Chasing data on two wheels – by Mark Smith

I’m new to diabetes, I’m also new to cycling but I wanted to take a data focused approach to try to understand and improve my ability to cycle. How much is diabetes impacting me and how can I control it? I wanted to use my time with a Continous Glucose Monitor (CGM) to understand and learn as much as I could. Time for some graphs I think….

I like data, control and precision

Let me introduce myself. I’m 30, I was diagnosed with Type 1 diabetes a year ago, shortly after becoming a Dad. It’s a lot to learn all at once, and I have been writing about my experiences as a way of processing them and learning.

I have always been an active person but since my diagnosis I have taken up cycling. I find it easier to carry the testing kit and carbohydrate provisions I need when out exercising.  Plus the symptoms of my undiagnosed diabetes left me with less than enjoyable memories of running. A change of sport has done me good and we all know the benefits of exercise in controlling glucose levels.

I have also always been a numbers person. I like data, control, precision. This has really impacted how I have tried to deal with diabetes (in both positive and negative ways!) but when I got the chance to trial the FreeStyle Libre CGM for two weeks, I wanted to maximise what I got out of it. The aim is to be better at managing my glucose levels when I’m out on the bike, so I immediately signed up for a sportive during the CGM trial period.

For those people not into wearing lycra and telling yourself you are going fast while cars fly past you on country roads, a sportive is generally a long distance event that is not competitive. You get your time but there are no leader boards, winners or prizes. It’s a relaxed atmosphere, so a perfect setting for a data collection exercise.

Being new to cycling, sportives are also new to me, this was going to be my first. 61 miles of Wiltshire countryside to not race round. My focus would be on trying to fuel for the ride and better understand what my body is doing while I’m pedalling. Having done some research before the event, my nutrition would consist of glucose sipping a high carb drink, along with eating slow release carbs from solid foods at set points, including the start and half way point.

I would plan to check my levels as I ride, trying to keep above 7.0, then review the data against my performance once the pedalling is done. General advice is to consume 0.5-1g of carbohydrate per kg body weight per hour of relatively intense exercise. For me (80kg, planning to ride 3-4 hours) that means anywhere between 120 and 320g would be needed!

The first thing I learnt about was the impact of adrenaline. I was aware that it will put sugars up but the magnitude surprised me. My first thing in the morning reading was 7.1, too early to eat breakfast so off I went, an hour drive got me to event HQ. The anticipation of the event contributed to a reading of 8.6 when I arrived, having still not eaten, that then went up to 9.3 by the time I was in the start pen being told to follow the signs and not run over pedestrians.

Hitting the wall

Still, I ate before I set off (protein cookie, 30.5g of carbs with only 4g of sugar, a slow release base to keep me going). I kept up a steady rate of glucose sipping, making sure the quantity of drink remaining reflected the dropping number of miles. The distance flew by at the start as I enjoyed the atmosphere and took in the scenery. 30 miles and a big hill later and the miles were considerably longer, but still passing.

My levels ran quite high for the first half, the adrenaline along with me taking on plenty of carbs keeping them in double figures.  I certainly felt a drop in performance during the second half, my readings also dropped but at the time it’s hard to distinguish between that and the natural fatigue. Part of my desire to do this experiment was to see if there was a point where I had to start using fat as an energy source instead of carbs. Runners call it “hitting the wall”, using carbohydrates is a much more efficient process for your body so you feel it when you run out.

So what did the data say? Well the impact of the adrenaline is very clear, the rise at around 6.30am is despite consuming no carbs at all, in fact I ate very little until 08.30, 15 minutes before I set off. I was cycling from 08.45 until 12.15 and the range of readings is massive. I felt low on energy at around 11.30, hence why I did several readings (indicated by the white circles).

This was as I was coming up to the final feed station so I used a little extra of my drink to help me get there and then took on board at least 30g of carbs. It’s evident how vital this was as it brought my levels back up towards 8 rather than 5 to allow me to continue to perform physically. The low at around 11.00 definitely corresponds to when I felt most tired on the bike (even compared to crossing the finish line).


So how does this correlate to my performance? If we use heart rate as an indicator of effort, that remained fairly constant throughout. You can also see that the change from a rising glucose level to falling (about a third into the ride) correlates with the big hill, a struggle which really challenged me and looks to have significantly drained my glucose reserves.

So how well did my nutrition plan work? I tried to keep note of what I was eating and when, which I have illustrated below. The glucose sipping provided a vital steady supply of energy, although I should have supplemented my solid food at the start earlier on.

I didn’t stop at the first feed station, instead waiting another 10 miles before I ate (it’s like they put it earlier for a reason).

You can also see that I needed to take in much more carbs for the second half than the first and still my levels were dropping for a long period, although they stabilised towards the end. In total I consumed around 190g of carbs while cycling, about 0.67g of carbs per kg body weight per hour.

Overall I think I got a lot out of my data collection exercise, it went well overall but there are certainly bits I can improve.

The realisation of how much carbohydrate is required is now setting in, the thought of eating that much seemed excessive before the event.

Planning is definitely vital, much more than I found when running before my diagnosis.  In terms of enjoyment, it was a fantastic event, fully of friendly people and even the rain held off.

The first of many sportives for me I think, but it may well remain the most important.

You can read more about my experiences along with more in depth preparation for this event at

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