Cycling, carbs, the bonk and ketones - in that order

[irc]

Interesting topic. On my last 1 month tour I lost 12 pounds in weight so was obviously burning fat much of the time. I never experienced any bonk symptoms. It was fully loaded and slow paced.

I have experienced the bonk before usually on a very long (for me) day ride or walk. Needed to get some carbs in and rest for 30 mins before going on. Though also bonked once or twice on previous tours last time was in 2009. No idea why I haven't bonked on more recent tours. It all seems a bit random.

[Gearoidmuar]

..slow paced. Burning fat.

[horizon]

This is an excerpt from a cycle touring company's website (my highlights):
https://www.ride25.com/cycling-blog/bonking-birds-bees/

How do you avoid a bonk?

In cycling, there is never a truer word than the saying ‘if you’re hungry, it’s too late’. The best way to avoid a bonk is to eat little and often during your bike ride and crucially make sure you eat before you feel hungry. This means ingesting about 100-250 calories of some form of high carbs every 30 minutes, even in the first hour of your ride. Whatever you choose to eat, the main thing is not to forget to. You may think you’d never forget to eat, but you might be surprised at how easy it is to forget on our cycling holidays when you’re coursing past beautiful sights. Some people like to set an alarm on their watch to remind them, but just be aware that the more you ride, the more energy you will use up and therefore the more you will need to replace. We always encourage our riders on our cycling holidays to make the most of the cake at designated stops and also take a handful of sweets (which our support crew always carry with them!) to pop in your pocket or saddlebag in case you need them. It takes practice to get into the habit of eating whilst cycling, but the consequences of forgetting to aren’t worth risking.

This AFAIK is the standard advice for cycling (and this is touring not racing) but could not be further from that suggested upthread.

[JohnI]

For 10y I've not had the bonk once. I'm on a lowcarb high fat diet for that period, keeping 38lb off.

I've been on a lowcarb diet for the last year, for general health reasons. I haven't been doing a massive mileage during that period (due to injury), but I've been surprised that my energy levels are higher than before and I've had no problem cycling for up to 2 hours or so. I've also lost 5kg and my FTP is the same as before so that's a win-win.

Certainly if I was doing longer rides I'd increase the carb intake, but probably only on the day of a ride. It does feel almost as if I've trained my body to need less carbs, or maybe I was just eating too much previously. It does seem quite individual though and I'd be wary of any general advice.

[LittleGreyCat]

There are two different aspects to this:

(1) You start out using glucose as fuel, then switch dynamically to using ketones - this can be difficult to impossible for some people. You can have more initial energy but less stamina. The switch over point can be harsh.

(2) You start and remain in ketosis - that is you have been eating very low carbohydrate so that your body has to produce ketones for your basic daily energy requirements. This IMHO is a better way to operate. You have less initial energy but more stamina. You don't have the pain of transition between energy modes.

AIUI, the body uses glucose for heavy-duty, anaerobic exercise - you tear off at speed, keep up a fast pace and charge up the hills. Obviously, you then have to re-charge quite soon.

Alternatively, you set off at a moderate pace, use some glucose to get you going and then transition slowly to ketones. A sprint up the odd hill might involve a burst of glucose but then you are back in steady mode. So really this transition depends not so much on your existing diet/regime but on your approach to the riding itself - you pass seamlessly through the bonk.

I'm wondering though whether you would actually have to forego the carbs on the ride itself in order to allow the body to go into ketosis and this I presume is the harsh transition you refer to.

The harsh transition is between using glucose for energy and ketones for energy.
The best way to avoid this is to not eat carbohydrates at all (or say, less than 20g per day) so that you start the ride with your body fuelled by fats, and also fully adapted to being fuelled by fats.
If you change to a keto way of eating it can take a couple of weeks of "keto flu" before you are fully adapted so flipping between carbs and keto on a single ride is an acquired art and can be unpleasant.

