Rapid insulin – Novorapid, Humalog, Fiasp, Apidra.

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Rapid acting insulin helps with the digestion and absorption of food. They are used to treat many forms of diabetes but are most commonly used in type 1 diabetes.

There are lots of different types of rapid acting insulins but most are pretty comparable. They are a new age insulin known as analogue insulins but have still been around for quite some time now. Another name for these insulin is insulin aspart. In practice we tend to just call them rapid acting though.

This article is to explain everything you need to know about rapid acting insulins so you can become the expert.

What are Rapid acting insulins

Rapid acting insulin is used to counteract the glucose increasing effect of food. Without diabetes your pancreas will release just the right amount of insulin to help absorb glucose in your blood. Rapid acting insulins mimic this effect.

There are a few different brands with the most common being Novorapid and Humalog. Apidra is also an example of a rapid acting insulin. Fiasp was also released in the UK in 2017 and is the newest version.

Rapid acting insulin may also be called quick acting insulin. The terms are used interchangeably so try not to get confused.

How do they work

We’ve already mentioned rapid acting insulins are used to help absorb food. However, they aren’t used for just any food. Primarily they are used to help the absorption of carbohydrates.

Carbohydrates are any starchy foods like bread, pasta, rice, potatoes, cereal and oats. Sugar is also a carbohydrate and includes processed and natural sugar.

Therefore, if you’re eating any of these foods at meal times or at snacks, you might need to take some rapid acting insulin.

Rapid acting insulins can also be used to correct high glucose levels but more on that later.

Rapid insulin is almost always used in conjunction with a long acting insulin. I’ve done a full post on how long acting insulin works here. When used together, this is called a basal bolus insulin regimen.

Insulin peak

Timing with rapid acting insulins can be crucial. In fact it can be the difference between a stable glucose level and a high glucose level. The reason is to do with how they work once injected.

Rapid acting insulin stays in the body for about 4.5 hours. Most of the insulins take about 30 minutes to be absorbed and do not peak action until about 1 hour later. The exception is Fiasp which is absorbed much more quickly – around 7-10 minutes.

This can be problematic for most insulins if you take them at the wrong time or are eating quickly absorbed carbohydrates. Often I find patients eat a high glycaemic index meal (if you want to learn about GI click here) and the food starts to get into their blood stream before the insulin.

Therefore, official guidance is to take your rapid acting insulin 30 minutes before eating (unless taking Fiasp). This isn’t always the most practical advice though. In reality, many people do just fine taking their insulin at the beginning of their meal.

To prevent glucose spikes, aim to eat slow releasing carbohydrates, portion control your them and time your insulin as best you can prior to the meal.

How are rapid acting insulins administered

All insulins are given by an injection. An insulin injection is different from the types of injections you might be thinking of though. Insulin needles are very thin. In fact, they are smaller than a hair follicle. Over the years these needles have become smaller and smaller and now are only 4mm long. If you’re using a longer needle than this, it might be worth speaking with your medical team.

If you need more information on injecting here’s a guide to how to do it or watch this YouTube video.

Carbohydrate Counting

Most people placed onto rapid acting insulin will be advised to carbohydrate count. This is a system where you match your insulin dose to the amount of carbohydrates you eat.

This allows flexibility in your diet. Sometimes you may eat more carbohydrates compared to other meals. It makes sense then that you will likely need more insulin.

Carbohydrate counting gives you a more methodical formula for calculating how much more or less insulin to administer.

It is based on a ratio format. Everyones ratio will be different but an easy example is you take 1 unit for every 10g of carbohydrate you eat. Therefore, if you calculated you ate 50g of carbohydrates you take 5 units of insulin. There are lots of ways to work out your carbohydrate intake including weighing food and using additional help like carbs and cals.

If you want more information on carbohydrate calculating, click here.

Correction doses

Rapid acting insulin can also be used to lower high glucose levels. A correction dose is the extra insulin you give on top of your carbohydrate insulin.

This too works off a ratio principle but remember this is different to carbohydrate counting. It’s extra insulin on top of your carb counting dose.

An example might be something like taking 1 extra unit of insulin to lower your glucose levels by 3mmol/l. Everyones ratio will be different.

As an example let’s say your glucose are 18mmol/l. You want to lower these back to somewhere between 5-10mmol/l. 1 extra unit of insulin will drop your glucose levels to 15mmol/l. 2 extra units will lower your glucose levels to 12mmol/l. Finally, a third unit of insulin will drop them back to 9mmol/l and you’re back in glucose range.

Putting this all together and combining this with the carbohydrate counting example above let’s do a working example.

You are using a 1unit to 10g carb ratio and you’re eating 50g of carbs. You are using a correction dose of 1 unit lowers 3mmol/l. Your glucose level is 18mmol/l.

Take 5 units for the food. We can write this out like this:

1 unit to 10g so:

  • 10g= 1unit,
  • 20g = 2 units,
  • 30g = 3 units,
  • 40g = 4units
  • 50g = 5units).

