Yanche wrote:OK now for a little teaser... What's the difference between a food calorie and a physics calorie? A physics calorie is the amount of energy need to raise 1 gram of water 1 deg C. What the hell does this have to do with food? I'll let a few post some guesses or opinions. I was curious and found the answer. Along the way, I was surprised how many registered nutritionist I asked didn't know. To be fair the answer was in their first year text book that they misplaced or long ago forgot about.
The food calorie story....
The method used for calculation of the energy in foods is called the Atwater system. Developed in the late 19th century by Wilbur Atwater at Weslayan University. He was a professor of chemistry and wanted to know how much energy there was is food. So he measured it. It's remarkable what he did. He fed humans food and measured how much work they could do. They peddled a bicycle connected to a generator producing electrical energy. But the human also breaths, a significant amount of that food energy is used for breathing. So he invented a respiration calorimeter, a room in which the bicycle was housed. By measuring the breathing output of the human and the electric power generated he could calculate the energy in various foods. See the photo below of the room (photo take from a USDA public document). To a chemist (also physists and engineers) energy is measured in calories (metric system). So it was natural for Atwater to classify foods by calories. Think of it this way, water is heated to make steam, steam runs the steam engine, the engine does work. In the metric system we would use calories not BTU's the describe the amount. The convention calories has struck even though the US doesn't use the metric system. It's also true that food calories are really 1000 times larger. Correctly written with a capital C.
Atwater also measured All the output of his human test subjects. Yes the pee and poop too. Converted the output to energy. Found there was lots of other output when eating beans.
His original late 1800's paper is here: http://naldc.nal.usda.gov/download/IND43645383/PDF
Now you know the full story behind food calories. A century or so is rather a small amount of time to really understand a particular science topic. Newtonian physics had been around since the 17th century, it's well understood. We are still learning food science and how our bodies really react to the food we eat.