How to determine carbs amount to properly fuel a workout

I wanted to ask about how you guys calculate how much carbs you need to put into your bottle for a workout. I’ve been trying to find an appropriate thread but only spotted discussions on how to properly mixed dextrose and fructose but not how to find out how much one would need.

I started training about a year and a half ago mostly for health but very quickly realized that being fitter means more fun and not I can hand out with more cyclists. So while my goal is not racing I’m trying to get as many gains as possible from the time I spend training.

A year ago when my FTP was somewhere around 160W and 1.8W/kg every workout was a struggle. The only way for me to get through it was to add some carbs. Once I raised my FTP to 190-200W I also got to a point when I no longer needed carbs and was only riding on pure water. That was fine for my base rides and also allowed me to drop about 10kg.

A couple of months ago when I started using TR the workouts however the struggle returned. I started experimenting with fueling my harder sessions (SweetSpot and Threshold only) and found out the best results are when I just drink a sip of carb drink every few minutes for the whole duration of a session.

I’ve heard that athletes that are more serious would already know how much carbs they need but I don’t and I also have no idea how to learn that. My Polar watch is giving me a nice breakdown of fat and sugars used during each session based on some super scientific algorithms and I thought maybe I should use just that.

I could take a similar session from the past (same duration and power output). Let’s say my watch says I had burned 910kJ over 1h 30 and 60% of that was from the carbs. That gives me 136g of carbs for the entire workout and 90g per hour. Is that the proper way of calculating?

Assuming it is I have a few more questions:

  • Should I drink exactly the amount I burn or slightly less since my workout dring is not the only carbs I eat throughout the day?
  • If I get a value smaller than 60g should I only add glucose?
  • Why is everybody freaking out about the ability to absorb high amounts of carbs. I can dissolve really huge amounts in 500ml of water and could dring it in no time.

I will appreciate any suggestion as the concept of fueling workouts is pretty new to me and also kind of scary when I think about the amounts of ‘unhealthy sugar’ I would drink. I’d like to avoid making a mistake and taking twice or 4 times much than I really need.

You might be able to drink lots of carbs, but your body can only process a limited amount per hour. Think of it has a door and a window.
The door has a limit of 60g/h and the window 30g/h. Glucose can only go through the door but fructose can go through the window.

There are lots of podcast out there that talk about it, some highly technical, and others a little more easily digestible (pun intended).

If the carbs are in your stomach, it’s considered to be “outside the body”.

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Don’t overthink it.
It’s fair to say that generally people can absorb approx 80-100g of carbs per hour. (This is through utilizing both glucose and fructose mechanisms). You can shovel in 5 times more than that if you want but it’s not going to help you it’s just going to sit in your stomach.
So, if you get approx 4 calories from 1g of carbs then that’s about 400cals per hour coming in.
If you are working at sweet spot or threshold for a 90 min session then you might be burning 1000 calories.
So there will always be a deficit. It’s like pouring water slowly into a bucket that’s got a really big hole in the bottom. Eventually you’re going to run out and stop.
As for fat/carbs ratios you can ignore all that for the purposes of a workout. In general it’s a sliding scale of using more fat vs carbs at easy paces and more carbs vs fat at higher rates. When you’re really working hard you’re only burning carbs.

In summary. Eat a couple of gels or a bottle of drink mix during a workout and it’ll help a lot. Eat some food after your workout to begin replenishing your stores. It gets a bit more involved if you start doing 6 hour races or an Ironman, but save that for next year :slight_smile:


The Alacrity Fuel APP might be helpful.

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The Saturday app (formally Alacrity) makes it easy

Saturday app thread


You are not really trying to get in the same amount that you use, for intense rides that is also not possible.

Over the entire day you want to eat about as much as you burned. But when on the bike you are limited to what you can comfortably have in you stomach and also you don’t need to take in more than you have to, to complete the ride, and then just eat the rest afterwards as part of a nice meal.

I would not add any carbs or just a low amount for rides under an hour.

Then for longer rides, if going in Z2 I could still go for 2 or 3 hours with little (15g-30g) to no carbs and feel fine.

For intense rides longer than an hour I would start adding carbs, minimum of 60g or up to 120g per hour.

You need to experiment for the amounts that work for you on what type of ride.

I don’t really bother and just use table sugar for a nice mix.

Because it might not sit nicely in your stomach if you get in more than you can quickly absorb. That is also a bit of trial and error to see what the max is that works for you. The limit is typically around 100+ g per hour.