Given that insulin is absolutely deficient in people with Type 1 Diabetes, the regimen has to replicate the normal physiological insulin profile for the glucose levels to be controlled.
The daily insulin profile: normal vs type 1 diabetes
Normally, insulin is secreted by the ß-cells in the pancreas at a low, steady rate in the background all day.
This basal insulin controls the glucose produced by the liver (and, to a lesser extent by the kidneys) during periods of fasting and in between meals:
The basal bolus insulin regimen provides the two forms of insulin – daily dose of basal insulin and rapid-acting insulin doses to provide the meal-related bolus. An insulin pump provides the same pattern, but the basal rate can be adjusted to fit a more physiological profile of basal insulin – lower rates overnight, higher rates at dawn and lower rates again during the day.
None of the rapid-acting insulins or the bolus function in the pump are as quick in their action as endogenous insulin. Carbohydrate absorption occurs more rapidly than the onset of action of rapid-acting insulin, the post-meal glucose levels can be erratic simply because of this mismatch.
Basal insulin replaces the background insulin that controls the fasting and pre-meal glucose levels.
The options for basal insulin include:
Although the injected insulins have a “flat” profile, they actually peak at ~6 hours, and the duration of action is 20 – 26 hours, depending on the insulin. They are generally injected once daily but may be given twice daily.
Basal insulin is best given in the morning or at bedtime to minimize the risk of overnight hypoglycemia.
These are formulated to provide a mixture of bolus action insulin and intermediate-duration basal insulin.
Pre-mixed insulins include:
They are a convenient option for people who refuse to inject insulin multiple times every day. However, these cannot provide a physiological insulin profile, so the resulting glucose levels can never be ideal in people with type 1 diabetes.
They are not generally recommended in type 1 diabetes because of their variable absorption and because their fixed dosing does not allow for variation in the carbohydrate content of meals.
The insulin analogues include Apidra, Humalog and Novorapid and are preferred over the older Actrapid because their onset and peak action occurs much earlier.
Peak action is similar for all of them, occurring at ~1 hour. Their duration of action is 3-4 hours.
In the pump, the dose is delivered as a bolus with a similar onset of action and the duration is set in the pump for 3 or 4 hours. The insulins used in a pump are Humalog and Novorapid.
Estimating the dose of basal insulin
For most people with type 1 diabetes, the dose of basal insulin = body weight ÷ 4.
In a pump, this daily basal dose translates as the hourly basal rate = basal dose ÷ 24
The basal insulin dose will increase with weight gain, illness or stress and later pregnancy. It needs to decrease with weight loss, renal or liver impairment, dialysis and the first trimester of pregnancy.
Estimating the doses of bolus insulin
The dose of rapid-acting insulin that is given with meals depends on two things:
Patients can learn how to recognise and count carbohydrates with a dietitian and diabetes educator. These skills allow your patient to adjust their insulin doses freely to match what they eat and to lower glucose levels with confidence. They will learn how to use these calculations:
Remember, it takes 2 – 3 hours for the insulin to have its full effect. That means that if you are trying to lower the glucose level, check it again in 2 hours’ time. Any time earlier, the insulin will not have had time to work.
If your patient is not able to count carbohydrates, work with a dietitian to estimate their typical intake of carbohydrate at meal times and use this as the regular meal time dose. Your patient can use the correction factor to calculate the extra insulin dose that is needed to lower high glucose levels.
The other alternative is to set a dose for meals based on their weight and basal insulin dose. A correction dose can be added to this for higher glucose levels. An estimated dose for meals would be body weight ÷ 12.
Eg. For a 60 kg woman, her insulin regimen could be set up as:
How to start insulin
When your patient has just been diagnosed, start a basal bolus insulin regimen from day 1. If your patient is an adolescent or adult, there is a good chance that they still have endogenous insulin circulating, so the doses will need to be lowered to prevent hypoglycemia. This may persist for years after the diagnosis, so be guided by the glucose levels as to their own insulin requirements.
A safe starting regimen of insulin would be:
Refer your patient to a dietitian and diabetes educator to learn about carbohydrate counting and how to use their insulin safely to correct for high glucose levels.
How to change basal insulins
If you want to change the basal insulin, you may need to adjust the doses because these insulins are not all equivalent.
How to handle insulin
Side effects of insulin
Although insulin is an essential treatment for diabetes, it can cause side effects, particularly at higher doses. These can be minimized by keeping insulin doses as low as possible. This is best achieved by adhering to the lifestyle tips that improve insulin sensitivity.
Side effects include: