Suppose I’m sitting here eating lunch. Let’s say the label on whatever I’m eating says it has 500 calories.
It occurs to me that this can mean three things.
The food contains a higher number of calories (say 1000) but my body will only absorb 500.
The food contains 500 calories and my body will absorb 500.
The food contains 500 calories and my body will absorb somewhat less than that (say 250).
Which one is it? Is my digestion efficient enough that I absorb all possible calories from the food? (Seems unlikely.) If not, does the label indicate the number of calories the food actually contains or the amount I’m likely to absorb from it?
And if digestion is less than 100% efficient, does the efficiency vary depending on what I’m eating; say a candy bar vs a hamburger vs a plate of broccoli?
I remember hearing that fat takes the least but this link says the opposite. It’s very likely that simple carbs take less energy to metabolize than complex carbs.
There are two different bits here … what does the body absorb, and how much energy does it take to digest it.
The absorption question is mostly related to the insoluble fiber content of the food. Is fiber counted as a calorie source on the label? How much energy do we get from the fiber? Here’s how one source addresses those questions:
The other issue, the efficiency of digestion, is what is known as “the Thermal Effect of Food” or “Diet Induced Thermogenesis.” Truth be told the actual numbers likely vary by individual and by age and level of obesity but the standard is as follows:
The Thermal Effect of Food, that is the loss of energy required to get the energy out of the food, is not included in the label in any case.
They originally tested the number of calories in food by burning the food and measuring how much heat comes off. Of course cellulose (aka non-soluble fiber) burns quite well and cows can digest it, but us humans can’t.
The numbers on labels are rough estimates, as our food comes from (previously) living organisms, so any food that’s not refined to its constituents will exhibit a significant amount of variation.
Interesting but a bit incomplete. The Atwater system does take into account digestive losses but that means energy lost in urine and feces. That is they did a bomb calorimeter of the food and one of what came out and the difference was the calorie value of the food. It does not take into account the energy needed to process the food. That’s the part covered in the Thermic Effect of Food (aka Diet induced Thermogenesis or Specific Dynamic Action)
Some of the absorption will depend on how thoroughly you chew the food.
For one example, late summer, when delicious fresh sweet corn is available here in Minnesota, I tend to overindulge, and eat it rather fast, not chewing as completely as I probably should.
About 24 hours later, I can clearly see that many kernels of that corn have passed thru me quite unchanged. No doubt much of the calories in those kernels have not been absorbed by my body.
Clean the bowels and digestive tracts of 100 people, feed them an identical diet for 24 hours and then collect their shit until they are cleaned out. burn the shit and find out how many calories are left in it. Probably has been done many times I would imagine.
Yeah, I’m seeing it’s more complicated than it sounds. But it does sound like giving up a glazed donut for breakfast really doesn’t remove 250 calories from my diet but something less. Obviously “counting calories” is more complex than simply “counting calories”.
First statement is false, mechanical grinding contributes very minimally to digestion compared to the chemical work done in the stomach. And you have surely previously observed that things like corn and peas can easily pass through the digestive system unfettered.
There is more on this in “Gulp: Adventures on the Alimentary Canal” by Mary Roach.
It’s far more complex since the body’s hormonal response to different forms of calories can vary somewhat. Eating 100g of plain sugar will not produce the same energy storage/use pattern in the body as eating 100g of plain butter, more difference than the difference in thermic effect will account for. However if you were eating a glazed donut regularly for breakfast and remove it from the menu of course this will prove a noteable difference over time. Don’t get caught up in the numbers game (the scale and the food numbers) if you’re trying to lose weight… it’s actually all pretty blurry when you try get up close.
There is some of option 1 involved sometimes, too.
In a class, I did bomb calorimetery on regular and sugar-free chocolate, in which the sugar had been replaced with a very slow-digesting artificial sweetener. In the bomb, they gave off roughly the same amount of heat, but the calorie count on the package was significantly less. So there is at least some amount of “there are X calories here, but you’ll only be able to digest and absorb Y” going on.
I have read that we get relatively few of the calories in raw carrots, but we get them all when cooked. The book “Catching Fire” by Richard Wrangham on the invention (or discovery) of cooking makes this point very clearly. It is very difficult to get many calories from most raw foods.