The energy is food, or used up by activity, is measured in a unit called an

J, kJ

Explanation:

Energy is measured by J which stands for Joules and also kJ which stands for kilojoules.

There is also an older unit for measuring which is called kilocalorie or just calorie but most people are using joules for the measure

One kilojoule or one kJ is having 1000 J ( Joules) and it is easier to count with joules. One calorie is having 4.5 Joules (J).