> If you were, for example, to inject sugar directly into someone's blood stream would the result be fatigue every time?

Missed this the first time around. They do this, for research, it's called a glucose clamp test.

https://en.wikipedia.org/wiki/Glucose_clamp_technique

Unfortunately it's almost impossible to find someone willing to administer it to you. It's an outpatient hospital procedure lasting a number of hours, and almost no insurance would cover it, as far as I am aware.