Glycogen

bahzob

Following is a brain dump of some assumptions/questions about glycogen, gleaned from various sources. While there are many of these covering various of the topics below I haven't been able to find one that covers all nor find answers to all the questions I have on the subject.

So the purpose of putting this together is just to get this topic clear in my own mind out of general interest and because its a subject that is especially relevant to the sort of long distance events I do. It will also hopefully be a useful overview for anyone else interested in this topic. I am not especially wedded to any of the views therein so quite happy to be corrected where they are wrong.

I'd appreciate any corrections, answers to questions and/or pointers to good sources of further information related to the topic.

GLYCOGEN STORAGE

Carbohydrates are required to provide fuel, in the form or glucose, for protracted high intensity exercise.

In addition to fuelling exercise glucose is required for other body functions, the most important of which is maintaining brain function.

Low intensity exercise is fuelled principally by fat.

A small amount of glucose (4-8g) is constantly circulating in the blood. The vast majority is stored in the form of glycogen in two principle sites: muscles and liver. The body draws down from this glycogen store when glucose is needed. The store is replenished when the need ceases, principally from carbohydrates digested in various forms from food.

The proportion of glycogen is roughly 20% stored in the liver, 80% in the muscles. The absolute total maximum amount of glycogen varies between individuals in a normal range of
450-550g. (An average of 500g will be used from now on, 100g liver, 400g muscles)

At 4kcal/gram this puts the range of total amount of energy stored in glycogen at 2000kcal.

Training can increase the amount of glycogen stored.

Question: Any data on how glycogen stored varies between individuals?
Question: Any studies showing effect of training on glycogen storage?
Question: If training can increase glycogen storage any information on the extent to which this can happen?
Question: Is there a self administered means to estimate level of glycogen storage?

Total glycogen store can be increased by “super compensation” aka “carb loading” which is exercising to complete exhaustion up to the point where glycogen reserves are fully depleted then consuming greater than 500g of carbohydrate. This will temporarily increase glycogen and can be used for one day long endurance events. Aside from this special case there is no benefit in consuming carbohydrate in excess of that required to replace that used/support immediate neeeds, anything additional will be converted to fat.

For every gram of glycogen a further 3-4 grams of water are stored. This is the key factor in the rapid weight change that can occur at the start of a diet, especially one that has a significant calorie deficit. 2-3kg of weight will be lost due the reduction of glycogen for a calorie deficit of c.2000kcal which can be achieved as the result of a single day fast. Losing the same amount of weight in fat would require a calorie deficit of anywhere from 12000>25000kcals. Similarly this can also be a factor in large pre/post workout weight change.


GLYCOGEN USAGE

The amount of glucose needed during workouts varies as a function of the intensity of exercise, from less than 1% at very low effort to near 100% past lactate threshold. The huge majority of the balance comes from fat. The rate at which this ratio changes varies between individuals and can be affected by training.
As an example relationship, for one reasonably trained individual (me) of lactic threshold HR vs proportion of energy from fat

LTHR % Fat%
60>>>>100%
65>>>>74%
70>>>>60%
75>>>>40%
80>>>>28%
90%>>>>23%
95%>>>>20%

Some studies have shown that training in a fasted state is one means to increase the amount of calories derived from fat.
Question: Any good summaries of research into this, showing how variation differs between people and changes following training?

Sleep uses up some glycogen stores, principally liver. An 8 hour sleep will leave liver 50%??? depleted.

During exercise muscle glycogen will be used preferentially over liver glycogen.

However even if fully replenished the total 500g of stored glycogen is available for exercise because:
- Muscle glycogen is stored locally so, for example, exercising the legs will not use glycogen stored in the arms.
- The body has a safety system to ensure that glycogen resources are not totally depleted as this could be dangerous or even fatal. Specifically:
- Muscles signal exhaustion when glycogen levels reach 25% of maximum
- Liver signals exhaustion when glycogen levels hit 10% of maximum.

These factors means the actual energy reservoir is less than 1500kcal. Even this assumes all sources of muscle glycogen are used, which will not be the case in cycling. .

Question: When cycling I assume that glycogen stored in the arms is not a limiter while that stored in the legs is . Any data on what the proportion split is? Presumably this is one example where training has an effect as would expect cyclists to have low arm glycogen and high leg glycogen compared to, say,swimmers.
Question: How localised is muscle glycogen? I assume its only available to the immediate muscle group where it is stored. So if I tend to emphasise one muscle group over others (say quads) could the situation occur where quad glycogen is depleted while that in other leg muscles remains at a relatively high level?
Question: Can training adjust the threshold at which glycogen exhaustion is signalled?

