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Fed or Fasted Cardio


Fed or Fasted ?

It’s a common discussion in the gym. On one hand you've got a ripped guy telling you he does fasted cardio every morning and that’s why he’s so lean. On the other hand you’ve got a shredded guy telling you you’ll only break down hard built muscle training fasted so he never trains without eating. The question is who is right?

Broadly speaking, it’s commonly thought that cardio, i.e. cardiovascular exercise like walking, running, rowing, swimming, or cycling is more likely to burn fat when performed fasted. Furthermore, it’s probably safe to say that most people aiming for fat loss have tried it at some point. The thought process behind fasted cardio is that with less carbs in the system, the body will favour body fat as an energy source. The real question then is whether this is actually true or whether it’s another one of those “bro-science” methods driven into mainstream training from competing bodybuilders and physique athletes.

Science (not bro-science, real science) of fat burning

There are three primary steps involved in fat loss:

  • 1. Mobilisation
  • 2. Transport
  • 3. Oxidation (burning)


Getting stored fat out of the fat cell. A process that involves enzymes and hormones like glucagon and adrenaline.


Transport refers to the transportation of fatty acids within the bloodstream, which can be the problematic step when someone has stubborn body fat.


Oxidation loosely means to ‘burn’ fatty acids from within tissues such as skeletal muscle, heart and liver to produce energy.

It’s better to be fat

For people already fairly lean (around 12-15% body fat for men and about 18-22% for women), the problem with fat loss usually lies in mobilisation and transport. The problem lying in reduced blood flow to troublesome areas like the lower abs and lower back for men and hips/thighs for women. The body also goes through some adaptations when it is very lean that can make getting fat out of the fat cells more difficult.

For the very obese (around 35% body fat or more for men and around 40% or above for women), mobilisation is not a problem at all! A large amount of fatty acids are in the bloodstream, however, oxidation is impaired for some reason.

In the middle, those holding just a little more body fat than they want (around 15-35% body fat in men and around 25-40% body fat in women) usually have no problems with mobilisation or oxidation. In fact, these people can often lose fat quite easily in a calorie deficit. Some may have trouble staying in a calorie deficit, possibly due to a lack of self control or a limited understanding of nutrition fundamentals.

Back to the main question. Is there any reason for fasted cardio if there’s no real problem with mobilisation for those between 15-40% (our upper and middle groups) with mobilisation? The answer is no.


Why has fasted cardio become THE answer? Largely because the bodybuilding fraternity, many of which will have been well under 10% bodyfat, have utilised fasted cardio to help with the mobilisation and transport issues suffered by lean people mentioned above. People trying to get to the real low end of the body fat spectrum anecdotally found that fasted cardio did the job for them. This is purely anecdotal and isn’t true for those within a “normal” body fat range.

We are all different

It’s a fact, we are all different. Some reading this will already be low body fat, others high body fat and some will be in the middle. Some people will only be able to train first thing before work and others will never be able to train first thing. The good news is that for the majority of people reading this, those who aren’t under 10% body fat (yet!), it really doesn’t matter if you can’t do fasted cardio. You don’t actually need to and probably won’t gain anymore from doing so!

For most of us, the best time to do cardio is when we can fit it in! Doing exercise at some point in the day is far better than missing it altogether because you can’t do it fasted.

Glycogen Stores

Another consideration is the stored energy in your muscles/liver, otherwise known as ‘glycogen’, a polysaccharide of glucose. Glycogen is utilised during training which must be replenished after exercise. Generally speaking your body will look to use circulating glucose and glycogen as primary fuel sources for exercise. And it’s only when these stores are largely depleted, usually as a result of hours worth of training, when more fat will be oxidised and relied upon to produce energy. Even first thing in the morning, your body will not have used up ALL the glycogen stores from the previous days eating while you slept. Meaning whether you train fasted or fed, you aren’t likely to be utilising solely stored body fat.

Experiments help

Many studies have been conducted over the years to examine whether fasted cardio results in a greater about of fat burning compared to fed cardio. The majority of these experiments have not been conclusive. Both acute oxidation data and long term trials have failed to indicate that training fasted will lead to more fat burning than training fed and thus lead to more fat loss. These experiments actually indicated that there is little or even no difference between fasted and fed cardiovascular exercise when the aim is fat loss.

As stated above, very lean bodybuilders have anecdotally suggested that fasted cardio works best for them, but they are a select group who have a very different set of parameters to work with. For the rest of us whom rely only on food, water and supplements, there is a real threat of utilising mobilised protein for fuel when training fasted. Not something we want to do when the goal is to save muscle and burn fat.


Train how best suits your lifestyle. If that’s fasted as you can’t bring yourself to eat before a morning run, then do so. However, don’t choose fasted cardio because you think it’s going to burn more fat, science tells us that for the large majority of us, this isn’t the case at all.

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