Fuelling basics

Hello, I’ve searched for and read a few threads about carb intake for workouts but something just dawned on me while listening to another podcast. I really don’t know why I’ve not considered this before as I listen to a lot of ‘sciencey’ podcasts.

So, in very simple terms, 1g of carb can yield 17kj of energy. Therefore, as long as you eat adequately between sessions you could complete sessions up to 2 hours with either very little or no carb intake during the session.

2 bottles with 60g carb in each yields over 2000kj, even my 2 hour Tallac +3 tomorrow is only 1332kj, surely given adequate intake prior to the session this could be done without in ride carb intake?

Am I missing something?

The body isn’t that efficient in utilising the energy. Of the 17kJ, only about 25%, or 4kJ, end up being used for movement.


The KJ’s that you are measuring for your workout are KJ’s to the pedal not KJ’s that you are expending. The average body’s efficiency is ~25% (just for a round number) so you are really burning more like 5200kjs in that workout.

Damn, @splash beat me to it.


Ah yes, that’s the obvious part I was missing, I knew this too. Thanks for the reminder.

I realise it’s not so crystal clear ref burn rates etc but as a rough guide I really should consider this aspect a lot more.

So the 2 bottles of 60g is very likely to be required in order to prevent liver glycogen being used up and me avoiding a huge brick wall.

I really need to lose some weight so I think I’ll be doing my endurance sessions without taking on any additional carbs over the winter, I’m talking about rides up to 2 hours not 4 or 5 hour ones.

There’s also a difference between “could” and “should”! There’s about ~2000 calories of glycogen stored in the body if your carbs are topped up. A 2 hour SS ride will burn about 2000 calories for me and a good chunk of those will come from fat. So I “could” do those rides without taking on any extra carbs in ride as long as I’m not going into them fasted. But doing so is likely to impact the quality of the session since the body starts to try to conserve glycogen stores well before you reach depletion point if there’s no fuel coming in. And could also impact recovery for the next training session if you run stores down too low.

The lower the intensity, the higher the proportion of your energy comes from fat stores not glycogen stores. So yes, if you want to change body composition then fasted endurance rides is one way to go. But there is also a good argument that doing higher intensity sessions not only burns more calories in total but also does more to raise your FTP meaning you’re burning more calories in every session. Think the best approach is a mix - fuel your key higher intensity workouts each week, use the fasting approach for easy and recovery days. Though need to ensure the fasted rides aren’t impacting the quality of your key rides which means going into them fasted and then taking on carbs afterwards to get ready for the next day.

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Yes this is pretty much my conclusion too, I’ve tried fasted sweetspot rides in the past, too many in a short period just lead to failure. As you rightly say, increasing FTP naturally leads to higher kj use in all sessions regardless. Overall I think I probably am over using carbs, as in, using powders on too many rides.

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