# Digestion efficiency and calorie counts **URL:** \ **Category:** Factual Questions\ **Created:** [September 3, 2014, 7:00pm UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396 "2014-09-03T19:00:44Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![tanstaafl](https://avatars.discourse-cdn.com/v4/letter/t/ba8739/32.png) [@tanstaafl](https://boards.straightdope.com/u/tanstaafl)\ **Post date:** [September 3, 2014, 7:00pm UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/1 "2014-09-03T19:00:44Z")
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. 1. The food contains a higher number of calories (say 1000) but my body will only absorb 500. 2. The food contains 500 calories and my body will absorb 500. 3. 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? ---
**Author:** ![MichaelEmouse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/michaelemouse/32/101_2.png) [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)\ **Post date:** [September 3, 2014, 7:08pm UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/2 "2014-09-03T19:08:44Z")
Number 3. Yes, it does depend. Simple carbs take less energy to digest than protein. I’ve heard 30% of energy for protein a few places, including here: [https://healthesolutions.com/protein-takes-more-time-energy-effort-to-digest-than-fats-or-carbohydrates/](https://healthesolutions.com/protein-takes-more-time-energy-effort-to-digest-than-fats-or-carbohydrates/) 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. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [September 3, 2014, 7:54pm UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/3 "2014-09-03T19:54:13Z")
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](http://www.wisegeekhealth.com/do-fiber-calories-count.htm) addresses those questions: > [@](#): > > How much energy is in fiber, and therefore how many Calories are in fiber, is still unclear. According to a 2002 report by the Institute of Medicine (IOM) regarding dietary reference intakes, the energy yield of fiber when consumed by humans is somewhere in the range of 1.5 to 2.5 Calories (kCal) per gram (6.3 to 10.5 kilojoules per gram). > > Whether fiber Calories are included in food nutrition labels varies among countries. Some countries do not include fiber Calories. In the US, the Food and Drug Administration (FDA) does not require manufacturers to include insoluble fiber in fiber Calorie counts on nutrition labels, usually as a subset of the total carbohydrate count. According to the United States Department of Agriculture (USDA), the general practice within the food industry is not to include insoluble fiber in the total carbohydrate count. > When fiber is listed as a part of a food’s total carbohydrate content, it is counted at 4 Calories per gram — just like all other carbs. The reason fiber Calories are counted is because when fiber is processed, the bacteria in the large intestines ferment the fiber, producing chemicals that get absorbed by the body. So, indirectly, fiber contributes Calories. … The other issue, the efficiency of digestion, is what is known as “[the Thermal Effect of Food](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC524030/)” 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: > [@](#): > > Reported DIT values for separate nutrients are 0 to 3% for fat, 5 to 10% for carbohydrate, 20 to 30% for protein [16], and 10 to 30% for alcohol [6]. In healthy subjects with a mixed diet, DIT represents about 10% of the total amount of energy ingested over 24 h. When a subject is in energy balance, where intake equals expenditure, DIT is 10% of daily energy expenditure. … 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. ---
**Author:** ![iljitsch](https://avatars.discourse-cdn.com/v4/letter/i/77aa72/32.png) [@iljitsch](https://boards.straightdope.com/u/iljitsch)\ **Post date:** [September 3, 2014, 9:16pm UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/4 "2014-09-03T21:16:30Z")
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. ---
**Author:** ![Surreal](https://avatars.discourse-cdn.com/v4/letter/s/eada6e/32.png) [@Surreal](https://boards.straightdope.com/u/Surreal)\ **Post date:** [September 3, 2014, 10:56pm UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/5 "2014-09-03T22:56:04Z")
Actually the Atwater system _does_ take into account energy losses due to digestion, albeit somewhat inaccurately: [http://jn.nutrition.org/content/28/6/443.full.pdf](http://jn.nutrition.org/content/28/6/443.full.pdf) > [@](#): > > In this system, which has long been generally employed in the United States, calories are obtained by multiplying grams of protein, fats and carbohydrates by the factors, 4-9-4, respectively. On the other hand the Rubner factors, 4.1-9.3-4.1, have generally been used in Europe and elsewhere. **The principal difference between the two sets of factors is that the Atwater values take account of digestion loses which those of Rubner do not.** Atwater has pointed out that Rubner’s values for fat and carbohydrate are heats-of-combustion values and that when they are multiplied by the appropriate coefficients of digestibilty they become approximately the same as his own (Atwater’s). Applied to mixed diets Rubner’s factors give a caloric value which is about 3% higher than obtained by Atwater’s factors, according to Morey ('36) who has reviewed the bases of the two sets of values. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [September 4, 2014, 2:46am UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/6 "2014-09-04T02:46:16Z")
Interesting but a bit incomplete. [The Atwater system](http://en.wikipedia.org/wiki/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) ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [September 4, 2014, 5:09am UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/7 "2014-09-04T05:09:05Z")
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. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [September 4, 2014, 5:27am UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/8 "2014-09-04T05:27:37Z")
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. ---
**Author:** ![tanstaafl](https://avatars.discourse-cdn.com/v4/letter/t/ba8739/32.png) [@tanstaafl](https://boards.straightdope.com/u/tanstaafl)\ **Post date:** [September 4, 2014, 8:08pm UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/9 "2014-09-04T20:08:10Z")
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”. ---
**Author:** ![Nippless](https://avatars.discourse-cdn.com/v4/letter/n/df788c/32.png) [@Nippless](https://boards.straightdope.com/u/Nippless)\ **Post date:** [October 8, 2014, 11:00am UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/10 "2014-10-08T11:00:25Z")
> [@t-bonham@scc.net](#): > > 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. 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. > **[Man With Hole In Stomach Helped Scientists Understand Digestion](https://www.huffpost.com/entry/man-with-hole-in-stomach-digestion_n_3147203)** > > Man With Hole In Stomach Helped Scientists Understand Digestion There is more on this in “Gulp: Adventures on the Alimentary Canal” by Mary Roach. ---
**Author:** ![Nippless](https://avatars.discourse-cdn.com/v4/letter/n/df788c/32.png) [@Nippless](https://boards.straightdope.com/u/Nippless)\ **Post date:** [October 8, 2014, 11:06am UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/11 "2014-10-08T11:06:26Z")
> [@tanstaafl](#): > > 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”. 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. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [October 8, 2014, 2:18pm UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/12 "2014-10-08T14:18:36Z")
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. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [October 8, 2014, 4:00pm UTC](https://boards.straightdope.com/t/digestion-efficiency-and-calorie-counts/697396/13 "2014-10-08T16:00:36Z")
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.