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I was taught in school that nutritionists measure the calories in food with a bomb calorimeter.

Dummies.com make the same claim:

Nutrition scientists measure the number of calories in food by actually burning the food in a bomb calorimeter, which is a box with two chambers, one inside the other. The researchers weigh a sample of the food, put the sample on a dish, and put the dish into the inner chamber of the calorimeter.

They fill the inner chamber with oxygen and then seal it so the oxygen can’t escape. The outer chamber is filled with a measured amount of cold water, and the oxygen in the first chamber (inside the chamber with the water) is ignited with an electric spark. When the food burns, an observer records the rise in the temperature of the water in the outer chamber. If the temperature of the water goes up 1 degree per kilogram, the food has 1 calorie; 2 degrees, 2 calories; and 235 degrees, 235 calories — or one 8-ounce chocolate malt!)

This has never made sense to me, and so I remain skeptical. The human body doesn't obtain all the calories from all food. The food isn't burnt to an ash by the human digestive system. Presumably the bomb calorimeter would find that hay has a high calorific value, but the human body can't extract that.

Were my teacher and Dummies.com over-simplifying when they describe the technique?

Is the nutritional information on our food essentially flawed, because the model is overly simplistic?

Or am I being ignorant and cynical by suggesting that the technique wouldn't work?

I'm using the old-school term, because the actual numbers are irrelevant. 1 food Calorie = 1000 real calories = 4.2 kilojoules.

  • Do you have a citation for your claim that the food isn't burnt to ash? I don't see that in any of the links you provide, and I think it's the most significant claim you're making from a physics standpoint. – Tacroy Jul 9 '12 at 23:33
  • @Tacroy, dried human waste can be burned. – Oddthinking Jul 9 '12 at 23:54
  • What about "it's a good enough proxy"? – Sklivvz Jul 10 '12 at 6:04
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    "and the oxygen in the first chamber (inside the chamber with the water) is ignited with an electric spark". While I'm aware in advance that this is pedantic, I do want to point out that oxygen does not ignite or burn. Think about it. – Alfred Centauri Jul 12 '12 at 23:12
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    @Tacroy: Re-reading 9 months later, I can see the source of the confusion. When I ambiguously said the food wasn't burnt to ash, I mean by the body, not by the calorimeter. I apologise for writing unclearly. – Oddthinking Mar 24 '13 at 21:54
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This is how a bomb calorimeter works, and as you suggest doesn't translate to the same mechanism as the human body. It is, however, an upper bound on the amount of energy your body can extract.

The way that the calorie information on food is calculated is slightly different. This uses the g/100g of fat, sugar and protein to calculate calories based on the calories per gram of each of these substances. FDA guidelines, L12 Those values are obtained from bomb calorimetry for the fat and sugar, and a modified version to account of the fact that protein is not turned into N2 by the body but into urea, and so the chemical energy of urea is subtracted from the value for raw protein.Combustion of protein

This is as you suggest an overly simplistic model, and there is research to suggest these values are wrong and should be tweaked in light of how the human body metabolises food.(I can't find a link at the moment though, too much diet nonsense when you search.)

There is a similar question/answer here: https://physics.stackexchange.com/questions/3315/how-are-the-calories-in-food-calculated

This excellent article explains how the amount of calories you get out of food depends on how well cooked it is (more cooking increases available calories / reduces cost of digestion) and your gut microbes. Processed vs non-processed food can have a 10% difference in effective calorific content as a result!

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    Nick, I understand that, having worked out the energy in the ingredients of a sandwich, there is no need to explode the sandwich as a whole. I am not clear about the urea aspect: is N<sub>2</sub> given off when food is burned in oxygen? This still needs references re: upper bound, that the calculation is simplistic, the energy of urea is subtracted. The answers on Physics.SE take a wild-ride around FDA regulations for labelling, which seem irrelevant. (The fact the OP asked for there NOT to be references strikes me as rather wrong-headed.) – Oddthinking Jul 10 '12 at 8:05
  • Yes, N2 gas, CO2 and H2O are essentially what you get when you burn organic matter completely in sufficient oxygen. Living things metabolise N to urea instead of N2 though(the urea is then excreted in an experiment you can perform at home.), and so there is still chemical energy left in the urea which is the adjustment which is made in the FDA calories per gram of protein compared to a bomb calorimeter value. I remember finding a very good article on this in the past, but I can't find it at the moment. I will edit in references when I find them! – Nick Jul 10 '12 at 8:14
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This answer is strongly based on a reference provided by @TechZen, so credit there. However, I draw a very different conclusion from it, and I didn't want to edit the answer that substantially... I decided to go with a separate Community Wiki answer.

