The thermic effect of food (TEF) describes the increase in energy expenditure as a result of digestion, absorption, and storage of food.
In the course of this article, we will explore what the thermic effect of food is and if there is any practical implication.
The Thermic Effect of Food: The Science
Protein is a macronutrient that increases metabolism the most (in the course of digestion). This is true because protein has a thermic effect of 20-30%, with carbohydrates at 5-15% and fats at 3-4%. However, meals are rarely made from just one macronutrient, so mixed foods have an average TEF of 10%.
If two individuals – one with normal and one with a sub-optimal thermogenic response to the meal – increase their food intake, then the former will not gain as much weight as the latter. Some studies show differences between overweight and lean individuals. Much of the research indicates that lean individuals have a higher TEF than do obese people, while other studies could not identify such differences.
A review by Jonge and Bray in 1997 took into account 49 studies. Approximately 60% of the studies found that TEF was higher in lean individuals than in overweight individuals. A more recent review by Granata and Brandon in 2002 came to a similar conclusion: out of a total of 50 studies considered, 60% showed a higher TEF in lean individuals compared to obese individuals.
Tataranni et al. performed an experiment in a respiration chamber. The researchers came to the conclusion that the body weight has no association with the TEF value. At this point, it may be worth noting that the average body fat percentage of the individuals was 30% ± 10 (men) and 40% ± 10 (women). The participating researchers, however, found a link between insulin resistance and lower TEF, which also has a stronger relationship with obesity in the literature.
Numbers up to a TEF value of 30-35% were identified for protein. However, since carbohydrates and fats are ingested with the protein in practice, we usually consume mixed meals. That’s why, as a lean person, you may find it more beneficial if you increase your protein intake (as long as your protein intake is high enough, of course).
Let’s take a look at two studies by Antonio et al. where the high protein groups consumed 3.4g / kg and 4.4g / kg protein. In these two studies, there was no difference in improving body composition in the 4.4g / kg protein per day group and the low protein group. Furthermore, there were minor improvements in body composition in the 3.4g / kg protein per day group (compared to the low protein group).
To be more precise, the body fat percentage decreased by 1.8% more in the 3.4g / kg group compared to the low protein group. This can be explained either by a higher adherence to training (again compared to the low protein group), a higher NEAT in the high protein group or an over- / underreporting in the reproduction of the diet.
Another important point: in the 4.4g / kg protein per day study, the dropout rate was very high and some of the subjects said it was too difficult to follow such a high protein diet. In the 3.4 g / kg protein per day study, the dropout rate in the High Protein group was also higher, whereby the high dropout rates in both studies can be partly explained by the fact that there were more participants in the high protein groups.
In fact, the researchers distributed the participants unequally in the groups because they wanted to consider the potential adherence deficiencies of the high-protein groups.
The protein intake of bodybuilders goes up to 4.3g / kg, but it remains doubtful whether it has any benefits. The reduction of fats and carbohydrates could well have been counterproductive as it has a profound effect on hormones, vitamin intake, performance, recovery and so on. If you are overweight/obese, then it seems to be at least a good idea if you follow a relatively high protein diet, as the mixed meal in the study only has a TEF of 10% (versus 18.7% for protein)
Why are there studies in this regard that contradict each other?
As you have seen, it seems that not all studies come to an identical conclusion, but why is that the case?
First of all, methodological factors, such as Meal size and composition, digestibility, timing, measurements below 3 hours, short duration, measurement methods and equipment, environmental factors and heterogeneity of human obesity. Granata and Brandon mention that in both their and the review of Jonge and Bray, most studies that used TEF measurements of less than 3 hours had shown that obesity levels were lower. In studies that performed measurements over 3 hours, a decreased TEF value in overweight individuals was rarely reported.
Most studies use variable calorie loads based on body weight or lean mass (FFM), while other studies provide the same number of calories for all participants. However, there are problems with both methods, making the results difficult to compare and conclude. The amount of TEF depends very much on the calorie content of a meal. So, if meal size is dosed in relation to body weight or fat-free mass, overweight subjects will be given larger meals, which could lead to bias when compared to lean individuals.
On the other hand, we have the situation where both subjects receive the same amount of nutrients, so the TEF increases less in overweight individuals because the basal metabolic rate (BMR) of the overweight is higher. However, this was not the case when both lean and overweight subjects ate meals that were equivalent to 35% of their BMR. Jonge and Bray’s review speculates that such factors as BMI were used, rather than body fat, so there was no such large gap between the upper limit of the lean group and the lower limit of the overweight group. This could have led to an overlap in the body fat percentage and thus a misclassification between the two groups, which in turn could have reduced the chance of finding a potential effect on TEF with varying body fat levels.
Reasons for a low Thermic Effect of Food
If overweight people really have a low TEF value, what could be the cause?
Recent studies suggest that decreased TEF in overweight people is related to impaired glucose tolerance and insulin resistance.
From Jonge and Bray’s review, we know that greater insulin resistance and a higher body fat percentage lead to a lower TEF. The same researchers speculate that a decreased sympathetic nervous system and a higher age could play an important role in this.
Granata and Brandon seem to agree that higher age reduces TEF but suggest that sympathetic system theory is purely speculative. It is commonly believed that a reduced rate of non-oxidative glucose storage plays an important role in the case (which implies greater energy costs compared to glucose oxidation).
Other explanations that are also mentioned are decreased thermogenesis in brown adipose tissue and skeletal muscle. Tateranni et al. also mention reduced spontaneous physical activity in people with low TEF. Another conjecture is that overweight individuals have a decreased sensitivity to thermogenic hormones, which are normally stimulated by eating a meal. One reason for this could be a predominantly sedentary lifestyle of many obese people.
Unless it is true that overweight persons have a lower TEF value, the scientists do not seem to agree whether this is a consequence of excess weight or merely favors it.
Conclusion: Thermic Effect of Food
If you are slim, your TEF may go up to 25% for a mixed meal (based on the result of a study). However, since research in this area is anything but clear, you should rather assume 10-25%. However, research indicates a higher TEF in lean individuals, so perhaps you could – possibly – tolerate an extra ball of ice cream without having to feel guilty if you’re slim and slim.
If you are overweight or obese, then you should stay on the safe side and assume that your TEF value is lower (compared to lean individuals). A TEF value of up to 10% seems realistic.
Based on the available data, it becomes clear that we need more controlled research in this area to be able to draw more concrete conclusions.