How did Claude Bernard’s glycogen discovery show that the digestive system can build complex substances?

Claude Bernard’s discovery of glycogen in 1856 fundamentally changed the scientific understanding of biological processes. Before his research, it was widely believed that the digestive system functioned exclusively to break down complex molecules into simpler ones.1

By identifying glycogen as a complex, starchy substance synthesized by the body from sugar, Bernard demonstrated that the body is also capable of the reverse process: building up complex molecules from simpler ones. He observed that the liver stores this glycogen as a carbohydrate reserve, which the body can then break down into glucose as needed to maintain a constant blood sugar level. This insight not only highlighted the liver's role in metabolic regulation but also provided a key example of homeostasis, illustrating how the body maintains a stable internal environment despite external changes.12