Dr. Rudolph Leibel explains. This is one of the more fascinating discussions that I have heard. 

And, once you listen to it, you will have a better appreciation for this new calorie calculator. It's science-based and designed by  The National Institute of Diabetes and Digestive and Kidney Diseases. 

NIDDK is part of the National Institutes of Health (NIH) and the U.S. Department of Health and Human Services.