Recently Xylon (our resident storyteller and half of our podcast team), ran a marathon. His glucose from the marathon tells an interesting story about interpreting glucose data though! 

First and foremost let’s start with the basics. 
Xylon is an experienced long distance triathlete who’s done multiple Ironman triathlons and a few marathons too. So he’s no newbie. The marathon he was running (and this is the interesting part) was predominantly downhill as you can see below: 

Supersapiens Strava
Figure 1: Elevation map from Xylon’s Marathon

Astute readers will note the 300m (1000ft) or so of elevation GAIN. This, combined with the figure above, shows both how hilly the race was but also the significance of the downhill, which totaled 1235m (4050ft). This presented a much different challenge to a normal marathon, in that the eccentric muscle damage from running downhill could be a real issue late in the race.

Training for a Downhill Race?

He ran downhill. Regularly. 
Seems simple, and it is - but training too often can be needlessly overcomplicated. 

*A quick note, downhill running can increase injury risk (another reason to do it before the race!) so make sure you introduce it slowly. 

Fueling a Downhill Race

Xylon used a mixed strategy, that is, he used a variety of products in the race rather than sticking to only one type (or even one form). 

His Fueling was as follows:

Hour 1
- 10min gel 25g of carbs
- 35min electrolyte gel 7g of carbs
- 55min bar 40g of carbs
Total 72g

Hour 2
- 1h35 gel 25g of carbs
- 1h55 electrolyte gel 7g of carbs
Total 32g

Hour 3
- 2h10 gel 25g of carbs
- 2h30 electrolyte gel 7g of carbs
- 2h55 gel 25g of carbs
     Total 57g

Hour 4
Small cups of Coke 
     Total 50g

Xylon finished in 3:42 (his fastest ever time for a marathon - but we don’t award PBs for downhill races unfortunately).

This meant his intake was roughly 56g of carbohydrate per hour. This is right in the sweet spot for someone running around his pace. The only feedback we gave him was perhaps his second hour was a little light on fueling and he would have been better pulling some of his later fueling back but this is pretty minor. 

Now the let’s get to the interesting stuff!

Downhill Marathon Glucose Data

Looking at Xylon’s glucose data (Figure 2, below) and knowing his fueling above, some may say that it is pretty unexciting and what would be expected. You may wonder if his glucose was a little low at the end or if the downward trend was an issue if you’d read some of our other blogs (here and here).

Supersapiens Dashboard Marathon
Figure 2: Xylon’s Glucose Data from his Marathon 

To confirm, you may even use the dashboard and see his heart rate overlaid as we have done below in Figure 3.  Again, thinking that all looked pretty normal. 

Glucose and Heart Rate
Figure 3: Xylon’s Glucose and Heart Rate Data from his Marathon 

But then, being the Supersapien you are, you’d use the dashboard to overlay the elevation of the race with his glucose data (as we have in Figure 4 below) and have your EUREKA moment. 

Glucose and Elevatio
Figure 4: Xylon’s Glucose and Elevation Data from his Marathon 

Xylon’s glucose tracks really well with the elevation of the race. Of course it initially increases in what’s probably a mixture of excitement, his bigger intake in the first hour and some of the elevation early on in the race (as we cover in this blog). 

What does this mean though?

One of the drivers of glucose increases during prolonged exercise such as marathon running is accumulated metabolic stress and whilst Xylon’s heart rate data suggests it was no walk in the park the metabolic stress of running uphill as opposed to downhill is very different. As covered in this blog glucose is absorbed differently when exercising and so his intake is likely not playing a big role (though some of the sharp increases as the end may be the coke). So unlike our previous marathon data, linked above, which was mostly during flat marathons with a larger accumulation of metabolic stress, Xylon was more limited by mechanical stress (despite his downhill running in preparation) and his glucose reflects this and his intake. 

Take Away Message

Many factors impact glucose and glucose should be interpreted IN THE CONTEXT OF THESE. Just like heart rate data is impacted and should be considered in the context of activity, elevation, hydration etc, glucose data should be considered in the context of things like sleep, nutrition, stress etc. 

When it comes to endurance sports, the roll of elevation changes on glucose is something that surprises a lot of users but is very normal. There is no reason to be trying to maintain high glucose whilst descending a hill on your bike - but watch this space for some experiments we are doing ;)