How efficient is the human digestive system?

ElPumaMex

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 !

http://boards.straightdope.com/sdmb/showthread.php?t=456091

Another way to look at this question, is to see the human body as a Machine (thermal machine, for calories):
http://mb-soft.com/public2/humaneff.html
(look there for a very detailed explanation, for those of you scientifically inclined :happy: )

What do you all know about this?

ElPumaMex

The following portion of the second link, makes me think that the Nutrition tables already take into account the efficiency of the digestive system.

Is that right?

See:

...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!

ElPumaMex

an excellent insight into how Nutrition calorie levels are determined for food:

http://en.wikipedia.org/wiki/Food_energy
http://en.wikipedia.org/wiki/Atwater_system

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.

tameko2

That was super interesting reading. It seems like since high caloric foods almost ALWAYS mean high fat foods (Because fat is the most calorically dense macronutrient per gram) and those links seem to indicate that under normal conditions for a normal person fat is extremely well digested, I can't imagine such a thing exists.

Even if you take a bunch of fat and mix it with something mostly indigestible, you're still going to digest all of the fat out of it and let the rest out as waste.

And yes since calories are determined (on nutritional labels anyway) based on the old original testing people did to determine how many calories are in a gram of protein, fat, alcohol, carb then its more or less accounted for.

There is also the thermic effect of food (how much you burn just by digesting) but that's already calculated into your TDEE so...

tameko2

Oh wow I want the Cap'n to weigh in on protein being negative food energy unless you're starving, because that sounds like madness.

(edit - I have read that protein has a HIGHER thermic effect and fat has a LOWER but they just average it out to 10% which would not be true for a diet that leans heavily on either food perhaps. But still. Protein = negative food energy sounds weird.)

ElPumaMex

That was super interesting reading. It seems like since high caloric foods almost ALWAYS mean high fat foods (Because fat is the most calorically dense macronutrient per gram) and those links seem to indicate that under normal conditions for a normal person fat is extremely well digested, I can't imagine such a thing exists.

Even if you take a bunch of fat and mix it with something mostly indigestible, you're still going to digest all of the fat out of it and let the rest out as waste.

And yes since calories are determined (on nutritional labels anyway) based on the old original testing people did to determine how many calories are in a gram of protein, fat, alcohol, carb then its more or less accounted for.

There is also the thermic effect of food (how much you burn just by digesting) but that's already calculated into your TDEE so...

Glad you liked the topic !

treetop57

I've wondered about this ever since I heard that caloric content was measured by literally burning the food. Obviously burning is a very different process than digestion. Sounds like that is taken into account either in the calorie values listed for foods or in the calorie requirements . . . but only near the center of the distributions where the averages of averages average out!

CaptainMFP

Am swamped til this afternoon, but am giving a lab exam. Let me read through the comments, check my own references and I'll weigh-in...I know the issue of digestive efficiency/calories burned/etc. is a hot topic for a lot of people. If you're watching this, check back for my formal response in about four hours or so.

ElPumaMex

Am swamped til this afternoon, but am giving a lab exam. Let me read through the comments, check my own references and I'll weigh-in...I know the issue of digestive efficiency/calories burned/etc. is a hot topic for a lot of people. If you're watching this, check back for my formal response in about four hours or so.

Excellent ! Looking forward for your response !

CaptainMFP

Okay, let's talk digestive efficiency.

First of all, there are two separate issues in play.
(1) How efficiently are macronutrients broken into their subunits for absorption into the body?
(2) How efficiently are macronutrient subunits absorbed into the body?

Up-front facts:
All digestible carbohydrates are chains of monosaccharide sugars (like glucose and fructose).
All proteins are chains of amino acids.
The bulk of the fat we consume is in the form of composite molecules (like triglycerides) that contain fatty acids.

The absorbed subunits are monosaccharides, amino acids, and fatty acids.

For the sake of simplicity, I'm omitting from consideration water and the micronutrients (vitamins and minerals). While important, these are not used for energy in caloric calculations. (The question as I read it deals with efficiency related to calories.)

Finally, the second law of thermodynamics states that disorder tends to increase in a closed system. Let me translate this to raw biological terms: every time energy changes forms, some of it is "lost" as heat. Thus, there are no 100% efficient energy transformations. This rule applies as much to the human digestive system as anything else.

DIGESTIVE EFFICIENCY
Fundamentally, digestion consists of two components. Mechanical digestion (chewing, stomach churning, the action of bile salts in the small intestine) which breaks food particles apart and chemical digestion, in which enzymes drive the breaking of bonds to separate the molecular subunits (monosaccharides, amino acids, fatty acids) from each other or from associated "parent molecules" in the case of fats.

