How the pump mimics natural insulin secretion

A healthy pancreas contains beta cells, which measure the glucose concentration in the blood at every moment and release exactly as much insulin as is needed [1]. The release goes directly into the blood and takes two main forms. A small and relatively constant amount (with some slow variations) is released all the time. In addition, when you eat, a larger amount is secreted abruptly and released quickly. This system works like an automatic loop.

When blood glucose rises, the beta cells sense this and secrete more insulin, and when blood glucose falls, they immediately reduce the amount. The insulin reaches the portal vein, which goes first to the liver and only then to the rest of the body [2]. The liver is the main organ that regulates the amount of glucose present in the blood at any given moment. Only half of the insulin that reaches the liver passes beyond it, the rest is used up locally [3]. In this way, blood glucose stays fairly stable, without any effort on your part.

In type 1 diabetes, the immune system removes the beta cells, and the body thus loses its ability to produce insulin and to regulate its own blood glucose. The insulin pump tries to take over this role and to restore the natural pattern, using a background insulin flow and additional doses at meals [4].

The more closely insulin delivery resembles natural secretion, the closer blood glucose will stay to normal values. This pattern reduces the risk of large rises after meals and of falls between meals, giving you better control and more safety in your life.

The basal rates correspond to background secretion, that is, that small and continuous amount of insulin which the pancreas releases between meals and during the night. The pump reproduces this secretion by steadily delivering very small amounts of rapid-acting insulin [5].

The background need is not the same throughout the day, because the body asks for more insulin towards the morning and less at other times. You can program different basal rates by the hour, so that insulin delivery follows the natural rhythm of your body as closely as possible [6].

The meal bolus corresponds to the rapid release of insulin that a healthy pancreas produces when you eat [1]. This additional amount covers the glucose that enters the blood from food, in the form of carbohydrates [7].

When you use an insulin pump, you are the one who asks it to deliver this bolus before every meal. The device then releases the insulin dose that is most often set by you (sometimes with help from the pump) [4]. In this way, the insulin given at the meal is added on top of the background insulin, exactly as happens naturally.

The pump comes very close to natural secretion, but it cannot copy it perfectly. The insulin from the pump is released subcutaneously, not directly towards the liver the way insulin produced by the pancreas arrives [2]. For this reason, the insulin released by a pump needs time to be absorbed and to start working [8].

Beta cells respond within a few seconds and with great precision, unlike the insulin from the pump, which only slowly starts to act after a few minutes. Even automated systems, which adjust insulin delivery on their own, keep this delay [9]. The pump remains a good imitation of nature, but clearly not an identical one.

Even when you are not eating, the liver continuously releases glucose into the blood, in order to feed the brain and the other organs. The background insulin keeps this release under control and thus maintains stable blood glucose between meals and overnight [10]. Without this continuously released background insulin, blood glucose would rise gradually, even if you eat nothing (the liver has a large store of glucose).

In addition, your body would start to burn fat and in the end to form ketone bodies, which can become dangerous [11]. For this reason the basal rates need to be there permanently, not only temporarily. When the basal rate is missing for longer, the signs are not limited to a high blood glucose. If vomiting, difficulty breathing or drowsiness appear, go to the emergency room straight away.

When you eat, carbohydrates are digested and turned into glucose, which enters the blood quickly and puts pressure on blood glucose to rise. To cope with this rise, the body needs a larger amount of insulin, within a very short time [12].

This extra insulin moves the glucose from the blood into muscle, into fat tissue and into the liver, and temporarily stops the release of glucose from the liver [12]. The more carbohydrates the meal contains, the larger the mealtime insulin dose has to be [7].

The rapid-acting insulin in the pump needs a little time to start working. That is why it is good to give the bolus a few minutes before the meal. Its effect then overlaps with the rise in blood glucose caused by the food [13].

The mealtime dose is calculated from the amount of carbohydrates, and the pump can release the insulin immediately or in an extended way, over a longer period [7]. An extended bolus suits meals rich in fat and protein, which can raise blood glucose later [14]. In this way, the timing and the shape of the bolus help the insulin follow the curve of your blood glucose as closely as possible.