## FTP and calories burnt

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pmprego
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So... According to intervals/Strava, I burn around 2000kcal on my long rides. Let's assume that for some magic, my ftp increases by 50%.

- Would I burn 3000kcal assuming the same type of intensity?

- If so, will my body suddenly be able to store 3000kcal or do I have to increase the amount of food that I usually ride by 50% as well?

TobinHatesYou
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pmprego wrote:
Sat Jun 10, 2023 9:11 pm
So... According to intervals/Strava, I burn around 2000kcal on my long rides. Let's assume that for some magic, my ftp increases by 50%.

- Would I burn 3000kcal assuming the same type of intensity?

- If so, will my body suddenly be able to store 3000kcal or do I have to increase the amount of food that I usually ride by 50% as well?
If your power increases by 50%, then yes your energy (kJ) usage will also go up by 50%. You should still be able to go about 2 hours before bonking but you “during” nutritional needs will increase.

And your needs may not increase at the same rate as your ability to absorb and metabolize carbs into glycogen.

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maxim809
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pmprego wrote:
Sat Jun 10, 2023 9:11 pm
So... According to intervals/Strava, I burn around 2000kcal on my long rides. Let's assume that for some magic, my ftp increases by 50%.
- Would I burn 3000kcal assuming the same type of intensity?
The rough equation for kcal burned on a bike:

kcal = Average Watt * Hours * 3.6

So say you averaged 150w over 4 hours. (150w * 4hr * 3.6) = ~2160 kCal

If you doubled that to 300w averaged over 4 hours, it'd be ~4320 kCal
pmprego wrote:
Sat Jun 10, 2023 9:11 pm
- If so, will my body suddenly be able to store 3000kcal or do I have to increase the amount of food that I usually ride by 50% as well?
No, your body doesn't necessarily store 3000kcal all of a sudden from getting fitter. You store calories from eating.

9 calories = 1g of fat
4 calories = 1g of carbs

Your body uses both glycogen and fats when exercising. How much of each you use is based on your body's efficiency and the intensity level you are riding. This is a much longer topic. But in general a rider who isn't on a restrictive diet will want to fuel with carbs, especially during a long or intense ride. The details are based on your goals and the type of riding you do.

A pound of pure fat (453g) is nearly 4000 kcal. And your body can store lots of fat. Imagine having a 80,000 kcal reserve from 20 pounds of fat. For this reason, there isn't a big case for eating fats on the bike for most people.

In contrast, a typical person's glycogen stores are limited to roughly ~1 pound, or ~1800 kcal. Glycogen is being burned alongside fats during endurance riding. And high-intensity burns through glycogen even more quickly, and fats are not as much utilized. Carbs are typically what you want to eat on the bike, given the limited nature of glycogen, the limits of how quickly it can be absorbed, and how it is important for speed.

But once again, nutrition is entirely based on your situation and goals, so it's difficult to give specific recommendations.

RyanH
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AFAIK glycogen stores can only be used by muscles that are actively being used so while our body may store that much, not that much is available during cycling.

voicycle
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Explanations above are brilliant in terms of what’s happening as you get stronger.
In terms of what to do about it, FWIW I find that as I get stronger and start to push the limits of how much carbohydrate I can eat (or drink) on the bike, the carb content of my evening meal the night before the ride becomes more and more important. They say eat 2-4 hours prior to the ride, but I find it’s the food I eat 10-14 hours prior that makes the biggest impact.

Everyone’s different though so your experience will vary - my point is just that you can hit your real-time on-bike digestive limits fairly quickly and from then on pre-ride carbs become more and more important as you get stronger. You’ll have to start thinking about fuelling in terms of 24 hour and weekly intake rather than just ride food.

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pmprego
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Based on the answer so far, I guess I was not entirely clear in my questions given that some answer moved a bit away.

What I can gather is, yes, if I double the amount of watts put on the bike, the transformation too kj doubles given that we are using a simple linear mathematical formula.

I may get more efficient at transforming glycogen into "leg power" thus increasing the amount of kj before bonking (assuming only body stored glycogen).

I may get more efficient into using carbs during the ride into effective energy to the muscles thus for the same amount of ingested food, I extract more energy out of it.

It is unlikely that the intensity on the bike remains exactly the same, being more likely that the intensity drops (especially in certain parts of the ride) thus being able to use more fat than carbs, which basically means having more disposable energy in my body.

I'm trying to summarize the answers here. I think this was it.

voicycle
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Yes for the most part, except some clarifications with these two:
pmprego wrote:
Sun Jun 11, 2023 11:10 am
I may get more efficient at transforming glycogen into "leg power" thus increasing the amount of kj before bonking (assuming only body stored glycogen).

