A Calorie is the energy needed to heat 1 kg of water by 1 degree.
Given that the human body is mostly water, and produces lots of heat, a calorie is a perfect way to measure the energy content of food.
Given that the human body is mostly water, and produces lots of heat, a calorie is a perfect way to measure the energy content of food.
To explain further, since further explanation is obviously needed: There is the physical calorie, which is ONE THING, and then there is the vernacular, "nutritional" calorie", which is something ordinary people deal with day-by-day. They are VERY different, in that kerosene calories and celery calories are handled differently by the human body.
Again, very simple until it comes up against people who don't interact often with reality.
The human body is a gigantic chemical engine, but it operates in a different way from a (jet/propulsive) engine. It's like the difference between LiOn and NiCad batteries. You measure them both in volts or wattage output or hours, but they operate in fundamentally different ways -- you can't use the same chemical mixtures, even though those mixtures have the same potential energy as measured by some abstract unit.