Glycogen depletion: What's happening?

[CycleFast]

I did quite a bit of training last week, and I probably wasn't paying enough attention to my carb intake after exercise.

Last Saturday I did a 3-hour ride, I felt OK for the first hour or so, but then I gradually lost more and more of my power. I didn't bonk, but I could feel I was exercising on fat burning exclusively, I did not have any snap at higher intensities, and my legs got heavier.

The next day my legs/thighs actually both felt and looked smaller. It was like my muscles had collapsed and they didn't have any tension or snap in them.

Over the next couple of days I increased my carb intake a lot and can feel the tension, volume, snap and strength returning day by day, but it sure takes a few days to recover from this. I'm still missing my high end power, but my legs are coming around on lower intensities.

My question is: Is there anything else happening physiologically in my body/muscles (apart from the obvious glycogen depletion) that makes my legs recover more slowly than usual after a serious glycogen depletion?

My previous coach said that when bonking, you take away the brains primary (only?) fuel source, and that this could result in a number of hormonal disturbances it could take days to recover from.

Are there any similar/other disturbances to hormones, enzymes, etc when experiencing a glycogen depletion (but not bonking) that delays the recovery process?

Any insights or thoughts would be much appreciated!

 

[vio765]

your situation happened to me today. except my experience was more drawn out; a few hard days, not enough carbs, and the first hot day of the year. to get to the point: i believe that your old coach was right. this will take time to recover from. if possible, i would say have about 20% more carbs than you need for a day or two. and more sleep couldnt hurt either. this might or might not speed things up, but you will most likely come back in better shape than if you didnt. i was used to a low-med intensity for several months and used to 35-50% carbs per day. now it is getting hot, in weather and intensity. so now i have to bump things up to 50-60%. this works for me. anyway, happy cycling

I did quite a bit of training last week, and I probably wasn't paying enough attention to my carb intake after exercise.

Last Saturday I did a 3-hour ride, I felt OK for the first hour or so, but then I gradually lost more and more of my power. I didn't bonk, but I could feel I was exercising on fat burning exclusively, I did not have any snap at higher intensities, and my legs got heavier.

The next day my legs/thighs actually both felt and looked smaller. It was like my muscles had collapsed and they didn't have any tension or snap in them.

Over the next couple of days I increased my carb intake a lot and can feel the tension, volume, snap and strength returning day by day, but it sure takes a few days to recover from this. I'm still missing my high end power, but my legs are coming around on lower intensities.

My question is: Is there anything else happening physiologically in my body/muscles (apart from the obvious glycogen depletion) that makes my legs recover more slowly than usual after a serious glycogen depletion?

My previous coach said that when bonking, you take away the brains primary (only?) fuel source, and that this could result in a number of hormonal disturbances it could take days to recover from.

Are there any similar/other disturbances to hormones, enzymes, etc when experiencing a glycogen depletion (but not bonking) that delays the recovery process?

Any insights or thoughts would be much appreciated!

 

[Spunout]

Last Saturday I did a 3-hour ride, I felt OK for the first hour or so, but then I gradually lost more and more of my power. I didn't bonk, but I could feel I was exercising on fat burning exclusively, I did not have any snap at higher intensities, and my legs got heavier.

You bonked.

do you eat 300Cal every hour after the first hour on these rides?

 

[CycleFast]

You bonked.

It might be, it depends on the exact definition of bonking.

I've experienced bonking several times before, but the symptomes has then been the classical symptoms; dizzy, hunger for food, loss of muscle control, sweating, etc. Noen of these symptomes occured on the ride in question. It was only my legs that lost power, all other bonking symptoms were not present.

I would think it's possible to have "muscle glycogen depletion" without having low bloodsugar (which is the cause of bonking)?

do you eat 300Cal every hour after the first hour on these rides?

Normally I'm much better. I don't know why I was so sloppy that week, I'll have to improve...

_________
CycleFast

 

[wbkski]

I rode 120 miles today... the first 70 were with the lead group at a pace of about 22mph into the wind. There were 15 of us so taking turns was easy. I felt like a million bucks, but knew I wouldn't be able to keep that pace on the return. Sure enough, my return leg was slower by 2mph (with the wind) and the legs really started getting heavy as did my breathing. It was a struggle to finish. I made it home and drank a bunch of water and took a nap....something I NEVER do. I felt like I had just finished running 2 marathons. My son says I probably should have drank more and ate more.

