Once we've got our nutrition or fueling down pat, it's now time to transport and use that fuel properly through training - metabolic conditioning.
We should be conditioning our body's ability to metabolize, or break down (and use), the carbohydrates, fat, and protein we're ingesting as fuel in different situations, through training.
In the grand scheme of things we need to be able to perform work, or generate power output in all time durations and kinds of activities well, in order to be determined as fit (see article here).
There are 3 primary energy systems that are body has that allows us to perform work and generate power.
Phosphagen, or ATP-PC Pathway
Glycolytic, or Lactate Pathway
Oxidative, or Aerobic Pathway
We as humans are capable of performing large amounts of work and generating large amounts of power in the Phosphagen Pathway, which lasts up to 10 seconds. Imagine lifting something and throwing it really far or high, using your whole body, like flipping a tire.
The next pathway, the Glycolytic, is what we're primarily using in time durations from less than a minute to over a few minutes. What we're doing here is breaking down Glucose that we get from our existing stores in our body, as well as the fuel/nutrition we ingest, into smaller pieces to be used as energy to power our body. Look back on something like a 7 minute AMRAP training we've done in the last few weeks or months, where we're moving large loads long distances quickly in that time.
The third primary energy system we use, especially when it comes to time durations lasting for more than just a few minutes, is the Oxidative pathway, where we're using Oxygen to help breakdown glucose (carbohydrates) even more and even fat, to use as energy to fuel our body in lower-power activities. Running for an hour or even walking for 6 miles, would be examples of activities where we're primarily using the Oxidative Pathway.
The point is for us to be able perform work and generate power in all 3 (and potentially many more undiscovered) energy pathways.
We need to be able to not only lift heavy weights quickly in a short amount of time, but also even last just a few minutes, for example in a fight, and win (if that ever happened). We humans were persistence hunters and gatherers for thousands of years, so we walked/hiked/ran for miles and miles at a time, so we heavily relied on our aerobic energy system. So we need to, from an evolutionary perspective, to be able to walk long distances as well, at a minimum.
To be able to do well in all energy systems means we are fit. As a result of being fit, our health improves. To be healthy, we avoid chronic disease, and makes even the chances of us encountering accidents like falling, and recovering from medical procedures (resulting from accidents) easier and quicker, respectively.
Looking ahead, once we're able to use the fuel we're taking in, we need to be able to use that to control our bodies properly.