The body adjusts to exercise in the form of increased power of the cell as well as the architecture of delivering nutrition to the stronger cell which also requires more nutrition. This is what we mean by conditioning.
The more difficult concept grasp is the inverse. The body also must adjust in the form of a diminished flow of nutrition which is deconditioning. Both happen in a predictable manner yet deconditioning is wrongly assumed to be pathological.
Genetic Role of Deconditioning
Food has to be transformed into fuel the cell can use. Imagine a series of buckets. The stomach fills up and spills over into the liver, which spills over into the blood, which eventually spills over into the cell. If the cell is in need of more fuel, it will empty the adjacent bucket, which will then empty the next bucket upstream, etc. If the cell is congested, the first bucket upstream will get backed up, then the next one will get backed up, etc. The actual order of the buckets in a series is different based on your genetic makeup, but the fundamentals of the cells processing of energy is the same. One person might spill the excess over into fat cells and gain weight as a first line of diversion. Another person might just not digest the food at all, while another would use the lungs or kidneys more easily to divert the excess. These different diseases are all genetically designed forms of regulation.
During conditioning, the gradient shifts so that instead of a chronic excess, there is a slight deficit which has the opposite effect. The cell now wants more food, which it will get from one of the buckets first. The conditioning might not occur in the order you would have chosen but if you train properly we hope all the buckets are restored to their ideal function.