There seems to be a rise in popularity of fasting. Variations in fasting techniques have been studied and been shown to improve indicators of physiological health. Most notably these include inflammation, body fat, lipid levels, and insulin sensitivity.
Blood glucose normally provides the body with sufficient energy through glycolysis. Glycolysis is the primary step of cellular respiration, which occurs in all organisms. It is a metabolic process that converts glucose into pyruvic acid and results in ATP production. The ATP molecule carries energy within cells, and is the main energy currency of our cells.
During fasting, maintenance of blood glucose levels initially relies on glycogen stores in the liver and skeletal muscle. Most of our glycogen is stored in the liver, which plays the greatest role in the maintenance of blood glucose during the first 24 hours of a fast. After that, glycogen stores are depleted causing the body to utilize energy stores from adipose tissue and protein stores. During this stage triglycerides are separated into free fatty acids and glycerol that the liver then converts into ketone bodies and glucose. These then become the input source for subsequent energy production.
It does not mean that fasting duration must be at least 24-hours. Intermittent fasting, involving the restriction of calories during shorter set periods of time, has been heavily studied and been shown to provide similar benefits. You can read more about the topic here.
This diagram gives an overview of considerations for intermittent fasting: