Fasted training, or training while practicing an Intermittent Fasting (IF) intervention will help your body improve its ability to oxidise (burn) fat. Fat oxidation (fat burning) is NOT the same as fat loss. Fat loss is influenced by your energy balance, accounting for what you eat compared to what you have burned over the longer term, also know as a a calorie deficit.
Fat oxidation is not the same as fat loss. There are more effective ways to lose fat.
Fat oxidation is how your body uses fat as a fuel for exercise. Fat is an energy source and the body can and will use for energy when the intensity is low and the food consumed around exercise assists this. Improving your ability to use fat as fuel have twofold benefits for the body:
Firstly, metabolic flexibility is the technical term for how well your body can use carbohydrates and fats as fuel, without the over reliance on one or the other. This is something we want to achieve in our daily lives as this encourages variety in our diet and a more optimal range of carbohydrate and fats in what we eat (not too much of one over the other). If we chronically eat a very high carbohydrate diet, it is likely that our body will become less efficient at using fat as fuel, and visa-versa.
Metabolic flexibility = How well you use carbohydrate and fat as fuels. You are flexible in your metabolism!
Secondly, with endurance exercise where the athlete is conducting long term lower intensity exercise, such as an ultra marathon, a fastest known time or multi day event, fat adaptation can help maintain exercise for longer duration than carbohydrates, due to the larger quantity of stored fat over stored carbohydrates.
Fat adaptation can be improved through three separate avenues.
If your habitual diet is higher in fat and lower in carbohydrates (not devoid of), your body will naturally improve its fat oxidising efficiency as that is the main fuel source being consumed.
Endurance exercise alone can improve fat oxidation in the body.
Fasted training can, should the intensity be low enough, encourage fat adaptation through the lack of available carbohydrates (from a fastest state). The body therefore breaks down body fat for fuel.
A client recently asked me, "is it okay to go on a long-ish run or a 40min HIIT workout in a fasted state?"
Short answer: Yes.
Long answer: Yes, just consider the points below.
Endurance exercise improves fat oxidation if intensity is low enough. Usually the standard weekend long training session is there to help you increase your recorded distance, so you are naturally taking it a little easier on the pace to push a new distance, or perhaps your are tackling a similar distance but focusing on a lower heart rate, or a lower rate of perceived exertion (RPE, aka, how hard it felt) . This is usually heart rate zone 2, or at around <145 beats per minute (depending on individuality and training), or the pace in which you can easily hold a conversation. If the intensity rises above this, it is highly likely that your body will begin to use its limited glycogen stores. This is the store of carbohydrates in the muscle and liver and accounts for approximately 2000kcals.
Endurance training alone will help increase your ability to use fat as fuel.
As glycogen depletes, there is a limited capacity for energy without refuelling with more carbohydrates. I commonly see people trying to 'burn fat' while training fasted and doing high intensity training, when in actual fact, they are training while using their glycogen stores and nothing else as fuel. As much as this is fine, it is limited in its time capacity. Equally, you will need to replenish these stores through your daily diet, because doing this on a chronic basis will lead to over-training, under-nutrition and a host of bodily complications. from the gradual depletion of glycogen without refuelling the tank.
If you burn lots of carbohydrate in a more intense training session, then eat carbohydrate to replenish them.
Your body can break down protein to create carbohydrate for fuel through a process called gluconeogenesis, which can be achieved through eating a much higher protein diet, or by pushing your body to the point of there being no choice but to deaminate (break down) muscle protein. This can be maintained through a much higher protein intake, but is certainly not optimal for performance (this is an example of the body's extraordinary ability to adapt, which some take as a sign of the perfect solution).
In a fasted state, deamination has no choice but to break down muscle protein after longer and higher intensities. This is an in-extremis function with detrimental effects, it is clear that we don't want to be breaking down our muscle mass for energy requirements.
If you go too hard for too long in a fasted state, you could breakdown muscle protein to fuel training.
