Which is, let’s be honest, a surprise. And I’m willing to share my information as well.

There are several things I needed. One was aquired from a sports weekend with all sorts of medical personell. They were not only incredibly helpful, they also brought along testing equipment. In this particular case, a spiroergometry. Which I had no idea what it meant, but the very short version is apparently that “it” measures oxygen inhalation, carbon dioxide exhalation and plots those over your sportive work output (or in other words: you’ll be sweating on a stationary bike with a breathalyzer over your face).

As the oxydizing of fats and carbohydrates uses different amounts of oxygen and releases different amounts of carbon dioxyde, with those two values (moles of oxygen/carbon dioxide) it’s possible to calculate the percentage of fats and carbohydrates used for exercise. I don’t actually have an English language source for that, and wikipedia only lists a vague “this might exist” part on their part.


Summary in the next mini-chapter, skip there if this bores you.


I do realize this is not helpful right away. On the x-axis is plotted POWER! Which was me, struggling on a stationary bike, as mentioned above. On the left y-axis is plotted energy used per hour. In kilo-calories, because we don’t really like SI-units over here either. On the two right-hand axii (?) are plotted the percentage of energy supplied from fats and carbs and the heart rate.

Still no use without the colours:

1) Black is my heart rate. It starts low and goes to high. No surprise there.

2) Red and red dotted are carbs used for energy. Solid is in kcal/hour, dotted is in %.

3) Green and green dotted are fats used for energy. Solid again in kcal/hour and dotted in %.

4) Blue is total energy used.. which is kind of a pointless stat. Probably helps me loose weight. Pfft.. as if that was an issue. I can just go to the hairdressers instead.

Spiroergometry summary

So what did I gain from this? I learnt at which heart rates how much energy is gained from carbs. That is important, as those will need to be resupplied. Not instantly – some of them are donated from glycogen stores in muscles and liver, but those will be filled after sport finishes.

So basically: What you use in carbs, you should really eat (eventually) for a stable blood sugar.

Example for my work trip: half an hour at 225W power output (speedy, sweat inducing) will require about 250kcal from carbs.

That is a lot. 4 calories per gram of carbs means that this is 60g of carbs alone. I need to eat a full breakfast to make it to work at a stable blood sugar.

How does the basal rate play into this?

And this is where it gets really interesting. See.. in the mornings, after breakfast, I need to bolus 50% to get to work without a drop in blood glucose. My morning breakfast of 60g of carbs is therefor only “half treated” – I get 30g of carbs without insulin.

(Note: that is my napkin method right there. This summary works well, though, so feel free to use it as a rule of thumb.)

In the evenings (I have to get home from work – I don’t intend to stay there!) I need 60g of carbs without insulin.

Both of those were found out by testing. Or basically: drinking more and more coke before the trip and trying to arrive home at a stable 100mg/dl.

Basal rate changes

Pump users will likely have different basal rates during their days. Mine is much higher in the morning hours (from about 4.00h in the bloody night to 7.00h in the mornings) than in the afternoon. The difference in my case is 0.85 units/hour morning to 0.55 units/hour evening. Please note that the basal rate affecting sports is the one you had one hour ago, usually (which is why you’d reduce basal rate one hour before sports start – right?).

And there we are. The ratio of morning basal rate to evening basal rate (0.85/0.55=0.47) is almost equal to the inverse ratios of carbs used (60/30=0.5).


Yes, all this rambling is actually useful. The short version: If you have found out a carbohydrate requirement for a workout that works, you can apply it to different times of the day (with different basal rates).

The factor by which you increase or decrease the carbohydrate intake is the same as the inverse ratio of basal rates. Or in short: If your basal rate is lower, you need more carbs (this makes sense, because a higher basal rate usually means less insulin effectiveness).

Spiroergometry is not required, but it will tell you the ceiling. How many carbs can you possibly use during workout. This number is probably way higher than what some people post on the internet. Alexis posts here about getting 4g of carbs for her morning workout. That is waaaaay too low in my opinion.