Regular readers of this site know I’m not a fan of cardio exercise for fat loss. It’s great for some things (improving your cardiovascular health, mainly) but endless cardio isn’t needed to get or stay lean. My protocols here all request that you don’t count the calories burned from cardio. So it won’t surprise you that I’m no fan of fasted cardio either.
That said plenty of guys and gals reading this site love their cardio. Some compete in team sports whilst others just enjoy going out for a run, or a cycle ride in the fresh air. And if they’re living the intermittent fasting lifestyle they often believe that it will have magical fat loss capabilities that will set them on the path to single digit body fat. It won’t.
This post will tell you why not, and will show you how to make cardio exercise work for you in your fat loss regime, and what you really should be doing in the fasted state. Read on…
Why do people think Fasted Cardio is a Magic Bullet?
First up, we need to look at why people want to do fasted cardio in the first place. It’s another classic case of the “more is better” mentality that humans have in abundance. We know that cardio burns fat. We know that after an overnight fast we’re burning a greater percentage of our calories from fat. So we put two and two together and come up with ten.
I have a question for you. If I offered you $50 today, or $10 each and every day for a week, which would you pick?
You’d be an idiot not to pick the $10 per day for a week, unless you desperately need money today, because you’d be getting a total of $70 rather than $50. But this “idiotic mistake” is exactly what people make when they think fasted cardio is a magic bullet. Because fat loss isn’t just about the now – the fat burned during exercise – it’s also about the rest of the day. And that’s what we’ll look at in this article.
Do you burn more fat through cardio if you do it whilst fasted?
Simple question. Complex answer.
For low intensity exercise (think walking the dog), with VO2max under 50% then yes, you do burn more fat during the exercise when doing this in the fasted state. The addition of anything (notably carbs) in the diet causes less fat oxidation (burning) during the exercise – specifically by stopping the fatty acids from entering your mitochondria where they will get burned off.
However, this effect is not seen in higher intensity efforts (65-75% of VO2max – think jogging, cycling, swimming etc), and specifically it is not seen in people who are already trained. That is, if you’re used to doing cardiovascular exercise on a regular basis, you will not see a reduction in fat oxidation during exercise as a result of eating before or during your workout.
So if you’re untrained, like one of the guys on “The Biggest Loser”, then maybe fasted cardio makes more sense. But if you’re already relatively fit, and already perform cardio exercise on a regular basis then consuming carbs pre workout and during the workout has been shown to have no affect whatsoever on fat burning during exercise for the first 2 hours of your training. You can find the references here and here.
So let’s sum this up: cardio (high intensity) does not burn more fat during exercise in trained individuals. Therefore we need another rationale for fasted cardio to be justifiable. Let’s take a deeper look.
Who performs best? Fasted athletes or fed athletes?
This one ought to be straightforward, and is one of the few instances where your gut feeling is right. It’s been shown that fed athletes are able to perform better when doing higher intensity cardio than fasted athletes. In fact, in a two hour cycling workout athletes either completed it fasted or took in regular carbohydrate drinks throughout the workout. The guys who sipped the carbohydrate heavy beverages throughout the workout performed better than those who trained fasted. Yet the overall amount of fat oxidation between the two groups was similar at the end of the exercise. Reference here.
Now add onto this the fact that carbohydrates consumed during the workout will spare your liver glycogen stores. This is important for us because we’re not just looking to burn fat – we also want to gain and maintain muscle mass. This study here shows that muscle glycogen depletion can lead to increased use of protein for fuel during cardiovascular exercise – up to 13 grams of protein burned per hour of exercise. Now, an overnight fast will have very limited impacts on your muscle glycogen stores at all (because overnight you principally burn fat for fuel and don’t move around much) and will have only marginally depleted the stores in your liver. But if you choose to perform high intensity cardio exercise in the fasted state you are asking for trouble. Depleting your muscle and liver glycogen stores will push you toward burning protein for fuel – when the goal here is to burn fat.
