Your body burns energy a few different ways.
There’s your basal metabolic rate- the amount of energy it burns just to maintain itself. Then there are calories burned during deliberate exercise, as well as non-exercise activity thermogenesis- the calories you burn just moving round while living your life.
And then there’s the thermic effect of food (TEF)- the amount of calories you burn just to digest the food you already eat. You can think of TEF as a “discount” on the caloric “price” of the foods you eat.
The existence of the thermic effect of food has lead some phony diet experts to proclaim that some foods have negative calories, because they burn more calories to digest than they actually contain. In fact it actually is theoretically possible for such a food to exist, but in practice nothing really comes close.
So what percentage of the calories you eat get immediately expended digesting your food? It usually ranges from five to twenty-five percent, depending on what foods you’re eating. Here’s the five-minute breakdown.
TEF of Fats
The thermic effect of fat depends on your body fat percentage. In lean individuals, it averages 14.4%, and ranges from 11-18%.
In obese individuals, it’s close to zero- in fact, ingesting fat, on average, decreases obese subjects metabolic rate by a fraction of a percent.
Now you might ask how it’s possible that digestion could cost negative calories. It isn’t, really. It’s just that the researchers can’t measure how many calories are spent digesting food per se, but only the overall change in metabolic rate after eating. The actual TEF of fat in obese people must be positive, but what’s happening here is that obese subjects see other parts of their metabolism slow down after eating fat- they go into a “food coma.”
This goes to show you just how badly obesity fucks up your body. You have all that excess fat, and yet your body is highly reluctant to oxidize it, preferring instead to store more and more fat.
The type of fat does make a difference- maybe even a big one. Medium-chain triglycerides, like coconut oil, have a much higher TEF than other fats, and eating them increases fat loss. Omega-3 fats, like you’d find in fish, also provide an increased TEF in men with metabolic syndrome– meaning they offset the reduced TEF from being obese. It’s not clear if they offer an increased TEF for lean individuals, but eating monounsaturated fat instead of saturated fat does improve fat loss results in overweight people.
Bear in mind, this is from consuming pure fat. Not a very fatty meal, but actually pure fat.
TEF of Carbohydrates
The thermic effect of carbohydrates depends on your insulin sensitivity, and maybe also your body fat percentage.
It’s higher in insulin sensitive people, and lower in insulin resistant people. The impact of body fat percentage is less clear. That study showed that the TEF of carbs is higher in lean people than obese people, but this study showed the reverse- it was higher in obese people.
The different findings in those two studies seem to be due to differences in methodology. Regardless, obese people are more likely to be insulin resistant, and lean people are more likely to be insulin sensitive. There’s not a one to one correlation between the two, but getting leaner will almost always improve your insulin sensitivity.
It’s also higher in high-fiber carbs. yes, celery burns a lot of calories- it just doesn’t have negative calories.
For relatively lean individuals with normal insulin sensitivity, the thermic effect of carbohydrates is around 15%- very close to the TEF of fat.
TEF of Protein
The thermic effect of protein is around 20%. Unlike fat and carbs, this seems to be very constant and unaffected by body fat percentage or insulin sensitivity, or anything else for that matter. It’s just 20%, plain and simple.
That should be considered in comparison to fat and carbs though. If you’re lean and insulin sensitive, that’s 20% vs 15%- a nice but modest increase.
If you’re obese and insulin resistant, though? That 20% looks amazing compared to the roughly 0% TEF of fat, or the 5-10% TEF of starches and sugars. Bro bodybuilders tend to eat more protein than they really need, but obese people could benefit tremendously from a high-protein diet.
We’re not done yet though.
TEF of Mixed Meals
So far we’ve looked at each macronutrient in isolation, but what happens when you mix significant amounts of all three of them in a single meal?
Simply put, it’s way better. Logically, you’d think a mixed meal would be a the weighted average of it’s components, but in this case the whole is greater than the sum of its parts. The TEF of mixed meals is around 13% in obese subjects and 25% in lean ones.
That begs the question- what constitutes a mixed meal? Do you need to have a totally even mix of macronutrients, or just a certain minimum amount of each, and there are diminishing returns beyond that? Right now we don’t have enough data to answer that.
Here are my best-guess general guidelines:
Protein: at least 25% of the calories in each meal if you’re lean, and 40% if you’re obese
Fat: at least 20% of the calories in each meal
Carbs: at least 20% of the calories in each meal. If you’re on a ketogenic diet, you obviously can’t go that high, but try to include like 5-10 grams of carbs with every meal to boost the TEF.
The X-Factor: Degree of Processing
Processed foods have significantly lower TEF than unprocessed foods. It can be nearly a two-fold difference, in fact– whole wheat bread with cheddar has a TEF of around 20%, vs 11% for white bread and processed cheese.
Processing food breaks down many of the chemical bonds that your digestive system would normally need to break down- in effect, processed food is partly pre-digested for you. That could be helpful for some people with impaired digestive systems, but it sucks for anyone who’s trying to watch their weight.
The Final Word: Eat Unprocessed, Mixed Meals, and Be Lean
In practice, the TEF of mixed meals can be anywhere from ten to thirty percent. In practice, it’s usually 10-15% if your diet is unhealthy (but still somewhat balanced and consisting of mixed meals), and 20-25% if you eat a truly healthy diet.
Based on my synthesis of the research and a little educated guesswork, here are some general guidelines for estimating TEF based on what you’re eating and how lean you are.
The need to eat mixed meals is yet another reason not to snack between meals- since snacks usually consist of just one food item, they’re not mixed meals.
The good news is, there’s no particular food you need to eat to maximize TEF. Adding some MCT or omega-3 fats with every meal helps a little bit, but the main thing is to eat unprocessed mixed meals with a decent amount of protein. As long as you do that, you can be very flexible about what you eat while still benefiting from a high thermic effect of food.