I don’t normally eavesdrop.
However, in this case I didn’t have a choice.
I was in the gym finishing a workout, when I heard a personal trainer talking with another guy about how “natural sugars” burn fat and “junk like high fructose corn syrup and trans-fats” makes you gain fat.
He went on for 20 minutes about how eating the right foods primes your body for fat loss, and how eating the wrong foods ruins your efforts to be lean or fit. He was still talking when I left.
Not once did this person mention the word “calories.”
Our friend isn’t alone. Many people believe that eating certain foods makes you gain fat or makes it harder to lose fat, and that other foods do the opposite.
We’ve already thoroughly examined (debunked) the idea that certain foods make you gain fat. As many people are quick to point out, however, macronutrients also matter. In some cases, people go so far as to say that eating a certain macronutrient ratio can help you lose weight and/or fat despite eating excess calories.
In this article, you’ll learn exactly how overeating any macronutrient can make you gain weight.
How Your Body Uses Calories
When you eat food — calories — your body has two options:
It can either:
1. Store them
2. Burn them.
Nutrients that aren’t easily stored are burned first. Your body treats food as if it has an expiration date. It “eats” the stuff that’s about to go “bad” first, and stores the less perishable items for later.
Why Your Body is a Multi-Fuel Engine
No one eats macronutrients in isolation.
Maybe you’ll have a meal every now and then that’s almost all protein, carbohydrate, or fat (or alcohol), but over the course of an entire day, you’re going to eat a combination of all three.
Whenever you eat a nutrient that isn’t easily stored like protein or alcohol, your body decreases fat and carbohydrate burning in almost exact proportion to the number of excess calories you consume.
The number of calories you burn doesn’t change — just where those calories come from.
Let’s take a closer look at how each macronutrient can be either stored or burned, and how this affects your ability to lose fat.
How Excess Fat Can Make You Fat
You’ve probably heard the term “fat makes you fat.”
It’s true that fat is easier for some people to passively overeat, because it’s highly palatable and is higher in calories than protein or carbohydrate.1,2
However, another common claim is that at the same calorie intake, you’ll gain more body fat if you eat more dietary fat. This is (mostly) untrue.
Your body can store almost unlimited amounts of fat, so dietary fat is pretty easy to store. After a meal, most of the dietary fat you eat will get stored as body fat in your fat and muscle tissue (as intramuscular triglycerides).
It’s easier to just store fat than it is to oxidize it immediately.
In most cases, eating dietary fat doesn’t increase how much fat you burn. The exceptions are when you eat a massive amount of fat in a single meal3 or when you completely eliminate carbohydrates (which we’ll talk about in a moment).
Even in these cases, eating more fat doesn’t make you burn more body fat. Forcing your body to burn dietary fat instead of carbohydrates won’t make you leaner; it just makes you burn more dietary fat and less carbohydrate.4-8
How Excess Carbs Can Make You fat
Your body can store some carbohydrate, but only in limited amounts. Depending on your training status and muscle mass, most people can store around 300-500 grams of glycogen in their muscles and liver.9 When you eat carbohydrate, some of it is usually burned as glucose immediately, while the rest is stored as glycogen.
Carbohydrates are almost never directly converted into body fat under normal circumstances,10,11 which has led some people to believe carbs are less fattening than dietary fat at the same calorie intake. This is also incorrect.
Carbohydrate and fat oxidation are almost perfectly inverse — the more fat you burn, the less carbohydrate and vice versa.5,7,8,12-14
Even though carbs are rarely converted directly to fat (a process called de novo lipogenesis, or “DNL”), they do cause you to store proportionally more body fat.15
The obvious next question is “why not eat a zero fat diet?” You need some dietary fat, but even if you didn’t, this wouldn’t help you stay lean in a caloric surplus.
The problem is that your body can convert carbohydrate to body fat when you eat enough carbs. When you massively over-consume carbohydrates and limit your fat intake to less than about 10% of total calories, your body increases de novo lipogenesis and starts converting more carbohydrate into fat. People who are already overweight also have a larger rise in DNL after meals.16 This conversion rate is almost directly proportional to the amount of excess carbohydrate calories you eat.10,17-21
About 25% of the carbohydrate calories you consume are wasted in the process of DNL,22 so you usually store slightly less body fat when overeating on carbohydrates.19 The difference is very small, however, and you’re still going to store some body fat when you overeat carbohydrates for long enough.
How Excess Alcohol Can Make You Fat
Alcohol is almost never converted directly to body fat.
However, alcohol shuts down fat and carbohydrate oxidation in direct proportion to how much you eat.23
Your body has no way of storing alcohol. It tries to get rid of it as fast as possible to make sure it doesn’t cause damage, as high blood alcohol levels can be toxic.24
After a meal with alcohol, your body is burning essentially 100% alcohol and zero carbohydrate and fat.24 Any carbohydrate you eat will get stored as glycogen and/or fat, and any fat you eat will get stored as fat. Despite this, drinking some alcohol is still not going to make a difference in your body fat over time.
How Excess Protein Can Make You Fat
You’ve thought about it before — overeating mountains of protein in hopes that you won’t gain any fat. Unfortunately, it doesn’t work that way.
In a caloric surplus, the protein you eat will usually be used in three ways:
1. Structural repair and growth.
Eating protein stimulates protein synthesis.25 Your muscles and other organs increase protein turnover and repair damaged tissues. If you’re training, the amino acids will also be used to increase muscle growth or remodel your skeletal muscles.26,27
Excess protein is usually oxidized in your liver, i.e. burned for fuel.28
Despite what you may have heard, your body can store small amounts of protein. After you eat a meal, your body stores some of the amino acids in your muscles and liver as “labile proteins.”29 These are temporary storage depots that your body can use for other essential processes when you aren’t actively eating more protein.
Your body can store enough protein to cover basic functions like hormone production, but not enough to be a significant energy source. Eventually, these proteins get used just like the others you eat.
Just like alcohol, protein oxidation decreases carbohydrate and fat oxidation. When you eat more protein, you burn slightly less of the other macronutrients, and gain just as much fat.
Macronutrients Still Matter
Eating excess calories from any source will always make you gain weight. However, that doesn’t mean that your macronutrient choices are completely irrelevant.
High-calorie diets that have more protein generally cause a larger increase in muscle growth (even without strength training), but the subjects still gain just as much fat.30 Nevertheless, if you’re trying to gain muscle, eating more protein is a good idea.
In the short-term (i.e. 24-48 hours), you may also be able to limit fat gain by overeating carbs rather than dietary fat, assuming that you return to maintenance calories or a caloric deficit afterwards.17,19 The differences are generally minor, so don’t get too concerned with this.
However, if you eat more calories than you burn from any macronutrient, over time, you’ll gain weight.
Why this is Good News for You
You might feel a little disappointed — there aren’t any magic macronutrient ratios that can help you avoid gaining fat while overeating.
However, the bright side is that you don’t have to under or over consume a certain macronutrient to gain or lose fat. Macronutrients are important, but it’s useful to know that you’ll always gain weight roughly in proportion to how many calories you eat over your maintenance needs — regardless of which macronutrient they come from.
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