The Thermic Effect of Food: What It Is & Why It Matters (A Lot)

Is the thermic effect real? Yes. Yes, it is.

"A calorie is a calorie is a calorie," they say - it doesn't matter whether you eat a 200 calorie Snickers bar or 200 calories worth of chicken breast, they will have the same effect on your body...right? 🤔
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Hmmm, I think we all somewhat agree the above statement has its flaws…⁣⁣⁣
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But do we actually understand why?

In this 4-part series, I’m going to ‘debunk’ why a calorie is not in fact a calorie when it comes to how we actually process them as humans. 

But, before we dive in… let’s quickly define what a calorie actually is.

What Is a calorie?

Before we get into the thermogenic effect of food, let's get on the same page about calories.

The concept of the caloric content of foods was developed by Wilbur Atwater in the late 1800s. The Atwater System includes putting different foods in a device called a bomb calorimeter and burning them to see how much heat they give off. This process gave us the calorie values for each macronutrient that we use today:

• 1 gram of Protein = 4 calories
• 1 gram of Carbohydrate = 4 calories
• 1 gram of Fat = 9 calories
• 1 gram of Alcohol = 7 calories

1 Calorie is equivalent to 1 kilocalorie. The capital C in ‘Calories’ denotes kcal on food labels. In the states, Calories and kilocalories are generally used interchangeably to mean the same thing.

Basically, a calorie is just a unit of measurement. It allows us to measure energy, both in what we consume through food and beverages, as well as what we expend through daily activities. This is also referred to as ‘energy balance’.

So technically, it is true that all calories have the same amount of energy. One dietary calorie contains 4,184 Joules of energy. So in that sense, yes, a calorie is a calorie. BUT, when it comes to how your body actually processes these calories, it’s not that simple…

So let’s dive into the first reason why all calories are NOT equal.

What Is The Thermic Effect of Food? (Definition)

By definition, the thermic effect of food (TEF) is a measure of how much different foods increase energy expenditure, due to the energy required to digest, absorb, and metabolize the nutrients within that food.

We know that in general, food is made up of three main components -- carbohydrates, fats, and proteins -- also referred to as ‘macronutrients.’ Each macronutrient goes through a different metabolic process in your body in order to be broken down and used for energy.

Some of these metabolic processes are more efficient than others. The more efficient a process is at breaking down the food, the more of the food’s energy is used (or stored). The less efficient, the more energy (calories) are lost as heat.

Higher Thermogenic Effects of Food → More Fat Loss

So basically, the higher the thermogenic effect of food → the less efficient the process → the more energy it takes to metabolize that food → the more heat is generated (or ‘calories are burned’) in the process.

If your goal is fat loss, this is a good thing!

Here is the breakdown of the TEF for each macronutrient:

• Fat: 0–3%
• Carbs: 5–10%
• Protein: 20–30%

Protein Thermic Effect of Food

As you can see, protein has a much higher TEF than fats and carbs. Therefore, protein requires much more energy to metabolize than fats and carbs. [*]

Put simply, the higher the protein content of the food you’re eating, the more of a metabolic advantage you will have. I dive deep into other reasons why protein is so important in this post: Protein is QUEEN! and breakdown how to figure out how much protein you need in this post: How Much Protein Should I Eat?

Additionally, whole foods -- think 1) grew from the earth and 2) had a face at some point -- also require MORE energy to digest than processed foods. Again, another positive metabolic advantage AND one of the many reasons I drill WHOLE FOODS FIRST into my clients’ brains when their goal is losing body fat and improving their body composition.

Thermogenic Effect of Food Example

I’m going to layout a bit of math for you just to drive the point home… stick with me :)

Let’s take our snickers bar vs. chicken breast example:

200 calories of raw, skinless chicken breast has roughly:

• 5 grams of fat
• 39 grams of protein
• 0 grams of carbohydrate

200 calories of a Snickers bar has about:

• 10 grams of fat
• 3 grams of protein
• 25 grams of carbohydrate

Okay, time to do some math and figure out the TEF for both the chicken breast and the Snickers bar. Because the TEF of each macronutrient is a percentage range (sources vary for TEF which is why there is a range), let’s just take the middle ground for each.

So we’ll say:

• TEF of fat = 1.5%
• TEF of carbs = 7.5%
• TEF of protein = 25%

Based on these percentages:

The calorie yield of the chicken breast after going through it’s metabolic process:

• Fat: 5*9 = 45 kcal*1.5% = 0.675; 45-0.675 = 44.325 calories
• Protein: 39*4 = 156 kcal*25% = 39; 156-39 = 117 calories
• 44.325 + 117 = 161.325 

• Total Calories = ~161 calories 

The calorie yield of the Snickers bar after going through it’s metabolic process:

• Fat: 10*9 = 90 kcal*1.5% = 88.65 calories
• Protein: 3*4 = 12 kcal*25% = 3; 12-3 = 9 calories
• Carbohydrate: 25*4 = 100 kcal*7.5% = 92.5 calories
• 88.65 + 9 + 92.5 = 190.15
• Total Calories = ~190 calories 

This may not seem like a huge calorie difference at first, but just think about how this can add up over time with other foods you're consuming.

And of course, this is just an example and these numbers will vary person to person based on things like individual digestion and overall metabolic health.

But hopefully you get the point that, based on their macronutrient profile, different foods actually yield different calorie ranges after going through the metabolic processes of digestion, absorption, and utilization in your body.

AND on top of that, the above calculations don’t even factor in that it takes more energy to digest whole foods vs. processed foods. So when a chicken breast and a snickers bar go head-to-head in a TEF match, I think you can guess which one dominates. 😉

A Final Note on The Thermic Effect of Food

Some may argue that TEF only has a small effect (about 10%) on total energy expenditure (or the amount of energy you burn throughout the day).

While this statement is in fact true, that 10% is still going to add up over time and affect long-term energy balance. 

Want to learn more about the thermic effect of food and what's really going on with calories? Join my women's fitness program to learn the ins and outs of nutrition and exercise.

In part 2 of this series, we’ll continue the chicken vs. snickers match when we talk about the satiety index. Stay tuned!