I see an earlier comment of eating 300 calories per hour after the first hour. Is that good advise? And.. .is there any way to "de-bonk" once you're there?

I look forward to the replys....
Thanks everyone.
WBK

 

[alienator]

I rode 120 miles today... the first 70 were with the lead group at a pace of about 22mph into the wind.  There were 15 of us so taking turns was easy.  I felt like a million bucks, but knew I wouldn't be able to keep that pace on the return.  Sure enough, my return leg was slower by 2mph (with the wind) and the legs really started getting heavy as did my breathing.  It was a struggle to finish.  I made it home and drank a bunch of water and took a nap....something I NEVER do.  I felt like I had just finished running 2 marathons.    My son says I probably should have drank more and ate more.  I see an earlier comment of eating 300 calories per hour after the first hour.  Is that good advise?   And.. .is there any way to "de-bonk" once you're there? I look forward to the replys.... Thanks everyone. WBK

You're son is right. You also should never have gone to sleep just after having water. That's not enough. When done you should immediately get some carbs in you and then within 30 minutes to an hour take in some protein? Were you really sore and sub-optimal feeling when you woke up?

 

[wbkski]

Originally Posted by alienator .


You're son is right. You also should never have gone to sleep just after having water. That's not enough. When done you should immediately get some carbs in you and then within 30 minutes to an hour take in some protein? Were you really sore and sub-optimal feeling when you woke up?

Sore for sure... sub-optimal? More like sub-human. Felt like ****, but then again, I felt like **** before the nap too.

 

[daveryanwyoming]

Originally Posted by CycleFast .

It might be, it depends on the exact definition of bonking.

I've experienced bonking several times before, but the symptomes has then been the classical symptoms; dizzy, hunger for food, loss of muscle control, sweating, etc. Noen of these symptomes occured on the ride in question. It was only my legs that lost power, all other bonking symptoms were not present.

I would think it's possible to have "muscle glycogen depletion" without having low bloodsugar (which is the cause of bonking)? /img/vbsmilies/smilies/confused.gif

FWIW, in my wife's MS RD studies they introduced two concepts related to exercise and low glycogen/glucose conditions. In their terminology they discussed 'hitting the wall' and 'bonking' as two related but different concepts with the following definitions (my loose paraphrasing as I wasn't in those classes):

- Bonking: depletion of blood glucose and glycogen/glucose stores in the liver leading to problems feeding the brain as the brain requires sugars as it's fuel source and cannot directly use other macronutrients (though it can indirectly through ketoacidosis and catabolizing muscle protein to produce sugars but that is a relatively slow process). This is the classic tunnel vision, feelings of despair, mental bonking that many of us have experienced and it's mostly a brain related issue. You can turn this around fast by ingesting high GI carbs so eat immediately if you start to feel this coming on.

- Hitting the wall: depletion of muscle glycogen stores from your working muscles. When this happens we have no choice but to back down to lower intensity exercise that primarily burns proteins and fats. This can take a long time to bounce back from as glycogen synthesis rates are relatively slow and our muscles consume blood sugar very rapidly when working out so it's hard to ingest enough sugars to fuel both the muscles and brain. Still if this happens with or without the bonking you want to get sugars on board, just don't expect a fast recovery to full strength.

The two often happen together but not necessarily. If you've been working out a lot day to day or are on a low carb diet (not a good idea for endurance athletes) you can be partially or largely glycogen depleted before the ride even begins and run out of glycogen in the working muscles even though blood sugar levels are decent and the brain is fine. Of course continuing to workout in those conditions can rapidly lead to bonking as you burn through your blood and liver sugars an start to starve the brain. I suppose it's also possible to do the reverse during very high intensity workouts where you begin to starve the brain on acute blood glucose supplies even though the muscle glycogen is in good shape. That starts to look like diabetic issues of acute low blood sugar even if the root causes are different and your insulin levels are fine. But in either case ingesting simple sugars is a pretty good idea.