I may get more efficient into using carbs during the ride into effective energy to the muscles thus for the same amount of ingested food, I extract more energy out of it.
This is near the limit of my level of understanding so might need someone else to chime in and confirm:

I don't think you ever get more or less energy out of either stored glycogen or ingested carbs. The total kJs are fixed, but you can develop your ability to digest/store and access/burn more of them per hour (and to some extent you also develop an ability to store a larger total quantity, though the upper limit isn't usually much bigger than the baseline which is why you won't be able to store 3000kcal of glycogen).

Imagine you have a large container of water with a funnel at the top pouring into it, and another funnel at the bottom which pours onto a paddle water wheel or similar. The top funnel is your digestive system, the container is your glycogen stores, the bottom funnel is your work rate (FTP), and the water wheel is the pedals on the bike. By getting fitter you can widen the openings in the two funnels so you can move more mL of water per second, but 1mL of water going in the top funnel will always be 1mL of water in the container and 1mL of water coming out of the bottom funnel will aways be 1mL hitting the water wheel - the same quantity of water will always do the same quantity of work.

The opening in the top funnel (digestive) also has a much smaller maximum size limit than the one in the bottom funnel (FTP). For example, using the formulas that maxim provided, 100w for 1hr = 360kcal, and 360kcal = 90g of carbs. An untrained athlete will often struggle to process 90g of carbs per hour, and even the very best fuelling cyclists in the world currently top out at consuming something like 150g of carbs per hour. 200w is 180g per hour so you see how easily we can exceed our ability to replenish the fuel we're using and start to drain the container faster than we are filling it.
(The mismatch is actually even bigger because there's evidence to say that even if you can digest 150g carbs per hour, you probably can't turn all of that into useable fuel for your muscles at the same rate - it will get there eventually, but not at 150g per hour. I'm also over-simplifying by ignoring fat burn there, but the point is that power doesn't have to be very high before it becomes physiologically impossible to replenish all of the carbs/calories you're burning in real time.)
Most cyclists will max out their ability to replenish carbs at the same rate they're getting burned somewhere well below the average power they would hold for a social ride.

Final thing on the water analogy: the container isn't a fixed size, but it really can't change very much in the grand scheme of things. You'll see a nice percentage increase when you first start training but then you come up against an upper limit pretty quickly and it's not possible to extend much beyond that. Maybe think of it like getting better and better at filling it up to the very very top than actually increasing its size?

AND just to put a huge caveat on all of this, we should acknowledge that there actually is an outright efficiency thing going on here too. Humans are only something like 18-25% efficient at using fuel to do work. Something like three-quarters of the energy produced by the chemical reactions in our mitochondria is lost as heat. That percentage varies a bit from person to person and can potentially change a very small amount within an individual over time, but it's incredibly difficult to test, even harder to train, and the variance is so small that the field of endurance sports has decided to just treat it as a fixed number. That's one part of what the 3.6 represents in maxim's formula (it's also why the same power data might give you very slightly different calorie estimates in strava vs trainingpeaks vs another platform - everyone agrees to treat efficiency as fixed but they don't all agree on the exact same number.) If applied to the water analogy I guess that means somewhere between the bottom funnel and the water wheel 75% of the water evaporates, but that's just unnecessarily confusing since the 75% isn't a number we can do anything to change.
As far as we're concerned as amateur endurance athletes in 2023, 1 watt will always burn the same amount of calories, and 1g of ingested carbs will always eventually become 1g of carbs to burn (or can get converted into fat for long-term storage). All we can train is how quickly we can take it in and how quickly we can burn it, and in nearly all cases the limiting factor will be how quickly we can take it in.

pmprego
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Wow... That was a hell of an answer

Conclusion: just improve your ftp and ride

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Andrew69
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pmprego wrote:
Sun Jun 11, 2023 3:41 pm
Conclusion: just improve your ftp and ride
Exactly, but Im not sure anyone really touched on why a higher FTP has benefits in preserving glycogen stores.

Simply put, the higher your FTP, the more power you can hold while still burning a higher percentage of fat for fuel
For example, a ride where you average 200w for an hour and your FTP is 250w is going to burn a higher percentage of glycogen for fuel than if you averaged 200w and your FTP is 300w

AS far as glycogen stores goes, I heard a figure (which I cant recall, but will try to find) that people bonk far before glycogen stores are truly exhausted
The more you train, the more you can exhaust your reserves before you bonk, but regardless, you still have quite a lot of glycogen in reserve when the body calls it quits with exercise simply because the body has far more important uses for that glycogen than pushing pedals (conservering brain function for one)

voicycle also brings up an interesting point regarding efficiency and the large variation between cyclists.
Obviously well trained cyclists are more effcicient than someone just starting out and although I havent read this anywhere, I think its got to do with lactate metabolism where well trained cyclists have the physiological adapations to effectively reuse the lactate as fuel whereas those less trained do not so much of the potential fuel is wasted
And wouldnt you know that improving FTP also improves lactate reuse

So, in conclusion, just improve your FTP and ride