Metabolic Pathways


I have a question regarding the relationship between glycogen-optimization and fat-burning optimization over 18+ week training plan. From my understanding, how you fuel your training over an 18-week plan will determine your best metabolic pathways during an event. Also, I understand that training each of these pathways are two sides to the same coin, meaning you can only accomplish one at a time. First of all, is my understanding correct? If so, then how would you balance the optimization of these two pathways over a 18-week plan for a gran fondo event (9K+ climbing)? My guess would be to work on fat-burning optimization during base-training, then shift to glycogen 6-8 weeks before the event.

The goal is to have a strong aerobic engine (rely less on glycogen) for 80% of the terrain while also being able to process simply sugars during the VO2 efforts during the big climbs. I understand, specificity in training 6-8 weeks before the event is key, so this is the base of my above assumption.


I am afraid that your understanding is incorrect. Muscle can oxidize all three macronutrients, but “prefers” fat when ATP turnover is low and carbohydrate when it is high. In between, you will oxidize a mixture of the two (plus a little protein). Endurance training attenuates this progressive shift, allowing you to exercise at a higher intensity after training without fatiguing early due to carbohydrate depletion (low to moderate intensities, longer durations) or an imbalance between ATP production and utilization (high intensity, short duration). We do tend to burn what we eat, but performance improvements due to training are greatest when you simply provide the fuel for the work performed. This means eating a high carbohydrate diet, even though it seems counterintuitive to the fact that training increases fat oxidation (by slowing carbohydrate utilization).

TL,DR: Train lots, train hard, eat carbs, rinse and repeat. Most of all, don’t get confused by all the mumbo-jumbo and pseudoscience you can find on the web, pushed by coaches, ketoheads etc.


First of all, I’m not a keto fan. And… I realized both are used in different proportions based on your metabolism. That proportion is determined by your anaerobic threshold. My question is how to optimize fueling during training to promote each… think periodized nutrient. For example, if over a 12-week training program you fuel a Z2 ride with mostly simple sugars, your body will not be training to optimize fat-burning. This is the point of fasteded rides (not keytonsis)… to maximize your body to burn fat at sub anaerobic threshold. I believe the answer is to fuel rides based on your expected average heart reate compared to your specific AeT. So, more complex carbs (real food) and less simple sugars for endurance and tempo rides; and more simple sugars for interval work.

Also, I’m not simply interested in my 20-minute power (i.e. FTP) but also my ability to hold 90% of my FTP for 90 minutes… deisel engine.

Thanks. Appreciate reply and it help to clarify my thinking.

Train hard. Eat carbs. There are no shortcuts.

ETA. 90% for 90 minutes should be a cakewalk (provided you eat enough cake, anyway).


Gotta agree here. Don’t major in the minors. Its volume first #1. Then working hard #2. Then about everything else. For me that everything else is about lowering rpe. So I can do more 1 and 2. Carbs lower rpe so I can do more 1 and 2. So carbs are good:) all the time


Think OBNYD nailed it, although I would add a couple of points: 1) eat decent carbs (whole grains, fruit and veg etc.) - especially when bumping around at endurance pace; and 2) don’t overfuel (a little goes a long way)/don’t overcompensate post ride.

Less then 2 hours of zone 2 endurance, provided that you have had a good carb meal before hand and you shouldn’t have to fuel during the ride. There is not a real benefit to a completely fasted ride.

But it’s not about simply volume. It’s about the right volume and nutrition. Too much volume and you overtrain or cause injury. Undertrain and you leave stakes on the table. Eat too much and you put on weight, eat too little and you don’t provide enough fuel for your training. Why does everyone want to oversimplify? If it was that easy you wouldn’t see so many people get it wrong.

And for RPE I don’t get it. Not very quantitative. You can determine your training stress quantitatively very easily without having to guess through perception.

DenntyD - Thanks for your useful feedback. I usually take in 300 - 400 grams of carbs per day during light works and 400+ on days leading into big-ride days (3+ hours) and also fuel those rides with 50-60 grams per hour.

I found this as a useful resource:

You use less glycogen for fuel on your long rides by going at a lower % of your FTP. So you can either go slower or you can increase your FTP. The strategy for increasing FTP does not care about what you will use it for. You need lots of time around FTP and that requires lots of carbs.

A higher FTP than another person doesn’t mean you’re faster than that other person. It comes down to the demands of the race. I’m not a crit racer, mountain biker or cross rider. I ride really demanding fondos. Sustained power (steady-state) is more important to me. You can’t fuel a 5+ hour ride strictly on carbs.

I put in 600-750 TSS a week in my base phase and (49 year old). So, I don’t think volume or hard work is the issue.

You are misunderstanding my point. The way to use less glycogen during your activity is to go at a lower percentage of your FTP. So you can either go slower or you can increase your FTP.


You come here for advice and people give it to you, but then you disagree with them and say they are wrong


Your 5 hour power is very closely related to your FTP.

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You won’t burn strictly carbs in a 5 hour event. Intensity Factor will be in the 0.7s, maybe 0.8s if you’re a beast at sustained power. Pacing at Z2/Z3 with a few short Z4 efforts scattered in there.

If you’re able to fuel the event at 90+g/hour carb, there’s little to no chance you’ll bonk. You simply can’t ride hard enough over 5 hours.

That amount of climbing doesn’t really change your pacing and fueling strategy.

Not sure about this strategy for pacing a fondo. Do what you need to do to stay on a wheel, but if you’re a heavier rider, you can sag climb.

Extended VO2 efforts scattered throughout a 5 hour event will probably slow you down overall.

I’m 64 kg and only working towards PRs… overall time. Yes… 9K+ climbing with some 1 hour plus climbs. Think DK, WBR, Cookie Fondo, Levis. Also, shooting for a double this year.

I’ve been working really hard on building sustained power and currently at 3.5 w/k (88% of FTP) for 90 minutes. My current IF is ~.72 for a hard 5-hour ride and would love to get to ~.8.

Based on other research I’ve done I’m going to focus on fueling my evening midweek 90-120 minutes sessions without in-ride carbs (I already consume 350 grams/day) and sprinkle in a few fastened 90 minute SST rides in the AM. Hopefully, I can extend my sustained power to 120 minutes or more and increase to 90% of FTP. A .8 IF is a good target.

I’m going to go on record here and predict it will make absolutely no difference.


Keep in mind that whatever you’re gaining in metabolic efficiency might be offset by the loss of potential fitness gains.

In other words, unless you’ve actually hit your overall fitness ceiling, you’re better off raising your fitness and riding at a lower IF.

Efficiency gradually improves by itself anyway when fitness becomes stagnant, whereas fitness rarely improves without a concerted effort (except for off the couch riders).


I’m just calling BS when I hear it. Recommendations to eat cake and saying your 5 your power is closely related to your FTP. If you don’t know what you are talking about, don’t share misinformation. It’s just more bro-science.