Fasted vs. Fed Workouts: Which is Better?
By: Emily Rudow
Tags: Nutrition, Fitness, Running, Sports Nutrition, Diet

Fasted Workouts

What is a fasted workout?

Although it may seem pretty self-explanatory, let’s first elucidate what a fasted state encompasses. Simply put, a fasted state is when your body hasn’t eaten for several hours (usually 8–12 hours), but with some people, that timeframe can be even more condensed. It really depends on how quickly your body can digest meals. The most common time to work out in a fasted state is first thing in the morning when your last meal was the night prior and you can sleep off the fed state. However, if you are a person that prefers working out in the afternoons or evenings, it is still possible to workout fasted by allowing your body a certain gap in time from your last meal.

Benefits of fasted workouts

The first and most common benefit is that fasted workouts could potentially, burn more fat. In fact, several studies have shown that you can essentially burn up to 20% more fat working out in a fasted as opposed to a non-fasted state. Depending on your meal the night before and how late you ate, you should have a bit of glycogen left in your body to use to fuel your morning workouts while also burning fat. Did you catch my tone of uncertainty in the first sentence? The reason being is if you’re only doing fasted workouts for the potential weight loss benefit, you might want to reevaluate your stance — especially if you find your workout difficult to get through without eating before-hand. Here’s why.

In fitness YouTuber Jeff Nippard’s video Does Fasted Cardio Burn More Fat? (What The Science Says), he explains that just because you might burn more fat during the cardio session in a fasted state, it doesn’t mean that you will lose more fat overall.

He goes on to explain the results of a study titled, “Body composition changes associated with fasted versus non-fasted aerobic exercise,” where 20 young women were put on a 500 calorie deficit per day diet over a four-week period. The women were split into two groups: one group performed one hour of fasted cardio three times per week and the other did one hour of cardio in a fed state. After four weeks, there was no fat loss difference between the groups, meaning they were equally effective. To summarize, even though it is well established in the scientific community that fat oxidation increases post-fasted cardio, over time, there doesn’t seem to be any particular differences in body composition.

However, it’s important to note that there isn’t conclusive evidence on shorter-term effects. Here’s the takeaway: If you only do fasted cardio for the potential weight-loss benefits, it doesn’t seem to matter whether you’re fasted or fed so long as you’re in a calorie deficit at the end of the day.

Another perk of not eating before a workout is the whole issue surrounding digestion, as I previously mentioned. It can be a lengthy process to not only digest your meal but also be able to use that food for fuel. Your body is drawing blood to help with digestion vs. using that energy to kill your workouts. When working out fasted, you can avoid the issue of how much to eat and how soon before a workout altogether.

Another popular reason to use a fasting strategy is performance-related. When you start working out fasted, you’re teaching your body to access different fuel sources. Instead of using sugar or glycogen, your body is tapping into your fat stores to use as energy. In the Evidence Based Athlete’s article, Does Fasted Cardio Enhance Fat Loss?, Brandon writes:

Long term fasted cardio appears to lead to chronic molecular adaptations favoring fat oxidation. Mechanistically this implies that fasted training may help athletes like those that compete in endurance events since fat is an important fuel for those activities OR those that compete in a sport like CrossFit that requires some flexibility in fuel substrate utilization.

The preferential fuel substrate utilization may give endurance athletes more of an advantage in training or on race day.

Lastly, fasted cardio can help improve insulin sensitivity. Basically, the more your body operates in a fasted state, the less insulin your body releases. What does this mean? Well, in short, insulin can help you accumulate fat and high insulin levels are associated with diabetes. Brandon writes, “if you have blood sugar regulation issues including exercise-induced hypoglycemia, type 2 diabetes, pre-diabetes, etc. — this may very well help improve those conditions.” A quick disclaimer: This isn’t to be taken as medical guidance. Please educate yourself and consult an accredited professional.

