Most human energy and nutrition supply comes from breaking down carbohydrates. Glucose released by these processes is critical for human development, particularly the brain, and for maintaining health. Many serious clinical conditions can be traced to or be treated by regulating this glucose release, such as diabetes, nutritional disorders and certain types of obesity. This theme investigates the major mechanisms in the human breakdown of nutritional carbohydrates, by host human enzymes and those from the resident microbiome. These enzyme systems together are thought to be chiefly responsible for the metabolic activity that forms glycogen – energy stores – from nutritional sources. Regulating their activities offers a means to treat associated clinical conditions.