Written by Alex Leaf and verified by the Examine.com Research Team. Last updated on Dec 20, 2018

While regular overeating leads to fat gain, a lot of the weight you gain from just one binge will simply be water, especially if your starting glycogen stores are low and your food rich in carbs.

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Think of the last time you ate a big meal then weighed yourself the next day. Perhaps it was Thanksgiving, Christmas, or New Year’s, and your scale showed a gain of several pounds. But how much of that is fat?

Very little, it turns out.

There are about 9 food calories in a gram of fat, so in theory you could gain one pound of fat (454 g) by eating some 4,000 calories more than you burn. The actual number may be closer to 3,500,^[1] so if you eat 500 extra calories per day for a week, you might gain a pound of fat.

But not if you eat 3,500 calories in a single meal. There’s a limit as to how much food your body can turn into fat in just a few hours. What your body cannot process for storage during that time, it tries to burn (your temperature rises slightly) and to excrete (you end fighting with your family for access to the toilets).

And of course, digestion itself has an energy cost.

Thermogenesis

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 known under various names, notably diet-induced thermogenesis (DIT), specific dynamic action (SDA), and thermic effect of food (TEF).^[2][3]

DIT represents about 10% of the caloric intake of healthy adults eating a standard mixed diet,^[4] but your actual number will depend on several factors, which include your lean body mass and the composition of your meal. The energy required to digest each macronutrient (its DIT) can be expressed as a percentage of the energy provided by this macronutrient:^[5]

• Fat provides 9 calories per gram, and its DIT is 0–3%.
• Carbohydrate provides 4 calories per gram, and its DIT is 5–10%.
• Protein provides 4 calories per gram, and its DIT is 20–30%.

Also, if you ate too much, you may start feeling restless and consequently fidget more as a subconscious way to burn off the caloric excess. Burning calories through fidgeting is a kind of nonexercise activity thermogenesis (NEAT),^[6][7] “the energy expended for everything we do that is not sleeping, eating, or sports-like”.^[8] NEAT varies between individuals, which is why some people seem to gain fat more easily than others.^[6]

Overeating will increase energy expenditure through dietary induced thermogenesis (DIT) and nonexercise activity thermogenesis (NEAT). The magnitude of your DIT depends on the composition of the meal, whereas the magnitude of your NEAT depends on size of the meal and individual variables.

Glycogen

Your body has two ways to store the carbs you eat. It can transform them into fat, in a process called de novo lipogenesis (DNL), but first it’ll want to refill its glycogen stores in the liver and skeletal muscle.

Glycogen in the liver

Your body uses your liver for short-term storage of glycogen. When you fast (which most of you do overnight as you sleep), your liver is where your brain will first search for the glucose it needs to keep functioning. After a fast of 12–16 hours, when liver stores are 25–50% depleted, each kilogram of the average liver still contains 44 grams of glycogen (range: 14–80 g),^[9] so we can estimate the maximal capacity of the average liver at 60–90 g/kg.

Glycogen, moreover, cannot be stored on its own: it must be bound to water. In your liver, each gram of glycogen comes along with 2.4 grams of water.^[10]

^[11][12][9] 

When full of glycogen, the average, healthy, human-male liver is heavier by 289–432 grams (0.6–1.0 lb), whereas the average, healthy, human-female liver is heavier by 241–364 grams (0.5–0.8 lb).

Glycogen in the muscles

Glycogen also gets stored in your muscles.

^[13]

Of course, typical muscle mass varies greatly between individual men (22–40 kg, typically) and women (15–30 kg).^[14] By combining those numbers with an estimation of the muscles’ average glycogen content (11.7 g/kg), we can further estimate that, in their muscles, men carry 256–466 grams of glycogen, and women 175–350 grams.

As we saw, however, glycogen cannot be stored on its own: it must be bound to water. In your muscles, each gram of glycogen comes along with at least 3 grams of water,^[15] which can become 17 grams if you co-ingest a lot of fluid and a lot of carbs after exercising in a hot, dry environment.^[15]

Therefore, in normal circumstances, a man who carries 31 kilograms of muscles (68 lb) also carries in those muscles 361 grams of glycogen and 1,083 grams of water (0.8 and 2.4 lb). And if he drinks a lot while ingesting his carbs, as is often the case during a feast, he may end up carrying much more.

Likewise, in normal circumstances, a woman who carries 23 kilograms of muscles (51 lb) also carries in those muscles 268 grams of glycogen and 804 grams of water (0.6 and 1.8 lb). And if she drinks a lot while ingesting her carbs, as is often the case during a feast, she may end up carrying much more.

