UNDERSTANDING THE THERMIC EFFECT OF FOOD

The thermic effect of food (TEF) is the energy required to eat and digest that food. The TEF is calculated as the percentage of the total calories in a given meal/food will be required to process that food. Most articles just casually mention it and most metabolic calculators just pretend like the TEF of any given diet always ends up at 10% even though this seems like a desperately random number that they use simply because it makes for an easier equation.

When discussing the thermic effect of food (TEF) many websites will sprinkle in the following numbers:

Protein is 20%.

Carbs are said to be between 5–10%

Fats are said to be between 0–5%.

Occasionally these websites will discuss some of the mitigating factors that can change these numbers but usually don’t really add any proper numbers to them. So here are the proper numbers as best we can figure (this science is by no means settled). Please understand that none of these studies have looked at large enough groups of people to be able to make truly accurate forecasts of the general population so please take these numbers with a healthy grain of salt (pun intended).

• Mixed meals (meaning they have carbs, fats, and protein) appears to increase TEF by as much as 2 fold. Obviously you can expect some degree of variability but there hasn’t been anywhere near enough research to understand what elements control that variability.

• Processed foods vs whole foods in the case of a mixed meal (meaning carbs, fats, and proteins) will decrease TEF by about half. It will do so for individual macronutrients as well but it’s less clear exactly how much as it depends on what type and degree of processing has occurred. General rule of thumb is that as processing goes up, the degree of the TEF will go down. So guacamole vs avocados is likely to give you an almost identical TEF. White bread will give you a smaller TEF than will whole wheat sourdough. And fruit juice will give a much lower TEF than will the actual fruit.
• People with insulin resistance have a lowered TEF than do people with a healthy insulin sensitivity by around 25% in mixed meals.
• People who are obese have a lowered TEF than do people at a healthy weight by about 45%.
• When you combine insulin resistance and obesity you are looking at about a 67% or ⅔ drop in TEF when eating a mixed meal. This, at least in part, explains why obese people with insulin resistance (which is most of them) seem to have so much difficulty losing weight. The same plate of food will effectively have a higher usable calorie count than would that food have if they had a healthy weight and good insulin sensitivity.
• People who are obese appear to have almost no TEF when eating fat as where healthy people can have as much as 14% TEF. If you are keeping score this means that obese individuals see a decrease in their TEF from mixed meals and meals that are almost entirely fat or carbs but effect is much greater in fat than carbs. I have seen the thermic effect of fat listed on at least one website at almost 20% but can’t figure out where that number came from which is why I capped it at 14% which was the highest number I could find the literature. Whether it’s 14% or 18% it is still a big difference in the effect of eating a single avocado or serving of bacon.
• Older people have a lower TEF than younger people but only by about 2% as best I can tell. So technically a factor but not a big one.

