Is fasted cardio or fed cardio better for fat loss and why or is there a difference?
Colin: If all we are concerned about is burning fat and nothing else then fasted cardio is the winner. You will definitely be tapping into your fat stores much quicker fasted. Of course there is the risk of using muscle for fuel too, but as long as the cardio isn’t high intensity it shouldn’t be a problem. You can always take some BCAA’s before your workout if muscle loss is of concern. I do not recommend high intensity fasted cardio nor do I recommend fasted cardio for anyone trying to build muscle. However, since the question only asks if fasted cardio or fed cardio is better for fat loss, my answer is fasted cardio. If the question were which one should be done, the answer would be much more complex and things such as goals and body types would play a key role in that answer.
JC: I have to disagree. First and foremost, if you don’t eat, you just won’t have the energy to train at full intensity. If I am not mistaken, research has shown there to be no difference between fat loss from fasted cardio and cardio after eating. It seems like flawed logic, when most of the people who subscribe to fasted cardio are the same people who think they will catabolize after three hours without a meal. If you cannot go three hours without a meal, how can you go all night and then workout?
Exercising fasted does not tap into fat stores. To burn fat you need to release the fatty acids through lipolysis and then oxidize them for energy. I may have to dig through my notes to provide you a citing, but it is my understanding that performing cardio fasted increases lipolysis but it does not increase oxidation. That means more fat is released to be burned, but it won’t be. The thermogenic and energy effects of eating prior to cardio, or any workout for that matter, are what will keep you burning fat.
I have performed fast cardio and it is miserable. Much like with steady state cardio and HIIT, I can get more for less when I eat prior to cardio.
Matt: I’ve seen evidence that suggests it doesn’t matter and evidence that supports both but I can’t get my head around the fact that if you are in a fasted state after a full night of sleep and you are in a mild state of ketosis regardless of what your diet is, then it just stands to reason that fat will be that much more available for fuel. If muscle loss is a concern then simply stick to low intensity cardio. Get out of bed and go for a brisk walk for 45 minutes. It can be that simple.
This is what I was getting at with my answer to which is more effective for fat loss, HIIT or steady state: the advantage that fasted cardio has over HIIT in that it is easy. It may not burn as much as an intense HIIT session but if your plan allows for enough time to shed the fat, then this is the way to proceed. The energy expenditure is less, the ratio of fat being used is higher and mentally it’s actually relaxing to do.
The disadvantage that fed HIIT has is that it is intense. That’s why it has to be done in the fed state. It’s so taxing that it uses up a lot of energy. That’s also what makes it so effective of course.
HIIT on its own has been shown to be great at preserving muscle but during dieting when there is a finite amount of energy available, if you are to expend a portion of that on fat loss then it stands to reason that you won’t have as much in the tank to train with. This can indirectly lead to muscle loss.
I don’t expect what I just said to be popular with many but it’s something that is purely logical. There are no studies showing a correlation between the two but I’d love to see the results if there ever are studies performed.
JC: I don’t know about that. Think of it as a camp fire that has been left to dwindle throughout the night. Sure, the coals will be hot in the morning, but it won’t cook your eggs. You’re going to need to add fuel to that fire to really get things done.
Matt: If you’re fasted you’ll be in a mild state of ketosis, meaning your body is already using ketones (incomplete fats) for fuel. By doing low intensity cardio in this state you will accelerate the usage of ketones, meaning you’ll use more fat for fuel. The process of fat burning in ketosis is not very efficient, which is why it’s so effective.
I’m pulling this from a distant memory about the process of fat utilization but what you are talking about is the need for oxoloacetate (a byproduct of carbohydrate metabolism) in order to utilize fat for fuel. This is in fact correct, except in the case of fasted, ketone fueled, fat utilization.
JC: Ketones are a byproduct of fat oxidation. Those trying to lose fat by going on keto diet do so by dropping carbs and upping protein and fat. So yes, your body will produce ketones for you to use as energy. If you are in ketosis your body will convert amino acids to glucose for energy. This will either come from food, stores, or muscle. Personally, I want it to come from food. In conjunction with HIIT cardio, you stand to lose even more fat since HIIT not only increased oxygen transport (and fat oxidation), but has more muscle sparring properties.
Matt: Your body will only convert amino acids to energy if the exercise is intense enough that the ketones won’t provide enough fuel. That’s why I suggested only low intensity steady state when doing fasted cardio. To do anything more intense than that will absolutely rob your body’s protein stores for energy. We agree on that.
I also agree on principle regarding HIIT. I am just not sold as it being the ideal method to lose fat when all factors are considered.
Colin: I can’t disagree with JC that energy levels can be more of a problem with fasted cardio than with fed, of course that’s also going to vary from individual to individual. Personally I don’t seem to see that large of a drop in energy when fasted, but there is a small drop.
Dara: In my experience the research is very mixed on this and again it depends on your goals and where you are starting from and possibly your genetics. I won’t add to what has already been said regarding ketosis and fat use in the fasted state versus the fat burning benefits of HIIT because we’ve pretty much covered it. I will add that recent research has suggested that training in the fasted state improves your ability to use fat as fuel. From what I have read, it’s not the actual fat burned during the fasted cardio but the longer term metabolic adaptations that occur from training fasted that allow your body to burn off fat. So I would not recommend doing fasted cardio as your only form of cardio, but doing it a couple of times a week could result in positive metabolic changes that will allow you to get leaner through all of your other types of training.
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