How efficiently chemical digestion occurs depends on the size and composition of the meal as you'll see below...so let's look at each macro individually. (All numbers are taken from Saladin's A&P, 6th edition.)

(1) Carbs. Carbohydrate digestion begins in the mouth through the action of salivary amylase. It's digestion continues in the small intestine through the action of pancreatic amylase and some intestinal enzymes. The acid environment of the stomach causes salivary amylase to stop functioning, but if a meal is large enough, it can "hide" from the acid and function for up to 2 hours in the stomach. As much as 50% of the carbs we eat can be digested before reaching the intestine under the right conditions.

(2) Proteins. Real protein digestion begins in the stomach through the enzyme pepsin, facilitated by the acidic stomach environment. Only about 15% of protein is actually digested here with most digested in the small intestine by enzymes from the pancreas.

(3) Fats. Limited fat digestion (< 15%) occurs in the mouth and stomach. Most occurs in the small intestine through pancreatic lipase supported by bile (which doesn't digest fats but breaks them apart from each other). This enzyme (the lipase) is so efficient that it can digest all of a person's typical daily fat intake in as little as 2 minutes.

Now, all of this is mechanism. This is the "how" and the "where." The longer these enzymes have to operate, the more efficiently they can actually break down the food. The stomach is the big determiner for how long the digestive process takes. A typical meal of mixed macros takes around 4 hours in the stomach. Fatty meals can linger for 6 hours. Bigger meals take longer than smaller meals. Liquid meals can be processed in 1 - 2 hours.

At the same time, meal size creates another problem. The more food there is to be digested -- even with more time -- the less likely the enzymes are to act on all of it. Given the time frames above (like the action of pancreatic lipase), it stands to reason that smaller, balanced meals will digest more completely and efficiently than larger ones.

ABSORPTION
Most monosaccharides and amino acids are absorbed in the small intestine through a mechanism similar to that used in the kidneys to retain nutrients in the bloodstream. Sodium is actively pumped into the body from the intestine and it pulls (either directly or indirectly through water movement) other nutrients with it. This is applies to pretty much all amino acids, galactose, and glucose. (As an aside, fructose is NOT handled this way. It is absorbed through a pathway that essentially will result in more fructose entering when there is more in our diet. While the commercials are right...your cells can't tell the difference between glucose and fructose...well, one gets in a lot easier than the other. Thus, there is some merit to a discussion about sweetener composition and its effects on our health. But I digress...) Fats are absorbed passively as they diffuse through the membranes of our cells. Like the comments I just made about fructose, fats will enter largely as their proportion in our diet...more fats consumed and digested = more absorbed.

All of these mechanisms are limited and less than 100% of digested macros enter the body. Consider the following with reference to the composition of feces.

75% water
25% solid
Of the solid matter...
30% is bacteria
30% is dietary fiber
The rest is fat (10 - 20%), protein, and other undigested/non-absorbed particles, as well as digestive secretions like bile.

Given that 2+ hours in the small intestine are typically allotted for digestion and absorption, the efficiency is pretty good but not quite 100%.

NOW...finally... I have no numbers on overall efficiency. :noway:

<wah, wah, wah> :grumble:

HOWEVER, let me summarize the factors that influence digestive efficiency.

(1) Regardless of the actual energy content of food, < 100% of it will be harvested by the digestive process. (Again, 2nd law.)
(2) Enzymes for digesting macros are highly efficient, but how efficiently your macros are broken into subunits is a function of too many factors to reasonably account for in general. These include (but are not limited to), meal size, specific macro composition of the meal, solid/liquid nature of the meal, food pH, food temperature, and even emotional state. (Digestive upset shuts it all down!)
(3) Depending upon the macro, it will be absorbed either actively or passively. However, each of these has limitations and 100% of digested macro subunits will not actually enter the body. The specific efficiencies (at a macro level) will be linked to amounts and time spent in the small intestine.

To go back to the core question...that is, relating efficiency to calories...the caloric content of food is determined literally through the burning of food. A Calorie (or more appropriately a kilocalorie) is a measure of energy and so this is the appropriate unit and method for determining the energy content of food. Since the efficiency of digestion and absorption are functions of factors beyond the food itself, there is no reasonable way (in my opinion) to account for digestive efficiency in determining caloric content of food.

Also, with reference to the whole positive/negative calorie issue, consider the following points.

(1) If you were to determine the caloric content for a given food item and the specific amount of energy that is required to process that food in the digestive system, I have no doubt that cruciferous vegetables and some high protein products take more energy to process then they will yield. This does not make them "negative calorie" foods. When calculating resting metabolic rate (RMR) or BMR modified for activity, the digestive system's functions are taken into account. Hence, they are not negative calories...

