Fat oxidation in short time trials

Ok, this is a question that has been on my mind for some time. In shorter steady state aerobic race efforts (time trials), how important is fat oxidation really?

Let’s suppose we’re talking about time trials lasting less than 30mins, which just happens to be the usual format where I live. Looking at the graph below, utilization of fat seems to cut off at about 85-90% of vo2max.


As shorter time trials are probably raced at an IF of about 1.05 or so, wouldn’t that mean fat utilization is slim to none? FTP as a percentage of vo2max seems to be in the 78-85% range (https://blog.trainerroad.com/how-vo2-max-work-makes-you-fast-the-science-behind-it-all/)

The reason I’m asking is because whenever zone 2 training is mentioned, this seems to be the main selling point. If, however, fat oxidation is not very important for the outcome of the types of races I focus on, wouldn’t that to a certain degree steer my base training towards tempo and sweet spot that is way more reliant on glycogen?

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this is because people are obsessed with diet. All they can think of is fat oxidation

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Hardly at all.

However that doesn’t mean that zone 2 training has no benefits for time trialling and indeed general fitness. I do a fair amount of tempo when my time is crunched, but if I have 4 hours on a Sunday and I did hard intervals the day before, a long zone 2 ride will probably be on the menu.

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Not really, at least not in the cycling community. It’s more that as you get into longer distance events, particularly things like Ironman, fuelling becomes a major limiter. So being able to get more energy from fat can have a big impact on performance. Which is a reason to do those longer rides, maybe some fasted riding, even if your body composition is already optimal.

If your body composition is decent and your races are all short then I don’t think there is such a strong argument for those kind of rides. In fact I’d say that crits and <1 hour TTs are great events to focus on for those who have limited training time, as you can get great results off relatively low volume and lots of quality.

High volume Z2 does more than just FO. Mainly mitochondria density which is where most energy is produced. So it will pay off later when you do intensity.


Long rides in Z2 are considered the single most potent stimulus for increases in muscle capillarization. Muscle capillarization is central for the delivery of oxygen and nutrients to the exercising muscle, and thus, capillarization is vital for exercise capacity. A high muscle capillary density means a large muscle-to-blood exchange surface area, short oxygen diffusion distance, and high red blood cell mean transit time. Furthermore, removal of waste products from the working muscle is enhanced.

In an age where something like Keto is the avantgarde, there is this strong focus/obsession on fat oxidation.

And the imporance of Z2 for any aerobic distance does not stop with muscle capillarization. A fast engine has many parts, some of them benefit from Z2 training. Track cyclists, XC skiier, all with fairly short competition durations do a ton of Z2.


According to this table from Overton, high intensity exercise is the most potent stimulus for increasing capillarization?

Great discussion ongoing, thanks for the replies. What I should perhaps have clarified is that I’m well aware that there are other benefits to Z2 than just increased capacity for FO. Maybe a better question would be: how much of the reaped benefit from Z2 training is due to increased FO, as opposed to the other physiological changes that take place?

@sryke have you ever done some digging into the training of pursuiters? I also remember reading somewhere that they do indeed train lots in Z2, but just how does their zone distribution compare to road cyclists? I would find this quite interesting.

Is it possible to specifically increase the capacity to oxide fat? How would you do that? What muscular adaptations are involved? What limits the rate of fat oxidation anyway? Why can’t we exercise at 100% of VO2max without oxidizing carbohydrate?

I’ve totally had this question too and am hoping…but far from certain…that long zone 2 rides will ultimately improve aerobic capacity. Look, you can’t do sweet spot or higher intensity all of the time, right? You’d just burn out in a few months. But you need a certain amount of volume…say 10 hr/wk…so you have 2-3 days of intensity and fill the rest in with zone 2. That’s my current understanding fwiw.

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Can you do high intensity every day? For weeks? Months? Consistently without breaking down? Stimulus = frequency x intensity x duration.

Couple of reasons for that but one main reason is that fat can’t get into the muscles quick enough. The transport is limited and not as efficient as with carbs. However, if you’re trained you’ll have plenty of intra-muscular fat. The increase of this store is one of the key adpations of training. And this is why you see a higher fat oxidation at higher intensities in trained individuals

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