However, that is not to say that its not possible to do high intensity training in a fasted state if your glycogen stores are replenished and the exercise is less than about 45 minutes. Ultimately, the decision here lies on your own preference of whether you want to eat before a workout or not (for example, first thing in the morning). Temper this with the knowledge that fasted high intensity training will not make you burn body fat, nor will it aid in fat oxidation, but you will be training on stored glycogen and there is an associated risk with that if adequate refuelling does not take place.
Fasted training is a tool. You only need to use the tool for the right job. Don't put a picture on the wall with a chisel.
An overnight fast decreases the contents of your digestive system and does not impact your muscle glycogen stores, only liver glycogen is likely depleted .
It is important to note here that if your chosen training is to improve performance, then carbohydrates will need to be consumed prior to exercise, which is why you won't see athletes heading to the Olympics on an empty stomach.
Performance demands carbohydrates, equally, exercise lasting longer that 90-120 minutes in a fasted state has been shown to decrease performance . A systematic review of meta analysis looking into the difference in performance between groups of fed and fasted participants showed 'pre-exercise feeding bolsters prolonged aerobic performance, while seminal evidence highlights potential beneficial metabolic adaptations that fasted exercise may induce in peripheral tissues' . Which translates to eating before training improves performance, but there are positive adaptations to utilising fat as fuel in some fasted aerobic exercise.
Performance is fuelled with carbohydrate. But you can use fasted training to get better at burning fat as fuel, IF YOU NEED TO.
In the case of endurance exercise, if your average morning session is an easy pace run of low intensity for less than 90 minutes, then training in a fasted state is likely to illicit greater fat oxidation and improve your metabolic flexibility. This is a benefit in the case of ultra endurance or low and slow efforts. This can also be achieved through eating a higher fat pre-exercise meal which naturally limits carbohydrate intake .
Carbohydrate is the preferred energy source of the body, when you eat carbohydrates, the body automatically puts a higher preference on carbohydrate as fuel and increases the rate at which it is burned. This is true no matter the intensity if you consume higher amounts of carbohydrate. 
So with this long answer the verdict is relatively simple:
If you WANT to train fasted, then train fasted.
If you don't feel like eating before training (time or preference etc), then train fasted.
Ask yourself what the training benefit is first, if you are looking for performance, then it is best to eat prior to training. If your goal is an easy session and less than 80-90 minutes, then by all means use that session to increase your metabolic adaptation and train fasted, but remember that this can also be done by eating a small meal that is higher in fat.
Are you doing this for fat loss? If so remember the longer term calorie deficit is the first step. Fasted training does not lead to fat loss.
Recover with a meal of ample protein and some carbohydrate after (more if the intensity was high).
Fat adaptation through eating fat seems like an easy win, just eat a pre-exercise meal high in fat and away you go. The issue here is the caloric density of fat. At 9 kcals per gram, it is over double the density of carbohydrates and protein, this therefore needs to be taken into account when deciding to consume fat prior to exercise. The larger the meal the more calorie dense food consumed and therefore the longer you will need to wait to aid in digestion. Fat is slower than carbohydrate to digest, but a smaller portion can offset this and ensure that you get some fat consumed prior to your longer training session while reducing the risk of digestive discomfort. Leave at least an hour after eating before commencing exercise (some people can 'eat and run', and this comes down to self discovery and experimentation to find what works for you).
If you eat fat before exercise, keep the portion small.
It should be noted here that this is not the optimal solution for an endurance athlete competing or aiming for higher intensity, hence why you don't find World champion Iron-man triathletes on a high fat diet or in a fasted state at the start line. However, for those longer duration and lower intensities, this is a solution to ensure adequate fuelling. There is a relatively niche requirement for this outside of preference, so consider your actual requirement when deciding this intervention.
One area of training that is recommended to incorporate into the longer term training program is that of train low, recover low and sleep low. These are three separate tools to help increase fat adaptation. The train low theory does not suggest training in a low calorie state, but rather in a low carbohydrate availability state. This can be achieved by consuming more fat in your habitual diet or pre-exercise (see above), or depleting your glycogen levels the day before your train low session. For example, this could be a high intensity or lactate threshold interval session that depletes glycogen, where you don't ingest carbohydrates after, and the next morning is a scheduled long and slow endurance session, but fed on higher fat and moderate protein before exercise.
