Physiology behind the bonk?



kmavm

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May 16, 2005
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"Bonk," like "overtrain," is one of those cycling buzzwords that gets thrown around a little too lightly. Let me tell you, friends, I just had a real, no-fooling, bonk. I contemplated lying down by the side of the road, even though it was raining. I had to remind myself where I was, and how to get back home. Zonel 1 required a maximal mental effort. The borders of my vision were a little bit blurry. Etc.

I realize this is roughly a consequence of poor diet. I'm trying to lose weight while training intensely at the same time, which is admittedly dicey. But what, exactly, happened here? Is "bonk" a cutesy word for "hypoglycemia"? If I waited long enough, would my body's anti-hypoglycemia measures kick in, even without feeding? Muscle glycogen depletion? Liver glycogen depletion? Both? Neither? Related to this question, how best to recover from a bad bonk?

There's also a lot of folklore about how bad for you bonking is. Obviously, it can't be *good*. But, I hear claims that, e.g., it takes two weeks to recover from a bonk, and that seems impossible for something that's basically caused by a nutritional status. I've also seen Ric, whose opinion I take very seriously in such matters, say that there are long-term metabolic consequences to bonking; I believe he said it could predispose one to diabetes and/or insulin resistance. I'm curious about this, and any other long-term (or serious short-term) health risks I might be running by trying to train in a somewhat glycogen-deprived state.
 

WarrenG

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kmavm said:
"Bonk," like "overtrain," is one of those cycling buzzwords that gets thrown around a little too lightly. Let me tell you, friends, I just had a real, no-fooling, bonk. I contemplated lying down by the side of the road, even though it was raining. I had to remind myself where I was, and how to get back home. Zonel 1 required a maximal mental effort. The borders of my vision were a little bit blurry. Etc.

I realize this is roughly a consequence of poor diet. I'm trying to lose weight while training intensely at the same time, which is admittedly dicey. But what, exactly, happened here? Is "bonk" a cutesy word for "hypoglycemia"? If I waited long enough, would my body's anti-hypoglycemia measures kick in, even without feeding? Muscle glycogen depletion? Liver glycogen depletion? Both? Neither? Related to this question, how best to recover from a bad bonk?

Low levels of sugar in the brain lead to the brain issues and low levels of blood sugar and glycogen are what reduce your exercise performance to a death march crawl back to your kitchen or nearest food supply.

IME, I have recovered within 2 days from bonking. In most cases the lack of glycogen and other sugars has reduced my ability to ride/ski harder than otherwise so once the blood sugar levels and glycogen levels have been restored recovery is quick.

I commented to my coach that the day after bonking I feel surprisingly good and I asked if maybe there was some hormonal response that facilitates a fast recovery. He said this is possible and that many riders that he has coached report the same surprising performance the next day. I mentioned that based on how the brain loses some function during bonking that maybe I killed off a few brain cells with each episode of bonking. He replied that he has worked with some pros who were able to be good bike racers in spite of an apparent lack of brain cells.
 

dhk

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You've got it, at least to my level of understanding. Bonk is caused simply by glycogen depletion of the muscles, blood and liver stores. Bet your blood sugar level when you had to get off the bike to rest was way under 100. Not sure of the units, maybe mg/dl?, but somewhere down around 50, you pass out. The brain gets priority of course, your working leg muscles give up first. Appears your body's self-preservation mechanism worked very well.

If you rest long enough, even without eating, your liver will restore the blood sugar by converting energy stored in your fat cells. If that ever runs out, protein in the muscles is used. Believe the ability to convert fat to glycogen is limited to something around 250-300 kcals/hour, which corresponds to a power out level on the bike of about 75 watts max.

If you're on a restricted-carb diet, recommend only easy riding so that you stay in the fat-burning aerobic zone. Trying to do high intensity without adequate carbs is no fun, as you found out. It doesn't seem productive either.

Note, above is just my explanation of the bonk. I'll gladly defer to an expert here.
 

Doctor Morbius

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Mar 15, 2004
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kmavm said:
... I realize this is roughly a consequence of poor diet. I'm trying to lose weight while training intensely at the same time, which is admittedly dicey. ...
Very difficult to do at the same time. Perhaps you could consume extra carbs before, during and after your intense workouts and cut back on low intensity and non-riding days. Zig-Zag your diet based on your workload. It may take a little longer to lose the weight this way but your training will be better off.
 

kmavm

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Doctor Morbius said:
Very difficult to do at the same time. Perhaps you could consume extra carbs before, during and after your intense workouts and cut back on low intensity and non-riding days. Zig-Zag your diet based on your workload. It may take a little longer to lose the weight this way but your training will be better off.
Yeah, I've experimented with this sort of pattern, and have mostly been successful. It just let me down badly yesterday :(. It was weird, too: I did the 2x20s pretty well, with a stronger effort on the second rep, and was just casually spinning back to work. I felt a little twinge of the bonk coming on, but figured, eh, I'm 15 minutes out, what's the worst that could happen? And then, just spinning along in Z2, Ka-Boom. I felt like **** the rest of the day, too.

