Insulin is a hormone produced in the beta-cells of the pancreatic islet cells. It serves as the body’s key hormone to normalize blood levels of glucose and other nutrients after food intake i.e. it has the ability to reduce blood glucose concentrations. And furthermore, it plays an instrumental role in the metabolic regulation after food intake, by steering the storage of nutrients and energy.
Storage Hormone Insulin
Insulin secretion is triggered by raising levels of glucose in the blood and the pancreas, and to a lower extent by other substances such as amino acids. Also some gastrointestinal hormones like GIP, GLP-1 or CCK or hormones from the parasympathetic nervous system promote the release of insulin, whereas the release of hormones like adrenalin or noradrenalinupon stress or during sports have inhibitory effects on insulin secretion.
The main functions of insulin include regulating blood glucose levels after food intake, and promoting the storage of nutrients and energy upon their uptake in the body. On one hand, insulin promotes the uptake of glucose into cells and its cellular utilization. On the other hand, it inhibits the mobilisation and the formation of new glucose (gluconeogenesis). Apart from that, it promotes the formation and storage of fat and the synthesis of protein. Therefore, insulin is a storage hormone.
Insulin functions at different sites in the body. In particular, muscle and fat cells with their glucose transporter 4 (GLUT4) depend on the effects of insulin for uptake of glucose. Since muscle and fat tissue represent a large share of the body weight, their glucose uptake leads to a significant drop in blood glucose concentrations after food intake.
- In muscles, insulin triggers the uptake of glucose into the muscle cell. The muscle utilises part of the glucose in its energy metabolism, promoted by insulin, while the utilisation of fat is suppressed at high glucose availability. The other part of glucose is stored in the form of glycogen in muscular depots. Apart from that, insulin promotes the uptake of certain amino acids such as alanine and the formation of new protein thereof.
- In fat tissue, too, the uptake of glucose is facilitated by insulin. In fat cells, insulin promotes the utilisation of glucose as well as its transformation into fat. At the same time, it promotes the uptake of free fatty acids from the blood and their storage in the form of fat in fat tissue, whereas the mobilisation of fat is inhibited by insulin.
- In the liver, glucose can be taken up independent of insulin via GLUT2. Here, insulin also supports the formation of glycogen (storage of glucose), fatty acids and triglycerides, and inhibits fat mobilization. At high blood glucose concentrations, insulin promotes the utilisation of glucose in energy metabolism. At low blood glucose concentrations, the liver releases glucose into the blood independent of the insulin level.
Relevance of carbohydrates and their insulin release in weight management
As carbohydrates have varying effects on blood glucose and insulin, it’s important to make the right choices to support weight management and body composition. A modern diet leads to high blood glucose levels over the day, as most carbohydrate-based foods are medium to high-glycemic. Carbohydrates with a high-glycemic index release high levels of insulin, which serves as a storage hormone and inhibits the fat burning process. In contrast, low-glycemic carbohydrates such as isomaltulose release lower levels of insulin, helping steer the metabolism towards fat-burning.
Reduced insulin levels have less of an inhibitory effect on fat mobilization from adipose tissue and on fat utilization in muscles. This promotes the mobilization and oxidation (rather than the storage) of fat. Over time, increased fat storage in adipose tissue can result in body weight changes. While greater fat storage in non-adipose tissue has been associated with the development of insulin resistance, the lower storage of fat has a positive impact on body weight, body composition, and insulin resistance.