My gut is not a calorimeter.

March 27, 2010 9:34 AM

Prompted by this comment, I'm here to ask what the deal is with "calories" as a measure of "nutritional content" (whatever that means). I understand that the calorie value we see on food packaging is arrived at by burning the food in a calorimeter, and that this would be an effective measure of the total chemical energy in the substance. How well does this actually approximate the energy our body can put to use (mechanically, or for other biological processes)? Isn't this measure essentially useless for people trying to lose weight? Why do we put so much emphasis on it then? Are there some foods which have an astronomical number of calories which our digestive systems can do little with?
posted by phrontist to science & nature (14 answers total)

Calorie counts are as accurate as we need them to be. It's true they don't tell the whole story and there is some individual variation, but they serve well as an estimate of how much food energy is being consumed.

You are correct that there can often be more to effective weight loss than mere calorie count - but that being said, if you restrict your caloric intake, you will lose weight. Losing weight in a healthy, long term manner is more complicated than just calorie restriction, but calorie restriction is a key component of almost any weight loss plan.

Yes, there many "foods" that have high calorie counts we cannot digest - grasses, wood, etc.
posted by Pogo_Fuzzybutt at 9:59 AM on March 27, 2010

The measure is very fucking far from useless for people trying to lose weight. Knock your caloric intake down to 1300/1500 calories/day, and you are virtually certain lose weight.

Yes, there are things that have are dense in chemical energy that you can't digest very well. Mineral oil. Sawdust. Coal. Cardboard. But they're not food, because by and large people aren't that dumb. About as close as you'll come with actual food is stuff like celery that is energy-poor to start, so it doesn't meet your first criterion, but that's also mostly indigestible cellulose.
posted by ROU_Xenophobe at 10:00 AM on March 27, 2010

Here's an article by a company that makes calorimeters.

They mention briefly at the end how scientists can measure how much energy a person actually derives from digested food, but no details. I remember seeing a science show about this years ago that went into detail exactly the question you are asking and, yes, some foods with a high calorie count aren't digested well by us, so the calorie count is way off. (Sorry, I can't find links to this show).
posted by eye of newt at 10:00 AM on March 27, 2010

Insoluble fiber is precisely what you're asking about. It is chock full of calories according to a calorimeter, but we can't use any of them — we just crap the insoluble fiber right back out again. But we know this, so we don't use calorimeters to measure the calorie content of high-fiber foods without making some sort of adjustment.

(Others are right that there are also lots of non-foods that fall into this category. Grass, wood, propane, gasoline, phosphorus etc. are all 100% pure indigestible calorie-rich material. The calorimeter definitely predicts the wrong outcome from eating these. Predicted outcome: lots of useful nutrition! Actual outcome: somewhere between "screaming diarrhea" and "death." Same story, though — in practice we know better than to believe what the calorimeter says here.)
posted by nebulawindphone at 10:03 AM on March 27, 2010 [2 favorites]

I think the comment your basing your question off of isn't that good to begin with. Someone saying "What you need to understand is that there are no such things as calories." would be like the person telling you "there's no such thing as time man."

If you're eating foodstuff that is meant to be eaten as food then the caloric information known for that is defined and broken down pretty well for the fats\proteins\and carbohydrates.

You get into an area where the measurements on boxes can be pretty inaccurate when you start talking about super processed foods.

If you're eating vegetables, meats, eggs, etc, it's pretty well known what you'll be getting from them.

If you want to learn a lot about nutrition and how your body uses it, the bodybuilding community has a lot of great science and articles.
posted by zephyr_words at 10:10 AM on March 27, 2010 [1 favorite]

I found more information. It turns out a guy name Atwater came out with the calorie counting system including how to adjust it for how well your body can actually digest it. Bot more recently a scientist by the name of Livesey is challenging this, saying that high fiber diets show higher calorie counts than what our body actually digests. Google these two names and you'll get lots of info.

