Negatives of fasted training?

Hi all,

What are the negatives to fasted training?

As a father to an 8 month old I’m finding the best time to train is at 5am to balance work and family commitments however I’m aware that this is pretty much all fasted. I’m wondering what negative/positive body adaptations I’m going to making as a result.

I’m training for an century event next May and I’m following a custom base plan which incorporates running as a bit of cross training and because I like running too. As I get closer to the event I’ll start to follow a build/specialty phase with less running.

Main negative is if/when it starts to impact the quality of your workouts, particularly longer or more intense ones that are burning a lot of calories.

Training at 5am doesn’t have to mean training fasted though. You don’t burn that many calories while you’re asleep, so if you have a good meal with plenty of carbs the night before you’re still going to have decent glycogen stores when you wake up. You can start topping those up as soon as you wake up and then fuel during the workout as well. I think best strategy is to fuel the more intense workouts with carbs. Then if you want the benefits of fasted rides do it on easier/recovery rides, have a low carb meal the night before, and maybe just a black coffee before the workout. Other thing that seems to be quite popular with the pros these days is starting an endurance ride fasted, and then starting to take on carbs maybe 60-90 minutes into it so that you finish strongly and recover well for the next session.


My experience is that there isn’t any below about 40 minutes. From 40 to 70 minutes you can still meet the targets but with a raised perception of effort. After about 70 minutes you can’t push through it and power actually falls if there are hard intervals.


This is a really personal thing IMO. Most of the research suggests that the benefits of fasted training are achieved by shorter, lower-intensity spins- anything beyond that and you’re potentially limiting your output.
However, as the above posters have said, your diet for the other 24 hours of the day makes an impact, and it is also something you can adjust to over time. Really all you can do is give it a shot and see if it works for you.
Personally I see a pretty big performance decrease for anything above tempo, but I find a high-carb meal the night before and a gel/banana when I’m warming up helps mitigate most of that without adding any additional time to my morning.

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I’ve had a lot of positive experiences with fasted training. Muscles seem to recover more quickly. I stop getting hunger pains for the most part while riding. There is certainly benefit to getting your body to learn to use ketones for energy.

That said, I’ve also had my fair share of bonking. As some of the others have said, it’s mainly about duration. I’ve been able to go up to 2 or so hours on fasted rides at sweet spot to FTP without issue. If I try to push much longer than that or if I am riding above threshold for any period of time, I notice the lack of energy.

You aren’t going to do any lasting damage with fasted rides for sure. It’s more a matter of if you will bonk during the rides and not be able to hit your goals. Best thing to do is play around with it and see how you feel.

One thing to watch out for is making sure that throughout the day at some point you are consuming enough protein for muscle recovery.

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I think the big thing is to make sure your muscles are not glycogen depleted going into the morning workouts. A lot of what I have read combines ‘fasted’ training with glycogen depleted training and most of the downsides (from my perspective) have to do with training in a depleted state. There are some advantages to Z2 in a depleted state (fat adaption), but I don’t think it is recommended for Z3 or above and would likely make hitting higher power targets difficult.

I also work out around the same time and aim to be on the bike 15min after getting up. Normally I don’t bother eating anything if it is under 90min regardless of intensity (recovery up to VO2 work.) If its over 90min or a real high calorie workout I might have a granola bar or two or a sports drink while I’m riding. And I need my morning coffee so I’m drinking that while I ride. If I’m going to be on the bike for 3+ hours I generally try to quickly have some oatmeal before I start and let it digest while I’m riding and then fuel on the bike.

VO2 workouts at 5am suck! But I don’t think its from fueling, I think it is just hard to go from waking up to that kind of intense interval in 15-20min. Be kind to yourself as you adapt to the new schedule, but believe that you can do it, the RPE might just be higher than you are used to. And maybe extend the warmup a couple of minutes if you aren’t quite ready.