Chronic glycogen depletion is a real problem for folks working out hard, long and or frequently. Critical half hour refueling after workouts is a really good idea as your body syntesizes and stores glycogen at much faster rates immediately following exercise. In that first half hour to hour after a hard or long workout your body can convert carbs to glycogen up to twice as fast as it can after that useful refueling window closes. So get some high GI carbs along with some low fat protein on board immediately after long or hard workouts if you plan to workout again soon. You can do this with commercial recovery drinks, with something like a fruit smoothie with some added lowfat yogurt or protein powder (emphasis on the carbs but some protein has been shown to be useful as in 3:1 or 4:1 carbs to protein) or with a lot of folks favorite, low fat chocolate milk which has been shown in some studies to be as good as the commercial recovery drinks and is widely available in convenience stores and supermarkets.

If you do deeply deplete your muscle glycogen stores it can take several days to fully replenish those stores so it's best not to go there. Also muscle glycogen is not mobile between muscle groups. IOW, your arm muscles could be fine on stored glycogen even as you deplete the glycogen in your primary leg muscles while cycling, that glycogen in your arms does not help your legs as the glycogen in your arm muscles cannot move to your leg muscles where it's needed.

In terms of on the bike fueling, most folks can only ingest and process roughly 250 to 300 Calories per hour while working out. Attempts to force down more fuels usually leads to GI distress and cramping which can end your ride in a hurry. But riding hard can burn through sugars at a much faster rate. You burn roughly 360 Calories per hour at 100 watts average power, 720 at 200 watts AP and 1080 at 300 watts AP. The closer that average power is to your FTP the more those calories come from sugars and the less from fats and protein. So it's a deficit spending game, you cannot ingest enough to cover everything your burn unless you ride at a very low intensity.

You can steer things towards burning fats and preserve muscle glycogen stores by managing intensity relative to your FTP. For instance if you limit your riding to say 70% of FTP you'll burn a higher percentage of fats and preserve some glycogen and we have plenty of fats on board so that's a good thing but it means pacing back and going slower. So in addition to raising your FTP to deal with hard fast sections of riding or to TT faster or to climb a long hill faster a huge benefit of increasing FTP is the ability to ride relatively fast and hard but to do so at a reasonable fraction of your FTP so that you preserve muscle glycogen and avoid hitting the wall or bonking.

For instance if two riders of the same size and weight have FTPs of 250 and 300 watts respectively and the race pace in the group forces them to hold an AP of 200 watts then the rider with a 250 watt FTP is riding at 80% of their FTP and the 300 watt FTP rider is only riding at 67% of his FTP. The second rider is doing a much better job of preserving muscle glycogen and is much less likely to bonk or hit the wall at this pace. So in addition to freshness and recovery concerns or the ability to deal with the fast and hard sections of the race the rider with the higher FTP has extra endurance from a fueling standpoint as they can rely more on fats at the same pace. IOW, it pays to be fit in many ways.

-Dave

 

[dsb137]

Interestingly, the percentage of glucose used for fuel is rather variable, and there are in fact people who never burn more fat than carbs even at low intensities. Personally, I didn't think that was possible, shows you what I know. The contribution of fat burning can be trained, directly, and as Dave showed, indirectly by virtue of raising your power output.

It can also be affected by your diet, the more you eat of something, the better you get at burning it for fuel, in an over simplified sort of way...

In Lim's Feed Zone Portables he has a table (pg. 18) where he compares the glucose storage for cyclists depending on diet carb content. For a 75 kg. cyclist eating a 'moderate' carb diet (Lim defines moderate as 40-50% carbs), at 15% body fat, they would have 1306 calories of glycogen stored. Whereas, the same cyclist on a 'high' carb diet (Lim defines high as 60-75% carbs) would have 2285 calories of glycogen stored.

Whether the person on the high carb diet would burn more at any given intensity than the moderate carb dieter is up for debate.

The recommendation for 300 kcal per hour is based on absorption rates, and commonly works out to 30 - 60g of carbs. This is also highly personal, not everyone can stomach the same amount of carbs, and different methods (liguid vs. solid vs. gel), as well as different types (sucrose, destrose, maltodextrose, etc.) can have a great influence on the outcome.

As they say, YMMV...

BTW, the 'central governor' types would argue that the brain down regulates performance long before actual muscle glycogen levels reach the point of being a limiter.