No, the bomb calorimiter values are not the definitive measure of calorific values.

In this conference paper, the techniques used to adhere to FDA Food Labelling guidelines are described and their limitations discussed.

For regulatory purposes, five options have been provided for the calculation of caloric content of foods. They are

  1. specific Atwater food factors,
  2. general factors of 4, 4, and 9 calories per gram of protein, total carbohydrate including dietary fiber, and fat, respectively,
  3. same as 2. except that insoluble dietary fiber content may be subtracted from total carbohydrate content,
  4. specific factors for particular food ingredients petitioned by manufacturers/users and approved by FDA as appropriate, and
  5. bomb calorimetry data after subtraction of 1.25 calories per gram of protein.

This demonstrates that the nutritional labelling does not rely on bomb calorimetry data without regard for the factors the question was concerned about. If a bomb calorimeter would give an unfair over-estimate, the manufacturer can use a more reasonable evaluation of the components. If even that is unreasonable, (I assume olestra would be an example,) they may request a ruling for a different technique.

The paper gives examples of how the different techniques (plus UK labelling regulations) give different answers.

It also explains how chromatographic techniques can be used to measure the different types of sugar levels of foods, such as honey.

Ignoring the water content, wood would consist of largely insoluble dietary fiber (which the paper demonstrates can be defined physiological, chemically or gravimetrically, and that the guidelines stipulate the last definition.)

The paper does point out there are some unanswered questions, such as whether all soluble dietary fibers, by the legal definition, have the same physiological effects.

In conclusion, the textbooks were the ones over-simplifying the process. The nutrition scientists and the FDA realise it isn't a gold standard for calorific content.

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The book is correct, the bomb calorimeter is still the official measure of the caloric content for food. You still see it in all the textbooks and the government required nutritional labels are all legally required to based on it.

You are correct that the calorimeter merely measures the total energy in the sample that can be released by complete oxygen combustion of the sample. Since the body doesn't burn food but instead processes it biochemically, the nutritional calalory is only a very rough guide to the energy content in food.

For example, if you put a chunk of dry wood in calorimeter it will tell you the wood had a great deal of energy in it. However, since all that energy is bound up in cellulose and humans can't digest cellulose, the actual nutritional caloric content of wood is zero. Celery is likewise the mostly wood and cellulose. Notionally, it provides something like 11 calories a stalk when burned but in actuality, it takes more calories to chew and other wise process celery than you extract from it. Celery is calorically, anti-food.

Different people also process different foods more effectively or not at all. The common example is the processing of lactose (milk sugar) in adults. Only about 20% of the human species presently has the gene to produce lactase which allows them convert lactose into absorbable simple sugars. For those people, milk provides a great deal of calories almost as many as the calorimeter reports. For the other 80% of humanity, 90%+ of the lactose remains as lactose until it travels to the large intestine where it is eaten by microbes who convert it to energy, CO2, Methane, water and embarrassing social situations.

Alternatively, diabetics, especially Type 1, can absorb carbohydrates into their blood but their cells cannot "burn" the carbohydrates without insulin.

The best guide today to actual nutritional calories is arguably the glycemic index which measures how much carbohydrates/sugars appear in the blood a set time after consumption. However, given the great range of variation from person to person even that is practically only a rough guess for any given individual.

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    Welcome to Skeptics! Please provide some references to support your claims. – Sklivvz Jul 11 '12 at 19:44
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    I've added "some references" although they change nothing. Do I need to provide a reference that humans can't digest wood or can I assume that is obvious? – TechZen Jul 12 '12 at 21:23

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