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.