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TOPIC: How efficient is the human digestive system?

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February 7, 2012 2:16 PM
Being that we try to track our calorie intake, and estimate the best we can on how much we "burn" through exercise and daily activities, I am curious to find out how efficient is the human digestive system to absorb all of the calories we so meticulously log at MFP !

If the body’s digestive system is found to be VERY efficient, say 90% or more, to absorb the calories registered in the nutritional tables, then we don’t have to worry about tracking this digestive efficiency.

But assume for a second that for some high-caloric foods, the digestive system is VERY inefficient (50% or less). Wouldn’t we all want to know that?

I have found some interesting articles, to get the conversation started !

Another way to look at this question, is to see the human body as a Machine (thermal machine, for calories):
(look there for a very detailed explanation, for those of you scientifically inclined happy )

What do you all know about this?
Edited by ElPumaMex On February 7, 2012 2:17 PM
February 7, 2012 2:19 PM
I believe if this were true " But assume for a second that for some high-caloric foods, the digestive system is VERY inefficient (50% or less). Wouldn’t we all want to know that?" we wouldn't have a weight issue.
February 7, 2012 2:29 PM

I believe if this were true " But assume for a second that for some high-caloric foods, the digestive system is VERY inefficient (50% or less). Wouldn’t we all want to know that?" we wouldn't have a weight issue.

From the second link, here is some text to explain what I mean:

Back to the matter at hand. Our ACTUAL diet is a mixture of carbohydrates, proteins and fats. The AVERAGE energy content for that mixture is generally around 5 Calories per gram, which is around 2200 Calories per the pound of food that we actually digest. So when anyone refers to a daily diet of 2200 Calories or 2000 Calories, there are a lot of approximations involved!

We actually all ingest more than a pound every day, generally around 22 to 30 ounces of actual food content (after subtracting the water in it). Note that just a quarter-pound hamburger and its fries and drink are far more than a pound (including the water)! In fact, for comparison, a single 16-ounce drink is one pound (but in that case, it is nearly all water). What's the deal? Well, not only do we ingest food each day, but we also excrete and eliminate it; it turns out that a significant amount of the volume of food we eat later leaves us, where our body does not even try to process it! An average person might take in around 25 ounces of food values each day (which involves around 3400 Calories in a Physics sense), but then excretes around 9 ounces per day, which, in a Physics sense, contains around 1,200 Calories of chemical energy. Yes, the body did not even try to digest or absorb that last amount, along with an assortment of organic materials (and a LOT of dead bacteria) that the body no longer needed. The remainder, the amount that the body actually digests, is therefore around 16 ounces, or one pound, or around 2,200 Calories, the usual Nutrition description of a healthy food intake. So it is important to make sure whether any data is referring to the Nutrition value of a food or the actual Physics value. Nearly anything you will find regarding food ONLY refers to the Nutrition value, as few people seem to even care about the Physics perspective of all of this! (Humbug!)

In other words, the common descriptions of the (Nutrition) Calorie content of foods does NOT include the materials that the human body will not even try to digest!
February 7, 2012 2:35 PM
by the way, that text from the second link makes me think that the Nutrition tables already take into account the efficiency of the digestive system.

is that the right interpretation?
February 7, 2012 2:39 PM
an excellent insight into how Nutrition calorie levels are determined for food:

Conventional food energy is based on heats of combustion in a bomb calorimeter and corrections that take into consideration the efficiency of digestion and absorption and the production of urea and other substances in the urine. These were worked out in the late 19th century by the American chemist Wilbur Atwater.[2] See Atwater system for more detail. This method of estimating the food energy has several defects, the most serious of which is that protein is not oxidized in the body as in the bomb calorimeter, with the possible exception of severe starvation.[3] In normal conditions, the protein is metabolized in processes which require energy such as protein synthesis or replacement, synthesis hormones, nucleic acids, etc.[4] Thus, the food energy derived from proteins is probably negative. However, the idea that protein contains 4 kcal/g is so ingrained that it is used universally, as in food labels. It is only approximately correct in cases of severe starvation.

Each food item has a specific metabolizable energy intake (MEI). This value can be approximated by multiplying the total amount of energy associated with a food item by 85%, which is the typical amount of energy actually obtained by a human after respiration has been completed.[citation needed] In animal nutrition where energy is a critical element of the economics of meat production, a specific metabolizable energy may be determined for each component (protein, fat, etc.) of each ingredient of the feed.


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