Do we absorb all of the calories we eat?

Answer

There is a difference between the total energy of food and energy available in digestion that is stated on side. The energy on side is an approximation and simplification anyway.

Full info can be found here: https://www.fao.org/3/y5022e/y5022e04.htm

Answer

No. Especially carbohydrates because of fibre. Foods are burned in the lab whole to count the energy produced. If you eat nuts or kale for example, some of the fibre (non-soluble) will be removed whole and soluble fibre will be absorbed and broken down. Some of the fibre will be also turned into short chain fatty acids. So you never really absorb all of the calories from a carbohydrate, unless it’s pure glucose or starch. (Fibre is non-starchy polysaccharides and resistant starch)Even protein and fats aren’t absorbed 100%. There are specific percentages for all macronutrients (Atwater factors).

Maybe this book can explain better: ‘Why Calories Don’t Count: How We Got the Science of Weight Loss Wrong’ by Giles Yeo

Here is a talk he gave for The Royal Institution:https://youtu.be/GQJ0Z0DRumg

Answer

>Excluding the thermogenic effect of food

okay, this makes it a bit harder. I would still say no; dietary fibers (soluble & non-soluble) for example are carbohydrates, so they’d contain about 4 calories per gram, but only tend to have around 2:

Nie, Y., & Luo, F. (2021). Dietary Fiber: An Opportunity for a Global Control of Hyperlipidemia. Oxidative medicine and cellular longevity, 2021, 5542342. https://doi.org/10.1155/2021/5542342

>Foods rich in DFs induce […] a relatively low glycemic effect compared to an equal amount of available carbohydrate (usually from white bread or glucose). Fully fermentable RS, for instance, has been estimated to contribute about 8.8 kJ/g, whereas glucose contributes 17 kJ/g [125].

(see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8052145/#sec4title)

they can also lead to worse energy availability of other contents of the diet:

Baer, D. J., Rumpler, W. V., Miles, C. W., & Fahey, G. C., Jr (1997). Dietary fiber decreases the metabolizable energy content and nutrientdigestibility of mixed diets fed to humans. The Journal of nutrition, 127(4), 579–586. https://doi.org/10.1093/jn/127.4.579Miles CW, Kelsay JL, Wong NP. Effect of dietary fiber on themetabolizable energy of human diets. J Nutr. 1988 Sep;118(9):1075-81. doi: 10.1093/jn/118.9.1075. PMID: 2843615.

Answer

Fecal waste is definitely a thing but not really that much for more processed (mechanical or chemical) foods.

I cannot find it now, but there is a study of the calories excreted between groups given ground peanuts and whole peanuts. The whole peanut group excreted significantly more calories because chewing didn’t do as good of a job as mechanical grinding, and more food passed undigested.

I remember a study a long time ago about whole fat dairy products leading to higher amounts of fat being lost in waste than some other sources of fat. Since dairy products keep getting connected to body composition improvements, besides the proteins, an increase in calories not absorbed may have something to do with it?

Answer

I don’t think. I’m not an expert but.. by what I have been reading, for example, glucose is absorbed by every cell of your body while fructose is processed by the liver into fat so then,.. your body can use it. The same with fats,… if your insulin is low, you can pretty much eat what you can/want (without carbs) and don’t store fat. So… the question is not perfect… more important than the whole calories is what you eat and when. If what I said is wrong I gladly accept your correction

Answer

I appreciate all the responses that explained the difference between the calories measured by a food vs. the very different and less efficient way that they are absorbed by the human body.

Can we get a quantitative sense of this? Suppose a food, let’s say a piece of banana, has 100kCal as measured in a bomb calorimeter. Typically, would 95kCal be absorbed by the body, or 80kCal, or more like 50kCal in most cases? Or is this impossible to estimate because it varies too much from person to person and from time to time (e.g., depending on what else is eaten at the same time, perhaps)?

What if we thrown in the question of how much the gut microbes feed on - of the 100kCal, would they consume typically 1kCal? 10kCal? More? Do they consume only portions of the food (e.g., fibre) that we can’t use, or do they “compete” with us for calories that we could have absorbed too?

Thanks for any order of magnitude numbers to help visualize all this.

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