In the absence of carbohydrate from digestion once glycogen levels reach their threshold exhaustion will set in and the absolute levels of sustained effort will be severely reduced, down to the 60-65% LTHR rates where fat forms the main source of energy. Even this effort requires some carbohydrate and the remaining glycogen will be used up, albeit more slowly, to satisfy this. This is sometimes known as the “wall” or “bonking” effect.

In the continued absence of carbohydrate, even continued low effort effort the risk exists of reaching the point where glycogen is fully exhausted and there is insufficient carbohydrate to supply the glucose needed for brain function. This can have severe implications on performance, in extreme circumstances up to and including death.


GLYCOGEN REPLENISHMENT

Following depletion caused by exercise glycogen will be replenished once exercise ceases. Principal source will be from carbohydrates digested from food.

Replenishment is quicker in the 2 hours following exercise. It is further improved if these carbohydrates are combined with a small amount of protein.

For this reason if rapid replenishment of glycogen is a primary concern (e.g. days with multiple workouts) recovery products containing a combination of high GI carbohydrate and protein can be beneficial.

Across a 24 hour period, provided a sufficient carbohydrate excess (i.e +c.400g) is consumed glycogen will be replenished even if fully depleted If rapid replenishment is not a primary concern there is no benefit in terms of glycogen storage of consuming recovery products vs normal eating. (Though there may be other benefits).

Provided sufficient carbohydrate can be digested to replenish used glycogen other dietary components do not matter, over a 24 hour cycle.

From the point of view of glycogen storage/usage/replenishment high/low fat, high/low protein diets have no effect so long as enough carbohydrate is consumed and digested.

Glycogen can be replenished from other sources including fat and protein. This process is less effective and will not by itself be sufficient to replenish fully depleted glycogen over 24 hours.
Question: Any numbers/other information on the amount of glycogen that will be derived from non-carbohydrate sources and the circumstances that will affect this process?

team47b

That's a lot of info and a lot of questions.

couple of points occur to me, the brain uses glucose as it's first choice of fuel but also has uses glycogen, it's stores are replenished first before liver or muscle after exercise.

Low intensity exercise is fueled by fat for the slow twitch muscle fibre and intensive fast twitch muscle fibre needs glucose.

Muscle glycogen does not break down into glucose for the blood stream so is exclusive to the muscles use, so you can't allocate muscle glycogen to your system only liver glycogen does this.

Muscles have 1 or 2 percent glycogen to muscle mass, so increase in muscle mass will increase glycogen stored.

Consuming more carbs especially simple carbs will increase insulin produced, one of insulin's jobs is to store fat, it also triggers hunger, so more insulin is not a good idea.

Not all calories are equal, generally, more energy is required to process protein than carbs, and more energy is required to process carbs than fat.

Early part of sleep uses glycogen and later in the night you burn fat, so important to sleep longer if you want to burn fat :shock:

First 30 minutes are optimum for glycogen replacement, best done at four parts carbs one part protein. High GI is not needed in fact will only trigger more insulin.

Following exercise, your muscles have an increased ability to absorb blood glucose. This is because of what is called “non-insulin-dependent glucose uptake.” The exercise one morning can affect blood glucose levels even up to the following morning, tested by me on me using a blood glucose monitor

cyco2

bahzob wrote:

From the point of view of glycogen storage/usage/replenishment high/low fat, high/low protein diets have no effect so long as enough carbohydrate is consumed and digested.

I read this as.. there is no need to call on the use of glycogen at all if you were to replace blood glucose during exercise.

The trick is not to over or under do it. Over doing it brings on insulin or produces fat. Under doing it may produce bonk. Also, I do believe that pre loading carbs with a big meal the day before is a more to do with self gratification because it can all be done with proper energy drinks and food on the ride.

BenderRodriguez

cyco2 wrote:

I read this as.. there is no need to call on the use of glycogen at all if you were to replace blood glucose during exercise. The trick is not to over or under do it. Over doing it brings on insulin or produces fat. Under doing it may produce bonk.

I had thought that, in reality, the body simply cannot 'top up' muscle glycogen with carbs, let alone protein / fats, anywhere near as fast as those stores can be depleted during hard exercise, otherwise one could ride at a threshold, or higher, level of intensity for ever.

Think of the body's glycogen stores as being kept in a leaky bucket that can only be topped up using a teaspoon. The harder you go, the bigger the 'leak', but you are still limited to using a teaspoon to top the bucket up.

To counter the above problem you need to

1) Start feeding early, so at least you are processing as much replacement fuel as you can from the off.

2) Get fitter, so that at any given pace more fat-derived fuel is being burnt.

