I'm writing a speculative fiction story that includes superhumans. The focus is on individual characters and their daily lives, and I'm trying to maintain some consistency by not throwing all the laws of physics out the window. However, this creates some problems in the daily life of these individuals - where does their super-energy come from, given that they appear to eat a normal amount of food?
For example, a superhuman with super-strength is able to lift heavy objects. This is fine according to the laws of thermodynamics, as long as a proper amount of energy is used to perform it. To lift a 15,000kg semi 2 meters off the ground, my superhuman needs to contribute a minimum of 60 kcal. In a typical day, this character might perform an action like this 50 times, for an additional energy expenditure of 3,000 kcal - that's more than double the normal energy expenditure of a similar human. On intensive days, this energy cost might be closer to 20,000 kcal.
How can I justify this discrepancy between the energy my superhumans expend and the energy that they consume, assuming they eat a normal amount of normal food? Specifically, how could a superhuman obtain the 20,000 kcal expended in an intensive hero day without eating proportionally more?
Shoutout to the Sandbox for helping me develop this question!