Now that was entirely pedantry. The microbiome isn't excluded from the law of conservation of energy either. Different microbiomes may process food differentl

Now that was entirely pedantry. The microbiome isn't excluded from the law of conservation of energy either. Different microbiomes may process food differently, but it cannot extract more energy than is available. A calorie is a unit of measure of energy, specifically the energy required to increase the temperature of 1g of water 1C. The fact that it is inconsistent based on temperature and pressure doesn't invalidate its definition. It is approximately 4.184J. Your example of diet soda is completely off the point as well. A lot of poisons have 0 calories, that doesn't make them good diet food. Olestra is another bad example, because the rats in the experiment didn't gain weight from the olestra but from a feedback effect that resulted in further overeating. But you're right in that calories don't tell you whether something you eat is poisonous. Calories are a good enough metric, but they require regular reevaluation in order to make decisions.