3) Strictly pace yourself. This can be hard when you feel fresh but if you are doing a 6 hour ride, blasting away for the first two or three hours will see your tank running very low and no amount of feeding once you crack will replenish those muscle glycogen reserves enough to let you resume your former pace. Not so bad if it is an easy ride to the finish, but if you have to climb a 1000m col you are going to crawl at 'fat burning' pace whilst feeling pretty dreadful. You might even have to take a 'cafe stop' to give you body a chance to top those glycogen reserves up a little, as even at a 'fat burning' pace, you still need some carbs.

Another thing is that you need to consider the 'water' part of the glycogen / water equation. Without taking on enough water, those carbs probably won't be able to used very effectively and might even promote further dehydration.

Also, I know that the research suggests that you can swig down a can of Coke, or eat a handful of sweets or whatever when exercising without this causing a subsequent 'insulin crash'. However, I feel that there might be a limit to this. Down one and half liters of Coke and some jelly sweets in one go because you have totally cracked might be hard to resist, but this will release just so much sugar into the system in one 'hit' that I would not be surprised if, exercise or not, some sort of 'rebound' insulin crash is possible.

BenderRodriguez

bahzob wrote:

- Muscles signal exhaustion when glycogen levels reach 25% of maximum
- Liver signals exhaustion when glycogen levels hit 10% of maximum.

Do you have a reference for this? I am interested as it might be relevant to some of the stuff I have read relating to the possibility of a 'Central governor' setting the limit on performance. After all, if you are 'exhausted' even when you have 25% of your muscle glycogen remaining, this suggests your should actually be capable of further work.

BigFatBloke

BenderRodriguez wrote:

bahzob wrote:

- Muscles signal exhaustion when glycogen levels reach 25% of maximum
- Liver signals exhaustion when glycogen levels hit 10% of maximum.

Do you have a reference for this? I am interested as it might be relevant to some of the stuff I have read relating to the possibility of a 'Central governor' setting the limit on performance. After all, if you are 'exhausted' even when you have 25% of your muscle glycogen remaining, this suggests your should actually be capable of further work.

The central governor theory is very interesting. The body constantly communicates with the brain via all sorts of methods - hormones and chemicals etc etc.

For example, sometimes if a part of the body is failing to do something the brain sends messages via a hormone to stimulate the part of the body which is failing.

I'm sure it is more complex than glycogen levels alone.

A complex subject. I don't think the Central Governor Theory should be discounted out of hand.

bompington

Surely none of this matters as long as you pedal in perfect circles? ;-)

BenderRodriguez

bompington wrote:

Surely none of this matters as long as you pedal in perfect circles? ;-)

Similarly, aerodynamics are irrelevant if you can put out enough power.

dilemna

OP gives no information of his riding routine(s). To be asking such detailed questions about respiration and metabolism he/she has got to be semi-Pro or some one competing at a high level or Ironman which would mean they should be consulting a coach and sports dietician/nutritionist. If none of this then the OP is a WUM with "Time Waster" stamped on their forehead. Just eat a healthy balanced diet and if doing long rides then up your food intake accordingly. Don't allow yourself to become dehydrated. Steer clear of Coca-Cola or Pepsi or sweet drops as they destroy your teeth. Simples.

johncp

dilemna wrote:

OP gives no information of his riding routine(s). To be asking such detailed questions about respiration and metabolism he/she has got to be semi-Pro or some one competing at a high level or Ironman which would mean they should be consulting a coach and sports dietician/nutritionist. If none of this then the OP is a WUM with "Time Waster" stamped on their forehead. Just eat a healthy balanced diet and if doing long rides then up your food intake accordingly. Don't allow yourself to become dehydrated. Steer clear of Coca-Cola or Pepsi or sweet drops as they destroy your teeth. Simples.

Or maybe he just has an enquiring mind and likes to understand how things (bodies in this case) work :roll:

amaferanga

Johncp wrote:

dilemna wrote:

OP gives no information of his riding routine(s). To be asking such detailed questions about respiration and metabolism he/she has got to be semi-Pro or some one competing at a high level or Ironman which would mean they should be consulting a coach and sports dietician/nutritionist. If none of this then the OP is a WUM with "Time Waster" stamped on their forehead. Just eat a healthy balanced diet and if doing long rides then up your food intake accordingly. Don't allow yourself to become dehydrated. Steer clear of Coca-Cola or Pepsi or sweet drops as they destroy your teeth. Simples.

Or maybe he just has an enquiring mind and likes to understand how things (bodies in this case) work :roll:

Nah he just likes to look clever. He tried the same questions on Wattage and was basically ignored so he's seeing if people here think he's clever instead.

BenderRodriguez

amaferanga wrote:

He tried the same questions on Wattage and was basically ignored so he's seeing if people here think he's clever instead.

So asking questions, and thereby admitting one's knowledge is lacking, is the best way to look 'clever'? :roll:

At the risk of also being accused of trying to look 'clever', I too would like some answers to the OP's questions!

