A lot of people swear by fasted cardio for fat loss. I've traditionally done the opposite. But I've been starting to question my stance on this lately.
So, here's the common argument for fasted cardio:
When you are fasted, your blood sugar levels at the lowest they'll normally be throughout the day. If you then do some exercise in this state, then your body can't rely on sugars for energy, so it'll have to resort to burning fat instead.
And here are the main points I've used to refute this argument:
1. A calorie is a calorie. If you're going to eat 2,000kcal in the day regardless, and you're going to burn 300kcal through cardio regardless, then your weight will change by the same amount whether some of those 2,000kcal are consumed before training or after training. The total calorie consumption and expenditure is the same either way, so it wouldn't matter when you do the cardio.
2. Just because the body doesn't have much sugar to work with, doesn't mean it will use fat instead. It could just use up what little sugar is there, and then you'll topple over experiencing the joys of hypoglycaemia. This actually reflects my experience with fasted cardio.
My experience and my understanding of thermodynamics appears to mute out the arguments in favour of fasted cardio. So why am I starting to consider the possibility that I might be wrong to dismiss fasted cardio?
The main reason is because I've always been one of those guys who thinks that low intensity steady state (LISS) cardio is a waste of time. 'Cus, you know, higher intensities burn more calories per minute and condition your heart and lungs to higher levels of exertion. However, I've more recently come to acknowledge the therapeutic effects of LISS, and the fact that any activity (so long as it isn't harmful) is better than no activity. This has opened my mind up to the totally insane possibility that doing something as simple as going for a leisurely walk might actually be good for you.
By opening my mind to the idea that LISS isn't inherently stupid or pointless, I've also been able to take a step back and consider energy pathways. As a general rule, the lower the intensity of physical work, the higher the percentage of fat is used to fuel the activty, and the the higher the intensity, the higher the percentage of sugar is used as fuel. Traditionally, I've always trained at intensities that use too much sugar to enable safe fasted training. So, instead of my body using extra fat stores to prevent hypoglycaemia, it'll use up the small amount of sugar available and then crash and burn when that sugar is no longer sufficient. Fun times.
On the other hand, if you do some light activity, then perhaps the mobilisation of fat for energy will occur at a more rapid rate than the depletion of blood glucose -- enough to keep it safe while increasing the amount of body fat used before breaking your fast.
The implications of this could be that you lose weight at the same rate as if you'd done the same amount of cardio later in the day, but you lose fat at a greater rate.
Alas, I can't prove whether or not there's merit to fasted cardio, this is just me thinking. Regardless, I think I'm gonna go for a walk now.