This site says: https://www.cptips.com/nutrtn.htm

"In order to avoid the "bonk" (the shift to fat metabolism with an accompanying deterioration in performance), supplemental carbohydrates need to be eaten during the early stages of rides that will be more than longer than 1 to 2 hours in length to supplement (and thus spare) the body's own glycogen stores."

I thought if I was doing aerobic exercise that I started burning fat after 20 min.

They are saying 2 hours after glycogen stores are depleted that I will start metabolizing fat, thus zapping me of riding energy, but If Im metabolizing fat I don't care how much energy I have to ride. Again though I thought I was metabolizing fat after 20 min of aerobic exercise.

Actually, the trick is to keep enough glucose in the system to keep mental function. Lipid/Protein metabolism provides 5X the energy of Glucose metabolism, but you need the glucose to keep your brain working due to the fact that the CNS can ONLY metabolise Glucose. The Liver used to produce reserves to glucose from Glycogen stored in the muscles and liver. To understand this better you need to read up on the Citric Acid Cycle( Or Kreb Cycle) for ATP production in cellular metabolism as well as the Lipid and Protein Cycles (Both anaerobic). Usually, when you are in a hard bonk, it's more a deterioration of mental function that keeps you from being able to function.....you can't think straight! You aren't able to take in enough calories, by the way to overcome the deficit in calories in vs calories burned on a long ride, and that's why pre-ride loading and post ride recovery is so important if you are doing something longer than a couple of hrs.

Fat and glycogen metabolism don't turn on or off at some time or level of exercise intensity. Instead, the ratio of fat to glycogen metabolism changes with exercise intensity.

This article explains the percentages:

https://www.brianmac.demon.co.uk/esource.htm

Thanks for the 2 informative responses.

Yes, well put. I would just add the the ratio also changes with exercise duration as well. I wouldn't focus on trying to burn a higher ratio of fat. Just burn calories. Yes, if you do slow exercise you will burn a higher ratio of fat to glycogen, but at the higher intensities you burn more overall calories and end up burning at least the same amount of fat. Mix up your riding to include both the long slow burn and the high intensity rides.

Oh, yes you will! If you are truly out of your glycogen stores(aka Bonked), you cannot ride. Period. You won't feel a little tired. You will be more exhausted and incapable than ever.

The general rule of thumb is to eat 250 -300 calories an hour for the long rides. You will likely burn around 700-1000 calories an hour. If you don't eat, you will bonk. If you bonk, your ride will get cut short. You will burn fewer calories. You may even have catabolized muscle and will take days to recover before you can ride again = less burned calories for the whole week!

I have been riding to lose weight for quite a while. I sympathize with you. I know it goes against every fiber of your being to eat while exercising. We want to lose fast and eating seems counterintuitive. I assure you, you have to eat for those long rides. You will still lose plenty of weight anyway, and not eating will just backfire.

If you read the earlier part of the article, "bonk" is mentioned a few times in ways that make it clearer.

This paragraph is specific to what happens when you bonk - what it means is that when you bonk, you go from a mixture of carb and fat metabolism (with the ratio dependent on how hard you are exercising and how well trained you are) to all-fat metabolism, because you have no carbs left.

Many cyclists describe bonking as the "outrunning of availible glucose" due to prolonged high intensity efforts. Athletes often have the will-power and capacity to work themselves into medical emrgencies due to very low blood glucose levels.

Decreasing intensity, switching to fatty acids or or other energy substrates to supply glucose is known as "getting tired," or running out of gas. It is not bonking.....

This thread starter, as well most of the public generally confuse any change of blood sugar at all as a bonk. Trained athletes seldom suffer, nor react to slight dips and highs in blood sugar, they just keep working.

The term "bonk" gets thrown around very liberally and means different things to different people. I've heard people talk about "muscle bonk" and "brain bonk" separately, referring to two different types of glycogen depletion.

For the endurance athlete, there are two different types of glycogen depletion:

1. Muscle depletion. The glycogen stores in your muscles run out, so they must rely on fuel carried into the muscle from the bloodstream. The athlete still feels fine, but it is very difficult to work beyond about 75% of max. Legs feel like they "just won't go". I get this all the time on long training rides. In marathons, this depletion is called "hitting the wall".

2. Liver depletion. This is the more serious one, where blood sugar levels starts to drop rapidly. I had this only once on the bike, and it came upon me quickly. I felt disoriented, had trouble forming sentences, and simply couldn't continue on the bike. I had forgotten to eat. I sat on a rock, took in some carbs in liquid form, and felt fine after 15 minutes.

wow Im learing alot here, thanks guys

Yeah this one truely sucks. I had that happen to me twice. I couldn't stay on the bike. Barely made it to the curb in a shade and just layed there. Thankfully first time I was with a group and someone had a gelpack. Second time I was close to home, so I was able to make it back... barely.
I started to cary a gel pack in my saddle bag, in case this happens again.