This is, I think, what some people aren't really appreciating.
I am not talking about some fuelling by glucose and some by ketones, but virtually all energy coming from ketones for all the ride (and every day as well).

[cotswolds]

Bonk recovery can be very quick ….., I wonder if it’s too quick for absorption of even simple carbohydrates. Could recovery be redistribution of carbohydrates from glycogen stores to muscles?

For personal medical reasons, I've had to find out more than I wanted to know about this and concluded this isn't well understood. (There may be more recent research I haven't seen.)

Some snippets of what I found:

Oral carbohydrate sensing

Studies on trained athletes holding a sports drink in the mouth for 5 – 10 seconds show performance benefits of 2 – 3 %

Believed to be unidentified carbohydrate sensors in the mouth (i.e. it doesn’t just work for sugars)

fMRI studies show an unidentified area of the brain lights up when carbohydrate is present in the mouth – that area is believed to communicate directly with muscles

Known as ‘carb rinsing’ amongst endurance athletes – when footballers, tennis players, etc, are seen spitting out liquid, it’s probably a sports drink.

Effect greatest when the body most carbohydrate depleted.

Reference
Oral carbohydrate sensing and exercise performance, 2010, Asker E Jeukendrup, Edward S Chambers

Carbohydrate in the mouth enhances activation of brain circuitry involved in motor performance and sensory perception
Clare E. Turner, Winston D. Byblow, Cathy M. Stinear, Nicholas Gant
Centre for Brain Research, University of Auckland
https://doi.org/10.1016/j.appet.2014.05.020

The presence of carbohydrate in the mouth appears to activate a novel energy signalling pathway capable of improving human performance.. This signalling phenomenon... has been shown to attenuate declines in motor function associated with fatigue.

Another reference
Carbohydrate in the mouth immediately facilitates motor output (Gant, Stinear, & Byblow, 2010)

[Jdsk]

Bonk recovery can be very quick ….., I wonder if it’s too quick for absorption of even simple carbohydrates. Could recovery be redistribution of carbohydrates from glycogen stores to muscles?

For personal medical reasons, I've had to find out more than I wanted to know about this and concluded this isn't well understood. (There may be more recent research I haven't seen.)
...

Totally agree.

And the studies on effects in the mouth are fascinating.

Jonathan

[Jdsk]

...
The bonk is caused by a rapid drop in blood glucose, not necessarily to hypoglycaemic levels.
...

I think that there's a lot more to it than that, and I don't understand why a falling blood glucose on its own would have these effects.

The technology for non-invasive measurement of blood glucose has recently improved enormously, so it's much easier to study...

Jonathan

[Jdsk]

...
Just one more thing I can’t remember; do the different muscle fibres have different preferences for metabolising sugars or fatty acid?

Are you thinking of the different types of skeletal muscle fibre, or the difference between skeletal, cardiac and smooth muscle?

Thanks

Jonathan

[531colin]

...
Just one more thing I can’t remember; do the different muscle fibres have different preferences for metabolising sugars or fatty acid?

Are you thinking of the different types of skeletal muscle fibre, or the difference between skeletal, cardiac and smooth muscle?

Thanks

Jonathan

Sorry, yes, I’m thinking about the different types of skeletal muscle fibres.
(I continue to be fascinated by the breadth of subjects we discuss here!)

[Gearoidmuar]

There's fat burning as well. It's not all glucose and ketones. Training with low carb ramps fat and ketones up.

[briansnail]

he more you will need to replace. We always encourage our riders on our cycling holidays to make the most of the cake at designated stops and also take a handful of sweets (which our support crew always carry with them!) to pop in your pocket or saddlebag in case you need them. It takes practice to get into the habit of eating whilst cycling, but the consequences of forgetting to aren’t worth risking.

Do not forget one essential.The water bottle as sometimes its water and not food that the body wants.In winter it should be warm as its hard to drink cold water in winter.
*****************************
I ride Brompton and Hetchins 531