Ketosis!

Many people, even biologists, don't know about ketosis, the alternate method our bodies have for supplying energy.  Most people (especially in modern society!) utilize glucose as their cells' primary energy source.  But, in the absence of carbohydrates, our bodies are able to burn fat.  The fats that circulate throughout our body during fasting or very low-carb dieting are known as ketones:

Source.

The metabolic changes necessary to metabolize ketones instead of glucose depend on human physiology and activity:

These graphs describe Peter Attia's experiments with different exercises while in ketosis, and the results to his blood glucose, ketones (B-OHB) and lactate levels.