Is Training Fasted A Bad Idea?

You may have been hearing recently that it’s a bad idea to workout fasted, on an empty stomach. 

And that its better to have eaten something beforehand.

This is typically directed at females more than males and the argument is that not only are women more sensitive to the increases in cortisol in the morning (and therefore shouldn’t be exacerbating it by exercising on top), but that they are also more prone to tapping into their muscle stores and breaking it down for fuel if they are in a fasted state.

Let’s dive in.

The breakdown and utilisation of fuel substrates (carbs, protein or fats) for exercise is largely dependant on the intensity and duration of the session.

The lower the intensity (sitting, walking, light exercise), the more the body will be obliged to use fats for fuel. Fats take longer to breakdown into ATP (our body’s energy currency) and so when energy demands are low, i.e. low intensity exercises, this is the body’s preferred fuel.

As exercise intensity ramps up and energy demands increase, we will slowly switch over to a more glucose dependant state. Glucose is a lot easier and quicker to be broken down into ATP. 

Protein finally, is more of a structural molecule. Its main job is to build things. DNA, hormones and neurotransmitters, enzymes, muscle tissue, connective tissue, transport vehicles for molecules like cholesterol among a whole host of things.

So unlike carbs (which is primarily used for energy) and fats (which functions as an energy reservoir as well as having other important roles within the body i.e. cholesterol & hormones), protein doesn’t really function as an energy fuelling strategy. 

But what about when we are fasted and don’t have glucose readily available to us?

Again, depends on the intensity and duration of exercise. 

For slower, lower intensity activities, the body will still have adipose tissue (body fat) to tap into. 

The other avenue the body will go down if intensity ramps up is to tap into our glycogen stores (stored glucose). We have about 400g of glucose that we can store within our muscles in an average human body and then a further 100g in our liver too. 

At this moment in time, I’m not aware of any sex based differences that would suggest that females would go against this normal energy metabolism pathway.

Furthermore, when you consider that females tend to use more overall body fat as fuel than men, likely because women generally have higher body fat levels as well as more type 1 muscle fibres (which are slower to fatigue and therefore more prone to using fats as energy) than their male counterparts, this again doesn’t suggest that women have a higher tendency to use protein for fuel.

Am I saying that protein will never be used for fuel by the body? No.

The 400-500g of muscle and liver glycogen isn’t an infinite source of calories to pull from which is why endurance athletes will consume quick absorbing carbohydrates like gels and gummies throughout their events.

In a hypothetical scenario where a training session/event lasts for multiple hours without the ongoing consumption of fast-acting carbohydrates, there could potentially be the situation where the body starts pulling energy from protein sources like muscle tissue.

But we’re talking about a very specific and unlikely situation here.

The other point to consider here is that exercise (resistance training in particular) is a catabolic process. Catabolic meaning to break down.

And so breaking down protein is not only a normal part of exercise, but our day to day life in general.  

Protein structures like muscle break down and then they remodel and rebuild.

Those of you who are familiar with calories in vs calories out will know it as the energy balance equation.

Turns out there is a similar equation for protein, it goes like this;

Protein balance = rate of protein synthesis - rate of protein degradation.

As in if you are in fact breaking down more protein (muscle tissue included) than you are consuming/building on a consistent basis, then yes, you probably will start losing muscle mass. Which isn’t a great thing.

But, if you’re consuming enough protein to meet your demands, then muscle breakdown during exercise isn’t something to be concerned with.

(This is why those engaging in regular strength training should consume more protein).

Having said all of this, I think it’s important for me to close this out by giving you a 30,000 foot view summary as well as my closing position.

Across my own anecdotal experience as well as the experiences of all of the clients that I’ve trained, I’m pretty certain that if you consistently train fasted but decided to train fed tomorrow, that you could have an amazing session.

At the same time, if you are used to training fed all the time, you could also feel great if you chose to train fasted tomorrow. 

Switching over to the other group across a small handful of sessions alone is not a large enough sample size to really know whether it’s the right thing for you or not.

Maybe you get a placebo effect. Maybe you get a nocebo effect. There are just too many confounding variables. Sleep, hydration etc.

To really know whether it suits you or not, I feel you would have to commit to a switch for at least 2 weeks before you can be quite sure.

Importantly, this would also give you a chance to see whether it actually suits your lifestyle or not.

However, if we just looked at this question purely from a performance point of view, it would 100% make sense to train in a fed state over fasted.

Having energy readily available to you is vital for performance. I doubt many olympians are competing after a 12 hour fast.

So, unless you are planning on running marathons everyday and at the same time pay no regards to your carbohydrate intake, I think either way will be fine, as long as you are meeting your protein demands (which you should be doing anyway).

But if high performance is your bag, then you should be biased towards training fed.

I guess the million pound question is, what is more important to you? Sustaining high level performance or establishing a routine that suits your lifestyle?