• People who exercise regularly have a greater TEF than do sedentary people by somewhere in the realm of a 10–20% increase to the TEF (that is really just a ballpark guess). So this means that a mixed meal with say a TEF of 20% for an active person might get as low as 16% simply on the basis of being sedentary. Unfortunately we don’t really know what the differences are between say weight lifting and endurance training are here so we are stuck using the incredibly generic term “regular exercise”.
• Breaking a diet up into much smaller meals appears to lower TEF with the TEF dropping off to near zero if the meals get below 500ish calories (this is my best guess rather than rigorous science). Conversely, one large bolus meal will actually increase TEF on a calorie for calorie basis (reference, reference). This gets further compounded by realizing that this is particularly true of protein and hydrolyzed starches (rice, potatoes, pasta, etc.) (reference). This gives some insight into how traditional bodybuilding diets, which are designed to put weight on do so by increasing the quantity of the meals instead of making the meals bigger. Keeping the meals of moderate size helps to keep the overall TEF down to the extent possible considering the high protein content of their diets. Put another way, this helps us explain an existing phenomenon we already see in the real world of how bodybuilders and other athletes that want to gain weight or struggle to maintain weight must eat more meals instead of bigger ones.
• One thing nobody has yet tested is the difference between the TEF of say a boneless skinless chicken breast and that of a ribeye steak at isocaloric values. In other words, does the naturally added fat in beef protein increase the TEF when void of any carb sources? This is commonly done in nature in most forms of meat, (even poultry has this naturally occurring fatty covering we call skin) but we still haven’t yet investigated it scientifically. What this actually means is that the most common way in which the human race eats meat, eggs, poultry, and fish has a yet unknown effect on TEF when simply eaten by itself. So as absurd as this sounds, I can’t tell you if eating a ribeye steak has a higher or lower TEF calorie for calorie than eating a boneless skinless chicken breast. I don’t know about you but as someone that eats steak and eggs for nearly every morning for breakfast I find this really annoying. I suspect the answer is yes but I have no evidence to back this up.
• Another ignored finding is that protein powder, unsurprisingly, lowers the thermic effect of protein from a baseline of 20% in the case of milk down to as low as 12% (reference). To be fair these protein drinks were only 50% protein, 40% carbs, and 10% fat. But because all the nutrients were processed the overall effect seems to be much lower than they would be otherwise. Again, in the interest of scientific honesty I can’t guarantee these numbers but am guessing at what they imply which is protein powder probably knocks down the thermic effect of protein from 20% down to around 15%. There is another form of whey protein called hydrolysates that are effectively predigest proteins. Unfortunately no one has ever tested their effect on the TEF (at least none that I could find) so any number I give you would be pure speculation. The best I can tell you is that it is likely to be lower than 15%.
• There does seem to be some evidence that omega 3 fats have a higher TEF by 50% according to the abstract. Unfortunately I could only find the one study and I don’t know how reliable it is because you can’t see what the control diet was that they were comparing it to unless you pay for the year long subscription to the Canadian Journal of Dietetic Practice and Research which is essentially $25 for the right to read a single study. If someone can access this article and can break it down in the comments I would be grateful.
• There is a decent increase in TEF from MCTs (medium chain triglycerides) which almost entirely come from coconut and palm oil but by how much this will increase TEF in a real world setting is not at all clear. I say this because the study I linked to used approximately 45 grams of pure MCTs which would be roughly 80 grams of coconut oil or 3 fluid ounces of coconut oil. That is the coconut oil equivalent of ¾ of a stick of butter. And unless you are doing the keto diet that seems a bit extreme. In any case cooking with coconut oil vs butter or olive oil will probably increase the TEF of that meal but by how much is not at all understood and would likely change greatly based on the overall meal and how much coconut oil you are cooking with.
• A few websites claim overtly or more under wraps that fiber adds to the TEF but I actually couldn’t find evidence of that effect. The one study I could find done to look at the thermic effect of fiber found no thermic effect. To be fair the fiber was in the form of a supplement (derived from barley) instead of a whole food. I imagine, much the like ribeye with all its naturally occurring fat, that whole fruits and vegetables with their naturally occurring fiber adds something to the TEF but we have no idea what that is.

What all of this information really does to the initial numbers surrounding the TEF for the 3 macronutrients are as follows.

• Protein 15–30% (and if its a whey hydrolysate protein powder it is almost guaranteed to be lower than 15% but we have no idea by how much).
• Carbs are 5–10% but it could be lower than 5% depending on if the carbs are in liquid form and who is drinking them but again we don’t know how much lower than 5% it would go.
• Fat’s thermic effect is 0–15%. The degree to which obesity changes this number is staggering as at least half of it and even possibly all of it appears to be based on obesity. An unknown and likely small amount of that difference any individual would see could be coming from the addition of omega 3s or MCTs but it is unclear how much that be and would almost guarantee to be different to each individual’s biology and dietary context.

