Google “10,000 calorie challenge” or search for #cheatday on any social media network, and you’ll find endless examples of fit people eating ungodly amounts of food without getting fat.

You probably know someone who’s able to put away platters of food without putting on weight. 

What gives? 

Are they blessed with ”skinny” genes?

Do they have a “fast” metabolism?

No and no. 

While it may seem like some people are simply immune to gaining weight no matter what or how much they eat, their bodies follow the same rules as yours. 

The unsexy explanation for their seeming freedom from fat gain boils down to their eating and exercise habits (including the ones you don’t see), not some inborn trait that allows them to gorge with impunity. 

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Overeating and Fat Gain

In reality, there are three explanations for why some people can (seemingly) eat whatever they want without gaining weight: 

1. You probably aren’t observing these people at all times or carefully measuring their calorie intake, and it’s likely their average food intake is less than you think.
2. You also probably aren’t observing these people’s exercise habits, and it’s likely they’re exercising more than you realize. 
3. While overeating for a few days (or weeks) will almost always cause some fat gain, several changes occur in your body that prevent all of the calories you eat from being stored as body fat. 

The first two points are straightforward.

People witness someone regularly eating large amounts of food without gaining fat, and they assume this is due to some biological blessing like a “fast metabolism.” 

Dollars to doughnuts, though, they’re basing this assessment on a few snapshots of the person’s daily routine. Maybe it’s a coworker or classmate who eats large lunches, or a friend who gluts themselves at holiday parties, or a roommate who seems to snack nonstop throughout the day. 

In reality, most of these people don’t eat as much as you might assume at first blush, because they eat less the rest of the time. 

For instance, that friend who seems to go hog wild at lunch may not eat breakfast or only has a light dinner. That roommate who always has a snack in their hand? Maybe they rarely eat large meals. The person who binges at parties? Probably eats smaller portions the rest of the time. 

The scientific term for this gap in your knowledge is known as a selection bias, and it affects all of us. You see skinny Dave devouring an entire pizza at the New Year’s Eve party (wow, he eats so much), but fail to notice that he eats significantly less the rest of the week. You notice lean Lisa munching on fruit and nuts all day, but don’t realize she rarely eats large meals.

This same principle applies to activity levels. While some people do eat considerably more than others without gaining weight, they’re also much more active. 

Someone who lifts weights two or three times a week and walks their dog a few laps around the block doesn’t need to eat nearly as much as someone who hits the gym five days per week, participates in a sport like Brazilian Jiu Jitsu, and goes rucking, hiking, and cycling on the weekends. 

In other words, most people who seem to eat a lot without gaining fat don’t have an innately “fast metabolism.” Instead, they probably eat about as many calories as they burn over time, and you’re overestimating their calorie intake or underestimating their activity levels. 

All of that said, there are a few quirks of human physiology that prevent all of the calories we eat from being stored as body fat, and this partially explains why the aftermath of overeating isn’t as severe as we sometimes assume. 

To wit, here are the main factors at play:

• You burn more calories digesting food.
• You store some macronutrients as fat more easily than others.
• You (may) move more when you eat more.
• You don’t absorb every calorie you eat.

Let’s look at each in turn. 

You burn more calories digesting food.

When you eat a meal, your body has to expend energy to digest and process the food. This is known as the thermic effect of food (TEF).

Research shows it accounts for approximately 10% of your total daily energy expenditure. That is, about one in ten calories you consume is burned digesting your meals.

How much energy you burn through TEF depends on the size and composition of the meal.

Smaller meals require less energy to digest, so they cause a smaller increase in energy expenditure. 

Large meals, on the other hand, require far more energy to digest, so they cause a much greater rise in energy expenditure. (This is also part of why you feel warmer after eating a large meal—your body is expending energy to turn that food into usable calories). 

Some macronutrients also raise TEF more than others:

• Protein has the highest TEF of around 20-to-30%.
• Carbohydrate has a TEF of 5-to-10%.
• Fat has a TEF of 0-to-3%.

What this means is that meals higher in protein and carbohydrate cause a greater increase in TEF than meals high in fat.

How processed or unprocessed your food is also changes its effect on TEF. In one study conducted by scientists at Pomona College, they found a processed-food meal of white bread and American cheese increased TEF about 10%, whereas a whole-food meal of multi-grain bread and cheddar cheese increased TEF about 20%.

Another study found similar results when people burned 92 extra calories per day by eating whole grains instead of refined grains for six weeks. 

Thus, let’s say you overeat 1,000 calories. Right off the bat, you’re losing about 100 of those calories to the thermic effect of food. If the meal is high in protein or carbs and mostly composed of whole foods, you might burn closer to 150 or 200 calories. 

You store some macronutrients as fat more easily than others.

How many calories you store as fat also depends on what percentage of those calories come from protein, carbohydrate, or fat. 

