Cycling with diabetes – an exercise in calories

20 Feb 2021

Reading Time: 8 minutes

So, what’s that with the title up there I hear you ask?

To say that 2018 was an interesting year would be a gross understatement. Beside the decision to shut shop on making kit (you can now grab it here from the now not so new kid on the block), our young man started school, yours truly spent six months working out the next step, which resulted in this and also learning that Part A of my genetics stopped playing nicely with Part B, resulting in a case of Type 1 Diabetes.

Now, before you start making all the assumptions about that (like we all do/did), Type 1 and Type 2 are very different beasts, so much so I’ve heard it said that they should be called different things. While it’s still not known what triggers either, Type 2 is generally lifestyle driven (with a 50% inheritance rate), and Type 1 is genetic (with about a 5% inheritance rate). Generally Type 2 can be controlled with change in diet, exercise and a pill, but Type 1 requires daily insulin to control blood sugar, something that is usually done by the pancreas. Type 2 can kill you over time if you don’t control it, Type 1 (T1) can kill you quickly (or slowly) if you don’t do what you should do, when you are supposed to do it.

And that was the start to 2018.

So cycling with diabetes eh?

Note: None of the below is medical advice, nor is meant as such – diabetes T1 or T2 is an immensely complex issue and I am generalising it greatly. What I am describing here are purely my own tests based on things I have read, my own trial and error, as well as discussions with my oncologist. Do not take this as gospel without your own reading or discussions with your medical advisor.

When all this came about, I was about to get back on the bike after a spell off. But when the news came in (through a rather amusing set of events), that plan went out the window as I started to do my reading on this new friend of mine. Turns out hopping on a bike and going for a blast is no longer as simple as it used to be – do it wrong and you can pass out mid pedal stroke….

Clearly that is not an option if a good time is to be had.

Coming to grips with the new normal, it dawned on me that I’d better work out what’s needed to be done in order to prevent myself from passing out mid flight, which would sorta suck. To do that, I had to properly understand what happens when you exercise and at its most basic, it goes a little something like this (a VERY basic generalist synopsis):

  1. When you exercise, your muscles demand more blood flow.
  2. The increase in blood flow the muscles as able to pull more sugars from the blood to keep them fueled.
  3. The type of exercise will also determine what happens to blood sugars – anaerobic tends to raise it, aerobic tends to lower it.


The complications for T1 come from having insulin floating around the blood stream (through that injection) as opposed to being ’normal’, where your pancreas spits it out as needed. It’s floating around in your system because in a nut shell, the body fights against its natural ability to produce insulin, so you don’t (which is why T1 is classed as an autoimmune disease). If left untreated (a.k.a that injection), blood sugar levels start climbing until the point that bad shit starts happening to you up to, and including, dropping dead.

Cycling with diabetes, with cycling being an aerobic activity and all, is high on the list of activities that don’t play nicely together. As you ride and your muscles pull sugars out of the blood at an increasing rate, that insulin floating around to keep the levels low is also making the muscles more sensitive to insulin, meaning they potentially may be metabolising sugars somewhat more than usual; and very quickly you end up with a too low blood sugar count. See afore mentioned bit re. passing out mid flight…. and that’s a shit.

In this day and age, it would seem that there’d be plenty of information on how to combat this, but the reality is the whole thing is an ongoing work in progress. Diabetes and exercise is immensely complicated made worse by the simple fact that everyone’s different, so the outputs and results vary from person to person, depending on a million different things. There are still no firm strategies other than the most basic broad brushstroke approaches and it’s seemingly made more complicated by the fact that correlation between the different units of measure is almost non existent; we can generally work out how many grams of carbs equate to 1 unit of blood sugar but there is no correlation, in time or otherwise, of how long it will take to drop 1 unit.

Over time, I found several websites that gave interesting insights into more than the most basic levels of exercise with T1, this one about a pro cycling team in the US, and this about a ‘new’ way of managing exercise and T1. Using base info from the two, a fluid trainer and time, I figured I could work out how to make this happen, and to do that, I thought coming up with baseline measurements to be the best starting point…

The first was how many points my blood sugar would drop for a given amount of riding.

The second was how many carbs (because carbs fuels blood sugar, NOT sugar) I needed to offset that drop.

And finally, I needed a marker that I could use to tell me when I have chewed through the blood sugar points, a.ka. a gauge.

Right now, there is no single system outside of a CGM (constant blood sugar monitor) that tells you the rate of blood sugar consumption and a CGM is not something I am all that interested in, as it’s 1. expensive and 2. connected to you at all times.

So I had some testing to do.

Luckily I have always been good at following the numbers and while I disavowed HRMs some time back, I realised that a HRM might provide the information I needed to work this out. So off I went and got me a new Polar H10 monitor and paired it to Polar’s Beat app on my phone. While not 100% perfect, it’s been long regarded that Polar’s ‘Own Index’ is a fairly decent measurement of VO2 outside of a lab environment, so I could get close enough values for HRav and time, which seemed like they may be good points to combine with blood sugar count.

