Fiasp – some further observations… Or why an artificial pancreas can’t do everything!

I’ve written about the various curveballs that Fiasp has thrown at me on multiple occasions, and for the past few weeks I’ve continued to observe another one. Most T1s are aware that if you have to administer a lot of insulin in a single shot, insulin absorption can be an issue. Dr. Richard Bernstein talks about splitting any dose above 7u across multiple sites to improve its absorption and research backs this up. Indeed, experimentation with a “Sprinkler needle” has shown much better absorption. I initially got the impression that with Fiasp, this effect can be magnified.

When I first started to use it, large carb loads needing loads of insulin seemed simple. Just take the insulin just before eating and it would do the job.

As time has progressed, I’ve written extensively about needing to adjust the amount of insulin I take, variance in sensitivity and carb ratios and how I was needing more and more insulin with meals.

What’s become apparent is that the large carb loads and insulin needs result in far worse postprandial highs than lower ones do.

Looking first at the days with a large, single, meal with a lot of carbs, the two traces are below:

Lunchtime on both days saw large carb loads with a lot of insulin, and limited effect. Bringing the levels down ended up proving much harder than I would have liked. Even with a loop, you can see that I decided to intervene to try and reduce the levels because they were uncomfortably high.

In the first of these, the amount of insulin needed was in line with the bolus estimation shown on the picture. Net insulin used over the period was 27.8u, where the estimate was 26.67u. In the second, the estimate was 16.7u, while the actual used was 26.1u. In the first case, then, it would appear that my carb count was pretty accurate. In the second it would appear that it was way off. And yet I’ve eaten the exact same meal with the same carb count and seen nothing like the reaction, managing to allow SMB to handle it all in the past.

There’s clearly an issue here with either my carb estimate or the insulin action. It could even be both, to complicate things even more.

In comparison, the data from a day with a higher carb count, but with a lot better outcomes, is below:

Overall, there are more carbs consumed than on the two days in the first example, but requiring a whole lot less insulin. And let’s be honest here, whilst the meals in the first two examples are Katsu Curry and Fish and Chips, both if which I’ve had a lot of success with with SMB, they’ve completely blown me out of the water here. Whereas the roast chicken dinner with roast potatoes, gravy stuffing and apple crumble (then going back for repeats) hasn’t had the same effect.

This begs the question, what’s different between the examples shown.

So let’s take a look at the days in question:

Days 1 & 2:

  • Slept for 5 hours overnight
  • Both work days
  • Both fasted until lunch
  • Both days walked ~10,000 steps (approx five miles)
  • Time at office mostly sedentary

Day 3:

  • Slept for 8 hours overnight
  • Weekend
  • Fasted until lunch
  • Walked ~6,400 steps
  • Time spent standing and sitting in varying amounts, but no significant additional activity

The things that really stand out here are the differences in sleep and the fact that the third day is a weekend, whilst the first two are work days, so stress may also be playing a part. These are factors alongside the obvious ones of carb amounts and trying to fit too much insulin into too small a space that may also play a part.

Of course, at this point you would be within your rights to jump up and accuse me of Apophenia, or a tendency to attribute meaning to perceived connections or patterns between seemingly unrelated things. And you’d probably be right.

If I look back at the data, and the amount of food being eaten, whilst I’ve had some success with Fiasp at limiting the post-prandial rises with large carbs, with rare exceptions of early testing eSMB, it’s been where I’ve stuck a fair old insulin dose in before/at eating and let eSMB manage from there. It’s rarely been where I’ve eaten a load and then let it get on with it. Where I’ve eaten more normal amounts, eSMB has done well though (~80g carbs max).

And where I’ve had success with eSMB and large carb loads, I’ve been in the midst of more exercise (generally there’s been a few gym sessions or equivalent a week involved).

So while I’ve definitely seen step changes in ratios and requirements, I think some of the issues I’ve seen recently are not aligned with those.