If you drink a lot and ingest a lot of carbs, as is likely during a feast, your muscles might gain several pounds of water weight.

Glycogen depletion

Compared to sedentary people, athletes have more muscle and can better synthesize and store glycogen.^[16][17] A small study found the maximal storage capacity of its subjects (three male collegiate athletes) to be 629–1,146 grams, with an average of 810 grams.^[18] That’s way more than the 341–593 grams (85–127 in the liver, 256–466 in the muscles, as we saw previously) carried by the average man.

Note that, to reach those numbers, the athletes followed a specific protocol: the first three days, they depleted their glycogen stores with exercise and a low-carbohydrate diet; then, for each of the next seven days, they consumed 3,500–5,000 calories, of which 80–90% came from carbs (760–990 grams). On the first day of this week-long binge, all the extra energy served to refill glycogen stores; the athletes didn’t gain any fat. On the second day, fat synthesis amounted to only 30 grams. On the third day, to only 45 grams.

At the end of this week-long binge, the athletes had gained 4.6 kg (10.1 lb) on average, of which 1.1 kg (2.5 lb) was fat. Only half of this fat came from the enormous amount of carbs consumed; the other half came from the proportionally little fat consumed.

Glycogen depletion through diet and exercise on the days leading to a feast can help buffer the caloric load of any carbohydrates being eaten, and therefore help minimize fat gain.

Making fat from carbs

As the study we’ve just examined demonstrated, once your glycogen stores are full, the carbs you ingest can be stored as fat. This process, called de novo lipogenesis (DNL), takes place when you regularly consume more calories than you burn.^[19] DNL is your body’s least preferred way to use carbs; your body would rather, in order of priority, burn them for energy, store them as glycogen, or even burn them off as heat.^[20]

^[19]

In one study in healthy men, a single meal of bread, jam, and fruit juice supplying 480 grams of carbs (93% of the calories in the meal) resulted in a DNL of only 2 grams over the following 10 hours.^[21]Most of the carbohydrate (346 g) was converted into glycogen, while the remainder (133 g) was burned directly. During the same 10 hours, 17 grams of fat was burned for energy, so 7 grams more than the sum of the meal’s fat (8 g) and DNL fat (2 g). In other words, the subjects burned their body fat — they “lost weight”.

Another study had healthy adults consume 150% of their caloric requirements for five days, with the surplus energy coming entirely from carbs (684 g/day).^[22] The resulting DNL was ten times greater than experienced during a maintenance diet (100% of caloric requirements), though it still amounted to only 5 grams of fat per day. Note that, after a year, 5 grams per day makes 1,825 grams (4 lb), which is a lot, but note also that the study participants ate 50% more than normal.

When you consume carbs, your body first burns them (as energy or heat) or stores them as glycogen in your liver and muscles. When those stores are full, your body can make fat out of the carbs, through a process known as de novo lipogenesis (DNL), but only if you regularly consume more calories from carbs than you burn.

Excretion and water retention

As we saw, glycogen must be bound to water to be stored in your muscles and liver, which can thus carry a lot of water weight. Blood volume, and total body water in general, can vary quite a bit with exercise, medications, and dietary factors. Aside from carbs, the main dietary factor is salt (sodium), which pulls water with it wherever it goes and thereby causes water retention.^[23][24] 

And of course, there’s the water in your bladder, as well as the content of your rectum. The average morning pee weighs half a pound,^[25] and the average poop weighs a third of a pound.^[26] You can bet those numbers will be even greater after a feast.

When you binge, your weight increases simply from having more food traveling through the intestinal tract. You may also be carrying around more water if you ate more carbohydrate and salt.

Bottom line

When you binge, a lot of the immediate weight you gain isn’t fat, but water, especially if your binge is rich in salt or carbs, and in the latter case, especially if you are low on glycogen (which carbs replenish, and which must be bound to water to be stored in your muscles and liver). In fact, depleting your glycogen stores in the days leading to a feast can reduce the fat you’ll gain from that feast.

How much fat a meal can make you gain depends on different factors — including its size, of course, but also its macronutrient composition.^[27] Some of the calories in the food you ingest will be used to digest the rest of the food. Protein is especially calorie-costly to digest, and it can be used to make muscle (your muscles are your body’s protein stores) rather than fat, especially if you exercise regularly.

The occasional feast won’t make you fat, but overeating on a regular basis will, though how quickly and to what extent depends on individual factors, starting with genetics.^[28]