New submitter ami.one writes: Cynthia Graber and Nicola Twilley explain how we are still using a century old method for measuring the calories in our food and the calories spent in different human activities. Essentially, there is a very big difference between burning stuff in a bomb calorie-meter and the extremely complex ways our body extracts energy from food. In fact, the exact process of digestion is yet to be understood sufficiently at a micro level, and years from being replicated to any close degree. Plus, the way our bodies spend calories for a given activity is hugely different from the way a car consumer gasoline and dependent on a number of parameters — some of which are not even known currently. Therefore, balancing calories in to Calories out is not so stupidly simple as it seems to the underweight layperson . Update: 01/28 22:09 GMT by T : Sorry for the duplicate post; it was a long night.