How’d I do it?

I’ve spent close to 4 months on the indoor trainer, religiously tracking numbers in order to try and work out the system. And it was pretty basic with some simple measurement points:

Start, middle and finish blood sugar values.
Time spent.
A side note on intensity as per HRav.

Over a session, I found I religiously dropped 2 blood sugar points, so where I’d start at 7mml, I’d finish at 5. If I started at 6, I’d finish at 4. It was that constant. At first I put it down to intensity over time, so began tracking HRav + time more closely as that seemed like a possible way to develop a system. But over the months I realised that the calories used were the simpler indicator, especially as I started spending more time per session.

And that was the key. At almost exactly 500 calories (as measured by the Polar algorithm), I’d be down 2 points.

The kicker to realising this is the key, was that regardless of the time it took to hit the 500, the drop would be the same, independent and regardless of HRav. So in an intense session where I chewed 500 calories in 35 minutes, the drop would be 2 but in an easier session, where it took 50min to hit the 500, the drop was the same. And I ran it enough times to show that this count was a constant.

So while calorie count per se does not have a lot to do with blood sugars, calories consumed proved to be a fairly good dial to know what the outcome was going to be.

The next step, probably the most important, was to work out how many carbs I needed to offset the drop.

I started introducing measured calories as an input, either before or during a session. This was a bit hit and miss, as there is no way to work out a correlation with the numbers I had, but maybe I had a magic fairy on my side as I worked it out with next to no effort!

I’d start at lunch, by having two slices of toast that would put my sugars at the top end of ‘acceptable’, as set by my oncologist. This would ensure I had ‘room to move’, without too much fear of things getting messy.

Then , using orange juice as the carb base (because it’s absorbed quickly), I started with a simple 20 grams carb count, which was about 250ml of juice. I decided to drink the juice 20 minutes into a session originally, as that would give the muscles enough time to burn through a chunk of their reserves before starting to seriously pull the sugars out of the blood. As it turned out, 20 grams was the perfect amount to offset a two unit drop, meaning that I am currently using 10g of carbs for every unit of blood sugar. Most interestingly, it did not seem to matter all that much when I took in the carbs, before or during, as the outcome was the same i.e. over 500 calories of expenditure, my blood sugar would end up pretty much where it started.

And that was good news!

I now had a way to reasonably work out how to manage things on the bike based on a single, trackable number. With a measured amount of pure carb replacement in a marked bottle, I’d be able to keep my levels more or less constant in a scaleable fashion. This of course means going back to bottles when off road, but that suits me fine – I don’t like packs.

Of course this is not the whole story, there’s the matter of post ride drop as the muscles continue to pull sugars from the blood in recovery over several hours, but this is a whole lot easier to manage than while being on the bike.

This took me the best part of four months to work out but the end result has been pretty solid and consistent; and a good way to get back into bike shape after being off it for so long! I suspect over time as things change so will the numbers, but as long as I keep doing baseline measurements it will be easy enough to keep on top of. I have to stress that this is a device dependent system, meaning different systems will deliver different numbers, so while the numbers work for my H10, if I were to switch monitors, I’d have to re-run the test to develop a new baseline. I’ll also need to re-calibrate carb intake for longer duration rides to see when sessions that span multiple hours, if the calorie/hr consumption remains constant or if there is a ramp (up or down), which would mean changing the intake over time. The end result though is that I can now get back to riding, and that has to be a good thing, no?

So cycling ‘seriously’ with diabetes is totally doable, you just have to accept the fact that you are now riding by the numbers… all the time. At least I have a valid reason to using a HRM and computer!

And of course, there’s the wild upside…

Every rider I know bonks at some point. Unless very well versed, most riders don’t take in anywhere near enough carbs over the course of a ride. At a reasonable, evenly paced two hour ride ride, I figure I need at very least 60 grams of carbs per hour… with bottles full of water though, as I used to do in the past, well, I’d get none! As I now HAVE to keep fueled, see afore mentioned bit re. passing out I, in theory, should never bonk again!

(Let’s not mention the fact that I CAN NOT ever bonk again!!)


Copyright 2023 Gerard Thomas. All rights reserved.

I've run mtb events, distributed some legendary brands, ran my own cycling clothing brand, designed bikes and was a GM and head designer for a famous but sadly now extinct mtb marquee; and after 20 odd years I decided riding bikes was more fun than working with them.
Over that time though, I wrote (and some wrote for me) a lot of stuff about bikes, on blogs and the like. Some was good, some, well... not so much. Rather than loose it all when I shut everything down once and for all, I have kept some of my favourite, and more popular pieces here for... prosperity?
I also am working on new pieces as well...

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