Then add 3-4 units on top of this to lower your glucose levels from 18mmol/l to 9 or 6mmol/l.

  • 1 unit drops from 18 to 15mmol/l,
  • 2 units drops to 12mmol/l,
  • 3 units drops to 9mmol/
  • 4 units drops to 6mmol/l).

Therefore, your total dose for this meal is 5+3-4 units = 8-9 units.

Repeat correction doses as needed only at meal times or 4.5 hours apart. If you regularly take extra insulin to between meals or within the 4.5 half life of rapid acting insulin the insulin will begin to stack. This often results in hypo’s and can feel like you’re then chasing your glucose levels all day.

How many times a day do I take it

Now this varies depending on how often you eat and/or need to correct your glucose levels.

Conventionally for 3 square meals a day and no/small snacks, you’ll need to take this 3 times per day. Unless you eat a lower or zero carbohydrate meal and require no correction dose. Then you may even take fewer doses.

How much insulin do I need

The dose varies depending on how sensitive you are to your insulin and what your ratio is. If you are carbohydrate counting it also depends on how many carbs you eat and whether you need correction doses.

Some people will be on set doses of rapid acting insulin. From experience this rarely works out very well unless you eat the same amount of carbs at each meal and thus match your carb to insulin through dietary manipulation.

How often do I need to test my blood glucose levels

In a word, regularly. You should never inject insulin blindly and so knowing your glucose level is paramount for safety. This allows you to adjust your dose based on your current glucose reading and your food intake. Even without carbohydrate counting you should do this.

The obvious times to do this is before meals. This is because rapid acting insulin lasts 4.5 hours roughly. This is about the perfect time between meals and it helps you see the effect of your last rapid insulin dose.

Even if you don’t eat, testing at regular time intervals throughout the day is a good idea.

If you snack you should take a glucose level before injecting for these in case it effects your decision about your dose. It can be a lot of finger prick tests but the introduction of Flash sensors like the Freestyle Libre or CGM sensors like Dexcom have made it easier to test.

How do I know if my dose is correct

This is quite easy to figure out but it might take a bit of work. You want to look for patterns and not react off just one result. For example, if you use a 1:10g carbohydrate ratio and every day at breakfast you notice your glucose levels rise between 5-10mmol/l by lunch, its highly likely your breakfast ratio is too low. Therefore, you can change the ratio or simply increase the dose.

Just keep in mind if you then change your meal, the insulin required might change.

Advanced techniques

There are some advanced techniques to help manage your rapid insulin. With continuous glucose monitors we can now see what happens to your glucose levels between meals and overnight.

One common problem is patients glucose levels are in target before a meal and in target 4.5 hours later at the next meal but in between they are sky high. Due to the glucose levels being in target before each meal when you tested, we know this isn’t an insulin dose problem. If you increased your dose to account for the high level in between tests, you’d only likely hypo 4.5 hours later.

Instead the problem comes down to your insulin timing or food choices. Most rapid insulins take 30 minutes to get into your system and 1 hour to get going. So if you eat something that is rapidly absorbed, like cornflakes covered in milk, you’ll likely expect a high glucose between meals. The food is getting into your system before the insulin.

We 3 have choices to fix this. First you can inject earlier. Some people even 30 minutes is too late before the meal and so this isn’t always very practical. Particularly if the meal is unpredictable or your at work and can’t inject that early.

Second you can change the type of carbohydrate you eat. Instead of choosing quickly absorbed carbohydrates like Cornflakes, choose a lower releasing option like overnight oats. Adding protein and fat to the meal also helps to slow down the absorption speed.

Finally, from my own personal experiencing, lowering the total carbohydrate at the meal really helps. Reducing the potential to spike in between meals is reduced if there is less carbohydrate in the meal to increase your glucose levels. You’ll find scrambled egg on 1 slice of toast will likely be a much easier meal to control than if you choose a high carbohydrate, high glycaemic index, meal.

How often should I adjust my dose

Basically every meal unless you have a set routine. Keep in mind, adjustments to your background insulin may also effect your rapid acting insulin requirements. So your dose will be a fluid situation with regular adjustments needed.

Benefits of rapid acting insulin

The biggest benefit is they are flexible. You can eat whatever you want and match your insulin dose to the food. This is the most physiologically similar insulin to how your pancreas would normal release insulin for food.

You can also easily see if your dose was correct. As rapid insulin lasts only 4.5 hours, you get feedback quite quickly about whether your dose was appropriate.

Negatives of rapid acting insulin

The obvious drawback is you have to test your glucose levels a lot and take multiple insulin injections each day.

Carbohydrate counting and correction doses is also more effort than the set doses of some other insulin regimens. Although I’d argue you’ll always achieve better glucose control with background and rapid acting insulin (basal bolus regimen) as opposed to other insulin regimens. Especially, in type 1 diabetes.

Due to rapid insulins short duration and high insulin peak, they do put you at risk of hypos if you miscalculate your dose. This can often be frustrating for the insulin user. Avoiding hypo’s by fine tuning your doses often goes a long way to improving your glucose control.

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