A better way to understand the relationship of how much "fat" is metaboilzed due to exercise would be to look at what the "total amount exercise" you perform divided by the "total amount of time" you take.
It's really kind of simple: Effort/Total time = Fat to sugars ratio

To expend 1000 calories in an hour would require almost all the calories to come from glycogen and glucose. To expend 1000 calories in 4 hours, you would still need to use glucose and glycogen for "life support", but much of the muscle energy could come from "fat". Perhaps as much as 60-65%.

Now here's the part to "re-confuse" you - if you look back the first sample, where the guy uses "1000 calories per hour guy" while exercising, but no fat, well guess what - he uses "fats" during the next 3 hours to maintain and rebuild glycogen and glucose stores. So on balance, as long as you exercise, you'll either burn fats while you exercise or after you exercise.

More confusion, you do all these "energy" conversions almost all the time, unless you have a disease or are dead.

I hope you read the articles suggested, and reread them if you have to, this is good stuff to know.
https://www.brianmac.demon.co.uk/fatburn.htm

https://menshealth.about.com/cs/fitness/a/walking.htm

In this link he says the same thing. In fact he says the high aerobic intensity for 15 min burned off 9 times more fat than the moderate intensity for 45 min!

Whats the point of even riding long distance then?

Im thinking of cutting down my 46 mile ride to a 16 mile and do it much faster. What do you think about this?

How long were you riding before you hit liver depletion?

Well it depends on what you're trying to accomplish. If the goal is solely to burn calories, then that strategy might work. However, I think over the long haul, you're going to have trouble mentally and physically maintaining enough intensity on 16 mile rides to get the same calorie burn as on a 46 mile ride.

If the goal is to be fit, and a stronger cyclist, you need to mix longer endurance rides, with shorter more intense efforts.

Even if the goal is just losing weight, I think over the long run mixing longer slower efforts, with shorter more intens efforts will lead to more weight loss, than just all short super intense efforts because its more sustainable. Besides longer rides can be fun.

Nobody can answer that question but you. This is a biking forum so presumably you like biking. Yet, if you're considering cutting a 46 mile ride to a 16 miles, that suggests you don't actually like biking all that much. It seems you're focusing only on fat. Perhaps you should be posting in a forum more pointed to your goals? You have people here who ride 15 hours a week, or 100-200 miles at a pop now and again. If you're only goal is to lose weight, that's not really resonant with many of the people who post here.

Um this is the training and nutrition part of the forum!

I don't know the last time you went biking 47 miles in the woods but its tough and it takes almost 4 hours. Obviously I like to bike since I spend 1/4 of my waking hours 3 times a week on the bike. But the reason I am doing it is to lose weight. I don't see how biking and weight loss don't go hand in hand. Your post makes me think that you don't feel Im worthy of being in a bike forum becuase Im cycling to lose weight. So Ill post a picture of me 10 years ago and you can tell me If Im in the right forum.

Yes it's Training & Nutrition, but it's a sub forum of BikeForums. One presumes that the training/nutrition is meant to be used to make biking better, or more enjoyable. If you want to lose weight to enjoy biking more, then by all means this is the right place.

I assumed your 46 mile comment was in reference to road riding, not offroad riding. Incidentally, I rode 47.5 miles offroad this past weekend and yes, I agree it's tough. And it took me longer than 4 hours to do. Personally I don't see biking and weight loss as hand in hand at all. I like to bike to bike. And it does help maintain my weight or help me lose weight, depending on the time of year. But the reason I bike is because I like it, not to lose weight. Weight loss is something I'll be focusing more on in the winter and spring. But it will be done with an eye on being a more efficient biker.

My apologies that you interpreted my post as you did. I was merely pointing out that it seems biking is a secondary item to the weight loss, which suggested to me that perhaps you should be in the biking sub-forum of a weight loss website. Semantics, perhaps, but it was the idea of dropping time on the bike which led me to blah blah blah. Not important. Enjoy.

I see, Im biking to lose weight, instead of losing weight to enjoy biking more. Blashphemous! I should be drawn and quartered! LOL.

Fun for me was jumping over garbage cans. Fun was NOT riding to the city to do the jumping if you get my drift. I don't get much thrill from plain MTN riding unless my life is in danger. I own a motorcycle (crotch rocket) so you can see Im a adrenalin junkie.