HTH,
Dave

 

[wbkski]

This is exactly the type of information I was looking for. Dave... it's apparent you know your stuff when it comes to cycling, so, may I pose this to you... I have road race on Saturday...72 miles and its down in Nephi (you may have done it at some point in time). So... as it is a "race" and not a ride, it will be balls out the entire way. Not unlike the first leg of my ride yesterday. What type of rides would you do this week leading up to race day? I have a crit I can do ( or not do) on Tuesday night, but all the other days are open. I'm planning on doing a recovery ride today as soon as I hit the "send" button on this post. I workout everyday in the gym...light leg weight exercises and stretching.

I'm not the type who can go more than a day without working out... just not in my DNA. So...any ride/workout suggestions you or others may have would be appreciated.

Thanks for the great posts everyone...
WBK

ps... garmin says I burned 2700 cals while Mapmyride and Strava said I burned almost 5K in calories. Any idea who is closer to being correct? And...do you eat to replace those cals during a ride?

 

[alienator]

With respect to calories burned and estimates by Garmin, MapMyRide, and Strava, none of them are accurate. If you want a really close measurement, you'll want a power meter.

 

[An old Guy]

Originally Posted by CycleFast .

I did quite a bit of training last week, and I probably wasn't paying enough attention to my carb intake after exercise.

Last Saturday I did a 3-hour ride, I felt OK for the first hour or so, but then I gradually lost more and more of my power. I didn't bonk, but I could feel I was exercising on fat burning exclusively, I did not have any snap at higher intensities, and my legs got heavier.

The next day my legs/thighs actually both felt and looked smaller. It was like my muscles had collapsed and they didn't have any tension or snap in them.

Over the next couple of days I increased my carb intake a lot and can feel the tension, volume, snap and strength returning day by day, but it sure takes a few days to recover from this. I'm still missing my high end power, but my legs are coming around on lower intensities.

My question is: Is there anything else happening physiologically in my body/muscles (apart from the obvious glycogen depletion) that makes my legs recover more slowly than usual after a serious glycogen depletion?

My previous coach said that when bonking, you take away the brains primary (only?) fuel source, and that this could result in a number of hormonal disturbances it could take days to recover from.

Are there any similar/other disturbances to hormones, enzymes, etc when experiencing a glycogen depletion (but not bonking) that delays the recovery process?

Any insights or thoughts would be much appreciated!

I don't think you can tell that "I was exercising on fat burning exclusively."

Eating 300cal/hr to solve the issue is foolish advice. Your speed indicates you are most likely burning much more than that. Even eating 300cal/hour your problem would have occured before the end of your 3 hour ride.

Sometimes people have bad days or weeks unrealed to glycogen.

---

Just for reference I eat nothing on 3 hour rides. I don't deplete my glycogen. Glycogen recovery happens overnight unless you work hard to prevent it.

 

[daveryanwyoming]

Originally Posted by wbkski .

... What type of rides would you do this week leading up to race day? I have a crit I can do ( or not do) on Tuesday night, but all the other days are open. I'm planning on doing a recovery ride today as soon as I hit the "send" button on this post. I workout everyday in the gym...light leg weight exercises and stretching....

Tapering is personal and depends a lot on what you've been doing up till now, what sort of training loads and what sort of fatigue you've been carrying. But I would NOT back off from your normal training too much. Do not go into the event over rested after several days off the bike, that will likely hurt you more than steady training but backed off enough that you're relatively fresh (relative to recent training weeks) on race day. But per the rest of this thread you should avoid huge rides in the days leading up to the race if that's a normal part of your plan. IOW, train fairly normally, back it all down maybe 10% to 15% but don't do any huge glycogen depleting rides in the final three or four days before your race. That and of course get plenty of sleep, stay hydrated and well fueled as you approach your race.

FWIW, I'm not a fan of weight work for racing cyclists and definitely not in-season as important events approach but if that's your normal routine it's up to you. I'd personally drop those gym workouts during the racing season but some coaches do like gym work but even then I'm not aware of any that suggest them during the actual race season.

...ps... garmin says I burned 2700 cals while Mapmyride and Strava said I burned almost 5K in calories. Any idea who is closer to being correct? And...do you eat to replace those cals during a ride?...

As Alienator says unless your Garmin is driven by a power meter, none of those estimates is likely accurate. How far did you ride and how fast did you ride for the distance and how much climbing was there during your ride?

