How Basal and Bolus Fit Together
Reviewed by CDE Mehandi Sharma · Updated
A body without diabetes releases a small amount of insulin continuously, plus a burst whenever food arrives. A Type 1 regimen copies both.
The continuous part is called basal, or background, insulin. It covers the glucose your liver releases through the day and night, independently of what you eat. It is usually a long-acting insulin taken once a day, sometimes twice.
The burst is called bolus, or mealtime, insulin. It is a rapid-acting insulin taken with food, sized to the amount of carbohydrate in that meal.
There is a third use of the same rapid insulin: a correction dose, to bring down a reading that is already high.
The split is what makes the whole thing diagnosable. If your fasting and pre-meal readings are drifting but meals behave, that points at basal. If you wake up fine and go high after specific meals, that points at bolus. Knowing which is which prevents you adjusting the wrong one.
Premixed insulin regimens blur this and need meals at more predictable times in exchange for fewer injections.
Doses are worked out by your team and revised regularly — this is not something to set once and leave.