BigFatBloke

madasahattersley wrote:

The questions are totally aimless. Anyone (if anyone) who needs answers to questions like that has a team of physiologists doing the research for them.

If you are not interested in the subject and don't want to take part in any discussion why bother commenting?

Personally I find what he is saying and asking interesting.

If I remember the max carbohydrate you can ingest and utilize whilst riding is 70g and that is if you get the sugar / fructose balance correct.

Also interesting is how just washing some carb around the mouth and spitting it out seems to have a performance benefit. Tricking the Central Governor into believing there is some carb on the way?

bahzob

Thanks for some of the responses so far. As for the others well it just goes to show people are a bit sad that's all (and most likely ignorant to boot, I'm guessing they don' know the answers to questions I have here either)

I was quite open in my introduction that this is a subject that I am not trying to "look clever". Just the opposite, I'm happy to admit there's a lot I don't know about this subject. As I thought I made clear in my OP my reason for asking is several fold
- Curiosity, which in itself is a good enough reason in my book. While a lot of information is available on glycogen I haven't been able to find a single source that covers all the topics I raised/answers all the questions I have.
- Competition: My favoured events are 5+ hours long (including multi-day/up to and over 1500km). Next season I'll be targeting a 270mile 12TT, doing my first 24 and helping out at this years LEL inspired me to knock a big chunk off the 93hours I did the last one in . For these glycogen is a key limiter if you want to go as fast (which I do), along with the related topic of how to measure/affect the balance of energy derived from fat vs carbs.
- Training: Glycogen has an impact on training, especially if you do consecutive hard efforts. One frequently stated goal of training is improving glycogen usage but I haven't been able to find much apart from anecdotes on how to actually do or measure this.

Other comments:
- As also mentioned in my OP to keep things simple atm I'd prefer to ignore considerations of feeding to replenish during exercise, since, as shown in some of the comments above this is a whole extra topic in itself. I don't plan to ignore it, just add it in once the basics are clear.
- Source for the information on thresholds is herehttp://www.thesportjournal.org/article/glycogen-replenishment-after-exhaustive-exercise I am guessing it makes sense from an evolutionary viewpoint and it explains the phenomenon I guess we have all experienced of being able to summon up a big effort even if you are completely knackered if the right stimulation occurs.

BenderRodriguez

One glycogen-related issue I would really like to learn more about is how to estimate how long the contents of one's 'tank' will last for a given level of effort, and thereby ensure that you don't 'completely 'empty the tank' before the finish when on a long mountainous ride, or taking part in something like the Marmotte.

Currently, my 'tank' seems quite big enough to fuel two 'at threshold' (around 173 bpm) efforts of an hour plus with a minimal break between them and not 'run out of gas'. For example, ride two Alpine cols with 15 minute descent between them. Similarly, as part of a 3- 4 hour ride I have no problem riding 5 or 6, 20 minute plus climbs, again at approximately a 'threshold' level of intensity.

Anyhow, at the weekend I did a bit of an Alpine epic, finishing off with the Cormet De Roseland out of Beaufort. I had already climbed about 1800m by this point, equating to about 2 1/4 hours of climbing at what I had though was a relatively easy sub-threshold level of effort, averaging about 167 Bpm, and I had been on the bike for over 3 hours in total. So, off I go setting a threshold level of pace (after all this is the last climb of the day...) only momentarily thinking 'Mmm, that is a long way to go' as I pass the '20km to the top' sign. You can guess what happened next: after 10km of this and perhaps 45 minutes later, the lights go out and there is still 10 km to go to the top. I had to stop at a cafe by the lake for 10-15 minutes and then was able to grind to the top in my bottom gear.

So, my 'tank' seems to last for no more than 3 hours or so when riding at between 167 and 173 bpm, even when taking on a bit of food and having some short recovery sections as well. Clearly, when I hit the bottom of the Cormet de Roseland, I probably hadn't much left in the tank and should have eased right back. My feeling is that, had I done this, riding at 160-odd Bpm right from the bottom, I would just about got to the top of The Roseland riding 'on fumes'.

Question is, suppose I was taking part in the Marmotte (part of my plans for next year) and I wanted to eke out my reserves so that I got up the Alpe at a decent pace? OK, so there would be some time to refuel on the long descent off the Galibier, but even so, taking my weekend ride as a guide, I feel that trying to ride at 167 Bpm up every climb would have been too much, no matter how easy this felt for the first couple of hours. So, how much slower would my pacing strategy have to be? 160 Bpm? More than this? Less? There has to be an easier way to have an idea of what will happen when riding in such 'uncharted waters' other than ride a 8 or 9 hour route at a given intensity in order to discover if this will result in you blowing up!