Given that the fed group could exercise at a greater intensity AND had similar total amounts of fat burned during their exercise bout we can safely say that fasted cardio is a waste of time. Moreover, for the protein-sparing reason I highlight above, we don’t want to run the risk of doing cardio in the fasted state if maintaining or gaining muscle mass is the primary goal.
But what about my $70? You’ve only looked at fuel burned during training. What happens for the rest of the day?
And herein lies the kicker. We’ve established that fasted cardio doesn’t burn more fat in trained individuals, and that it’s not optimal for those of us that are looking to maintain or gain muscle mass. But what happens during the rest of the day, post exercise? Does that give us a reason to perform cardio in the fasted state or not?
The problem here comes again from the fact that low intensity exercise burns more fat during the exercise. Or at least it burns a greater proportion of fat during the exercise. This fact means people often overlook the fact that total calories burned are more important than the proportion of calories that were burned from fat. We seem to believe that fat gets burned and won’t come back. Which is painfully untrue – you can burn what you like on the treadmill, if you eat twice as many calories in the kitchen you’ll still get fat.
When you compare people training at 50% of their VO2max and people training at 75% of the VO2max, you see after 3 hours that the higher intensity training led to greater fat oxidation at the 3 hour mark than lower intensity training (reference). When you feed these trainees pre-workout you see that the post exercise fat oxidation levels go up even further: in the 2 hour post exercise window fed (glucose plus milk) trainees had a 25-30% greater oxygen consumption (fat burning proxy) than those that trained in the fasted state (reference). But when you scale this up and look across a 24 hour period you see no difference in total energy expenditure or fat oxidation between low intensity cardio and high intensity cardio when an equal amount of calories was burned during the exercise bout (references here and here).
By doing higher intensity exercise you are not increasing 24 hour fat burning in your body.
The same number of calories burned in low intensity exercise would have the same effect on fat loss. That’s not to say that higher intensity workouts are always bad – you’ll burn those calories more quickly, so you spend less time in the gym. They’re also better for cardiovascular health. But from the research available today the following statements are true:
- Fasted cardio work is inferior to fed cardio work for both fat loss and cardiovascular improvement
- Fasted cardio increases the risk of protein oxidation (muscle wastage), but does not improve fat oxidation during workouts in trained subjects
- Fed cardio enables greater total work output
- High intensity cardio is not better for 24 hour fat oxidation, when the same number of calories is burned during exercise (i.e. it’s total calories burned that matters, not how quickly you burned them)
- Given that total work output (total calories burned) is more important than intensity, we should seek to do high intensity work in the fed state and low intensity work in the fasted state
That last statement probably needs more elaboration. Given that performance in cardio is improved by feeding, you should choose to do your cardio workouts in the fed state. You’ll be able to work out harder and longer than you can when training fasted. The extra work you can put in whilst fed will ensure a greater total calorie burn, but the food you have consumed pre and during workout won’t affect the fat burning within your body. Put simply: do your cardio training when fed for the biggest total calorie burn.
If you insist on doing cardio, do it in the fed state. Your workouts will be more productive and you don’t need to worry that you’re missing out on some sort of “magic fat burning window” by doing cardio in the fed state – because this window doesn’t exist.
If you insist on doing some training in the fasted state then make it low intensity, long duration cardio. This will be less likely to cause muscle loss, but will be principally burning fat.
If you don’t currently do high or low intensity cardio, don’t worry. You can still lose body fat without endless exercise. If you feel the need to add either into your workout schedule then follow the advice given above.
Ultimately the best choice of cardio for you is the one you can stick to. So if you can get outside and take the dogs for awalk in the fasted state – do so. If you prefer to hit the road and go running or cycling, make sure you do it when you’re in the fed state. Do something you enjoy and that will become part of a long-term lifestyle change – ideally something that isn’t a chore for you.
Let me know your favourite cardio exercise in the comment section below: