The Laws Of Supply & Demand
Together your body & mind are the twin parts of an amazingly complex biological machine which is constantly regenerating & developing from birth until death. This ever-evolving machine learns from, and adapts to the demands placed upon it, optimizing the chances of your continued survival in your current environment.
If your environment makes no demands, your body has no need of toned muscles, strong bones, quick reflexes and a rapid mind. For reasons of economy it drops all functions to a lower level. If you continue to take on fuel at a rate more suitable for a higher level of function, your body will either store or excrete the excess in various ways.
Conversely, if your environment becomes more demanding than your current level of function, your body rapidly starts shifting resources into muscle growth, weight reduction, bone strengthening, and increased alertness, to name but a few. If you continue to take on fuel at a rate suitable for a lower level of function, your body will use any stored reserves to make up the deficit.
Supply & demand. Simple in principle, and the principle applies to every aspect of your physical & mental health. The key to success is to make the new demands just that little bit greater than your current ability, letting your body adapt to a gradually higher & higher level of function until you discover the right balance for you.