SBezza

If 12 Hour TT's and longer are the races you want to do well in, get the body used to burning fat primarily, these events are of a duration and intensity that you will be burning a big percentage of fat still. You can do them harder but fatigue would take over rather than the lack of glycogen, though go too hard too quickly and you will burn through the limited supply obviously. Fatigue is more of a limiting factor IME in a 12 hour TT, because as long as you keep eating you are likely to be OK glycogen wise, though it will become depleted eventually.

If you get used to doing 5 hour+ rides on minimal food then when you come to race these events you will be better adapted to running on a large percentage of fat (it does work). Obviously you need to eat whilst racing, but you will never take in enough calories per hour to what you are expending, the body just can not process the food quick enough, so you are very unlikely to replenish glycogen whilst racing, you will just be topping up your blood glucose levels and sparing what glycogen you have. I personally wouldn't bother trying to go as long as possible without food whilst training, just gradually increase the time without eating when doing long rides (obviously take food with you on these rides, just try and extend the time before eating any of it).

I found doing fasted training first thing in the morning good for getting used to glycogen depleted rides, as to whether it helped with glycogen sparing or not I will never know as I have never been tested, but I can quite easily do a hard endurance ride of 4-5 hours without food if I wanted to. Downside to this is recovery, as if you plan to do these sort of rides on back to back days, you might find you can't eat enough between rides to give you a decent recovery, so I would suggest these sorts of low food rides are done away from competition time.

BenderRodriguez

SBezza wrote:

If 12 Hour TT's and longer are the races you want to do well in, get the body used to burning fat primarily, these events are of a duration and intensity that you will be burning a big percentage of fat still.

Problem is, something like the Marmotte is not a constant 'fat burning' effort like a '12'. Rather, it involves extended climbing followed by a shorter (downhill) recovery period, then more extended climbing, so to do a decent time one needs to ride as close to one's threshold as is possible on all those climbs without using up all of one's reserves way before the end.

Agreed, becoming a more efficient 'fat burner' is part of the solution, but even here the ideal is to be able to use one's capacity to burn fat at an ever-higher level of work to increase one's sustainable pace, leading back to the same 'glycogen reserves' issue!

bahzob

BenderRodriguez wrote:

One glycogen-related issue I would really like to learn more about is how to estimate how long the contents of one's 'tank' will last for a given level of effort, and thereby ensure that you don't 'completely 'empty the tank' before the finish when on a long mountainous ride, or taking part in something like the Marmotte.

I am very much of the same mind. To get a handle on this I think you need to know 3 key variables;
- Glycogen level: Which is more complicated than the total number often cited since not all the total will actually be used in reality
- Rate glycogen is burned: Which will be a result of the intensity of effort and your "efficiency" in terms of burning fat vs glycogen for a given effort level (i.e. like the example I gave in the OP)
- Glycogen replenishment from fat: Some events like the Marmotte include extremes of very hard long effort (climbs) and relatively long rests (descents, group rides to next climb). Not sure about this but I am guessing that some glycogen will be replenished during the rests.

Like already said, just from the pov of curiosity it would be interesting to have a model that predicted how long/hard you could go for a given effort. This may give some hints on pacing, in some circumstances you may be find you can push hard because you know you comfortably have enough to get to the next feed. In other circumstances going slower may result in quicker times because otherwise you end up bonking.

If you don't have a sense for this then you can end up eating too little or too much, both of which end up affecting performance.

bahzob

SBezza wrote:

If 12 Hour TT's and longer are the races you want to do well in, get the body used to burning fat primarily, these events are of a duration and intensity that you will be burning a big percentage of fat still. You can do them harder but fatigue would take over rather than the lack of glycogen, though go too hard too quickly and you will burn through the limited supply obviously. Fatigue is more of a limiting factor IME in a 12 hour TT, because as long as you keep eating you are likely to be OK glycogen wise, though it will become depleted eventually.

If you get used to doing 5 hour+ rides on minimal food then when you come to race these events you will be better adapted to running on a large percentage of fat (it does work). ...

Yes. And this is exactly the sort of training I do. However, especially compared to threshold power training, it's pretty hit and miss. There is no on bike metric like FTP to measure your glycogen level and you can't monitor your fat burning efficiency like you can watts. Both, I think, require specialist lab tests. I like training with a goal in mind and a method to measure progress so don't like this .

Also without this it becomes very hard to really assess why you fail on long events. "Fatigue" is a bit of a catch all term. I know from my fasted training rides that reaching the end of your glycogen reserves is often not signalled by a massive bonk. Rather what happens is a slow but steady decline in steady power til it hits a new stable level. So for e.g. whereas it was easy to hold 240W for hour upon hour now you can only hold 200W (which is I guess an indicator of your "fat burning power", the level you can put out when glycogen is depleted..

