Fasting refers to a prolonged period of time without ingesting any food. Whether or not you should train fasted is a difficult question to answer. It depends on a number of factors including what your fitness goals are and what your initial body composition is.
Scientific research
In theory, low glycogen levels due to being fasted cause the body to shift energy utilization away from carbohydrates, thereby allowing greater mobilisation of stored fat for fuel. A study published in Pubmed found that Overweight individuals after 8 weeks on an alternate day fasting schedule lost 2.4 kg in body weight as well as 1.3% in body fat. This suggests over a period of time that there will be increased weight loss while performing exercise in a fasted state.
In fact another study in Pubmed found that obese individuals when performing an alternate day fasting protocol lost 8kg in 8 weeks. (1)
However there is conflicting evidence from other studies when looking into fasting. A study published in the journal of Sports Nutrition found that subjects regardless of being fasted vs fed found no difference in fat mass by the end of the 4 week study in healthy individuals (2). This is also backed up by a number of other studies published in Pubmed when looking at healthy individuals being subjected to a fasting protocol of some degree.
The main correlation that can be seen here is that if the participants are initially overweight or obese then the fasting seems to help them lose weight, however if they are already fit and healthy then it has less effect.
There is also the issue that due to depleted glycogen levels in a fasted state sporting performance will be impaired. In fact a study in the National Library of Medicine showed that during periods of prolonged fasting both aerobic and anaerobic performance were affected in elite judo athletes (3).
So this brings up the question of whether you would want to sacrifice exercise performance for the chance to lose fat mass? Well ultimately it depends on your fitness goals.
Who is it best for?
In the case of a beginner to exercise their initial goals might be just to lose some weight, then training fasted may help them achieve their goal quicker, however for an athlete who is looking to improve performance then training fasted before each session would not be recommended.
Training fasted has been shown to improve muscular endurance and therefore it can be recommended before certain lower intensity sessions throughout a structured training program where the end goal is to perform a long duration event. Training fasted would not be recommended if the goal is to build muscular strength or power. Intakes of protein and carbohydrate prior to strength training have been shown to decrease muscle protein breakdown and increase muscle protein synthesis during and after training leading to increased muscle mass gains over time. Strength training performance is also hindered in a fasted state with the total volume of reps performed in a session significantly decreased in a fasted state.
Is it worth it?
Ultimately there needs to be more extensive research with bigger study sizes to get clearer evidence around this subject as there is a lot of conflicting research. However what is pretty clear is that there is increased fat oxidation in a fasted state which in theory should lead to increased weight loss over time. Therefore if your goal is to lose weight then I would suggest to give this method a try. The best way to implement it would be to wake up and do your exercise, while making sure to stay thoroughly hydrated. The best form of exercise to do when fasted would be lower intensity lower duration cardio. However if you want to improve sporting performance then I would be more cautious about trying it, as numerous studies show depleted glycogen levels lead to a drop in performance levels. Although it may have its place when correctly structured in an elite athletes training plan.
References:
Comentarios