If you’re new to the fasting game, a quick word of advice: Like any adjustment to a new diet or training regimen, your body is going to need some time to adapt. For so many years, you’ve been eating before your workout, so it’s going to be a bit of a shock to the system. When I got back on the fasting train after a long break, it took around 3 days of fasted workouts to get my body to adjust again. I found myself suffering energy-wise one day, and then experiencing severe hunger pains the next.

My Experience

After the end of a long relationship with a person I cared deeply about, I wanted a big change. My workouts felt monotonous; performing the same routine day in and day out. My diet was okay, but I was eating out a lot (as in visiting my favourite Indian restaurant about three times a week…yikes). I was also consuming alcohol too often and needed to cut it down.

That’s when I started researching everything I could on diet and nutrition. One of the first diets I wanted to try out was IF—specifically, doing my workouts (strength and cardio) in a fasted state. I read a library of articles on how fasted cardio can help you burn more fat.

I figured I had nothing to lose, so may as well give it a go. I started implementing new exercises I learned from YouTube and also doing my workouts in a fasted state. Within a month I started noticing material differences with my physique; specifically, more vascularity and leanness.

When I first jumped on the fasted workout train, I did suffer for more than three days, as I mentioned above. It actually took me a few weeks to fight through the hunger pangs and random bouts of low energy in my workouts. Slowly, however, my body acclimatized, and I started to reap the many rewards of working out fasted.

My mind felt more alert and clear. I felt more energetic and didn’t have to wait around for my food to digest. I could just hop to it whenever I was ready to workout. Gone were the days that I would be able to feel the oats sloshing around in my stomach while I ran.

I leaned out and started to build muscle, letting go of the long-held belief that I would lose mass if I worked out in a fasted state. I also took branched-chain amino acids (BCCAs) before and during my workout, which helps preserve muscle.

The fasting regimen isn’t perfect. There are days where it doesn’t work for me and I need to workout in a fed state. I’m flexible and would recommend that you try to be as well. Basically, I’ve figured out a mental checklist of what needs to happen in order for me to have a successful fasted workout:

Eat a good amount of carbs the night before (~50–75g). I usually eat my last meal around 8–8:30 p.m.

In order to have enough glycogen to do my workouts with zest, I need to work out within two hours of waking up or I get too hungry and my workouts suffer.

My workouts are under two hours (any more and I need to eat). These are for my regular workouts (strength, cardio). If I’m doing a run for two straight hours or longer, I’ll definitely need to consume some intra-workout calories.

I curb my appetite in the morning with coffee and a mixture of BCAAs and creatine.

My menstrual cycle fucks up everything, so approximately a week before, I get very hungry. If I ignore my body’s signals, I’ll get bitchy, so I tailor my fasted workouts around my hormones.

When you shouldn’t work out in a fasted state

Like anything, there are of course some stipulations and some warnings that I would be remiss without mentioning.

If you’re a runner, then there’s the obvious (and dreaded) “bonk,” which basically means that your body has depleted its glycogen stores in its entirety and it’s now switched over to fat-burning mode. The bonk is also synonymous with the wall. If you haven’t experienced it yourself, every kilometer starts to feel like hell. In a marathon, this is typical at about the 30K mark, but it varies. If you’re planning to do a race or a long training run then do yourself a favor and eat beforehand. Also, carry nutrition on you in the form of gels, chews, or whatever else you can squeeze into your belt or pockets.

Similar to the point above, if you’re trying to optimize performance, you might want to consider working in a fed state to give your body the required energy to push through high intensity and tough workouts.

Then there’s the health perspective. In my article, My 2 Week 16:8 Intermittent Fasting Experiment, I outlined words of caution for particular groups of people:

Pregnant or nursing mothers, the elderly with chronic conditions, children, those struggling with eating disorders, or individuals with either type 1 or type 2 diabetes need to be closely monitored when adopting an IF regimen.

I know I’m being redundant, but I need to stress the importance of doing your own research aside from this article.