Now one article even took the time to create a really interesting infographic with how different types of people might have different TEF percentages depending on what is eaten and so on. The only problem with it was that I can’t figure out how they got many of their numbers. It seems a bit murky and many of the biggest claims it made weren’t given any references like when they claimed that in-shape healthy people could have as high as an 18% thermic effect of fat (a very bold claim that I couldn’t find in the literature).

So instead of an infographic that has numbers that aren’t likely to be accurate for the general population let’s divide this up into things that tend to increase the TEF and those that tend to decrease the TEF. For simplicity purposes I am also going to assume that you don’t sit around eating individual bowls of single macronutrients. That being said I have met a few people that resorted to eating nothing but plain boneless skinless chicken breast for a week but I don’t think that is sustainable or advisable. An interesting aside, thanks to the carnivore community it does seem you can eat nothing but ribeyes for the rest of life and be quite healthy, we just don’t know what exactly is happening to the TEF.

Here are the things that will increase the TEF of most any given mixed meal or snack on a per calorie basis in roughly the order of largest to smallest magnitude.

1. Eat more protein
2. Eat more whole foods and less processed foods
3. Exercise regularly
4. Get or stay lean
5. Improve your body’s insulin sensitivity (probably by reducing dietary carbohydrates)
6. Increase the size of your meals and decrease their frequency
7. Eating a meal that is naturally rich in omega 3s, particularly EPA and DHA
8. Using coconut oil in place of butter or vegetable oil
9. Simply being younger (if you are a senior citizen that figures out how to do this please contact me immediately)

So that was the easy part. Now we have to figure out the hard part. How much, if any of this actually matters? First we’ll need to dispel a couple of conclusions that many people come to which actually don’t seem to be true and let’s talk about the few conclusions that actually do make really good sense in light of what we’ve learned.

Myth 1: Because carbs have a higher TEF than do fats we should focus on low fat high carb diets. This is just silly for the following two reasons. First, remember that insulin resistance decreases our TEF so any increase in the TEF you get from eating more carbs will likely contribute to insulin resistance, which will consequently lower your TEF. It’s a classic vicious circle. Higher carb foods also tend to be less satiating and thus lead to increased consumption. This is why 300 calories worth of raspberries will stave off hunger for a much shorter period of time than 300 calories worth of fresh avocado. This is despite both of them having almost identical amounts of fiber and the raspberries actually have substantially more water. Carbs just aren’t very satiating when compared to protein and fat. This doesn’t represent sufficient evidence that high carb low fat diets can’t work. It simply means that any increase in the thermic effect of low fat diets is very unlikely to play a substantial role in successful weight loss vs other weight loss diets. Ultimately whether or not high carb low fat diets are optimal for weight loss is a much bigger discussion than the scope of this article.

The second myth this busts is the notion that snacking or including many smaller meals somehow kickstarts or revs up our metabolism. In actuality fewer meals that are large in size are better at increasing the TEF of each calorie one eats in a given day than a bunch of small meals. This is partially why bodybuilders and other strength athletes that want to gain weight usually eat a greater number of moderate sized meals throughout the day. The other reason, and probably the larger reason is that it is a more effective strategy for increasing one’s overall caloric intake. This would help to explain why many people seem to be able to start losing weight just by reducing their meal frequency down to one or two large meals a day without even trying to limit their food choices. But don’t think of this as a silver bullet. For some people going too long without food makes their hunger response so strong that by the time they finally eat their plate is chock full of all the bad foods for most any diet like donuts, pizza, sugary drinks and so on. This works so long as the eater can still exert necessary control over what they are eating.

