The Difference Between Basal and Bolus Insulin

Vials of insulin.
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In order to understand the role of both basal and bolus insulin, it is important to first understand how the body naturally uses glucose and insulin.

When food is eaten, it is digested and converted to glucose (sugar) so it can be used for energy. Virtually every cell in the body, including your brain, needs glucose to function properly. The hormone ​insulin is needed to carry that glucose into cells in all parts of the body so that it can be used for energy.

Some of this glucose is stored in the liver as a reserve fuel (called glycogen) that is released when glucose is not available through food. So, between the glucose that is consumed through food and what is gradually released from the liver, the body gets a constant supply of glucose. This also means that there needs to be a constant supply of insulin in the body to keep the amount of glucose in balance.

Since more glucose is produced after a meal, the pancreas secretes more insulin. When the amount of glucose is lower, such as between meals or at night, there is less insulin needed -- but there is always at least a small amount of insulin present in the body at all times.

Defining Basal and Bolus Insulin

Basal insulin is the background insulin that is normally supplied by the pancreas and is present 24 hours a day, whether or not the person eats. Bolus insulin refers to the extra amounts of insulin the pancreas would naturally make in response to glucose taken in through food. The amount of bolus insulin produced depends on the size of the meal.

In a person with type 1 diabetes, the pancreas no longer automatically makes insulin regardless of the intake of glucose. The beta cells that produce the insulin have largely shut down. Both the basal, or long-term background insulin, and the bolus, or quick bursts of insulin needed at mealtimes, must be obtained through injections or an insulin pump in order to process all of the glucose taken in through food or released by the liver.

Types of Basal and Bolus Insulins

Long-acting basal insulins, such as NPH, Levemir, and Lantus, begin working in 1-2 hours but are released slowly so they can last up to 24 hours, providing that background insulin that is needed around the clock.

Fast-acting bolus insulins, such as NovoLog, Apidra, Humalog, and Regular, generally begin working within 15 minutes. The exception is Regular, which has an onset of about 30 minutes. Each of these bolus insulins is designed to be taken just before a meal and have a duration of up to five hours for NovoLog, Apidra, and Humalog, and seven hours for Regular.

This means that a person with type 1 diabetes would have to take multiple injections of a bolus insulin each day to cover their meals and snacks, along with a basal dose to keep the background insulin in check.

Basal and Bolus Insulin With Insulin Pumps

The person using an insulin pump would typically receive a constant low dose of fast-acting insulin that would act as the basal background insulin. Before meals, the pump user would give a larger dose of fast-acting insulin to cover the meal about to be eaten. This satisfies both the basal and bolus needs using the same fast-acting insulin.

Whether injecting with a syringe or using an insulin pump, the actual dosing and type of insulin(s) used would be determined by a healthcare professional.

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