Training with Diabetes: Dawn Phenomenon & Glycogen Metabolism of Swimming

I’m working with an athlete who has recently been diagnosed as a diabetic and is training for an Ironman. He is also new to triathlons and all the training and logistics that come with it.

Last week I got an email from him about two problems with his diabetes management:

1. High glucose levels on his early bike sessions
2. Hypoglycemia during swim workouts done at lunch time

1) Hyperglycemia early in the morning is very common in diabetics. Also known as the Dawn Phenomenon, it is the result of decreased insulin sensitivity and changes in the levels of various hormones in the early morning hours (GH, Cortisol, Glucagon), which can lead to the overproduction of glucose by the liver and to the underutilization of glucose by peripheral tissues. 

Since the bike workouts are only about one hour, there is no need to have a big breakfast before these. So he was instructed to get a big breakfast after those workouts, taking a couple of extra ultra-fast insulin with it, and check blood glucose levels 2 hours after the meal to make sure it was all absorbed.

2) Hypoglycemia during swim workouts done at lunch time

For the second issue, there is also a link between the situations that are causing the hypoglycemia during the swim.

First of all, swimming brings with it a higher metabolism rate and a higher number of muscles involved in the work compared with running or biking. When you swim you are using many small muscles, which make the glycogen consumption a bit higher than during biking or running, thus making swimming the discipline that causes a bigger drop in blood glucose levels for the first hour of training.

Another reason is that on some days this athlete is training in the morning (when the dawn phenomenon occurs) as well. So by the time he gets in the pool, not only is his metabolism higher but his insulin sensitivity is high too.

I remember that back in my training days it didn’t matter how high my blood glucose levels were before a workout – if I didn’t eat anything before the session I would end up with a hypo.

The advice for my athlete was:

1. If blood glucose level is >180: Take a bottle of maltodextrin/sports drink (with 200+ calories) to the pool and drink it halfway through the session.
2. If blood glucose level is <180: Eat a small snack before training AND take a bottle of sports drinks to the pool, drinking it halfway through the session.

Connecting the two problems:

It is very unlikely that you will get an HIPO by training on an empty stomach in the morning, unless of course the workout is longer than 1 hour or you woke up with an extremely low blood glucose level. Swimming is the discipline that has a stronger impact on your blood glucose levels if you were to do them all at a similar intensity. 

One way to manage those two problems is by working on the training schedule. This athlete has only access to a pool at lunch time and has a group bike to train with in the morning. But if you can swim in the morning and run/bike at lunch/evening, that would keep your blood glucose levels steadier.

Plus, swimming in the morning is always best for those without a swimming background since you get to train on a fresher body, struggle less to float in the water, therefore getting more quality out of the sessions.

Vinnie Santana, Certified ironguides Coach – Bangkok, Thailand
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