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 10th
edition p 143
