Understanding The Thermic Effect Of Food
Takeaway Points:
A little-known aspect of diet is the thermic effect of food (TEF), which refers to the number of calories we burn in the process of digesting and absorbing our food.
Two foods with the same number of calories can have very different TEF, causing you to keep more of the calories from the low TEF food.
Cooking and processing foods are the primary ways that we can make modern foodstuffs, which are more resistant to going bad and are higher in calories. This can be both a blessing and a curse.
In general, high TEF foods are more filling and help us to more easily manage the number of calories we're eating. Low TEF foods, on the other hand, serve us well in food-scarce survival scenarios.
Recently I started watching through Doomsday Preppers, a show about the lifestyles of disaster preppers. The people interviewed on this show are absolutely obsessed with absurd doomsday scenarios, and are often convinced that some kind of apocalypse is very real and going to happen very soon. While many of them knew quite a bit about self-defense and the process of setting up fortress-like, self-sustaining home structures, many of them didn’t know very much about diet.
One prepper is a young woman who’s in the process of learning how to catch and prepare fish. On camera, she eats a piece of raw meat, cut directly out of the fish, and claims that this is actually a healthier way to eat it because it preserves more of the nutrients. While this claim isn’t entirely untrue, it’s very misleading - and demonstrates a very clear misunderstanding of how we eat and why we cook food. I took to a small rant on twitter, but I wanted to treat this to a full post so that I can cover the topic in more detail.
One of the interesting things that many people don’t know about food is that, while the calories in, calories out (CICO) model of energy balance is correct, there’s a lot of variables that people don’t know about. Two calories from different food sources can have very different effects on the body. At the end of the day, all calories are not equal.
In one study, participants were separated into two groups and given either “whole food” (minimally processed) or “processed food” meals of the same calorie value. In this case, they used cheese sandwiches, with the whole food sandwiches being made of multi-grain bread and cheddar cheese and the processed sandwiches being made out of white bread and processed cheese product.
What they found was that while the two meals felt equally filling, there was a huge difference in terms of how much energy they burned in the process of digesting the meal. The whole food group burned almost 50% more energy during the process of digestion, and this meant that they absorbed a smaller number of net calories.
How do we explain this?
It’s important to understand that the act of digesting food requires energy. It’s a complicated process of breaking down foods into their component nutrients, so of course that burns calories of its own. We call this the thermic effect of food, or TEF for short.
TEF varies widely depending on the kind of foods you eat. It is well known that protein is the most filling macronutrient because it's harder to process than fats and carbs - since more energy is lost during the process of digestion, that means that it has a lower calorie density and that you’re eating a greater amount of food, which tends to be more filling. In contrast, fats and carbs are much more easily digested, meaning you keep more of the calories that you eat. In general, TEF burns around 10% of the total calories of the food that you eat, while protein may have a TEF as high as 20-35%.
Why does this matter?
TEF explains why, in the study above, a whole food sandwich resulted in fewer calories at the end of the day. Whole, minimally processed foods are harder for the body to digest - so more of the calories they contain are effectively lost due to TEF. In contrast, highly processed foods are very easy to digest, so more of the calories are kept and stored.
Where does cooking come into this?
Cooking is the OG in terms of processing. It’s the very first way that we learned to process our food. Cooking breaks down the nutrients in our food slightly, making it easier to digest. Some micronutrients may be lost in this process - but this doesn’t matter very much, since the macronutrients (calories) are what count the most. We call them macro and micronutrients for a reason - without calories you die very quickly, without micronutrients (vitamins and minerals) we may not have optimal health but we can probably keep surviving for quite a while.
Think about it this way - cooking kills bacteria, makes our food last longer, and also makes it easier to digest, meaning that we get more calories out of each piece of food. In a survival scenario (most of history), this is great - it means that hunters, gatherers, and subsistence farmers don’t need to find or harvest as much food each year, and can more easily store the food that they do have. Add in other basic processing methods like fermentation (cheese, beer) and smoking (jerky), and civilization can start to store calories to defend against famine.
In essence, cooking does some of the process of digestion for us - it’s a way of allowing the energy in wood (via an exothermic - fire - reaction) to shoulder the energy cost of digestion so that we can keep more of the calories in our food.
Now that can become a problem in a modern situation, where food scarcity is less of an issue, and overeating can be as much of a problem as undereating. Raw food diets are sometimes trendy - the reason given being that it’s because you maintain the micronutrients that could be destroyed during cooking. But of course, the real reason that a raw food diet may have positive health effects, or cause weight loss, is primarily because raw foods are harder to digest, and have a higher TEF than cooked foods, leading to fewer calories absorbed.
There’s some other interesting, related tidbits as well. Certain kinds of starches are classified as “resistant starches”, and are actually more resistant to digestion when cooled as opposed to when they’re hot - meaning that, as your food cools down, it actually contains “fewer” calories.
A classic example of the effect of TEF is in the potato. The potato (white or otherwise) is actually a very healthy food, is one of the most satiating foods in existence, and is an excellent staple. Yet when we cook the potato (break down its starches), mash it or fry it, cover it in oil, butter, bacon bits, cream, cheese, or other energy-dense foods - suddenly, it’s much more calorie dense and easily digested, and commonly blamed for modern dietary ills.
I’ve written before about how, in a survival situation where your energy demands are high, high-calorie, processed foods would actually be a far better option than traditional “health” foods. Again, this is because in a survival situation, our calorie needs (and the types of foods that are best) would be very different from a minimally active modern environment with less food scarcity.
This lesson doesn’t seem to have been known by one of the other preppers, who was forcing his son to eat some random wild lettuce they found in the woods because he claimed that it was high in nutrients. His son certainly didn't seem to enjoy the experience.
One other random tidbit is about fiber - a major source of calories for many herbivores, and the stuff that we find in plant products primarily. Cows, for example, can get huge eating nothing but grass all day because they can process the fiber in the grass, while our stomachs can’t. Foods which are high in fiber but low in carbs tend to have very few overall calories, in part because they’re harder to digest, and this also makes them more filling for the same reason as protein above - more wasted digestion energy means more food mass to fill up the stomach.
This has led to the claim that some foods (like celery) are “negative calorie” foods, because presumably the thermic effect of these foods would be higher than the number of calories that you get out of the food. This isn’t strictly true - you’re always getting some amount of calories - but this helps explain why these foods are so low-calorie.
Some people may have genetic issues which make it easier or harder for them to process and absorb certain kinds of foods - and this has a corresponding impact on the kinds of diets that they should follow.
The thermic effect of food helps explain why some calories aren’t quite equal. Two foods can contain the exact same number of calories, but due to ease of digestion and absorption, still result in very different amounts of calories digested and kept by the body, at the end of the day. This is also why, as a general rule, it’s recommended that we primarily focus on whole, minimally-processed, home-cooked meals for the bulk of our diet.
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