From a power standpoint your burn roughly 3.6 Calories per hour for each watt. That's an estimate because without lab testing to determine your Gross Metabolic Efficiency (GME) you can't really pin it down closer than that but it's still a very accurate estimate within +/- 5% or so. So if you know your average power you get a very good estimate of dietary calories burned as:

3.6*AP*hours

For instance I rode a recent 100 mile ride with some team mates at a fairly quick pace. We came in just a hair under five hours and my AP was 208 watts on a somewhat hilly course at 160 pounds plus bike and kit.

So roughly I burned 3.6*208*5 =~ 3750 Calories in five fast hours on the bike. My lab tested GME is a bit less than the swag of 23.9% that's buried in that estimate so I burned closer to 4000 Calories.

So unless you are a really large guy in very hilly terrain and rode a very long ride it's almost certain you did not burn 5000 Calories on your ride. The Garmin estimate could be close and if your Garmin is connected to a Powertap or Quarq or SRM or similar PM then it's probably very close.

-Dave

 

[maxroadrash]

Dave-
This calculation works out to within about 2% of what I get just using the Kj count off my PM.
Does that square with your experience?

Originally Posted by daveryanwyoming .




From a power standpoint your burn roughly 3.6 Calories per hour for each watt. That's an estimate because without lab testing to determine your Gross Metabolic Efficiency (GME) you can't really pin it down closer than that but it's still a very accurate estimate within +/- 5% or so. So if you know your average power you get a very good estimate of dietary calories burned as:

3.6*AP*hours

For instance I rode a recent 100 mile ride with some team mates at a fairly quick pace. We came in just a hair under five hours and my AP was 208 watts on a somewhat hilly course at 160 pounds plus bike and kit.

So roughly I burned 3.6*208*5 =~ 3750 Calories in five fast hours on the bike. My lab tested GME is a bit less than the swag of 23.9% that's buried in that estimate so I burned closer to 4000 Calories.

So unless you are a really large guy in very hilly terrain and rode a very long ride it's almost certain you did not burn 5000 Calories on your ride. The Garmin estimate could be close and if your Garmin is connected to a Powertap or Quarq or SRM or similar PM then it's probably very close.

-Dave

 

[daveryanwyoming]

Originally Posted by maxroadrash .

Dave-
This calculation works out to within about 2% of what I get just using the Kj count off my PM.
Does that square with your experience?

It should work out within 0% as that is literally the calculation for kilojoules and the only swag I'm taking is that kj = dietary calories (which equal kilocalories or Calories)

It breaks down like this:

There are 3600 seconds per hour or 3.6 kiloseconds per hour

joules = watts*seconds

kj = watts*kiloseconds

kj = watts*3.6*hours

Calories = kj*4.184 (these are Calories of 'combustion' or the Calories of energy released by digesting foods but only a fraction of that is delivered to the working muscles where we measured power, most of the rest is burned off as excess heat which is why we get so dang hot riding the trainer)

but actual Calories based on watts measured at the cranks (as opposed to watts of combustion in the actual digestion of foods) = Calories (combustion) * GME

GME for humans is in the range of 19% to 26% or so with a mean of around 23% and a normal distribution. It takes a lab test with gas exchange equipment (for instance during VO2 Max testing) to pin down exact GME but it's convenient to assume GME is 23.9% which happens to be 1/4.184 so you get:

Calories burned = watts*3.6*hours*(4.184/4/184) = watts*3.6*hours = kj

So if your actual GME is below 23.9% then you'll burn a few more Calories for the same kj of work performed and if your particular GME is above 23.9% then you'll burn fewer Calories for the same kj of work performed but the variance across the human population is very small. BTW, the statistics of GME in highly trained endurance athletes is roughly the same as in the general human population so the general swag holds across the board though individuals may be a few percent higher or lower.

-Dave

 

[wbkski]

Dave... I used your formula and it comes out to 3762 for that ride. That's about 578 calories per hour. Should I be consuming that, or more, in calories using GU or something similar during the ride? If I am, then I can see where my deficiency was immediately. My ride consumption consisted of a couple of hits of EFS mocha, 1 honey stinger waffle at 33 miles another at 68, a candy bar at 68 and basically no food after that. I went through half of my tall water bottle to the 33 mile point, half again to the 68 mile point, and two full bottles on the way back.

 

[daveryanwyoming]

Originally Posted by wbkski .