The thing I've changed lately has been shifting my source for carbs from fruit to low-GI grains. My elaborate theory being that fructose can only replenish liver glycogen, while glucose (which is all that starch metabolizes to) can be taken up by the muscles as well. To accomplish weight loss while still being able to do some intense training, I figure the "ideal" state would be to have topped off muscle glycogen, but depleted liver glycogen. The low liver glycogen (and consequently low blood sugar) would keep adipose tissue pumping out FFAs and keep the liver in gluconeogenesis mode, while the muscles would hopefully have enough glycogen packed away to tolerate some vigorous training. It seems like this is the opposite of what fructose accomplishes (packing liver glycogen, but not getting any glucose into the bloodstream, and hence into the muscles), so I've moved over to grains.

Seems like something's wrong with my theory, or I just need to slow down a tad. Probably a bit of both.
 

benkoostra

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I certainly don't have a scientific knowledge of this stuff, but my personal, subjective experience is that weight lose and training for performance improvement are not really compatible. IF you really want to lose weight, you have to bonk, in that you must burn off your glycogen stores and start consuming fat. If you go hard while you still have sugar and then bonk it hurts a lot, but if you don't push, the transition is much easier.
The good part is that as you lose weight, you will get stronger and faster, just form having worked on your base.
 

mises

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May 27, 2005
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dhk said:
You've got it, at least to my level of understanding. Bonk is caused simply by glycogen depletion of the muscles, blood and liver stores. Bet your blood sugar level when you had to get off the bike to rest was way under 100. Not sure of the units, maybe mg/dl?, but somewhere down around 50, you pass out. The brain gets priority of course, your working leg muscles give up first. Appears your body's self-preservation mechanism worked very well.

If you rest long enough, even without eating, your liver will restore the blood sugar by converting energy stored in your fat cells. If that ever runs out, protein in the muscles is used. Believe the ability to convert fat to glycogen is limited to something around 250-300 kcals/hour, which corresponds to a power out level on the bike of about 75 watts max.

If you're on a restricted-carb diet, recommend only easy riding so that you stay in the fat-burning aerobic zone. Trying to do high intensity without adequate carbs is no fun, as you found out. It doesn't seem productive either.

Note, above is just my explanation of the bonk. I'll gladly defer to an expert here.
Typically you won't pass out until you are around 30 mg/dl or less. I have been close to 20 without losing it. In my experience the bonk commences between 60-70 mg/dl.

There is probably some supercompensation in the replenishment of muscle and liver glycogen which could account for the following good day as well as the cascade of hormones released when your blood sugar gets too low.

Once bonked you are essentially in a catabolic state and will consume muscle tissue to keep things going. That is the only long term effect I can think of, other than getting hurt riding around with severe brain fuzz.
 

benkoostra

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mises said:
Typically you won't pass out until you are around 30 mg/dl or less. I have been close to 20 without losing it. In my experience the bonk commences between 60-70 mg/dl.

There is probably some supercompensation in the replenishment of muscle and liver glycogen which could account for the following good day as well as the cascade of hormones released when your blood sugar gets too low.

Once bonked you are essentially in a catabolic state and will consume muscle tissue to keep things going. That is the only long term effect I can think of, other than getting hurt riding around with severe brain fuzz.
Why would lack of sugar cause your body to consume muscle instead of fat?
 

kmavm

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benkoostra said:
Why would lack of sugar cause your body to consume muscle instead of fat?
Because maintaining blood sugar is acutely important to survival, while maintaining muscle mass is more of a long-term benefit. A tight range of blood sugar is important to your CNS functioning, so your liver keeps the blood sugar level high by hook or by crook. "Gluconeogenesis" is the relevant term, and it happens by disassembling all sorts of organic molecules, including proteins and fats:

http://en.wikipedia.org/wiki/Gluconeogenesis
 

mises

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benkoostra said:
Why would lack of sugar cause your body to consume muscle instead of fat?
The way I remember things is glucose can be converted to fatty acids but the reverse is not true. To reestablish normal blood glucose protein is used, and continuing to ride in a glycogen/glucose depleted state can produce significant catabolism.

There are also more tissues that need fuel in your body than just muscles and the brain in particular in unable to use the same fatty acid shenanigans to fuel itself (at around 20% of resting metabolic requirements it's a lot of overhead) and in the absence of circulating or stored glucose protein is the only source for fuel.

Get rid of your brain and it would be no problem to [slowly] ride around bonked (and based on studies in flies your reproductive success would improve too :) ).
 

EndorphinJ

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Apr 8, 2006
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Are you diabetic? There is no way you have been close to having a blood sugar of 20 w/o losing it. Unless of course you are diabetic or hypogleciamc. I am type 1 diabtic and am in danger of low blood sugars during exercise. Clif bars, gu etc. benefit athletes but are necessary for me! You can work yourself too hard but not cause low blood sugars.


mises said:
Typically you won't pass out until you are around 30 mg/dl or less. I have been close to 20 without losing it. In my experience the bonk commences between 60-70 mg/dl.

There is probably some supercompensation in the replenishment of muscle and liver glycogen which could account for the following good day as well as the cascade of hormones released when your blood sugar gets too low.

Once bonked you are essentially in a catabolic state and will consume muscle tissue to keep things going. That is the only long term effect I can think of, other than getting hurt riding around with severe brain fuzz.