Net calorie deficit is still a net calorie deficit whether it comes from fat stores or glycogen.
But for the record if you see my prior posts in this thread, I do my Z2 rides unfueled. I’m trying to get my carb utilization as high / efficient as possible for a race coming up, but at the same time I see the benefit of being able to rely on fat stores and oxidize fat better. Outside of “training” and biking I’d be much more low carb in general…
The calories have to come from somewhere. If you burn more than you take in, it’s not magic, that is a net deficit. Of course that all goes out the window if you blow things up afterwards and eat a bunch more off the bike.
Think about it this way: Let’s say the calories you burn in my Z2 workout 100% come from fat stores and fat oxidation, then the carbs you take in aren’t used and ultimately get used to replenish a portion of those fat stores (but not all of them). If the carbs aren’t burned or used to replenish glycogen, then they get converted to fat.
My point was you only need 200 kCal from carbohydrates. Which you can do with 50g an hour. Then no deficit. Unless your body fat is something like 5% you’ve no need to worry about fat deficit, as most of us have it to excess.
I’m not disagreeing you can change where the energy comes from. But, math is math, and it’s still a deficit. If 100% of it comes from fat - you still lose fat even taking in those carbs.
The other angle is what I said before: if you’re actively trying to get your body better at carb utilization and how many carbs you can safely take in without GI distress. That’s where I am now. My goal is to not have my body rebel at 120g/hr over longer marathon events (Next one, about 9 weeks out so I’m getting ready to add in long Z2 rides in the heat each week where I’m taking in that level)
Then again, I’m trying to have my cake and eat it too which is why I do plenty of easy Z2 workouts fasted and generally don’t eat first thing when I get up unless I feel I need it for a workout.
Extending on what I just wrote above: It’s still a net loss, and what about if you want your body getting better at carb intake and utilization as opposed to fat oxidation, and your primary goal is fueling the workout appropriately and not fat loss? In the off-season I’ll probably change my tune and try to drop 5-10 lbs, but that’s not where I am personally right now.
Sounds like you’re actually recommending what I’ve said I’m doing, so hey, I’ve got that going for me 
Just so anyone doesn’t lose site of what I wrote earlier that I do:
Short Z2 rides inside - Fasted or No Additional Fueling
Long Z2 ride outside in the heat, Any sweet spot, threshold, VO2, Indoor or Outdoor, etc. - 120g / hr.
But, in my situation I’m also not sweating carb-fueling a short Z2 ride inside either as long as net KCal in is <= net KCal burned. Especially so if I’ve had a lot of other workouts stacked on back to back days or a double on a day.
50g per hour when even if 70kg and 10% body fat , you have 7 kg to use. It’d take you over 140 hours to burn through it, and that’s if you’re at the lean end. Losing some fat when working out is neither here nor there.
You’re totally right.
My point was that while you may not “fail” the workout, it can very easily become a miserable experience due to hypoglycemia, and require greater psychological and physiological effort than necessary, to get the same training stimulus.
You’re right though. At 90 minutes z2, absolute failure is unlikely even with zero fuel.
I’ve not reviewed the mouth rinse literature in a few years because I’ve been unable to find a scenario where spitting out the fuel is more optimal than just swallowing & digesting it.
IIRC, the mouth rinse literature I read was usually investigating the 30-60min workout duration window. I don’t recall seeing anything 90min long. Very fuzzy memory on this though.
Let me turn the question on it’s head: is there evidence that fueling Z2 rides with e. g. 80-100 g/h is detrimental in any way, especially in the long term?
Yes, frequent eating especially carbs literally rots your teeth…
There are studies showing that it significantly depresses your rate of fat oxidation. Thus making you more dependant on carbs even at low intensities
I brush my teeth and according to my dentist, I’m in the top 3 of all his patients when it comes to the state of my teeth. 
Dumb question: why should I care whether I burn carbs of fat during exercise?
Post exercise my body can replenish my glycogen from carbs or fat. If you burn 40 % fat and 60 % carbs during exercise, you don’t have to replenish fat and carbs in the same ratio. Your body can convert carbs into fat and vice versa.
If you want to perform well at ultra distance durations you want as much of the latter as you can. If you don’t ride ultra or not concerned with performance, it doesn’t matter.
True, if I ride ultra events, yeah, correct. Or if I wanted to do long rides in the saddle, some people surely prefer fueling with regular food rather than carbs. I get that. But for most disciplines of cycling, that seems like a worthwhile trade-off.
Anything but Ultra events, if you’re concerned about performance you want to be burning carbs, not fat. Even at 8-10 hour marathon distances you perform better by fueling with carbs and being able to utilize more.
I’m not even sure it’s the other way around for ultras.
(Edit, Metabolic flexibility definitely helps, but it goes both ways)
If you think you are ‘over fueling’, then you probably are. From my experience, most (many?) people DO ‘over fuel’. I’ve seen people in local spin classes (back before I got a smart trainer) snarfing gels for a 40 minute ride, and gulping ‘electrolyte’ concoctions. Post ride, I saw people chugging those Muscle Milk type protein drinks, and are probably wondering why they aren’t losing weight, or possibly don’t care.
I’m no expert, but often use just a banana on longish trainer rides. I have used gels for social (race) events on Zwift. If I feel bonky, I know I didn’t consume enough, but there are times when even I feel like I may have added too much fuel. I was just tested and my A1C is pretty low, safely away from the pre-diabetic stage, thankfully, but I do try to be very conservative on what and when I fuel. I’d say use supplements sparingly. Only for rides over an hour, or over a certain effort. If I swamp my body with sugars, am I training my body to use fat for energy, or to anticipate the reward of a large sugar bolus that exceeds the needed amount? Good question… One I’ve been thinking about myself. I have lots of gels and powders and don’t use them nearly as much as I didn’t use them before.
Fuel in, activity out.
Metabolic flexibility should be developed via training modifications, not diet modifications.
Any metabolic “flexibility” developed from the latter is lost ability to burn the other.
There is no convincing evidence that dietary modifications purported to enhance such flexibility are the things that enhance performance.
The folks that are making nuanced dietary mods to change the fuel they burn, are usually doing lots of reasonably intelligent things in their training and also optimizing every other area of their life for performance.
I hope there is someday evidence against what I’m claiming here. I am not holding my breath.
Some people are just lucky and never get cavities no matter how poor their diet or oral hygiene, probably in part due to genetics and or oral microbiome but most people aren’t so lucky (fortunately 
)
I personally see the negative there is I am watching every calorie I eat, and if I dont get it as sugar, thats better for me. I already eat SO MUCH sugar on the bike. Yes, like you I brush my teeth, so not too worried about that, but its more of the opportunity cost. Am I potentially shortchanging my body of helpful nutrients by getting too many UNNEEDED sugar calories?
Key word there being unneeded. I personally do not want to try to fuel with fats or proteins on my intense rides or workouts. If those metabolic systems contribute, great, but I’m plenty happy sucking down sugar water out of my hydration pack and having sticky hands and lips from gel packs.
I do brevets/audax of 300 or more km, i.e. ultras, and I’ve gradually worked my way up to consuming 85 g carbs/hour (counting everything I consume). Yes, there are plenty of people who just eat regular food and drink water. Everyone needs to work out what works for them. But I’ve noticed that I can still ride strongly after 30 h on the bike in a way that I just couldn’t when I wasn’t samming carbs. It’s also made a massive difference to my post-ride recovery.