Dave... I used your formula and it comes out to 3762 for that ride. That's about 578 calories per hour. Should I be consuming that, or more, in calories using GU or something similar during the ride? If I am, then I can see where my deficiency was immediately. My ride consumption consisted of a couple of hits of EFS mocha, 1 honey stinger waffle at 33 miles another at 68, a candy bar at 68 and basically no food after that. I went through half of my tall water bottle to the 33 mile point, half again to the 68 mile point, and two full bottles on the way back.

I'm pretty sure I already mentioned this above, but DO NOT try to consume more than 250-300 Calories per hour on the bike or at least experiment with stretching that intake a bit during long training rides.

Most riders will experience GI distress when trying to consume more than about 300 (a lot of published advice puts that around 280) Calories per hour while riding. That includes all calories including what comes from gels, solid food, energy drinks, etc. Try to push 500-600 Calories per hour during a race and I'd expect to find you at roadside with a really bad gut cramp before long.

So yeah, it's a deficit spending game we cannot replenish all the calories we burn as we burn them unless we back the power way, way down and in racing that pretty much means we've been dropped. Getting fitter does allow us to burn more fats as the race intensity relative to our FTP becomes lower (we burn almost 100% sugars as our intensity approaches FTP but most folks burn a higher percentage of fats as intensity drops relative to FTP). So getting fit helps a lot but we will always be burning through our limited glycogen and glucose stores in long races and cannot fuel at high enough rates to avoid digging into our glycogen and glucose stores.

Download this free doc if you want more info: http://www.hammernutrition.com/products/endurance-athlete-s-guide.fh.html

It's pretty good advice in general but they're in the business of selling supplements so read it with a critical eye. I certainly don't buy into all their sales pitches for supplementation but their discussions of fueling, hydration, and electrolyte maintenance are pretty good and all point to the idea that it's a deficit spending game and attempts to avoid those deficits are almost always counter productive.

-Dave

 

[wbkski]

Originally Posted by daveryanwyoming .


I'm pretty sure I already mentioned this above, but DO NOT try to consume more than 250-300 Calories per hour on the bike or at least experiment with stretching that intake a bit during long training rides.

Most riders will experience GI distress when trying to consume more than about 300 (a lot of published advice puts that around 280) Calories per hour while riding. That includes all calories including what comes from gels, solid food, energy drinks, etc. Try to push 500-600 Calories per hour during a race and I'd expect to find you at roadside with a really bad gut cramp before long.

So yeah, it's a deficit spending game we cannot replenish all the calories we burn as we burn them unless we back the power way, way down and in racing that pretty much means we've been dropped. Getting fitter does allow us to burn more fats as the race intensity relative to our FTP becomes lower (we burn almost 100% sugars as our intensity approaches FTP but most folks burn a higher percentage of fats as intensity drops relative to FTP). So getting fit helps a lot but we will always be burning through our limited glycogen and glucose stores in long races and cannot fuel at high enough rates to avoid digging into our glycogen and glucose stores.

Download this free doc if you want more info: http://www.hammernutrition.com/products/endurance-athlete-s-guide.fh.html

It's pretty good advice in general but they're in the business of selling supplements so read it with a critical eye. I certainly don't buy into all their sales pitches for supplementation but their discussions of fueling, hydration, and electrolyte maintenance are pretty good and all point to the idea that it's a deficit spending game and attempts to avoid those deficits are almost always counter productive.

-Dave

Awesome! Thanks again for all the help. Are you still racing USAC events?

 

[daveryanwyoming]

Originally Posted by wbkski .

Awesome! Thanks again for all the help. Are you still racing USAC events?

Sure, I've raced about a dozen times this season so far. Major setback at the moment with six weeks of international business travel in three different trips since April. Living on the road is no way to stay race fit but business is very good so racing is on hold at the moment.

-Dave

 

[An old Guy]

Originally Posted by wbkski .

Dave... I used your formula and it comes out to 3762 for that ride. That's about 578 calories per hour. Should I be consuming that, or more, in calories using GU or something similar during the ride?

I am going to butt in here and make a comment.

When riders around here do group rides of 100-200 miles 600+cal/hr, everyone eats a lot - much more than 300cal/hr.

I currently drink 880cals every 2 hours. When I was young, I would drink 44 pop (lot of sugar calories) and a 250cal candy bar every 2 hours - during a 5 minute stop.

I have been away from sugar too long and cannot handle that much sugar. I suspect you can train your body to handle anything.

Most of this 300cal limit comes from looking at people who are not trained to eat food and ride.