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How much fat is it possible to gain in one day?

We’ve all had those days where we eat everything in sight: pizza, burgers, cake, beer. Maybe it was 4th July or Thanksgiving, or maybe it’s a weekly occurrence. You feel heavier the next day and feel like you’ve gained 20 pounds; but how much fat is it actually possible to gain in one day of excessive overeating?

A study published in the American Journal of Clinical Nutrition in 1995 measured the fat gain in a group of participants after overfeeding them 50 percent above their daily caloric needs for 14 days. The extra food came from either fat or carbohydrates. After the trial, the participants gained three pounds on average, equating to 0.2 pounds of fat gained each day.

Considering these participants were eating 50 percent higher than they’re maintenance calories (averaging 1,400 calories extra per day), 0.2 pounds of fat gain is surprisingly low. But here’s the thing with fat gain: it takes a while.

‘So why do I weigh seven pounds heavier after a cheat day?’

When you eat in excess of what your body a few things occur. You gain a very small amount of fat which causes an increase on the scale, but the main increase on the scale is coming from water retention.

The type of junk food that is common to binge on (pizza, burgers, fries etc.) is high in sodium and refined carbohydrates. The combination of carbs plus salt results in a lot of water retention. Every gram of carbohydrate bonds with four grams of water when stored as glycogen; so if you’re significantly overeating, that’s a lot of carbs and water stored, making your weight jump up.

As these foods are also high in salt they promote water retention. This is because the body needs to maintain the homeostasis between sodium and water, so if excess salt is consumed, it holds onto excess water to try and balance it out. The water retention from salt alone can give you an extra seven pounds on the scale.

Now to go back to the point in question of how much fat you can gain in one day, the answer is: it depends.

Pseudoscience dictates that one pound of fat equates to 3,500 calories. So it would seem logical that you’d have to overeat 3,500 calories in one day to gain one gram of fat. However, it’s not that simple. When you eat significantly more on a particular day, a number of physiological changes happen in your body to try and burn the excess calories to reach energy balance.

Firstly, the more you eat the more you burn. The thermic effect of food (TEF) is a component in your basal metabolic rate, and the more food you eat the harder your body has to work to digest it; meaning your metabolism is elevated.

This also depends on the composition of the food you’re intaking. If you’re overeating with a cheeseburger, the TEF is elevated by around 30 percent. This is because different macronutrients require different amounts of energy to digest. Protein elevates TEF the highest at 30 percent, while carbs elevate it by 10 percent and fat barely affects it at a maximum of 3 percent. This diet induced thermogenesis was outlined in a study published in the journal Nutrition and Metabolism in 2004.

What this means is that if you’re eating an additional 2,000 calories high in protein and carbs, up to 600 calories of this will be burnt immediately rather than stored. But if you’re eating an excess of 2,000 calories worth of fat, you’re going to store all of it as excess fat. Obviously most foods are a combination of all three macronutrients, but if you are going to overeat, try to make sure you’re still getting the protein in to boost TEF.

Secondly, if an otherwise healthy individual binges on one day, their bodies are likely going to increase non-exercise activity thermogenesis (NEAT) and even exercise activity thermogenesis (EAT). NEAT is the random movements you make without even realizing: the gestures you make when you’re talking, twitches, brushing your teeth, etc. EAT is the exercise you do in a day that increases the total amount of calories burnt in a day, affecting energy balance.

A study that examined the role of NEAT in overfeeding was published in 1999 and gave a valuable insight into how our body fights to maintain energy balance homeostasis when it’s randomly overfed. The healthy participants were overfed an additional 1,000 calories per day for eight weeks and the researchers monitored their energy expenditure.

The researchers found that the participants greatly increased their total daily energy expenditure, which was two thirds due to the increase in NEAT; subconscious movements that the participants weren’t even aware of. The participants who gained the least amount of fat were also the ones with the highest activation of NEAT. According to this particular research, NEAT accounted for an additional 700 calories burnt per day.

You also don’t absorb every calorie consumed. For example, insoluble fiber is found in an array of food items like whole grains, wheat, legumes, nuts and fruit. Insoluble fiber and nondigestible carbohydrates are not absorbed by the body but rather pass through to the gut and bulk out stools. Fiber also reduces fat absorption, guiding more of it out in stools. However, the reduction of calories from fiber is minimal.

Ultimately, if you have a national holiday, birthday or special event coming up and you know you’re going to want to overeat: enjoy it, don’t stress. Even if the scale says you’ve gained ten pounds the next morning, we hope you now realize that it’s physiologically impossible to be entirely made up of fat. As shown in a study published in 2014, the participants gained seven pounds after two weeks of overfeeding, but it turned out only half of this weight was actually fat.

It’s hard to say how much fat can feasibly be gained in a day of overfeeding because everyone’s different, with different metabolic rates, digestive enzymes, muscle mass etc - and consuming foods with different compositions - which all affects how food is digested, stored or burnt.

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