Recover low and sleep low can be grouped together as a similar system. In the example above, the high intensity session was glycogen depleting, recovering with adequate protein and increased fats but limited carbohydrate has been shown to up-regulate metabolic gene expression, increasing your adaption to training and fat oxidation [7}. Sleeping low consists of sleeping in a low carbohydrate state, with depleted glycogen, which increases fat oxidation during the overnight fast. This can be followed by light intensity endurance or recovery training the next day having ingested higher fat and/or protein pre-exercise.
It is tempting to merge both fasted training with a low carbohydrate protocol, but this encourages an increased risk of negative training impacts through recovery, over training or fatigue. Again, this situation demands the question: What is the training outcome? Is this required?
Don't mix training with low carbohydrate availability with fasted training. There is no benefit here and risk of malnutrition from over-training-under-fuelling. Don't run the car fuel tank in the red all the time, you might breakdown.
An example of how this could be applied is similar to the above scenario. In the case of an endurance athlete training for an ultra marathon of 50km, for example, there may be place in the training program for high intensity workouts in the week that are completed while fuelled on ample carbohydrate (for performance). One higher intensity, glycogen depleting session could be completed one evening, with recovery consisting of high protein and higher fat content before bed - a recover and sleep low protocol. The next morning could consist of a low protein and fat combined pre-workout meal before a 2hr long run at a low intensity, followed by a carbohydrate and protein rich meal or series of meals for the remainder of the day and the maintenance of habitual diet.
It is likely to show benefit of fat adaption with this athlete for their ultra marathon by completing this at most once a week so not to impact the training adaptation of the remainder of the program. This is practicing how the body will adapt to potential long periods between feed stations on the ultra marathon, and the ability to keep going in a long and slow state while fuelled on fat.
"Is it okay to go on a long-ish run or a 40min HIIT workout in a fasted state?" The answer here is certainly a yes. But fasted training or training to increase fat adaptation must be met with the question of: What is the training outcome?
Careful planning and consideration of such training and nutrition interventions can be incorporated into the weekly program. For the majority of recreational runners, there is no harm in fasted training if you choose to do so over a few training sessions per week. Chronic repetition of this intervention is unnecessary for performance and metabolic adaption improvement.
As with a lot of nutritional interventions, if it works for you, then it works. Trial and error, experimentation and exploration are key areas to understanding what you CAN achieve. But must always be met with the questioning of what is NECESSARY.
Ask yourself WHY, test it for yourself, see how you react.
As always, when in doubt, have an honest and open conversation with an evidence based nutritionist, get the knowledge to back up the experience and find what works best for YOU.
Stay safe, stay informed.
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  Jeukendrup AE. Periodized Nutrition for Athletes. Sports Med. 2017 Mar;47(Suppl 1):51-63. doi: 10.1007/s40279-017-0694-2. PMID: 28332115; PMCID: PMC5371625.
 Van Proeyen K, Szlufcik K, Nielens H, Ramaekers M, Hespel P. Beneficial metabolic adaptations due to endurance exercise training in the fasted state. J Appl Physiol (1985). 2011 Jan;110(1):236-45. doi: 10.1152/japplphysiol.00907.2010. Epub 2010 Nov 4. PMID: 21051570; PMCID: PMC3253005.
 Aird TP, Davies RW, Carson BP. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 2018 May;28(5):1476-1493. doi: 10.1111/sms.13054. Epub 2018 Feb 23. PMID: 29315892.
 [7} Hearris MA, Hammond KM, Fell JM, Morton JP. Regulation of Muscle Glycogen Metabolism during Exercise: Implications for Endurance Performance and Training Adaptations. Nutrients. 2018 Mar 2;10(3):298. doi: 10.3390/nu10030298. PMID: 29498691; PMCID: PMC5872716.