The definition of a healthy person differs per person.
This is mostly related to the energy needs of their body. We measure energy in calories or Joules. 1 kcal = 4,7 kJ, you can heat up 1 liter of water by 1 degree Celsius with 1 kcal.
People that play sports or have a practical profession have a higher energy need than other people, there are also differences between males and females.