FAQ: Basal Insulin vs.
Bolus Insulin

I have heard the terms Basal & Bolus insulin used, what do these terms mean?


In looking at the normal non diabetic state, the pancreas continuously produces and releases into the circulation a small amount of insulin 24 hours a day. We recall that insulin is involved in transfer of glucose from the blood into the cells where it is burned to produce energy, carbon dioxide and water. Our cells continuously need energy, particularly brain cells and heart muscle cells so that they can keep on functioning (it would really spoil your day if the heart muscle cells stopped contracting because they ran out of glucose). This background insulin level sufficient to keep basic functions of the body working is called the basal rate, it is usually about 1 unit/hour. When using an insulin regime of long and short acting insulins, some of the long acting insulin gave a basal insulin level.

Another use of long acting insulin is to signal the liver to stop producing glucose. Normally in times when we are not feeding (such as overnight) the liver produces glucose to supply basic needs of the cells for energy. The basal insulin then transfers this glucose (energy) into the cells. When the blood glucose level starts to rise, this triggers release of insulin from the pancreas. This elevated insulin level then signals the liver to turn down or turn off glucose production. If glucose levels are high enough to cause a high insulin level, then further glucose production is not needed. One of the defects in Diabetes is that the liver becomes relatively insensitive to the insulin signal to decrease glucose production, so glucose levels increase overnight and we end up with a high fasting glucose in the morning. If we give an injection of long acting insulin in the evening to signal the liver to reduce glucose production then we end up with a lower fasting glucose in the morning. In the early phase of your illness, you were still making almost enough insulin for your body needs so if you fasted, your glucose levels would not significantly increase. At later stages of Diabetes, even if you fast, you would likely find that glucose levels progressively increase. The amount of insulin that will keep glucose levels steady and prevent this increase is the
basal insulin.

Basal insulin needs (while fairly steady) tend to be lowest during the early hours of the night and highest in the early hours of the morning (4-7 AM) this increased basal need before awakening is called the Dawn Phenomenon. With the insulin pump we can program the pump to take into account our changing basal needs at different times of the day.

If you stressed the system by eating which increased the need for insulin, then your pancreas could not keep pace with the increased need, and blood glucose would go up after meals (post prandial hyperglycemia). The increased insulin needed to cover the carbohydrate portion of a meal is called the
bolus insulin. The amount of bolus insulin needed depends on the degree of insulin deficiency. We usually need 1 unit of insulin for each 10 to 15 grams of carbohydrate eaten. This ratio is called the carbohydrate/insulin ratio and using this ratio we calculate the bolus insulin dose to be taken with a meal.

To the bolus dose we also add a high glucose adjustment dose, so that if glucose levels are high we give enough insulin to correct the high glucose and bring the glucose level down to normal within a few hours.

Dr. J. Robin Conway M.D. 20 Mar 2004