Here's Livesey's paper.
posted by eye of newt at 10:16 AM on March 27, 2010

You might want to know 200 calories looks like when it comes to various foods. Note that 200 calories of M&Ms is a small handful, not nearly enough to fill you up if you're hungry, while 200 calories of broccoli would likely leaving stuffed (and not wanting broccoli for a long time).

Compare calories with GI number and you get a better picture. In my understanding, GI number is basically how fast your body can turn the sugar in the food into energy. Food that's processed (those M&Ms) has already basically been chemically digested, and so your body runs through it quite quickly and easily. The broccoli, on the other hand, takes a lot longer to process and therefore leaves you feeling fuller longer.
posted by Brittanie at 10:45 AM on March 27, 2010

The comment you're linking to is pretty much nonsense. You'll note that protein is not a type of carbohydrate.

Anyhow, it seems that wikipedia has the answer to your question.

The amount of food energy associated with a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry.[5] However, the values given on food labels are not determined this way, because it overestimates the amount of fuel that actually enters the blood through digestion because it also burns the indigestible dietary fiber so that not all food eaten is actually absorbed by the body (fecal losses). Instead, standardized chemical tests or an analysis of the recipe using reference tables for common ingredients[6] are used to estimate the product's digestible constituents (protein, carbohydrate, fat, etc.). These results are then converted into an equivalent energy value based on a standardized table of energy densities.

posted by ludwig_van at 10:59 AM on March 27, 2010 [3 favorites]

I wasn't endorsing koeselitz's comment, quite the opposite - it just reminded me of a longstanding question I'd had.

ludwig_van has it - they don't use calorimeters.
posted by phrontist at 11:06 AM on March 27, 2010

Goddamnit. The stupidest comment I've made here in years, and somebody has to go and use it as the basis of a question.

Anyhow, yeah: fully retracted. Forget about it. Amino acids – proteins – are almost carbohydrates, in that they have the proper number of carbon, hydrogen, and oxygen atoms; but they aren't bonded correctly to be carbohydrates. That's why a process is needed to turn one into the other. I was being a silly fool. I am sorry.

And be careful what you say on ask.metafilter – the next time any of you say anything that's not quite true, rest assured that I'll make sure you never forget it. ;)
posted by koeselitz at 12:19 PM on March 27, 2010 [2 favorites]

Except Wikipedia is wrong, or at least incomplete.


Wilbur Atwater, the inventor of the bomb calorimeter, realized that we don't burn calories as efficiently as his machine, so he did measurements with people and developed tables of general conversion factors to adjust the calorie counts. His method of counting calories is used to this day in various forms for all food packaging, because it is simple, and precise, if not entirely accurate.

Livesey has been pushing to improve the accuracy, to measure net metabolisable energy (NME), but because of the overwhelming inertia involved in changing how the entire industry documents its calories, it hasn't happened (as far as my Googling shows).

So we end up picking the 300 calorie (or kilocalorie, if you are a scientist) brownie over the 300 calorie granola bar, thinking that you will gain the same either way, and end up storing 25% more fat in our bodies.
posted by eye of newt at 4:16 PM on March 27, 2010

To approach this from a slightly different angle, you're right in that the number is not entirely helpful for people trying to lose weight. The book Good Calories, Bad Calories by Gary Taubes is about how a food's effect on insulin levels matters far more in terms of weight gain or loss.
posted by Nattie at 9:13 PM on March 27, 2010

Amino acids – proteins – are almost carbohydrates, in that they have the proper number of carbon, hydrogen, and oxygen atoms;

You're still wrong. In addition to having carbon, hydrogen, and oxygen (albeit with different ratios than in carbohydrates, unless by unlikely coincidence), proteins also have nitrogen, and nearly all proteins also have sulfur.
posted by DevilsAdvocate at 10:23 PM on March 27, 2010

Thanks, DA. Thanks a lot.
posted by koeselitz at 10:36 PM on March 27, 2010 [1 favorite]