Obligatory disclaimer: I am not a medical doctor, and the content of this website was created for informational purposes only. Such content is not intended as a substitute for medical advice, treatment or diagnosis.
We’ve previously talked about how the body responds to long term caloric restriction, and we’ve also dipped our feet into some of the consequences that restriction places on metabolism. This time, however, we will lean what happens when no caloric restriction takes place, and we eat and drink as usual.
Remember, metabolism encompasses all the chemical reactions that sustain an organism’s life, and some of those reactions deal with how food is converted into our body’s energy. Furthermore, if decreasing our caloric intake impacts our metabolism, then it stands to reason that eating also impacts our metabolism. Let’s learn if that is indeed the case and what actually takes place. For this, we’ll use the following reference: Secor, S. M. (2009). Specific dynamic action: a review of the postprandial metabolic response. Journal of Comparative Physiology B, 179(1), 1-56.
There have been many investigations into the changes in metabolic rate after eating a meal, and because of this the Thermic Effect of Food has amassed a large number of names. Among them we find Specific Dynamic Action (SDA), Heat Increment of Feeding (HIF), Diet Induced Thermogenesis (DIT), Thermic Effect of Feeding (TEF), and the Thermic Effect of Food. This is because different researchers focused on different aspects or mechanisms of the same thing, and so came up with several names. We won’t follow the example set by our reference and just refer to the Thermic Effect of Food as the Thermic Effect of Food, after all, how many people will look for Specific Dynamic Action? Alright, so what is the Thermic Effect of Food? According to our chosen reference,
[the Thermic Effect of Food] is the accumulated energy expended (or heat produced) from the ingestion, digestion, absorption, and assimilation of a meal.
In other words, all the Thermic Effect of Food is just all the energy required to process whatever it is that we have eaten.
So now that we know what TEF is, how is it measured?
- If TEF corresponds to the change of metabolic rate after a meal, then we must know what the metabolic rate was before the meal, right? So the first step is to establish what the Baseline Metabolic Rate is. For warm-blooded animals, endotherms, the Basal Metabolilc Rate (BMR) is that Baseline Metabolic Rate.
- The animal is fed a meal to satiety or so that the meal is a set percentage of body mass.
- After feeding, the metabolic rate is measured continuously or periodically. The results of such measurements are plotted on an xy graph, where x is the time after feeding, and y stands for the metabolic rate. We won’t get into how the actual measurements are done.
A picture is worth a thousand words
The reference that we’re following has an amazing image of the sorts of graphs that can be obtained with the previous method. I think that image is so good at helping understand what TEF is that I’ll post it as well.
If that graph confuses you, don’t fret, because its rather simple to explain. Prior to meal ingestion the metabolic rate corresponds to the BMR, which explains why its value is so low. After meal ingestion, the metabolic rate rises in order to process the meal itself, such increase in metabolic rate reaches a peak and then slowly declines until the metabolic rate returns to its BMR value. So how does that graph tell us the TEF? TEF, which is called SDA in the graph, corresponds to the area under the curve. If you’re into calculus, that last statement must have made your heart beat with joy.
So from that graph several variables can be identified: BMR, the value of the postprandial (after meal) peak, the time that it takes to reach the peak, the duration of TEF, and TEF itself. Those variables let us know quite a lot about TEF and its behavior.
That’s great and all, but how does this apply to humans?
There have been several studies on how TEF is impacted due to exercise, pregnancy, stage of the menstrual cycle, stress, age. Even the effect of watching horror vs romantic films has been studied! I wonder if they played Hereditary in that study:
Anyway, from what Secor says human TEF is modest, as the metabolic rate shows a 25% increase above fasting rate that lasts between 3 to 6 hours. However, one would expect that meal size, meal energy content, body composition, and body size would have an effect on TEF, right?
Well, indeed! For instance, the greater the amount of energy in food, the greater SDA will be. This makes perfect sense, it takes more energy to process a meal that has more energy. On the other hand, meal composition is also important,
[TEF] is affected by the interactions among the relative amounts of proteins, carbohydrates and lipids
Wikipedia cites some numbers corroborating this, but Wiki’s references are older than the paper I’m using, so take them as you will:
- Carbohydrates: 5 to 15% of the energy consumed
- Protein: 20 to 35%
- Fats: at most 5 to 15%
What if energy content remains the same, but meal size differs (like eating a chocolate bar vs eating a ton of lettuce)? Well, meal size is also a factor to consider since the greater the size of the meal, the greater the TEF will be as well. It is important to note that this occurs not only by having a greater TEF peak, but by its duration increasing as well.
One factor that I wasn’t expecting to see is meal temperature. For endotherms (remember, warm-blooded animals), food tends to be at a lower temperature than their body, so some energy is expended in heating the meal upon ingestion. Of course, warming a large-cold meal involves a greater amount of energy than warming up a small-already-warm meal.
Finally body matters. In terms of body size, a greater body size increases BMR. This makes sense, since there is more mass that requires energy to keep alive. This also means, however, that TEF is also greater with a larger body size. As to body composition, it’s not clear whether it actually has an impact on TEF; some studies have evidence that it does, others show that it doesn’t.
So that’s it, we’ve learned what the Thermic Effect of Food is, even if it’s just at surface level.
I hope you enjoyed the post, see you next week! If you liked this post and would like to see similar material, please visit the Rebuttals to Fatlogic section of the blog.