Our body is forced to spend dif­fer­ent amounts of ener­gy to digest dif­fer­ent foods.

Many peo­ple do not real­ize that food not only con­tains ener­gy in the form of calo­ries, but also requires ener­gy to be digest­ed and absorbed. That is, in order to process what you eat, the body must expend a cer­tain amount of ener­gy. This is called the ther­mic effect of food (TEF).

What is the thermal effect?

Gen­er­al­ly, it is often said that the TEF of the food we eat is about 10 per­cent. In oth­er words, our body requires 10 per­cent of the ener­gy con­tained in the food itself to digest and absorb it. How­ev­er, in real­i­ty, dif­fer­ent types of foods have wide­ly vary­ing TEFs, large­ly depend­ing on their under­ly­ing macronu­tri­ent com­po­si­tion.

TEF of various macronutrients

Pro­teins, fats and car­bo­hy­drates require dif­fer­ent amounts of ener­gy to be absorbed by our body. Pro­tein has the high­est ener­gy “cost”, requir­ing approx­i­mate­ly 20–25 per­cent of its ener­gy to be processed. This is one of the many rea­sons why eat­ing a high-pro­tein diet is recommended—calorically, you can con­sume more pro­tein than fat or car­bo­hy­drates with­out gain­ing weight. Car­bo­hy­drates, in turn, are much eas­i­er for the body to process than pro­teins, and the body only requires about 6–10 per­cent of their ener­gy to use them. Final­ly, fats only require about three per­cent of their ener­gy to digest.

What does this mean for weight loss?

The ther­mic effect of food can rarely be of deci­sive impor­tance in the diet. As you can see, the dif­fer­ence in TEF between fats and car­bo­hy­drates is neg­li­gi­ble. Essen­tial­ly, for every 100 calo­ries of fat you replace with car­bo­hy­drates, you’ll burn an addi­tion­al three calo­ries per day.

How­ev­er, pro­tein requires a much high­er ener­gy expen­di­ture than oth­er macronu­tri­ents, so eat­ing a high­er pro­tein diet and thus replac­ing some fat and car­bo­hy­drates with pro­tein foods will result in you burn­ing more calo­ries through­out the day. For exam­ple, if you replace 100 calo­ries from fat with 100 calo­ries from pro­tein, assum­ing pro­tein requires about 20 per­cent of its ener­gy for diges­tion, you’ll get a 17 per­cent dif­fer­ence in the ener­gy cost of pro­cess­ing the two macronu­tri­ents. That is, for every 100 calo­ries you replace with pro­tein, you will burn an addi­tion­al 17 calo­ries.

Does this mean that a protein diet is better than others?

Don’t rush to con­clu­sions. Even though pro­tein has a high­er TEF, get­ting too many of your dai­ly calo­ries from pro­tein alone is not a good idea. On the one hand, pro­tein is a more expen­sive plea­sure. In addi­tion, it is not very pleas­ant to eat it sep­a­rate­ly. But more impor­tant­ly, both car­bo­hy­drates and fats play impor­tant roles in the func­tion­ing of the body, even if they are eas­i­er to process.

Fat is impor­tant for reg­u­lat­ing hor­mones and also helps improve the taste of food. And car­bo­hy­drates are need­ed as a short-term source of ener­gy for the body — they help replen­ish glyco­gen stores in the liv­er and mus­cles and allow for more pro­duc­tive work­outs. In addi­tion, car­bo­hy­drate-rich foods pro­vide us with the dietary fiber and oth­er impor­tant nutri­ents we need for health. We pre­vi­ous­ly wrote about how low-carb diets can harm you. There­fore, in most cas­es, a rea­son­able per­cent­age of pro­tein (no more than 30–40 per­cent) in the diet is rec­om­mend­ed, with the rest com­ing from car­bo­hy­drates or fats.

Conclusion

In truth, the ben­e­fits of try­ing to manip­u­late TEF to lose more weight are ques­tion­able. You are unlike­ly to cre­ate sig­nif­i­cant addi­tion­al ener­gy pro­cess­ing costs for your body, and in fact it may cause more prob­lems than it is worth. You will have much more suc­cess in con­trol­ling your ener­gy bal­ance by reduc­ing some of your ener­gy in than by try­ing to manip­u­late your ener­gy out by hav­ing a high TEF.

It’s also worth men­tion­ing that the entire myth of neg­a­tive calo­rie foods is large­ly based on a mis­un­der­stand­ing of the ther­mic effect of food. For a food to tru­ly be neg­a­tive calo­rie, it would have to have a TEF close to 100 per­cent, which is sim­ply not pos­si­ble.

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