macro splits and nutrient timing.

Pretty simple right?

Ok, but that’s more so long term, what about short term?

“How much fat can I really gain in one day?”

The science around acute fat gain; major worry or unnecessary paranoia?

Buckle up ladies and gentlenerds, this is going to get a little bit science-y and even dip into a bit of the old mathematics.

When working out how much fat you can really gain in one day, there’s a number of factors you must consider (outside of how many calories you’ve actually overfed on);

  • Your TDEE (Total Daily Energy Expenditure)

  • Your existing levels of glycogen storage

  • Your dietary composition that day

  • Increases in NEAT (Non-exercise activity thermogenesis) variation

  • Your current body composition

Let’s look at each individually so you have a better understanding of everything involved in the process of weight and body composition change.


Your TDEE is itself composed of multiple factors. Four, to be precise;

  • Your basal metabolic rate

This is the number of calories your body would expend at complete rest. This will change based on your body composition, age, gender, the form of exercise you engage in, health, and body size.

  • Thermogenesis or the thermic effect of food

Some of the calories in the food you ingest will be used to digest, absorb, and metabolize the rest of the food, and some will be burned off as heat. This process is called diet-induced thermogenesis or referred to as the thermic effect of food.

  • NEAT

NEAT is the energy expended for everything we do that is not sleeping, eating or sports-like exercise. It ranges from the energy expended walking to work, typing, performing yard work, undertaking agricultural tasks and fidgeting.

  • Programmed or intentional exercise

This is simply the exercise you have consciously chosen to do; be that walking, going to the gym, playing sports etc. and the calories which you’ve burned performing that exercise.

We add up the calorie expenditure from each of these four contributors to determine our


Your existing levels of glycogen storage

Glycogen simply refers to our internal stores of carbohydrate; found predominantly within muscle tissue and the liver.

The more glycogen depleted we are the more of the food taken in will be stored as glycogen as opposed to fat. This is also affected by our exercise patterns which alter nutrient partitioning.

Your dietary composition that day

The macronutrient composition of your diet (i.e. the ratio of protein to carbs to fats) appears to have a very significant role on both acute and long-term fat storage (at least as it relates to the ratio of protein intake).

Studies have shown that protein overfeeding, or the consumption of a high protein diet, may not result in a gain in body weight or fat mass, both acute and long-term, despite consuming calories that exceed one’s normal or habitual intake.

In a study investigating differences in fat storage following overfeeding between carbohydrate and fats with a 50% calorie surplus it was shown that, after one day of overfeeding, those who overfed on carbohydrate stored only 10% of that as fat, whereas those who overfed on fat stored 60% of that as fat.

Even after several weeks, it was shown that carbohydrate overfeeding produced progressive increases in carbohydrate oxidation (breakdown) and total energy expenditure resulting in 75-85% of excess energy being stored as fat.

Alternatively, fat overfeeding had minimal effects on fat oxidation and total energy expenditure, leading to storage of 90-95% of excess energy. The researchers concluded that excess dietary fat leads to greater fat accumulation than does excess dietary carbohydrate, and the difference was greatest early in the overfeeding period.

Increases in NEAT variation

NEAT has more recently been stipulated to be the key determining factor in weight management / change. In fact, studies have shown that it can range up to 2000kcals a day between two people of individual size!

We also know that NEAT increases following overfeeding so that has to be factored in too (although would be incredibly difficult to measure given the individual variability).

Your current body composition

Ok, to simplify the science on this one; those with greater levels of existing body fat have more adipocytes (fat cells) to store fat. Those who are leaner are likelier to have less.

This has been shown to be set during childhood and adolescence, so it is likely that those who had higher levels of body fat in their youth may have a greater propensity to store fat.

This may not be much of a factor in acute overfeeding but hey, it’s interesting, and I’m a nerd. Don’t judge me.

So, now we know what factors in to how much fat we can gain in one day, what are the practical implications of this and can you please, for the love of god Jamie, just give us some numbers that we can be happy with and get up off the toilet so we can stop reading!?

Good Maths; a few numbers to show off how much fat you can practically gain in one session of acute overfeeding

Let’s look at an example of overfeeding; Easter Sunday and the consumption of a big ass Easter Egg which has been staring you down for what feels like several decades now (even though you likely picked it up from the shop yesterday as you fought some person for a twelve pack of toilet roll to go with the other several hundred you have in your toilet roll safe).

