The kcal (kilocalorie) is what most people mean when they refer to calories in food, and that's just the energy needed to heat 1 kg of water by 1 deg C.

Since a person is mostly water this means that, say if you weigh 100 kg, and eat 1000 kcal of food, that food contains (theoretically) enough energy to heat your body mass by 10 deg C. The units do give a kind of intuitive understanding of the energy in food, while also being based on metric quantities.