Fasted Workouts

The studies and Coach Mac C’s podcast did make me think. My belief was that training while fasted would promote the energy-from-fat burning machine to allow me to go long (bike or run). I still believe it does for me personally, but was not aware that this comes with other trade-offs besides making those early season training sessions harder than they needed to be. My goal is to go longer faster so I’m thinking of trying something different and only do fasted rides on the easier days (cruising, recovery etc) which was another take-away from the podcast - if I understood it right.


Thanks for the link, added to my queue!

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@Philip this topic is certainly a hot button in recent years and as the thread suggests, there are many on both sides of the fence. I would like to share, not from opinion or citing a study, but from medical and physiological processes and my own experience of being a Type 1 Diabetic and former Pro Cyclist. I have the fortune of being married to a smart gal who went to medical school and practiced in Internal Medicine and Sports Nutrition in several sports and for years worked with World Tour and Pro Continental teams in the US, UK and individual
Pro cyclist.

I have seen several posts in the thread stating they have practiced fasted rides or workouts but do not know if it is sustainable or what the side effects might be. We always want training to be a positive on the body and our goals. To that end, we should know why we do what we do both in training and nutrition. When training fasted we want to gain something positive from the training but are we losing too much in the process? Below are the medical cause and effects of training on low or prolonged timed fueling. Take or leave, just remember to always train with a purpose and for a positive result.

What is Fasting?
Fasting is a vague term and I have seen it used to describe as little as 6 hours to as long as 48 hours. Usually referring to the absence of eating it can also be used to describe abstaining from a certain food group or amount of food. Most refer to Fasting as an 8 hour period of time. It takes the body 8 hours or more to breakdown carbohydrates and store liver glycogen. If you have carbohydrates for dinner at a reasonable hour, go to bed and wake up and do an hour trainer ride before eating breakfast, that is fasted in the traditional sense, but you have glycogen stores on board so you do not have the side effects you can incur if you deplete and overextend the body past the glycogen stores and enter glycolysis.

Side Effects of Fasted Training

  1. Short term weight loss:
    You may experience weight loss short term when doing fasted training. Over time is this weight loss sustainable? Weight loss should be initiated by reducing caloric intake or raising physical caloric burn. Practicing the withholding of foods or substances will see the body want to “hold” those foods when they are reintroduced. When training fasted, if the body does not have the Carbohydrates and Glycogen stores it needs it will sound the alarm and go into survival mode and begin actually holding onto fat and turning muscle into fuel. You may see some weight loss at a rapid pace due to the bodies cannibalizing of tissue but when you do reintroduce carbohydrates, sugars and fat the body will hold these as stores to make up the deficits and try to ensure survival. Usually rapid weight gain is experienced when foods are reintroduced unless physical activity and calorie consumption are drastically increased. Taking in large amounts of protein and fat during fasting or low carbohydrate periods can cause an increase of fat in the liver which can have a high mortality rate.

  2. Low Blood Sugars/ Glycogen dump:
    If we do not provide our body with Carbohydrates for fuel, it will revert to it’s glycogen stores. Even if you have glycogen stores for use, you can still encounter low blood sugar levels which will decrease brain and muscle function. Once you reach a low blood sugar level you have “bonked” and need to replenish glycogen stores and take carbohydrates on board to resume normal muscle and body function. In some form or another, bonking becomes a collapse of the entire system: body and form, and function.The liver will be depleted of it’s glycogen (which fuels the brain) and will experience a big glycogen dump (recruited from muscle tissue) during the bonking process which is highly toxic and can take days to clear. With decreased cognitive function your bike handling, decision making and even attentiveness are compromised. The body has pushed past using the glycogen available and has now started the process breaking down to produce the fuel it needs.Most people will reach a state of bonking at a ride of moderate intensity with no nutritional intake in 90 minutes. During the period of low blood sugar or bonking your body also does not have the ability to adequately clear lactic acid which is another hit to the struggling muscles.

Consider the muscle-glycogen bonk, where the brain works fine but the legs up and quit. Then there’s the blood-glucose bonk, where the legs work fine but the brain up and quits. The total bonk of dehydration, training shutdown, gastric problems, and nutrition stoppage where you just don’t feel like you can hold anything down. All are bad and not pleasant!

  1. Muscle Loss:
    When we train, we are stressing the body, which is what we usually want to grow stronger. Just as your car cannot run without petrol or oil, your body needs fuel to function. If we do not provide the needed fuel it will begin to pull fuel sources from itself. The first place our body will search for fuel is the muscles. Deep within our muscles are the building blocks called mitochondria which hold glycogen. Glycolysis is the first step in the process of our bodies search for fuel. Glycolysis happens within the folds of the mitochondria inside the muscle. For the body to break down and extract the glycogen from the muscle it breaks down the mitochondria and therefore, the muscle tissue. We are essentially cannibalizing muscle to function. Of course, this process is the direct opposite of what we are trying to accomplish during our training. Another by product of our breakdown of glycogen and muscle is that we will not have maximal protein channeling and subsequent mitochondria response needed for adaptation during our recovery phase.