This feels very much like "fatigue" but root cause is not enough carbs. Pretty sure this is the case because the simple solution of eating some carbs at this point quickly restores performance which seems to show the muscles just lacked fuel rather than being really fatigued.

bahzob

SBezza wrote:

Downside to this is recovery, as if you plan to do these sort of rides on back to back days, you might find you can't eat enough between rides to give you a decent recovery, so I would suggest these sorts of low food rides are done away from competition time.

Agree. I think there may be value to doing these rides back to back and perhaps even going so far to follow a zero carbs diet.

This way you would estabish a controlled base line where you know you are riding a glycogen depleted state and the effort you put out could be one metric of your "fat burning power". This could even be measured by conducting an FTP test, which you would expect to be substantially lower than one conducted under normal circumstances.

Further if you train in this state you may or may not see this power change. This might be an indicator that your fat burning is getting more efficient.

But for sure best time to do this is outside of competition. This is why I happened to raise it now as I have just switched to training with 2014 in mind.

BenderRodriguez

bahzob wrote:

I know from my fasted training rides that reaching the end of your glycogen reserves is often not signalled by a massive bonk. Rather what happens is a slow but steady decline in steady power til it hits a new stable level. So for e.g. whereas it was easy to hold 240W for hour upon hour now you can only hold 200W (which is I guess an indicator of your "fat burning power", the level you can put out when glycogen is depleted..

I have found pretty much the same, although I would say that my decline in power is pretty rapid once my reserves run low! Oddly enough when I was a schoolboy the first sign I had done too much was usually a massive 'bonk' that left me hardly able to see, let alone ride a bike. I still recall once being forced to sit outside a shop eating before being able to get myself home. Not that unusual, but this particular shop was less than a mile from my house!

Perhaps the difference is that when I am running low on glycogen these days I often a have long climb between me and home, so the depletion of my muscle glycogen is very noticeable before my reserves have fully run out. When younger the ride home was always pretty flat, and I probably didn't ride as hard, so any decrease in power would probably have been much less noticeable and I would have been able to continue at a relatively low level of effort until the point I had nothing left at all.

bahzob wrote:

This feels very much like "fatigue" but root cause is not enough carbs. Pretty sure this is the case because the simple solution of eating some carbs at this point quickly restores performance which seems to show the muscles just lacked fuel rather than being really fatigued.

Personally, once I have hit the 'wall' no amount of eating seems to allow me to continue at a pace similar to that before I cracked. Perhaps if I had a prolonged break, say an hour, things would be different. In any case, isn't the rate of refueling much slower than the rate at which fuel can be burnt? If one can only process 50g or so of carbohydrate in an hour, this might quickly restore ones blood sugar levels, but there would still be very little in the tank to fuel any extended efforts.

SBezza

bahzob wrote:

SBezza wrote:

If 12 Hour TT's and longer are the races you want to do well in, get the body used to burning fat primarily, these events are of a duration and intensity that you will be burning a big percentage of fat still. You can do them harder but fatigue would take over rather than the lack of glycogen, though go too hard too quickly and you will burn through the limited supply obviously. Fatigue is more of a limiting factor IME in a 12 hour TT, because as long as you keep eating you are likely to be OK glycogen wise, though it will become depleted eventually.

If you get used to doing 5 hour+ rides on minimal food then when you come to race these events you will be better adapted to running on a large percentage of fat (it does work). ...

Yes. And this is exactly the sort of training I do. However, especially compared to threshold power training, it's pretty hit and miss. There is no on bike metric like FTP to measure your glycogen level and you can't monitor your fat burning efficiency like you can watts. Both, I think, require specialist lab tests. I like training with a goal in mind and a method to measure progress so don't like this .

Also without this it becomes very hard to really assess why you fail on long events. "Fatigue" is a bit of a catch all term. I know from my fasted training rides that reaching the end of your glycogen reserves is often not signalled by a massive bonk. Rather what happens is a slow but steady decline in steady power til it hits a new stable level. So for e.g. whereas it was easy to hold 240W for hour upon hour now you can only hold 200W (which is I guess an indicator of your "fat burning power", the level you can put out when glycogen is depleted..

This feels very much like "fatigue" but root cause is not enough carbs. Pretty sure this is the case because the simple solution of eating some carbs at this point quickly restores performance which seems to show the muscles just lacked fuel rather than being really fatigued.

