3 ms·

Deriving energy from fat is strictly less oxygen efficient than from glycogen; oxygen supply of course being the main limiter of aerobic endurance. This is why

Deriving energy from fat is strictly less oxygen efficient than from glycogen; oxygen supply of course being the main limiter of aerobic endurance. This is why as you increase intensity towards VO2max the balance of fat versus glycogen use shifts dramatically in favor of glycogen. So training in a glycogen depleted state (back then we called this bonking) is bad news if you want to increase your aerobic threshold, since the kind of training that has been shown most effective in causing the required adaptations involves repeated maximal efforts. The only training effect you can hope for is nudging your body towards using more fat at lower levels of exertion, thereby saving valuable glycogen. This isn't very relevant for most running events which are typically done at a consistently high VO2max percentage, but beneficial for say cycling races where you have low intensity (riding in the slipstream on the flat) mixed with very high intensity (up a steep grade).

deleted 9y ago
[deleted]
will_brown 9y ago
>So training in a glycogen depleted state (back then we called this bonking) is bad news if you want to increase your aerobic threshold These is a difference between burning glucose as the primary fuel source, depleting it, and bonking vs using fat as the primary fuel source to begin with (there is no bonking). But basically glucose store empty pretty quickly and if not refueled the body shuts down to conserve the limited glucose for the brain because that is it’s primary fuel source (the body shut down is the bonking). When in keotsis, the burning of fat produces a by product, ketones, which is a different source of fuel for the brain. So when in ketosis when burning fat as the primary fuel source (which is essentially limitless compared to glucose) there is no bonk because 1. You don’t run out of fat (like you would glucose), and 2. As you burn fat you produce ketones to fuel the brain (which isn’t happening when burning glucose as the primary fuel). For example my last marathon I ran in ketosis I did with only water and electrolytes (no fuel), I cant do that if I’m not in ketosis. Your point about being able to more efficiently increase the aerobic threshold while using glucose sounds right.

deleted 9y ago

[deleted]

will_brown 9y ago

>So training in a glycogen depleted state (back then we called this bonking) is bad news if you want to increase your aerobic threshold These is a difference between burning glucose as the primary fuel source, depleting it, and bonking vs using fat as the primary fuel source to begin with (there is no bonking). But basically glucose store empty pretty quickly and if not refueled the body shuts down to conserve the limited glucose for the brain because that is it’s primary fuel source (the body shut down is the bonking). When in keotsis, the burning of fat produces a by product, ketones, which is a different source of fuel for the brain. So when in ketosis when burning fat as the primary fuel source (which is essentially limitless compared to glucose) there is no bonk because 1. You don’t run out of fat (like you would glucose), and 2. As you burn fat you produce ketones to fuel the brain (which isn’t happening when burning glucose as the primary fuel). For example my last marathon I ran in ketosis I did with only water and electrolytes (no fuel), I cant do that if I’m not in ketosis. Your point about being able to more efficiently increase the aerobic threshold while using glucose sounds right.