Fasted Cardio & Training: Pros & Cons

Fasting is a buzzword in fitness/wellness culture that needs some unpacking, especially in regard to fasted cardio and fasted weight training. This article goes over what fasted cardio and weight training are, what the science says, and what you should know.

What Is Fasted Cardio?

Fasted training means an individual engages in either strength training or cardio training in the morning before eating any food after fasting overnight. The thought process behind this practice is that if you do cardio before you eat anything, you will tap into your fat stores instead of using glycogen stores, which would lead to more fat loss. However, this isn’t the case. When you engage in fasted cardio, you are burning more fat, however this doesn’t mean it is necessarily coming from your body fat stores. Fat burning is not the same thing as losing body fat. Fasted cardio increases fat burning during exercise, but this is compensated for by reduced fat burning during the rest of the day. On the other hand, fed cardio burns less fat during exercise, but this is compensated by increased fat burning the rest of the day. All in all, the net difference between fasted and non-fasted cardio is 0.

This is similar to the fat burning zone myth, which states that 60-70% of your max heart rate is the zone where you utilize more fat for fuel, and intensities above that percentage shift your body into using carbohydrates as fuel. But the reality is that your body compensates for this during the rest of the day post workout, and will balance out, so there actually is not more of a benefit to working out at that intensity. In essence, the body will recalibrate how much fat and carbohydrates you burn over a 24-hour period, so you burn the same amount each day regardless of what you burn during your workouts.

Benefits Of Fasted Cardio

The only benefit to fasted cardio is when it keeps someone constantly moving their body in a routine fashion.  For instance, many people have a morning routine of working out before they eat breakfast – and it might actually feel better for them to train with an empty vs full stomach. This is totally dependent on the individual and their specific body and needs. In general, our motivation is highest in the morning, so it makes sense for a lot of people to have a routine around working out before they eat breakfast.

Cons Of Fasted Cardio

There are a few drawbacks to fasted cardio. First, there is potential for more muscle breakdown during fasted cardio. Your overall training intensity will be slightly less for your workout than if you were fed. A study from 2014 was done on young females that looked at body composition and fasting. The study indicated that body composition changes associated with aerobic exercise in conjunction with a lower calorie diet are similar regardless of whether or not an individual fasted prior to training. So it doesn’t seem to make much difference overall in body composition if you train fasted or not.

What Is Fasted Strength Training?

Fasted strength training is similar to fasting in cardio, in that the individual engages with a strength training routine before eating any food in the morning and after fasting overnight.

Benefits Of Fasted Strength Training

Just like fasted cardio, the only benefit to fasted strength training is if it’s a part of your routine that helps with consistency.

Cons Of Fasted Strength Training

Just as your ability to train at a certain intensity decreases with fasted cardio, you’re going to notice even more reduced performance, especially if you are lifting pretty heavy loads. Optimally, your body needs protein before and after your workout in the morning to reduce any potential risk of losing muscle. Make it a point to eat enough protein every single day if you’re strength training. Enough protein is going to play the biggest role in reducing the risk of muscle loss if you decide to train fasted. And, you need to ingest protein pretty quickly after your training session if you are doing fasted cardio or strength training.

A recent study was published that looked at men fasting during Ramadan that were split into two groups: one group who trained while still fasting in late afternoon and another who trained in the evening after they were fed. They found that the group that trained in the evening after they had some fuel got significantly stronger. According to this study, you’re most likely better off training fed IF your schedule allows for it and it won’t impact your consistency.

The Bottom Line On Fasted Cardio & Training

According to recent studies, you’ll see more results in strength and performance if you’re training when fed. What matters most is getting enough protein throughout your day. Period.

Fasting in general is a flag for potential for disordered eating and/or behaviors. Even if you don’t have a tendency towards disordered eating now, incorporating fasting could be a quick way to develop one. We talk about this more on our podcast, Stronger Than Your Boyfriend, Episode 59: Fasting is Not Magical for Fat Loss.

If you’re newer to strength training or even cardio training, you should avoid fasted training at first. The longer you train, the more your body will learn how to adapt to different stressors. So, if you like working out early in the morning before breakfast, your body will be able to adjust accordingly.

Want to learn more? Listen to The Stronger Than Your Boyfriend Podcast Episode 118: Fasted Training.

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