What Are We Really Counting?

January 15, 2012 1:02 PM

How do you measure how many calories are contained in food?

Packaged foods are required to list their calorie contents. Some restaurant menus will detail the calories of each specific entree. TV advertising will say, "5 great meals under 550 calories!" How do they determine that? Is it something I can do at home? Is there a better measurement to use for maintaining ideal body weight?

posted by netbros to Health & Fitness (11 answers total)

try this: How Nutritionist Measure Calories--For Dummies. the basic answer is, they burn the food.
posted by miss patrish at 1:05 PM on January 15, 2012 [1 favorite]

not to imply anything about your mental capacity, you know. it was just the first link that clearly stated how they do it.
posted by miss patrish at 1:06 PM on January 15, 2012 [1 favorite]

Originally, you would burn it in a sealed container in a tub of water and measure the change in the temperature of water; a calorie is just a measure of the amount of energy the food contains.

Now, you generally look up how much protein/carbohydrate/fibre/... it contains and guesstimate.
posted by katrielalex at 1:07 PM on January 15, 2012

Well, assuming you don't want to invest in a calorimeter and burn food, something like this will be helpful.
posted by Decani at 1:09 PM on January 15, 2012

The USDA maintains a nutrient database that is the standard in the United States. You can download it for free or use one of the online services. Most of the commercial sites that I have seen use the USDA database.
posted by calumet43 at 3:01 PM on January 15, 2012

A bomb calorimeter is the technically correct answer, but there are issues with how dietary calories compare to chemical calories. For example, you can burn wood and get heat out of it, but if you replace the starch in your diet with cellulose (sawdust) you're not going to be able to digest that. I'm pretty sure they account for this. (Also not that a nutritional calorie is a Kilocalorie, so you have to add or remove three zeros from the end of things as you translate.)

If you're working with pure chemicals, you can actually add up the bond strengths of all the bonds in your starting material and ending material and work out how much energy is lost or gained during the process. That doesn't work so well with food.

For restaurants and the like, I think it's probably they take something like what Decani or Calumet43 link to and add things up.
posted by Kid Charlemagne at 3:22 PM on January 15, 2012 [1 favorite]

So, crazy factoid, Kid Charlemagne, they actually don't account for it all that well. I agree it's plausible that they'll cover the really obvious bases, like try to analyze dietary fiber content and subtract the expected calories-in-a-calorimeter caloric value from the experimental results, but there are lots of things they don't take into account. For example, note that beef is listed as 173 calories from fat and no carbohydrates.

First of all, the only way your beef is actually carb-free is if the cow was absolutely starving to death or if some Atkins disciple helpfully exercised it to the point of glycogen depletion before slaughter or if I'm a monkey's uncle. So that's rubbish right there.

Secondly, where are the other 124 calories coming from? Conveniently, the protein content is listed as 29g, and a common rule of thumb is that protein is worth ~4kcal/g. But a healthy adult (aforementioned Atkins disciple notably excluded) is not actually deaminating protein for energy. For reasons falling out of some simple but tedious biochemistry, protein is an inefficient fuel source whose metabolism generates some undesirable byproducts and makes you smell like you're on the Atkins diet (or diabetic). So while it's hard to measure this sort of thing, chances are that if you eat some beef the protein isn't going to be worth much energy.

This is just one of many, many special cases. Consider Olestra, for example, which in a calorimeter burns just like so many grams of any other fat but in your body turns out to pass through completely untouched. (Google "Olestra anal leakage" for more than you ever wanted to know about the dangers of calorie-free fat substitutes.)

The fundamental problem is that we're trying to measure a few hundred complicated, enzyme-catalyzed, intricately regulated pathways and our best strategy is basically "burn it with fire!" At some point the model (bomb calorimetry) diverges from the underlying phenomenon (human metabolism). The really cool thing is that it works as well as it does. We can pretty much look up a few table values, do some second-grade arithmetic, and pack about the right amount of rice into an MRE. And that's roughly the level of nuance the calorie count is intended to support.
posted by d. z. wang at 4:48 PM on January 15, 2012 [2 favorites]

Um, on post, to answer OP's question rather than ranting about this blind faith in calorie counts, assuming you're John Q. Public trying to shed a few pounds this year, yeah, just add up the table values for the ingredients and weigh yourself frequently. Measurement error in your kitchen and noncompliance in the journalling (eating something and forgetting to write it down) will almost certainly be the primary error sources. Hence the frequent weighing.
posted by d. z. wang at 4:51 PM on January 15, 2012

The burning method is fun, though, and it absolutely is something you can do at home. We did it as a high school science project. As far as I remember, we put a peanut in a clamp and set it on fire, with a vial of water sitting directly above it. We measured the temperature of the water directly before and after the fire (waiting until the peanut had entirely turned to ash before the second measurement). Then there's a simple formula which turns the number of degrees change into calories, taking into account how much water you used. (The less the better, if you are burning something as small as a peanut, otherwise you might get no measurable change at all).

The only thing I don't recall is how we ensured that the peanut actually burned. We can't have doused it in alcohol or lighter fluid, because that would change the calorie count. Maybe peanuts are naturally oily enough to burn? That would mean it would be hard to do with other foods at home.
posted by lollusc at 5:07 PM on January 15, 2012

There are four classes of chemicals from which your body can extract caloric nutrition: fats, carbohydrates, proteins, and alcohols. For the sake of food-calorie calculations, it is estimated/assumed that your body can extract 9 calories from each gram of fat, 7 calories from each gram of alcohol, 4 calories from each gram of carbohydrates, and 4 calories from each gram of proteins.

In my own weight loss (and gain) experience, these values seem pretty close to correct for my body. They do vary a bit from person to person, although the extent to which they vary is a subject of some debate.
posted by Juffo-Wup at 6:00 PM on January 15, 2012

The USDA has a free site specifically tailored to your needs. You can also track your progress here.
posted by The White Hat at 6:09 PM on January 16, 2012