THE PANCREAS – UNLOVED, BUT ESSENTIAL
The pancreas is an essential organ responsible for both the digestion of food and blood glucose regulation. It was first identified by Greek anatomist and surgeon, Herophilus, around 2300 years ago. A few hundred years later, Rufus of Ephesus, another Greek anatomist, gave the pancreas its name. “Pancreas” originally meant sweetbread, a name that is still commonly used in culinary circles for calf or lamb pancreas.
The pancreas is located behind the stomach in the upper left part of the abdomen. It is surrounded by other organs including the stomach, small intestine (duodenum), liver, and spleen. It is spongy, about 15–25cm (6–10in) long, 2.5cm (1in) thick, and is shaped a bit like a flattened pear or a fish extended horizontally across the abdomen. The bulk (95%) of the pancreas consists of tissues and cells that produce pancreatic secretions for the digestion of carbohydrates, fats and proteins. The remainder consists of little islands of cells called islets of Langerhans. These look a bit like small bunches of grapes and produce hormones that regulate blood glucose and help regulate pancreatic digestive secretions.
Food digestion Once food has been chewed in the mouth, then mulched and partially digested in the stomach by acids, it is released into the first part of the small intestine known as the duodenum. The pancreas then releases its own digestive juices and enzymes into the partially digested food, via a small duct connected to the duodenum. Pancreatic juices contain enzymes that help breakdown carbohydrate, fat and protein. They are activated once they reach the duodenum to prevent the protein-digesting enzyme trypsin from breaking down the proteins in the pancreas itself, or in its duct. Other enzymes produced by the pancreas and released into the duodenum include amylase (to break down starches and maltodextrins into sugars) and lipase (to break down fats into monoglycerols and fatty acids). The pancreas also secretes sodium bicarbonate, which helps to neutralise the stomach acids in the partially digested food.
Blood glucose hormones Two of the most important pancreatic hormones are insulin produced by beta cells and glucagon produced by alpha cells in the islets of Langerhans which manufacture and release these hormones directly into the bloodstream.
Insulin regulates the metabolism of carbohydrates, fats and protein by promoting the absorption of glucose from the blood into liver, fat and muscle cells. In these cells the absorbed glucose is converted into either glycogen (a kind of starch found in the liver and muscles) via a process known as glycogenesis or fats (triglycerides) via lipogenesis. Circulating insulin also affects the synthesis of proteins in a wide variety of cells and tissues. It is therefore an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules inside the cells.
Glucagon stimulates the liver to break down glycogen into glucose, which is then released into the blood. It also activates gluconeogenesis, the conversion of certain amino acids from proteins into glucose. Finally, it facilitates the breakdown of stored fat (triglycerides) into fatty acids for use as fuel by cells. It is therefore a catabolic hormone, promoting the breakdown of large molecules in cells into smaller molecules in the blood.
Pancreatic beta cells are sensitive to blood glucose concentrations. When glucose levels are high, they secrete insulin into the bloodstream and when glucose levels are low, secretion of insulin is inhibited. On the other hand, alpha cells secrete glucagon into the blood in the opposite manner to insulin: when blood glucose levels are low, or in response to vigorous exercise, secretion is increased, and when blood glucose levels are high, secretion is decreased.
The secretion of insulin and glucagon into the blood in response to changes in blood glucose concentrations is the primary mechanism of blood glucose homeostasis. In other words, the two hormones work in partnership with each other to keep blood glucose levels balanced. Optimal maintenance of blood glucose levels is critical to the functioning of key organs including the brain and nervous system, liver, and kidneys.
If the beta cells are destroyed by an autoimmune reaction, insulin can no longer be synthesized or secreted into the blood in sufficient quantities. This results in the development of type 1 diabetes. In type 2 diabetes, the destruction of beta cells is less pronounced than in type 1 diabetes and is not primarily due to an autoimmune process. The exact cause of type 2 diabetes is not fully understood but people have a reduced number of islet beta cells, and of those that survive there is a reduced secretory function, and there is also frequently (but not always) peripheral tissue insulin resistance (the insulin that is produced does not work as efficiently in the target cells as it should). Type 2 diabetes is also characterized by high rates of glucagon secretion which are less responsive to the concentration of glucose in the blood, but insulin is still secreted into the blood in response to concomitantly increasing blood glucose concentrations. As a result, insulin levels are typically much higher than they are in people without type 2 diabetes.
Alan Barclay, PhD is a consultant dietitian. He worked for Diabetes Australia (NSW) from 1998–2014 . He is author/co-author of more than 30 scientific publications, and author/co-author of The good Carbs Cookbook (Murdoch Books), Reversing Diabetes (Murdoch Books), The Low GI Diet: Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment, New York).