But before we dive into this, let’s first take a quick detour and address the age-old headscratcher; “how many calories is in a lb of fat?”

Well, fat itself is very energy rich, with a content of around 9kcals per gram.

So, a lb of fat (which is roughly 454g) would equal 4100kcals.

But Jamieeeeee, I thought it was 3500kcals?!

Well, the 4100kcals is based on the assumption that a lb of your stored bodyfat is purely fat, which it is not.

In fact, it’s only about 72 to 87% fat (when you take away the fluids and proteins in fat cells).

When we do the maths on this a lb of fat will have an energy content of anywhere (roughly) between 3400 – 3750kcals. Whilst it is a bit of an inexact science, this would be roughly how much you’d have to take in above your maintenance calorie intake for that day to gain a lb of fat.

What makes this even more fun is now knowing that your “maintenance” calorie intake during an overfeeding day is dramatically different than a “normal” day of eating. I love maths, I really, really do *screams internally*

Procrastinated enough, equation time…

I’m going to base this on your average adult female, expected to burn around 2000kcals a day. Makes my life easier, and if you don’t like well… I mean, I don’t care really just to be brutally honest.

I’m also going to assume that she’s eating her Easter Egg on top of her maintenance; so, taking the worst of the calorie culprits (looking at you Cadbury crushed nut XL egg), we’ll be adding 3000kcals on top of her 2000kcals giving us a total daily intake of 5000kcals.

Ok, so the first part of the equation, how much you typically burn in a day or your TDEE (which, in this example, is 2000kcals) subtracted from your calorie intake that day.

Next, let’s look at the thermic effect of food (TEF) coming in and assume in general the ratios for the lady’s diet that day fall somewhere in around 15% Protein / 45% carbohydrate / 40% fat.

The TEF (or how many calories it takes to metabolize these nutrients) is roughly 35% TEF Protein, 15% TEF Carbohydrate and 15% TEF Fat. Take the weighted average of these and you get a total TEF for your dietary intake that day of is around 18%.

So, what do we do next? Well we take that 5000kcals and find out what 18% of that is going to be, which works out to be roughly 900kcals; so, she’ll be burning 900kcals through thermogenesis alone on that day. Stick it into your equation folks

Neat, now that we’ve done that, let’s add in NEAT. This will never be exact because it’s virtually impossible to calculate this without a metabolic chamber (basically a confined room that researchers use to measure energy expenditure by continually measuring the ratio of carbon dioxide and oxygen in the room).

Assuming that NEAT can be as great as 2000kcals a day in some individuals, let’s just go super conservative and go with 300kcals; stick that into your equation.

For the heck of it, let’s say she fancies a walk that day too and that burns off an extra 200kcals; add it to the equation

Now, we can’t possibly hope to predict the storage dynamics of fat as it relates to your existing body fat storage capacity (based on your set number of adipocytes) or how your level of glycogen depletion will factor in so what I will do is go super conservative on it and take away an extra 33% of this and assume the remaining will be stored as fat (but know that if we could factor it in it accurately it would likely either have no effect or reduce the amount of nutrients / energy available even further).

This leaves us with this; the amount of available fat / energy which can be stored

To round this up, after taking everything into account, Barbara (yeah, why not, big Barbs), will have roughly 119 grams of fat available for storage which accumulates to a mere 0.25lbs of total fat gained.

Not that scary now that you see it like that right?

The consistency element of fat gain

Reversing this change in fat gain (if you did indeed gain any fat to begin with) would take no more than a few days of getting back into your old habits. In fact, you’ll probably be more likely to increase your adherence to dietary and exercise patterns and achieve weight / fat loss at the end of that week rather than net gain.

It is the consistency of overfeeding and or acute bouts of overfeeding that will lead to an eventual, notable, gain in body fat. Not a one-off session of indulging at Easter, or Christmas or even just simply once in a blue moon because you f&%king feel like it, that is going to cause you to gain significant amounts of body fat.


Hopefully this didn’t fry your brain too much.

Morale of the story; have the damn egg, it won’t do jack shit to your progress and is more likely to initiate or develop disordered eating / an eating disorder if you guilt yourself into not having it.

P.S. if anyone could tell me where to get a half decently sized dark chocolate egg for next Easter, that’d be massively appreciated.

Sincerely me and my hips, where the chocolatey goodness will be heading straight to 😉


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