1.6 What's next?
Digestion is only a preparatory step in metabolism. The really interesting part starts once the substrates have been absorbed and made their way to the liver. Their fate there will depend on the prevailing metabolic situation. The utilization of glucose, the most important single substrate in energy metabolism, is controlled by the hormones insulin and glucagon, both of which are secreted in the endocrine islets of the pancreas:
- If glucose is plentiful (typically after a meal), a significant fraction will be retained in the liver and stored there in the form of glycogen, which is a polymer closely similar to amylopectin. The stored glycogen is broken down again to glucose, which stabilizes blood glucose levels during prolonged intervals between meals. Once the glycogen stores are stocked up to the roof, glucose and amino acids will be turned into fat, which will then be forwarded to the peripheral fat tissue for storage.
- If it is available but in high demand due to exertion, the liver will pass on the glucose to the periphery for utilization.
- If it is scarce, the liver will use part of the amino acids it receives from the intestine to turn them into glucose, which it then passes on to the periphery.
Dietary fat is processed differently from water-soluble substrates sugars and amino acids. It is packaged into lipoproteins directly in the intestine, and it bypasses the liver because it is delivered into the lymphatic vessels rather than into the blood stream.
We will consider all these processes in turn. At the end of this class, we will have the tools to understand in depth what goes on in metabolic diseases such as phenylketonuria, lactose intolerance, and diabetes.