  2. Loss of Bone Density:
    We went over the steps above for our bodies recruitment of fuel sources and we can also add another to the list- Bone Marrow and Density. When we train we need minerals for the muscles and bones to work in correct patterns. If we do not provide the needed minerals our body will again search for these nutrients where they are stored. The body will drain the bones of calcium to keep the mineral levels appropriate for correct heart rhythm and bone function. It is not uncommon for cyclists or athletes to have calcium seepage issues but this is exacerbated in those practicing fasting or keto based diets. Low calcium levels can also cause arrhythmia issues.

  3. Fatty Liver or Liver Damage:
    When we train fasted or with decreased or non existent fuel sources we are running on what our body has left or on reserve. Once these reserves have been exhausted the body begins the hunt for fuel including a very important filter- our liver, which functions and holds glycogen stores for the body and most importantly, the brain functionality. When these stores are depleted the liver function is compromised. We decrease the function of the liver which helps carry away waste, breakdown fat and provides protein for the creation of blood plasma. If we lower the function of the liver we raise our risk of “holding” fat in the body and the blood stream as cholesterol and lower blood plasma.

  4. Toxicity:
    Training while fasted can mean that we use all on board reserves and stores from our body. When the body has used all Glycogen stores and needs more fuel you enter Glycolysis and have a Glycogen “dump” where the body gets carbohydrates from the mitochondria in the muscle for fuel. The process of breaking down the mitochondria for fuel creates cellular waste products and combine with the lack of liver function create a perfect toxic storm which then flows throughout our blood system and leaves it’s toxic deposits in our kidneys and heart along the way. Not a very pretty picture but it does happen and it has lasting effects for days. You may feel sluggish and legs heavy for 3 days post glycolysis dump

  5. Electrolyte imbalances:
    If we are not providing fuel and our body uses the reserves it has to fuel your training eventually we will see an imbalance of electrolytes and vital minerals as the body has to continue to make up the deficits. Usually the main ones lacking are: Sodium, Calcium, Magnesium and Iron The first 3 can cause a range of cardiac arrhythmia issues and short circuit muscular response in training causing cramping. Even if you are taking in electrolyte mix with no carbohydrates or sugars if the body has to go searching for fuel or use it’s stores your electrolyte balance will be off as the glycolysis process creates acidity and dehydration due to the work done by the body.

Not a lot of “pros” listed for fasted training. Training can be very personal and individual, I get it. Everyone does not respond the same but how our bodies function on the basic level is common to all of us and how our body uses and gets fuel to function is common to every athlete. The big questions to ask are:

  1. What are the goals of my training?
  2. Will my fasting cause my body to have more damage than gains?
  3. Will my fasted training cause my body to retain fat?
  4. Am I creating the mindset that I need to “earn my food” based on how long I have trained? This is a hard one to shake once we associate a workout with how much we get or need to eat. This may also make us not feel as though we should eat during our recovery week since we are not training as much and if we do not fuel our recovery cycle enough the body will not go through the adaptation process properly.
  5. Is there another way I can loose weight separate from my training?
  6. What does my body need to be the strongest/fastest athlete I can be?

Just some food for thought!!!


Thanks for the summary @Coach.Simon.B

Thank you for sharing.

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This should be a blog post, if it isn’t already, would be easier to reference and share.
Thank you.


Even a wiki … we using wiki’s?

There was some great stuff around the number of days for the body to recover glycogen stores, three days to even start building new Stif in our muscles and so on in some other posts as well - felt wiki like as well !!

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:point_up: What coach Simon said

In my years of training I’ve never seen any benefit of fasted, I look to do the best I can each workout and for me, I need fuel to do that.

For what it’s worth, assuming I’m eating properly, I can do most workouts up to an hour on just water, over an hour I’d look for a gel or electrolytes to keep me topped up.

I suppose if you are at the real pointy end of races then I can understand the potential marginal gain you may get over a competitor but for the majority of us I’d guess that hitting each training session fueled and giving the best we can would be for more beneficial overall.


If your goal is fat adaptation (which has numerous benefits in ultra-endurance racing) then fasted workouts have a place in your plan.

Personally I seem to perform best in a fasted state and my fastest times up a short hill (4-8min efforts) are early in the morning with just a cup of coffee in me. Longer efforts, different story. The feeling of strength you get from “fueling” before a workout comes mostly from blood sugar levels. The energy for a 4-8min effort comes mostly from glycogen stored in your muscles, so it has to do more with what you ate the night before, than with breakfast. Once you’re fat adapted you will be less dependent on that sugar high feeling. (I do need the caffeine kick, that does make a difference for me)


fasted training upto 3 hours have had a really positive affect on my endurance and weight loss. Key thing is to adapt gradually starting with an hour and anything over an hour stay in Z2


Hi Philip,

Really interesting topic, all I can say is that I have tried fasted riding up to 2 hrs on a morning commute. I built up to the longer rides. Didn’t effect me at all, and I felt I got generally faster and lighter. All that said it’s very anecdotal, which I hate, and not a controlled study (n=1), and the evidence is kind if on the fence as far as i can tell.

My view, try it, if you feel ok then don’t worry, otherwise stop. Different people may respond differently to this, like many other forms of training.

My rides to work were fairly steady when i did this, if thet matters, but I did sometimes gun it home (not fasted).