Diet-induced thermogenesis


Joel, a reader, wrote to me and asked which of the four macronutrients in food required the most kilojoules/Calories to be digested. The four macronutrients are protein, fat, carbohydrate and alcohol, the only ones that provide kilojoules/Calories in the diet.
His question related to the fact that we burn kJs to digest food, that is, you have to burn some kJs to actually digest and absorb the kJs used by your muscles, heart, brain, liver, kidneys, lungs etc. This is called diet-induced thermogenesis (DIT).
The textbooks will tell that an average 10% of ingested kJs form the DIT. For example, if you ate 10,000 kJs (2400 Cals) then about 1000 kJs will be used up to digest the food. So, which of the macronutrients “cost” the most kJs to digest and absorb?
And the winner is … 
It was the first time I had ever been asked that and I hadn’t clue, and that meant some detective work. You may have had your Basal Metabolic Rate (BMR) measured. After an overnight fast you lie very quietly in a room with no distractions, a pleasant temperature, breathing through a mouthpiece for 30 minutes. Clever people then determine how many kJs you need for your baseline metabolism to stay alive. The BMR is about 5% higher than your sleeping metabolic rate.
Now if you eat a meal and get re-measured under the same conditions you can work out from the difference how many kJs are needed to digest that meal. This time the test won’t be for 30 minutes because the process of completely digesting a meal and then absorbing the nutrients is likely to take 6 hours or so. Yes, you would have to pay me to do the experiment too. I mean, you can’t even watch telly during the test.
Although it would be nice to give you very precise numbers, nature is never that accommodating. Here is an approximation of the “cost” of digestion:
Fat: 0-3% 
Carbohydrate: 5-10% 
Protein: 20-30% 
Alcohol: 10-30%
Protein satisfies for longer
Mmm, so it doesn’t take much effort to digest fat. It is quite easy to break the bonds in a fat molecule and absorb the resultant fatty acids. Protein requires a bit of effort to break it apart with digestive enzymes before absorbing the amino acids. This may partly explain why high protein foods satisfy the appetite more than carbohydrate or fat.
Apart from digestion and absorption, where do the other calories go?
Just to complete the picture, aside from digestion, the rest of our kJs or Calories are burned to keep us alive (your BMR – heart beart, liver function, breathing etc.) and to keep us active (your daily activity: walking, running, popping off to the gym, cleaning your teeth etc). If there are any kJs left over, you know where they are stored.
Why do I keep mentioning both kJs and Calories (officially written with a capital C because it is an abbreviation of kilocalorie. Why don’t we therefore say Joules? Dunno, sorry). For those of you who went metric last century, or the one before, I remind myself that we have lots of good folk living between Mexico and Canada who still use Calories. Hate to leave them out of the conversation. Simply put, 4 kJs = 1 Cal. Close enough.
What does it all mean?
Not a great deal in reality. You don’t eat just a single macronutrient at a meal. It will be a mix of protein, fat, carbohydrate and alcohol if you have a glass of wine with the meal. Different combinations and different quality foods will take more or less time to digest.
You can bet that a meal of legumes, vegetables, mushrooms and nuts will take more kJs to digest than a donut, meat patty and a soft drink, even if they were matched for protein, fat and carbohydrate. I recently read that about 15% of the fat in nuts passes out the back end. That is fat neither digested nor burned, yet it is still “kJs in”.
The DIT is more an academic exercise than a guide to constructing your next meal. The figures are only approximations. Minimally processed foods are likely to require more energy to digest than a highly processed food. Anyway, thanks to Joel I’m a little wiser. Hopefully you are too. Eat well.
Selected references:
·      Westerterp KR. Diet induced thermogenesis. Nutrition & Metabolism 2004 
·      Shils ME et al. Modern Nutrition in Health & Disease 10thedition p 143 

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