Its nice that you ride to 'ride'. Im riding with a goal in mind so I will achieve said goal faster, you are not and thus more 'free' and perhaps more in the moment. They both are 50/50 value wise, and neither is 'better' than the other, both have their merrit.

Of course you are probably still in shape, I think thats the big difference. One of these days you won't and you will start biking to get fit and you will look back at this thread and laugh.

Also my 47 mile ride is on a flat gravel, sand, and dirt RR path with no big rocks or anything thats why I can keep a 12 mph average.

That's a bit extreme, but beaten with a club would be fine.

I still have a goal, or goals, but they're bike related. And I agree, neither is better or worse than the other at all. They just are. It's good to have goals.

I used to bike for the sole purpose of losing weight, which is why I bought my road bike 7/8 years ago. I bought a new mountain bike this year for biking. So hopefully I'll keep my previous self in mind and not let myself go too much. But yes, I may be biking to lose weight in the future as well.

Based on my experience: Performance Road Biking at your limit (and above) does not go well with weight reduction. Perhaps I am biased because of my age of 64. I will bike nearly 10,000 miles this year and have lost no weight but my speed average is up.

That's about right because when you're riding right at your LT, you're burning pretty much all carbs and no fat at all. The other problem is that you can't ride for 3-4 hours at a time at this pace anyway.

Back off to about 10-15% below and you'll burn close to the maximum amount of about 200-250 cal/hr from fat. The more fit you are, the higher the amount of fat burned at any given pace. Also riding at a slower pace will allow you to go for 3-4 hours at time, it's the hours after the 1st one that really burns off the fat.

I agree with you. Biking and weight loss works great for me. I was racing as a teenager, then continued just riding road bikes, then as life got more intense I stopped. Got fatter, out of shape and tried occasionally do something about it including running and dieting. Nothing worked. Got a serious lower back injury that had me in a wheel chair. I was forced to have surgery last year which went fine. I could not run anymore and started to ride a road bike again at my age (49). I have easily lost 25 pounds since I started to bike 6 months ago and I enjoy every ride I do too. I am again fast enough that I have even passed some younger fit guys . If I do it for the joy of being able to ride again or the fun does not matter, it is all good. Maybe someone gets some inspiration out of it and get back into the saddle. That's why I am posting this.

This is consistent with what a lot of literature says. They suggest losing weight in the "offseason" and the foundation/endurance perdiod, when you're riding LSD rides. When you're ramping up your efforts, it's not necessarily about burning carbs instead of fat. The idea that you burn no fat at high efforts is a myth. The problem is that in order to maintain/improve performance, you need to consume a lot of calories, which runs counter to any weight loss ideas. Your pre/during/post ride meals need to be substantial. You can't get back from a 50 mile pace ride and eat a rice cake and expect to be able to do it again anytime soon.

I seem to recall it was about 6 hours into a 12-hour MTB ride, which started at 4000 feet elevation and topped out around 14,000'. I bonked at arount 13,000' and never summited; attempting the summit would have meant descending in the dark.

This is interesting, but I wonder about even lower MHR percentages, like 55-65%. I did a 20mi hike a few days ago up in Point Reyes, and carried the Edge 305 with HR strap just to get the data in with the rest in TC. Just about the entire time (7h) was spent at 55-65%, only occasionally getting up to 70 and slightly above on the steeper grades (like going up over Mt Wittenberg). The amazing thing is how closely grade and HR% follow, they're virtually identical, until the last two miles or so (when I was starting to feel severe muscle fatigue) where they diverged a bit (HR up). Energy consumption was probably in the 5000 kcal range, but since HR was so low I went out on a limb and guessed I was going mostly on subcutaneous fat. I ate one clif bar all day on the trail and was just fine. A little diluted cytomax to keep salts in check and blood sugar up just in case it turned out to be a hot day (it was nice and cool). So the fat % must have been really big, at least 70% or I would have bonked or started feeling symptoms. I did carry enough food that I could simply take a 3-hour food break and siesta before taking the shortest route back, so it wasn't as reckless at it may seem.

It's a different activity for sure, but I'm still really curious what low-HR exercise is fueled by. I did have the worst case of DOMS I've had in many years BTW, man I could barely walk yesterday. The Edge estimated 6900 kcal; yeah right, even though hiking is more demanding than bicycling at the same speed, and more demanding downhill than on flat ground, that's still way the heck off!

I can't seem to find anything on this anywhere, most of the hiking resource still live in a world where you need to replete every calorie (instead of viewing subcutaneous fat as a fuel tank that can be repleted much later), that lactic acid buildup causes DOMS, etc.