Glucose
A simple sugar (monosaccharide) that is the body's main energy fuel, notably for the brain and red blood cells, and whose level in the blood (blood glucose) is tightly regulated.
Glucose is THE reference energy molecule. All cells can use it; the brain and red blood cells depend on it in particular (the brain consumes about 120 g of glucose/day). It circulates in the blood (blood glucose) and is stored as glycogen (liver, muscles).
An original angle (the brain and its alternatives): people often say « the brain needs glucose ». That is true, but with nuance: during prolonged fasting or ketosis, the brain can run on about 70% ketones, reducing its need for glucose. The body still makes the minimum glucose needed for survival (gluconeogenesis) from protein and fat. So there is no strict dietary requirement for carbohydrates: the body can produce them (which does not mean carbohydrates are useless).
Blood glucose regulation: after a meal, glucose rises, insulin moves it into the cells, and blood glucose falls back. Between meals, glucagon releases glycogen to maintain blood glucose. This system is remarkably precise (blood glucose is kept at around 1 g/L).
Spikes and rollercoasters: fast carbohydrates (sugars, white flour) cause glucose spikes followed by reactive drops (reactive hypoglycaemia), causing cravings, fatigue and irritability. Eating carbohydrates WITH fibre, protein and fat smooths these curves. The order of foods matters too: starting with vegetables and protein before starchy foods reduces the glucose spike.