There are a lot of different exercise methods, all with one specific goal. We employ them to get fitter, leaner, faster, and stronger. Fasted exercise is one such method.
Unfortunately, even though it’s a firm fan favorite, it carries plenty of misconceptions.
Purportedly, fasted exercise is more beneficial than other forms of exercise because it burns more fatty acids than glucose. Hence, people think that they’re using stored body fat – not glucose – as fuel.
To break down the rationale; fasted exercise evangelists believe that eating before exercise prompts your body to use the glucose from that meal in order to fuel the workout. So if you don’t eat, you’ll tap directly into body fat for energy.
Why the Misconception Persists
However, the misconception persists due to some confusion. Specifically, it is true that you’re burning more fat during fasted exercise, however, the issue with fasted cardio is the low fat oxidation over a 24-hour period.
Specifically, calorie expenditure may be higher during fasted – as opposed to fed – exercise, but it will be lower during the rest of the day as your body compensates by reducing the use of fat for fuel. This may be because fasted cardio doesn’t cause a sustained and increased metabolic rate.
Hence, you may burn more fat during fasted exercise, but you’ll end up burning less in the subsequent hours. And by the end of the day, you’ll even out.
Finally, similar to breakfast skipper who tend to overcompensate calories throughout the day, fasted cardio may lead to lower activity levels as the day goes on.
Effect on Appetite
Another reason for the 24-hour effect may be a person’s appetite. In fact, a 2012 Loughborough University study1 found that fed exercise “suppressed appetite to a greater extent than fasted exercise.”
The authors’ findings are in line with other “breakfast skipping research.” Specifically, it’s well noted in the literature that breakfast skippers tend to overcompensate skipped morning calories throughout the day.
The aforementioned study expands on those findings. Namely, breakfast skippers who exercise before their first daily meal are more likely than their fed counterparts to experience hunger throughout the day.
Effect on Muscle Mass
But fasted exercise isn’t only for fat loss. In fact, bodybuilders also adapted the regimen as a tool in their exercise arsenal. Fortunately, two studies elucidated what – if any – effect fasted exercise has on muscle mass specifically.
The first – a 2010 Massey University study2 – compared the muscular adaptation to endurance training conducted in both fed and fasted states.
The research revealed several interesting points. Chief amongst them wasn’t the difference between fasted and fed training, but the discrepancy between men and women.
Specifically, meal ingestion prior to daily exercise can modify some of the exercise training-induced adaptations when compared to no prior meal. However, the difference is further accentuated when comparing adaptations between men and women.
A second study3 subjected participants to fasted and carbohydrate fed exercise. Similar to the abovementioned research, both groups experienced “similar adaptations in peak Vo(2) whether carried out in the fasted or carbohydrate-fed state.”
Further, the authors noted that “there was a decrease in exercise-induced glycogen breakdown and an increase in proteins involved in fat handling after fasting training. However, fat oxidation during exercise with carbohydrate intake was not changed.”
Fasted Exercise and HIIT
High intensity interval training has recently become a popular form of exercise. Naturally, to maximize results, fitness enthusiasts combined fasted exercise and HIIT. If fasted exercise led to greater fat loss, and did HIIT, why not combine them?
It makes sense on paper, but a deeper look refutes the purported benefits. Specifically, unless you’re a top level athlete – training with proper nutrition and knowledgeable support – high intensity exercise and an empty stomach don’t mix.
A 2013 McMaster University study4 observed fasted and fed state subjects as they performed high intensity interval training. The researchers measured influences on body composition, muscle oxidative capacity, and glycemic control.
The authors found that “interval training in the fed or fasted state improves body composition and muscle oxidative capacity.” Hence, HIIT is beneficial. But there’s no difference in effect whether you’re fed or fasted.
The study concluded, “short-term low-volume HIT is a time-efficient strategy to improve body composition and muscle oxidative capacity, but fed- versus fasted-state training does not alter this response.”
A 2014 study5 continued where the McMaster researchers left off. This time, the researchers compared hypo caloric fasted and fed subjects performing aerobic exercises.
The findings mirrored those of the earlier study. Specifically, aerobic exercise during a hypocaloric diet does impact body composition. However, fed or fasted, made no difference.
The authors concluded, “these findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training.”
Let Your Food Digest
So what’s the best time for exercise?
Optimally, you don’t want to be fasted, but you don’t want to get on the bike right after a meal either.
You’re aiming for a period when nutrients are available and you have the greatest chance of fat oxidation. This time window is optimal for several reasons.
First, there’s the myriad of research indicating that there’s no difference between fed or fasted exercise.
Second, exercising on a full stomach is simply uncomfortable.
Lastly, there is some research evincing a “conflicting effect” concerning fed state training. Specifically, a 2017 University of Bath study6 found that exercising immediately after a meal isn’t advisable.
The aforementioned “conflicting effect” occurs because you’re forcing your fatty tissue to deal with food and exercise concurrently. To reiterate, if you’re exercising immediately after a meal, your fatty tissue “is faced with competing challenges.”
The authors concluded by stating that “this study provides the first evidence that the feeding status alters the response of adipose tissue to acute exercise.”
Confusion is inevitable with so many contradicting studies. After all, a plethora of earlier research indicates no difference between fed and fasted exercise, but the University of Bath study indicates a possible negative impact of training after a meal.
So how do the seemingly contradictory findings affect you? In relation to fasted exercise specifically, the best course of action is to find the sweet spot. Consider the full spectrum of evidence and compound it with your personal experience.
Aim for the sweet spot, the happy medium. A meal before exercise is beneficial since you’ll have nutrients to work with, but not right before, that way your fatty tissue won’t be conflicted.
Unfortunately, there’s no concrete number for how long you should wait. After all, we’re all different, and there are too many factors to consider when calculating the rate of digestion.
But if you’re looking for your sweet spot – the window between satiety and hunger, there a couple personal tips we can share.
In our experience, having smaller but nutritious and complete meals is key. Incorporate good fats, carbs, and proteins. A meal with those attributes, consumed at least an hour before working out, will leave you full at first, but light enough when it’s time to hit the gym.
Another piece of advice is to go for less solid meals. There are lots of shake combinations that will hit all your macros and keep you feeling light and ready for the gym.
Word of Advice
As with every aspect of nutrition and fitness, take everything you read with a grain. What works for us, may not work for you. What was so conclusive in research, may not be applicable to you.
Use a combination of your experience and learned knowledge to make the right choices for yourself.