If you train the right way, then you should be able to hold 240W say for the full duration, if that is a reliable figure to aim for in the first place, and the only way is to try. Fatigue is a bit of a catch all, but you can quite easily get tired without using up all the glycogen, as say one hour and 10 mins at a FTP type effort, you will be pretty well fatigued, but you will still have glycogen available. Glycogen depletion causes a lot more serious issues with dizziness and the general inability to put out any sort of decent power until food is eaten and processed, a drop of 40-50 watts is not glycogen depletion IMO, just fatigue. But again, if you were eating 60-90 gms an hour of carbs during an event, you would likely hit the wall in general tiredness before glycogen depletion hit IME

Training is a glycogen depleted state is hit and miss I will admit, and to be honest you need to be sensible about it, don't do it too often and if you have back to back days of long rides perhaps only do one of them as fasted. Doing lots of training consecutively fasted might end up causes more health issues that the performance gain you will get by being able to burn fat more efficiently. In the run up to my 12 hour TT, I didn't do any fasted rides (these were done in the winter/spring as an aid to weight loss), so even though I was training in a non fasted state I still was able to go for long periods without food if needed so the adaption is there without extreme measures of food group restriction. To be honest a zero carb diet is almost impossible, and I would think not the most healthiest, virtually all foods with the exception of meat contain some carbs

BenderRodriguez

SBezza wrote:

If you train the right way, then you should be able to hold 240W say for the full duration

And what changes might that training bring about? An increased overall aerobic threshold and with it an ability to burn a higher proportion of fats at an output of 240w than is currently the case perhaps?

SBezza wrote:

a drop of 40-50 watts is not glycogen depletion IMO, just fatigue.

And what, exactly, is the mechanism of such fatigue, if not glycogen depletion?

SBezza

BenderRodriguez wrote:

SBezza wrote:

If you train the right way, then you should be able to hold 240W say for the full duration

And what changes might that training bring about? An increased overall aerobic threshold and with it an ability to burn a higher proportion of fats at an output of 240w than is currently the case perhaps?

SBezza wrote:

a drop of 40-50 watts is not glycogen depletion IMO, just fatigue.

And what, exactly, is the mechanism of such fatigue, if not glycogen depletion?

Training for your targeted events brings about adaptions useful for that event, no point in say just doing 1 hour max sessions at FTP, if your target event was a 12 hour + TT for example, yes by all means do that training, but you also need to train in a way you stimulate the body to what that event will require to get the maximum performance. 240W was just a figure bazhob mentioned, some just wouldn't be able to hold that for 12 hours no matter what training they did. When doing a long ride and you see a pretty big drop in power, I would suggest hydration could be an issue as well as tiredness, but train correctly you can start doing the same duration without the drop in power, you won't drastically swing the amount of glycogen you can spare, but you can change it in your favour. Obviously if the ride is a fasted ride, then glycogen depletion will be a major cause as well, but I was mainly on about racing where you just wouldn't do it fasted in any shape or form.

Fatigue is tiredness, this can quite easily happen without glycogen being fully depleted, again try doing 8 x 5 mins Vo2Max intervals, if done right I would suggest near on impossible, but you would use only a small amount of fully stocked glycogen (well probably about 50-60%, but again not all). Muscle get tired when worked hard, and then begin to fail. Tired muscles just can't produce the same amount of force. Another example is try doing the 8 x 5 min Vo2Max intervals on a daily basis, you might find doing them might just be possible one day, but 2 days in a row the power might be a lot less, even though you would have likely fully topped up glycogen in between sessions. It is purely muscle damage that has caused the difference.

BenderRodriguez

SBezza wrote:

Fatigue is tiredness, this can quite easily happen without glycogen being fully depleted, again try doing 8 x 5 mins Vo2Max intervals, if done right I would suggest near on impossible, but you would use only a small amount of fully stocked glycogen (well probably about 50-60%, but again not all).

But we are talking here not about the fatigue induced by high-intensity efforts, but the fatigue that comes after 3 hours or more of working at a sub-threshold level. Assuming hydration is taken care of, what else is causing this drop in performance if not glycogen depletion?

Also, there is the fact that different levels of effort utilise the various energy producing system to different degrees, so there is almost certainly more than one path to 'fatigue'. What is under consideration here is not what causes one to be fatigued after a 200m sprint or other high-intensity effort, but why one cannot sustain, say 240 watts, for 5-6 hours when it is relatively easy to do so for 3 hours or so.

Perhaps it would be helpful to think of both the rate at which the glycogen can be utilised and the size of the stores available. Sure, for high-intensity efforts one might simply be unable to maintain the high rate of usage necessary and so become fatigued, but for lower-level intensities the capacity of the tank is of primary importance.

P.s. you say "Fatigue is tiredness". This is more of a tautology than an explanation!

SBezza

When you work out at any intensity, you gradually tire, how fast this happens is down to intensity obviously. When you do an hour at FTP, what stops you going at that same power for longer, it isn't glycogen is it (fully stocked you would have enough for 2 hours +), it is the muscle fibres getting tired and being unable to produce the same force.