(2) At a cellular level, simple sugars are our bodies purest form of energy. That's what cells mainly use carbs for. Fats are more diverse. They are useful for energy storage and as an energy source. They are also used to build our cell membranes, signal molecules in our bodies, and other cellular structures. Dietary amino acids are used to build proteins (all proteins in our cells, including enzymes). So while amino acids can be used for energy, they are a major structural component of our bodies. This is why people looking to bulk up need lots of protein (for construction, not energy) with a very high calorie diet (carbs and lipids to provide the energy needed to DO the construction). Again, since not all of the protein taken in is used as an energy source, far less than 100% of absorbed protein contributes to our practical energy intake. But this doesn't make it a negative calorie food...it does make it harder to consume too much protein, but it isn't an energetic sink, either. (Figure this...if you're taking in protein to build protein and you don't have adequate energy to build, where will your cells take it from? The protein!)

(3) I would dispute the claim that protein is not oxidized "except during starvation" as the one article claims. Amino acids are most definitely oxidized to contribute to the Krebs/TCA/citric acid cycle. A specific discussion of this will take longer than I can spare right now, but amino acids will undergo a process called deamination (which produces ammonia and other nitrogenous waste products) and be subsequently oxidized. If you have ever noticed a faint ammonia smell in your nose after an intense cardio workout, this is the excess nitrogenous waste in your blood from cells using more protein as an energy source due to insufficient carbs. (It's not a sign of ill health; just of insufficient carbs being available during your workout.) This doesn't make protein negative in my mind; it means we may be counting it to aggressively but how much it is or isn't an energy source is very circumstantial.

We can pick at the flaws of how calories are calculated, how accurate the BMI is, or how effectively this or than method of calculating body fat and energy requirements are all we want. At the end of the day, people need to make practical decisions about diet and health. If we take the numbers and calculations with a grain of salt, recognize their limitations, and pay attention to how our bodies respond under different conditions, we can use them all effectively.

I know this is long (hopefully not rambling) and it probably isn't totally satisfying from the perspective of a concrete answer. Unfortunately, the best answer to the question "how efficient is the human digestive system?" is this: it depends. :sad:

(Last word...I swear!...keep in mind my discussion here is limited only to carbs, fats, and proteins, their chemical digestion, and subsequent absorption. I have not addressed nucleic acids (DNA, RNA, and ATP are digested and absorbed) or micronutrients, and I have not addressed any post-absorption issues (where nutrients go and how cells process them). While interesting and important, these latter topics were beyond the scope of the question and will be considered another time if questions are asked.)

Class dismissed!

tameko2

Nope, it was satisfying and not at all rambling. And also confirmed my feeling that it was MADNESS to suggest no protein is digested. MADNESS I TELL YOU.

LabRat529

Fantastic response! Thanks. Most of that was new to me (or I'd forgotten it). Very awesome.

ElPumaMex

Wow, what a great response !

I have learned quite a bit from it, thank you very much ! :bigsmile:

Now, given that the short answer to the question was "it depends"... and that we know it will be less than 100% efficient...

Do you have any estimates on efficiency range for the digestive system, in terms of what the Caloric value used in the Nutrients listed on labels?

I am just looking for a rough number. For example: Range of 40 to 50%, or 80 to 90%.
A higher percentage would mean to me: "Dont worry about efficiency".
But if under certain conditions the efficiency could be as low as 50%, I definitively want to know under which circumnstances that may happen ! :glasses:

Again, thank you for your very valuable insight here !

CaptainMFP

Do you have any estimates on efficiency range for the digestive system, in terms of what the Caloric value used in the Nutrients listed on labels?

I can't give a specific number in good conscience since I have no specific data to work from. However, my gut response is this. As most of the mechanisms for uptake involve active transport and as the enzymes -- under typical conditions -- act in a highly effective fashion, I would expect the value to be much closer to 100% than to 50%. Probably above 75 - 80%. Put it another way -- I don't take digestive efficiency into account in regulating my own diet. I get what I feel is the best possible estimate of my caloric requirements, take caloric measures for food at face value, and set myself up for a deficit based purely on that information.

The one aside I'll offer (which is more about calories than digestive efficiency) is I like to be as precise as possible with my caloric estimates. I weigh damn near everything I eat and I don't use any of MFP's estimates for produce. (What is a medium banana anyway and who made that a serving?) I like the website http://www.fruitsandveggiesmorematters.org/?page_id=164 to establish my produce numbers per gram and then measure. My feeling is that as long as I get as precise an estimate of inbound energy as possible then the rest is just a matter of estimation and monitoring what my body does with the fuel it receives.

Hope this helps!
Cap

ElPumaMex

Thank you again Cap !

Saffyra

You are amazing! That was a fascinating read

annarouni

Oooh, that is one thorough answer, Cap. Thanks to OP for starting the thread.