I generally draw the line at IF .7 and 45 minutes. If a workout exceeds 45 minutes and has an IF > .7 I make sure my fuel is topped off and I am supplying carbs during the workout. I"ve done fasted rides up 1.5 - 2 hours (or so) w/o carb supllementing. I too often work out in the morning on waking as long as I have sufficient time (rouse myself, get kitted up, do the workout, get breakfast, cleanup, get to work). Those lower IF workouts I have had zero problems completing in a fasted state. I will keep something nearby in the event I feel a bonk coming on, but I’ve yet to tap into those.

When my general approach calls for non-fasted, I typically eat a banana or some other sweetish thing to get me started. Give it a bit of time and then supplement via some sort of carbs in my drink. Again, I keep gels and other food at hand.

I am also looking during breakie, lunch, and dinner to replace those workout calories, but it can be tough to do that.

As long as you discover and work within some comfortable limits I would say there are no negatives. In fact, it is often reported that there are definite benefits.


Main impact is that you’re hungry. :stuck_out_tongue:


As a relatively new father I think training in the morning is a great way to optimize your time and indeed, my preferred time to train.

All shorter sessions < 1:15 should be fine. When I am doing harder stuff, I do make sure to eat gels and have some bottles with carbs. Even if you could do harder workouts at other times, being consistent is always better than having to squeeze in your workouts at odd times of the day.

Edit: Make sure to eat enough the day before. So I’d avoid keto-type meals the day before a very long and hard VO2max session, for example.


@cartsman ftw

One potental negative could be the increase in cortisol. This may cause accelerated protein break down and hamper desired training adapations. Furthermore, bone health metabolism may be affected. It may be prudent to get adequate protein in post-workout. And get some carbs in pre-workout for more strenuous sessions. This keeps cortisol levels lower.

I don’t see the problem with getting in some easily available carbs pre-workout at 5 am. Probably not necessary for easy sessions.


I train at 5am as well due to family.

Usually for up to 1hr I use just water (regardless of intensty), however if I wake and feel more tired / lethargic than usual I may take a gel. Anything over 1hr is with a Bananna (or 2)

I try and eat something straight away afterwards, otherwise by mid morning I’m on the hunt for donughts :grinning:

I firmly believe in not being too rigid during training, there will be a time in a race or ride when you have run out of food, it’s good to know you can cope and don’t need your 45min gel.

I also train straight out of bed.
Coffee, hop on the bike, and then it depends on length and intensity.
Anything Z2 I do fasted, up to 2h.
Tempo/SS i tend to fast the first 30 mins then get a bottle of SIS. If it’s longer than an hour, I’ll also eat something on the bike.
Anything above that, I start eating after the warmup and eat consistently, every 15 mins or so, to ensure that I fuel at least 50% of the kcals expanded.
Recovery shake afterwards, and then my normal breakie after the shower.
I aim to have consumed the kcals of the exercise plus at least half of what I’d normally have for breakfast by then.

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We are in ‘exactly’ the same boat my good man. Same age child and same time wake up to train! My alarm goes off at 4:50am and I scoot off downstairs to make a black coffee. Down it whilst watching the TdF 10 minute highlight reel and am on the bike around about 5:15-5:20am ready for one of the General Build HV sessions that range from 1 - 2 hours in length.

I know, just like you, that I have to sacrifice sleep and a sit down brekkie before the sessions, in order to actually get out and train before everyone else wakes up (assuming the little one decides to stay in bed past 6:30am! Fingers crossed every morning on that one!) That way I am off the bike by 7:30am by the latest and I can help with the morning routine, organizing breakfast for everyone and also getting myself to a somewhat presentable level ready for work that day. It is a busy time for us early morning trainers! I am sure that we would have plenty of similar stories to swap. Notably, the shocking night sleep before hand if the little one decides to push out some teeth and having to still jump on the bike after what seems like only having a 45 minute nap the night before. Bulk coffee before I train on those mornings!

One thing I noticed when I started to do fasted training was that I was a mess - not only physically, but also emotionally and mentally. It just ruined me and I went into this huge sugar lull which ruined my mood and motivation until around lunch. Not good times.

