Does Fasted Cardio Work? The Science Says....It Depends

Lately I’ve had a lot of questions about fasted cardio and whether it’s beneficial for those trying to lose weight or for helping runners ‘tap into their fat stores’ to become more metabolically efficient. This is a tough topic and I’m generally of the mindset that we need to be adequately fueled to get the most out of workouts. But I can see the pros and cons to both. So I decided to take a deeper dive into the research to better understand the topic. Here’s a little more about what I learned.

What is fasted cardio?

Doing a cardio workout (so running for 30 min or more) without fueling (typically for 8-12 hours) prior is basically fasted cardio. The idea, similar to the popular diet trend intermittent fasting, is you will burn more fat vs. carbohydrates which are our preferred fuel source during cardio workouts. Essentially – we have endless stores of fat but limited stores of carbohydrate in our bodies. By exercising in the fasted state, we’re forcing our bodies to burn fat instead of carbohydrate.

For runners, the thought is this may simulate what it’s like in the back half of a long race when our bodies have run out of glycogen (the storage form of carbohydrate in our muscles) and we need to tap into our fat stores – making us more ‘efficient’ at utilizing fat for energy come race day and allowing us to go longer on less (and even break through the dreaded ‘wall’).  

Why do it?

In theory – this makes sense. By working out on empty – with no fuel in our tank – we are forcing our bodies to tap into our fat stores in hopes of targeting them for fuel vs. our more easily accessible/readily available carbohydrate stores. But does that translate to becoming more metabolically efficient OR result in body composition changes?

Unfortunately, there is not much science/research to support either claim.

Current State of the Science

Let’s look at some of the research. A 2014 study found that changes in fat mass and fat-free mass (i.e., muscle) following a hypocaloric (low calorie) diet for 5 weeks in 10 women working out 3 times per week for an hour in a fasted state vs. 10 women working out the same amount of time per week but in a fed state (total calories consumed were the same) resulted in no significant differences in body composition. Both groups did have significant weight loss and changes in fat mass from baseline.

A 2017 systematic review and meta-analysis to examined the effect of overnight-fasted versus fed exercise on weight loss and body composition. 5 studies were identified with 96 participants in total. The review revealed trivial to small effect sizes for body mass (weight) when comparing fasted to fed state aerobic exercise. Same with percent body fat changes.

So, does it work for weight loss? Ultimately if it creates a calorie deficit, yes it could work for weight loss and losing fat mass. Because it leads to a calorie deficit over the entirety of the day.

But not all fasted state exercisers are looking for weight loss. So, does it help with simulating the later stages of a race (think mile 20 of the marathon) – improving your body’s ability to ‘tap into it’s own endless fat stores’?

A 2018 systematic review and meta-analysis of 46 studies suggested that eating before exercise enhanced prolonged (>1 hour) but not short duration (<1 hour) aerobic (cardio) exercise performance. The review’s findings suggest pre-exercise feeding bolsters prolonged aerobic performance while initial evidence highlights potential beneficial metabolic adaptations that fasted exercise may induce in peripheral tissues - suggesting it may in fact help our bodies adapt - but more research is needed to make that conclusion and there are a lot of variables at stake.

And finally, a 1985 study investigated the effect of consistent training at the same intensity in a fasted vs. fed state on muscle metabolism and substrate selection (whether the body preferred carbohydrate of fat as it’s preferred fuel source) during a 6-week endurance training program with 20 healthy, physically active adult males. Similar improvements in VO2Max (a marker of an athletes aerobic endurance capacity) and time trial results were seen in both groups. Metabolic measurements varied with fasted state exercises having more breakdown in fat vs. those exercising consistently in a fed state and did not have a drop in blood glucose concentration during fasting exercise. So there may be some benefit to consistent training in a fasted state at a low to moderate intensity – but the study was small and is dated.

What’s the bottom line? At present, I feel that there is not enough research to support either fasting cardio for weight loss/body composition improvements or enhancing our metabolic efficiency for endurance athletes. And there may be potential negatives. Exercising in a fasted state may burn fat, but it also may deplete the muscles of protein because the body will need to use its own protein stores to build new muscles. There is no way to know for sure if our body is tapping into it’s fat or protein stores — and we definitely don’t want the latter. When we are tapping into protein and not fat, it becomes very hard to ‘lean out’ or lose fat and gain muscle. And we know pre-exercise feeding augments exercise performance compared with fasting conditions. So want to get the most out of your workout - especially if it’s a hard one over 60 minutes? EAT!

What do I recommend?

When an athlete comes to me wanting to try fasted cardio, I always start with what’s their end goal.

Are they trying to lose weight/improve body composition (lose fat/gain muscle or ‘lean out’)? Or are they trying to become more ‘metabolically efficient’ in advance of a big competition/race?

I see no problem working out in a ‘fasted state’ if it’s an easy day of cardio equal to or less than 1 hour of low intensity exercise like a slow jog (just make sure you drink some water!). Following the workout, I encourage them to refuel with high quality protein and carbohydrates (i.e., eat breakfast) within 30-60 minutes.  

If you are planning a workout longer than an hour or at a moderate to high intensity, you will need some carbohydrate for quick energy. Go without and you may end up feeling tired, weak, lightheaded, nauseous and of course, ‘hangry.’ Wake up early and don’t think you have time? Munch on some peanut butter toast or a banana while you get dressed/ready. Trust me - your body will thank you at the 1-hour mark.

Also DO NOT consider a strength training workout in a fasted state. The stress on our muscles is lower during cardio workouts. But during strength training, you are stimulating greater muscle protein breakdown. You want to maximize your muscle gains by making sure you are adequately fueled pre- and post-workout when it comes to strength training. In a future blog post I’ll share some suggestions on what a good pre- and post-workout meal looks like.