That same mechanism happens at lower intensities as well, you train to get the body to adapt, either by producing more power at a certain duration, or keeping the same power for a longer duration, or most likely a combination of both. There are many paths to fatigue no doubt, and glycogen depletion is one of them, but IME not the main cause. When racing a longer race, you will be supplementing that glycogen with food and drink, so you will be able to go longer without running out of glycogen, but if you haven't really trained for going long, you will still tire prematurely.

BenderRodriguez

SBezza wrote:

When you do an hour at FTP, what stops you going at that same power for longer, it isn't glycogen is it (fully stocked you would have enough for 2 hours +), it is the muscle fibres getting tired and being unable to produce the same force.

So, if you have enough for 2+ hours, what happens when you have done 3+ hours, given one can burn carbohydrate much faster than one can replenish it through eating?

Also, what exactly happens within a muscle fibre so that the force it can produce after 3 or 4 hours is less than it can produce for 2- 3 hours? Please don't just say 'the fibres get tired'!

By the way, I would agree that as long as glycogen is readily available, say for efforts of under 2- 3 hours, running out of glycogen is unlikely to be the primary cause of fatigue. However, it is a different story for efforts lasting much longer than this, unless they are of a low enough intensity to be sustainable via one's fat burning metabolism.

SBezza

It is 2 hours + at threshold intensity (the higher the intensity the less time it will last), the thing that stops you doing 2 hours at threshold is NOT glycogen. Surely anyone training seriously knows this.

So for a 3 hour ride, the intensity is less than what the body can do whilst burning near on 100% glycogen (this never happens at lower intensities, even threshold intensities you burn some fat), the thing that stops you going longer than say an hour at max intensity is fatigue (however you may want to represent this), which is the muscle fibres getting damaged and the body wanting to limit this damage, so muscles no longer use the damaged fibres, or can't use them. If you need more info on this then I suggest a google search . Even a 2 hour ride at what you think is the hardest you can go will be sub threshold, and as such will not be burning nearly 100% of glycogen, so you could go longer than 2 hours at sub threshold intensities without running out of glycogen, just tiredness takes over and limits how much you can do at that intensity.

So in a 3 hour ride, you are using a fair portion of fat which helps fuel you alongside the glycogen (fat always needs glycogen to burn however, you will never use 100% fat for fuel), hence this is why you can do fasted rides of 3/4/5/6 hours duration. Obviously you might do a 3 hour ride at a higher intensity (burn more glycogen), and run out quicker than someone who does a 6 hour ride (duration of the ride means you normally do this at a lower intensity than a 3 hour ride).

If you are eating enough you can ride for quite a long time at quite an effort, as long as you are also fit enough and have trained for doing that. I rode a 12 hour TT at an effort I have never done more than 3-4 hours at in training (low Z3 for me), and still didn't run out of glycogen, purely because I was eating and drinking during the race. Now I have possibly got a good fat burning/glycogen sparing mechanism, but that is only through the appropriate training over many years of training.

Glycogen depletion is just one aspect of getting tired, but it generally manifests itself with serious power drop off, and feeling very very unwell. There are many processes going on in the body that cause you to tire and this is just one of them, but not the major cause IMO. You have to be very poor at burning fat, or eating very poorly indeed to suffer major glycogen depletion on a ride as short as 3 hours IME. Most of the time people suffer is they are going to hard for that sort of duration and the muscles get damaged and then want to stop working, they then repair themselves and you get stronger, and then you can do that sort of ride just as fast/powerful easier as you get stronger and fitter.

mamba80

Jeez talk about over analysis leading to paralyisis

BenderRodriguez

SBezza wrote:

You have to be very poor at burning fat, or eating very poorly indeed to suffer major glycogen depletion on a ride as short as 3 hours IME.

Or are simply riding very close to ones threshold, for example by climbing two Alpine passes within that time, and so are burning predominantly glycogen for most of the time.

I guess you believe that all the stuff about marathon runners risking 'hitting the wall' due to glycogen depletion are also nonsense, despite also admitting that even when 'fully stocked' most people are only able to store enough glycogen to sustain an effort of 2 hours or so?

As to the effect of eating, if you are burning 600-700 kilo-calories an hour, and at such a high level of intensity are deriving this largely from stored glycogen, how on Earth can this be replaced at anywhere near the same rate when one can only process 50-60g of carbohydrate an hour, giving only 200 - 240 calories?

I also feel that it is not true there is no middle ground between suffering a full-on 'bonk' and riding along in an 'unfatigued' state. Sure, completely depleting one's glycogen stores will lead to the 'bonk', but before that happens local muscle glycogen stores will become depleted, so reducing the amount of power that can be sustained as the body struggles to shuttle what glycogen stores do remain from where they are stored to where they are being used.

BigFatBloke

Depletion of glycogen, ATP and CP is not the cause of muscle fatigue, it is the build up of H+ ion.