Another key takeaway is that the more overweight, obese, and out of shape we are, the smaller our TEF becomes and the harder it seems to be to lose weight, particularly when you are eating a mixed diet (that is a diet with carbs, protein and fats) as most Americans are. If you are obese, have insulin resistance, and don’t exercise the exact same meal may burn 67% fewer calories processing it than if another fit, lean, and insulin sensitive version of you ate the exact same meal. The effect to your biology is that a given 1000 calorie mixed meal will give the obese, insulin resistant, out of shape version of yourself an extra 75–150 calories. So just eating three 1000 calorie meals each day would be the equivalent of eating an extra 225–450 calories each day. I can’t say for certain that we’ve completely connected all the scientific dots to be able to say that this is exactly how it works. This means that the circumstances of their biology (obesity, sedentary lifestyle, and insulin resistance) literally make the food on their plate more calorie dense than it would be for a healthier version of themselves. Sadly this message is not taught to anyone probably due to a combination of ignorance and a fear that telling people the truth of their circumstances would somehow be a form of fat shaming.

This would also help explain why healthy lean people that exercise seem to have this ability to absorb dietary discretions in a way that those of us who are overweight or obese can only dream about. I actually see this as really helpful because it means that even if we think we have bad genes getting lean and in shape can actually help make anyone more resilient to weight gain in the future regardless of genetics and that is encouraging.

Special Note: I suspect that much of this adaptation that causes lean people to have higher TEFs comes not just from being lean and in shape but is what happens to us over time as we maintain our fat loss. The longer we maintain our fat loss the more our biological set point resets itself to the lower weight. I think that part of that resetting is actually the body increasing its TEF. But, full disclosure, I know of no science that has even tried to test this hypothesis nor do I know of any mechanisms that could help explain it. It’s just an untested hypothesis so take it with a grain of salt.

The final takeaway shouldn’t surprise anyone, which is that protein is the best macronutrient to focus on for both weight loss as well as muscle gain. Is it any surprise that protein, which seems to be nearing the top of almost everyone’s diet food list is also the one macronutrient which has a TEF that doesn’t meaningfully change if you are fat or not, insulin resistant or not. If you eat a chicken breast you will burn the same amount of calories digesting it as anyone else regardless of your level of health, fitness, or leanness. This would help explain why everyone out there doing the carnivore diet seem to all get almost universal weight loss benefits regardless of their starting point, body type, digestive problems, and so on. Whole food based protein and particularly animal protein seems to have an across the board universal fatloss capacity. And even our limited understanding of the thermic effect of protein helps explain that phenomenon.

The final review tells us the following three truths surrounding TEF:

• To maximize your TEF and the weight loss potential of your diet eat only 1 or 2 large meals per day of whole foods consisting of a healthy dose of protein.
• To minimize TEF and increase your ability to gain or maintain weight eat a greater number of small to moderate meals throughout the day with the addition of some processed foods like white rice to further reduce the TEF and potentially increase potential weight gain.
• The benefits of TEF are drastically reduced the more unhealthy we are and the more processed our diet becomes so if you are looking to lose a lot of weight you need to understand the importance of working that much harder to eat healthy whole foods, plenty of protein and get some exercise.

Cautionary Note: Some people will read this and think it is an argument that exercise will directly lead to weight loss through the increase of the TEF. This is simply not the case or at least not the case in any kind of universal way. Some degree of exercise does seem to help some people lose weight but it also has a tendency to make us hungrier. The best middle ground I can recommend is to go for short 10 minute brisk walks after meals and try to get some strength training in at least once a week and preferably two or three sessions a week.

The not so surprising conclusion of all this is that the basic components of our diet are far more important to improving our health and consequently our TEF than focusing on the minutiae of the diet. Whether or not to cook with coconut oil vs extra virgin olive oil, what type of exercise we should be doing instead of just finding some method of exercising and getting our bodies moving, or arguing over whether or not the increased thermic effect of carbs makes them more useful in losing weight than fat just aren’t helpful areas of the overall health discussion. So if you know you aren’t hitting your weight loss goals try returning to the basics. When I’ve worked with weight loss and weight gain clients in the past it is amazing how often my clients failure was rooted in the fundamentals rather than the minor aspects such as which non-caloric sweetener they are using. So I hope you incorporate one or two of the three fundamental takeaways of the Thermic Effect of Food and see if it improves your health and/or performance. Above all, keep learning about how your health works and what works to improve it.