What Does Insulin Do? Basal Vs Bolus

It surprises me how often questions come up online about what insulin does and the purposes of basal and bolus insulin. As usual, there is a tl;dr at the end, although this blog article is relatively short.

The Two Roles Of Insulin

The most well known role of insulin is its role in taking glucose from the blood and moving it into the cells of the body. In fact there are parts of the body which do not require insulin to access blood glucose, such as the brain (the thing that uses most of the fuel and is too important to go without it), and the liver (the thing that makes the fuel when fasting). For most of the rest of the body though, to use glucose as a fuel, requires insulin.

The second and less well know role of insulin is in helping regulate the liver. In “What Is Ketosis And Diabetic Ketoacidosis?” I talked about this in detail. In short, when there is less insulin in the blood, the liver becomes more active at producing fuels for the body and when there is more insulin the liver is substantially less active at doing it.

So What Does This Have To Do With Basal and Bolus Insulin?

For those that do multiple daily injections (MDI), the two roles of insulin are reflected in the two types of insulin used i.e. rapid- and long-acting insulins.

The role of a long-acting insulin is to mimic the slow and continual release of insulin from the pancreas to offset the slow and continual release of glucose from the liver, keeping the liver in check and blood sugars stable (Basal Insulin).

The rapid-acting insulin is designed to mimic the release of insulin by the pancreas to offset spikes in glucose from things like exercise, and carbohydrate intake (Bolus Insulin).

In the case of a pump, it is one type of insulin but there is a continuous release (the basal rate) and bolusing (sharp spikes of insulin release).

Consequences for Insulin Use

The big takeaway from all this is, if you are not doing things which cause glucose spikes, your blood sugars should be flat and in range. If, when you fast, your blood sugars and trending up or down, your long-acting insulin/basal rate needs adjusting.

Get your basal right and it will be a lot easier to manage your blood sugars.


Insulin serves two purposes in the body: moving glucose from the blood into cells (bolus insulin) and to assist in the regulation of the liver’s fuel production (basal insulin). Rapid- and long-acting insulins tries to reflect these two roles. A consequence of this dual-role is, at least in theory, if a person is not eating or undergoing anything else to spike their glucose levels, their glucose levels should be flat and controlled exclusively by their basal insulin. If glucose levels are not flat the person’s basal insulin routine/rate needs adjusting.

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