This is because not all macronutrients are processed the same way by the body.

Protein is used almost entirely for repairing, building, and regenerating cells, hormones, and other molecules in the body, and the excess is burned off as fuel in the liver. It’s never directly converted into body fat.

Carbohydrate is mostly stored in the muscles and liver in the form of glycogen. Despite what many people believe, carbs are rarely converted directly into body fat. 

Fat is chemically very similar to body fat, and thus your body prefers to store dietary fat as body fat and burn carbs and protein for immediate energy instead.

Now, you might be thinking you can outsmart your body’s metabolism by overeating carbs and protein and keeping your fat intake low, but this isn’t as effective as you might think. Eating too many carbs can make you gain fat, just not in the same way as overeating fat.

Chemically speaking, carbs are very different from the molecules that make up body fat (triglycerides), and they have to undergo an energy-intensive process known as de novo lipogenesis (DNL) to be converted into body fat.

The thing is, DNL only ramps up enough to make a difference in your body fat levels after several days of gorging yourself on carbs. We’re talking 700-to-1,000+ grams per day for most people. 

Even then, about 15-to-25% of the energy in carbohydrate is lost during the process of converting it into fat, so 100 calories’ worth of carbs might only turn into 75 or 85 calories’ worth of body fat.

Overeating carbs can increase fat storage in another way, though. As your body burns more and more carbs for energy, it burns proportionally less of the fat you eat, and thus more dietary fat is stored as body fat.

What this means is that the more carbs you eat, the more fat you’ll store and the less you burn throughout the day.

This is why most studies show that people gain similar amounts of body fat after overeating carbs or fat. 

In other words . . .

• Eating more dietary fat results in the direct storage of body fat. 
• Eating more dietary carbs results in some of the carbs being converted to fat (if you eat a ton of carbs for several days), and it results in any fat you eat being primarily stored as body fat.

And in case you’re wondering, alcohol is burned immediately because the body has no way to store it, and is thus never stored as body fat. It does, however, shut down fat burning in a similar manner to carbs, which means that while alcohol is in your system, you’re going to store almost all of the fat you eat as body fat. (This isn’t to say that alcohol is inherently unhealthy, though, as explained in this article.)

Now, if you’re willing to keep your fat intake very low, you can minimize fat gain by primarily overeating on carbs and protein. More on how this works in a moment. 

You (may) move more when you eat more.

There’s some truth to the idea that some people have naturally “fast metabolisms,” and can eat more than others without gaining as much weight. 

This isn’t because some people are blessed with fat burning genes, though. Instead, the reason some people seem to have fast metabolisms is due to how their activity levels change when they overeat.

Research shows some people will spontaneously and subconsciously move more throughout the day when they overeat, and this increase in activity can help them burn up to 700 additional calories per day.

That is, if people are fed 1,000 calories more per day than they need to maintain their weight, some will burn off about three-quarters of that just by fidgeting, tapping their legs, and moving around more throughout the day.

These activities are known as non-exercise activity thermogenesis (NEAT). One thing you should know about NEAT, though, is that it varies a great deal from person to person. 

When people eat more calories than they need to maintain their weight, some might actually burn fewer calories and others might burn almost 1,000 calories more per day. Women also tend to have a smaller rise in NEAT than men when overeating.

That’s why I say you may move more when you eat more—not everyone responds the same way.

That said, the average increase in energy expenditure works out to about 300 calories per day, which helps partly explain why people don’t gain as much fat as you’d expect after binging. 

You don’t absorb every calorie you eat.

As you may have noticed after a trip to the bathroom, not every calorie you eat is entirely digested. 

Some foods, particularly ones high in fiber like grains, nuts, seeds, and vegetables often make their way through the digestive system without being completely broken down. 

What’s more, these high-fiber foods can also interfere with the absorption of fat, meaning more gets excreted in your poop.

Does this mean that many of those extra cheat day calories just go in one end and out the other?

Nope.

The body is very good at extracting calories out of the food you eat, and roughly 95% of the calories you put in your mouth are absorbed during digestion.

That said, eating more or less of certain foods can change how many calories are absorbed. 

Specifically, eating more fiber-rich foods, particularly those high in soluble fiber, can slightly reduce how many calories are absorbed during digestion. 

For example, you only absorb about 70% of the calories in whole almonds, because the fibrous shell of the almonds prevents much of the fat from being absorbed during digestion. Not only that, but when you eat high-fiber foods like almonds with other high-fat foods, the fiber from the almonds reduces the total fat absorption of the entire meal. 

Thus, eating high-fiber foods reduces the absorption of calories from all of the foods you eat, not just the high-fiber ones.

This won’t help significantly reduce how much fat you gain when overeating, but it helps further explain why you don’t gain quite as much fat as you’d expect.