[toast0]

Calories in - calories out is true, but it's hard to measure either with any much accuracy. You can get close with calories in, but measuring calories out isn't easy: There are charts for different activities, but our bodies are not perfect efficient machines that always use the same amount of energy for the same tasks.

Additionally, our bodies can make subtle changes in response to caloric intake: If you ingest fewer calories, you may fidget less, possibly lowering your caloric use more than you lowered your input.