T Nation

Question about food

Just wondering if anybody knows this. How do they measure the Protein, Fat and Carbohydrate content of food? I mean how would somebody find out that 100 grams of cottage cheese contains 15.2 grams of Protein for example?

I’m not sure how it currently actually is
done, but a way to do it is combustion
followed by elemental analysis. The amount
of calories (heat produced) due to protein
can be calculated pretty accurately from
the amount of nitrogen, and I would think
that the amounts of fat and carbohydrate
can be determined from the balance of
the calories produced and the amount of
CO2 produced – fats and carbohydrates
differ in this ratio.

That’s very interesting, Bill. I seem to remember reading somewhere that the number of calories in a food is measured by actually burning it and using a device, I think it was called a bomb calorimeter, to measure the amount of heat liberated. I always wondered how valid that method is. I mean, I can have an awful lot of heat energy by burning wood, but it has little caloric value as food. Is there any consideration to whether the calories being measured are actually digestible?

That’s true, bomb calorimetry will not
determine the difference between usable
sugars and cellulose or fiber.

I don’t know how fiber is assayed, but
food labels today often do give a value
for fiber.

In practice I think, but am not sure, that
the label claims are NOT from analysis of
the actual product, but the sums of the
analysis of ingredients. In other words,
if I am selling a food bar, and I know how
much of each ingredient I put into it and
what the values for protein, carbs, etc.
for each were, I can just add them up and
make that my label claim.

This can be problematic for products such
as food bars that add ingredients like
glycerin, which they probably will not choose
to count as a carbohydrate. I am not sure
if they accurately include the calories of
the glycerin or not, in such cases.

I just read the nutrition label on the items I eat.

Mr. Roberts is right, they often find these results by analyzing the elements after it is burned. The other method that is often used is spectrophotometry. They set up a curve using standard amounts of protein, fat, and carbs (which all will have diferent wavelenghts at which they will be detected). The sample is then tested and the amount of each is detected, compared to the standard, and then you know how much of each is in the sample. This is used rather than the other method to determine actual caloric values because different polymers (such as cellulose for an alternate polymer of carbohygrates) will not be detected be they will have little to no absorption at the selected wavelenghts.