The reality is that under my more normal sedentary lifestyle, with the best will in the world, Fiasp has lulled me into a false sense of security. Thanks to seeing what I can only consider to be outstanding results with certain physical activity patterns going on, I think I’ve transposed that to how the insulin should work.

For me personally, at least, in the very large doses required for very large carb loads, and in the absence of a decent amount of GLUT4 activation from proper exercise, I see the high spikes that I’ve shown here.

I suspect that many people have encountered similar issues with getting too caught up in the hype. Whilst Fiasp does work well, and is fast, there are undoubted issues that people are seeing with the insulin, and if you don’t treat it and the diabetes with the respect it’s due, it will come back and bite you. I wonder whether the amount of variance that I and others have seen can also be attributed to factors that we haven’t properly considered?

But for now, I think it’s fair to say that my learnings are:

  • Keep the carbs to normal levels – less of the experimenting with giant carb loads (even if technically they are supposed to be slower absorbing
  • Get back in the gym – that’s good all round and will help on multiple levels
  • As always, keep an eye on what’s going on.

Even with an artificial pancreas doing a great job of having my back, I’m still a person with T1D, and no matter how much an almost closed loop helps, I still need to be aware of that in the decisions I make and the effects that they have.

Whilst an artificial pancreas with Fiasp makes a huge difference, it’s no substitute for an active lifestyle, which I may, recently, have been ignoring…


  1. Great article and some interestimg points. I have found similar results in relation to large meals and exercise has always helped improve my levels.
    One thing I have noticed with fiasp (compared to novorapid) is a reduction in absorbtion of insulin for patch sites that are approaching the 3 day mark. I.e. same meal requires more insulin on patch site day 3 than on day 1, this theory may also be supported by the fiasp blurb about site reaction and irittation.
    Another interesting observation and in agreement with your exercise observation is the contrast between correction doses during waking hours and while sleeping (well going back to sleep). I have found that during the night hours correction doses can have little effect. Not an unsurprising conclusion but perhaps more noticable than with novorapid?

    • As I’ve previously mentioned, I have moved to two days on sites, where I can go to three (or more) on Novorapid, although the limiting factor seems to be amount of insulin passing through the site. It’s pretty consistently 90-100 units that causes the site to become useless for me with Fiasp.

      Fortunately I’ve not been having to worry about correction doses overnight, as the loop handles that side of things extremely effectively.

  2. I’ve been using Fiasp since starting on a pump and am now thinking of trying Novorapid. Whilst Fiasp worked well for me in pens, after using Fiasp in a pump for a few months now it just seems to be too unpredicatable and often I have to keep adding additional bolus to bring BS down. I’m wondering if the Vitamin B3 additive is building up, decreasing insulin sensitivity and causing uncertainty in the reaction of the body to doses making everything unpredicatable. I think FIASP works well in pens where you rotate sites all the time, but using it with a pump for both basal and bolus insulin and more limited rotation of sites I think the compounded build up of the vitamin adds yet another unknown variable to an already complicated life of diabetes.

  3. I’m now four months into using Fiasp, had a few teething problems to start with and did find things quite frustrating. Now everything has settled down and as long as my basal is correct and I split all bolus’s there does not seem to be a problem with it unless I go over the top with carbs then I suspect that’s the same with all insulin’s

  4. Glad you are getting on ok with it Sue. I feel like it could be that it works very well if you are able to steer a tight line with it but perhaps if levels go particularly high when you already have a lot of IOB things get unpredictable. I’ve read over threads about the water effect and definitely seem to have experienced it. This morning it’s working great for me, haven’t gone above 8 since breakfast but if my levels rise I fear things will become unpredictable again. I wonder if is to do with the added vitamin remaining in the system after the insulin dose is used and therefore after a certain amount of bolus’ you have a greater ratio of the vitamin on board in comparison to insulin causing the desensitivity to any further bolus’ you give. Once things get back together normal and the vitamin ratio is reduced, things get nice and predictable again. Seems like it’s very unforgiving if ratios are not correct. I think I will give novorapid a try to compare as often Fiasp does not work as quickly and I still have to pre bolus 30 mins before meals.

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