The importance of the basal rate
Overall, the basal rate demand over a 24-hour period follows a certain pattern which is similar for every individual
The insulin demand over 24hrs in healthy people and diabetics are similar:
My insulin pump closely mimics what a healthy pancreas does: It provides a steady flow of insulin every hour of the day. This is what my basal rate profile looks like (for a normal day, without exercise involved, rate for every hour of the day):
My basal rate during swim training
With the effects that the Dawn Phenomenon has on my blood sugar, 60 or 90 minutes without receiving any insulin is a long time and despite the exercise I used to find it really hard to keep my sugar levels at bay, particularly after the swim training. The insulin in the pump is short-acting insulin and lasts for circa 4 hours. Not receiving any between 6-7am means that my body is short of a unit of insulin that would have usually lasted until mid-morning. I tried several options how to overcome the shortage of insulin in my body and to avoid elevated glucose levels for the remainder of the morning:
One option was to inject 50% of the insulin that I would normally receive through the pump between 6-7am to weaken the effects. However, this still proofed not to be enough to keep my glucose levels normal. Injecting 100% of the rate before getting in the pool sent me in a hypo (hypoglycemia = low glucose levels) after 35-45 minutes of swimming since the insulin reaches its peak after about 45 minutes of injecting. After some time of experimenting, I found that the best solution, although still not perfect, was to take the pump off for the swim without injecting any additional insulin prior to the swim start, but injecting insulin midway through the swim for those sessions that lasted longer than 90 minutes. This meant that 45 minutes into the swim, I would briefly get out of the pool, attach my pump, inject 80% of the insulin I needed, unclip the pump and get back into the pool to continue training. It wasn't ideal but at least it allowed me to finish the swim session with a reasonably normal glucose level and for the hours after that. At the end of the session I would still inject the remaining 20% and continue to have the pump running as normal.
For swim training lasting no longer than 60 minutes, I would usually inject 100% of the rate that I missed out on after the session. Due to the little time delay from injecting the insulin to when it starts to work in the body, I would usually wait for 20-30 minutes until breakfast (and of course inject for this again, a so-called bolus rate)
Some Diabetics might have had a similar experience to mine; others may not. I am not saying that everyone experiences the same but I have found it incredibly hard to manage glucose levels for hours after my swim training. Even a post-swim injection often wasn't sufficient to make up for the hour of when I missed out on the insulin. In addition to this, a carb-rich breakfast would then send my sugar levels sky high. I tend to have a relatively low carb but protein-rich breakfast anyway but, still, I often had to be really careful with what I had for breakfast.
Being able to keep the pump on during the swim is great but it still meant that I had to experiment with my basal rate and reduce it temporarily: The full hourly rate would send me into hypos during the swim because the combination of a high dose of insulin plus the intense exercise would lower my glucose levels too much. I already use a temporary basal rate when I train on the bike or run which, depending on intensity and duration, varies from 20-60% less of the standard rate.
To understand how much I had to reduce the basal rate for the swim training in the mornings I had to simply test it out over a few sessions. It seems to work well with about 30% less insulin of the standard rate. My post-swim and post-breakfast glucose has improved a lot too.
I did my first open water triathlon last weekend with my new pump and things went pretty well. I will write about this on a separate note. It will be interesting to see if I need to make further adjustments to my temporary basal rate during the swim under race conditions, particularly for the longer distance races and swim events I am planning to do over the summer.