I then started to try to eat breakfast before hand but ended up feeling sick on the bike because of the intensity of the sessions. No way am I waking up earlier either just to have some toast or cereal. So I had a dilemma - feel sh–t or feel sh–t.

So, I tweaked things a little - for me. If I know I am doing a bigger session, then the night before I will have pasta for dinner. That works sensationally well for me! Feel super strong in the whole session. If it is a lighter session then we will stick with veggies and lean meat and fish etc. So basically I tweaked my major meal the night before to suite whatever session I was doing. There is also now a placebo effect in there as well where I know I can train hard if I have had a heavy carb meal the night before. A mental win and a mental game.

The other thing I added was on the super hard sessions like the Anaerobic ones, I have a small banana or an apple or something light like that with my black coffee about 20-30 minutes before the session just to give me that little pep up. If it isn’t one of those sessions then I generally ride fasted off black coffee.

So what’s happened? I have dropped 12 kg since last Christmas riding fasted and with coffee before hand (some really good articles out there about how Team Sky used fasted riding with caffeine for their riders in pre-season) and I feel that my body has adapted. I feel like I am more efficient at using alternate fuel sources now during sessions and can ride solidly without the need for fueling up before hand. When I am outdoor riding over Summer, which seems like a distant memory given the COVID lockdown and exercise restrictions that we are currently in, the obvious preference is to have breakfast before hand but I do take solace in knowing that I have conditioned my body to be able to ride fasted.

Strangely enough, I also don’t really have sugar cravings throughout the day any more either. Before I started training fasted at 5am I would always crave chocolate after lunch and very much after dinner - to the point where I would be annoyed if I didn’t have it. Now, not at all. Don’t have such intense cravings for high sugar processed foods at all. I guess that this is part of what’s happened with the adaptations in my body.

So there you go. From one Dad to another, after an initial struggle I have found some real benefits to it.


One thing discussed elsewhere on here is to simply eat more before going to bed, so you are not as hungry in the morning.

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That’s really great to read and hear your own experiences. I’ll have to search for that article. It would be interesting to know why they did that.

Thank you to all for your insight. I too feel the difference after having a carb tea the night before. I don’t drink tea of coffee so I never get the caffeine hit but I definitely need a little snack (banana etc) before I start.

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I eat before getting on the bike in the morning. I am up at 4am, on the bike around 4:30. I prepare overnight oats before bed, so I pretty much just shovel them in and out the door :slight_smile:

My morning rides are usually around 90mins, sometimes a little more. I definitely find the harder ones a bit easier if ive eaten, especially towards the tail end

Just another option if you stomach can handle it, I have no issues with it

I’m not sure a first-thing-in-the-morning ride needs to be a fasted ride. It’s just that you don’t have the luxury of 2-3 hours to digest breakfast.
I do all my workouts first thing in the morning, but eat 100-120g carbs per hour on the bike from the start. Before the ride I’ll have a couple of slices sourdough toast (regular bread doesn’t work for me pre-ride) with honey, a black coffee, then jump on the bike. Once on the bike I have a bottle of energy drink (using Tailwind or Maurtens at the moment) for each hour of workout (plus a little extra for the cooldown) and I top up with gels every 15-30 mins depending on how hard the workout is. I do all the drinking in the warmup and between intervals so I can focus on the intervals, then chug between them.
Straight after the ride, if I’ve not had enough carbs to cover the energy used in the the workout I’ll have more toast, but then always a second breakfast of yogurt/nuts/granola/berries to get some protein and fat.
Pre-ride solid food may not work for everyone, in which case perhaps just take a gel just before you get on the bike. Or maybe it’s something you can train your body to get used to.
I’m not saying there’s not a benefit to fasted rides too - but rather I’m suggesting that morning workouts don’t force you into making every workout a fasted one.

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I tend to do lazy mountain every morning almost, in a fasted state. So far I’ve only noticed benefits, hardly any drawbacks since it’s very low intensity.