Gluconeogenesis

Gluconeogenesis is the manufacture of new glucose from things that are not carbohydrate: lactate, glycerol released from fat, and amino acids, principally in the liver with a smaller contribution from the kidneys.

It exists because some tissues need glucose regardless of what you have eaten. Red blood cells have no mitochondria and can use nothing else, and the brain, while it adapts to using ketones during prolonged fasting, still requires a substantial glucose supply.

It is why carbohydrate is not an essential nutrient in the strict sense, and why very low carbohydrate diets do not cause the collapse that a naive reading of glucose requirements would predict. The body simply makes what it needs.

It runs continuously and rises overnight, during fasting, during prolonged exercise and under the influence of glucagon and cortisol. That is also why fasting blood glucose does not fall towards zero after eight hours without food: production is being matched to demand.

In type 2 diabetes it is part of the problem rather than the solution. Insulin normally restrains hepatic glucose production, and when the liver becomes insulin resistant it keeps producing glucose it does not need to, which is a substantial contributor to raised fasting glucose. Metformin works largely by suppressing that overproduction.

The gym anxiety about it, that fasted training or a long gap between meals will convert your muscle to glucose, is overblown. Amino acids for gluconeogenesis come substantially from the circulating pool and from turnover rather than from dismantling working tissue, and adequate daily protein covers it.

In practice, understanding it removes a category of unnecessary worry about meal timing and fasted training.