Could Cardio On An Empty Stomach Aid Weight Loss, Or Is It An Empty Promise?

There are plenty of activities that are a bad idea while hungry. Grocery shopping can lead to frivolous purchases, negotiating can lead to unethical business practices, and trying to reason with a toddler without enough calories fueling your body can send you into a hangry dad rage. But when it comes to working out on an empty stomach, the rules are less black-and-white.

Studies show that aerobic exercise in a fasted state, referred to as fasted cardio (typically first thing in the morning before breakfast), can burn more calories and increase fat oxidation, compared to exercise after eating. Still, other research suggests that the long-term impact of fasted cardio is insignificant and any increase in physical activity and decrease in calorie consumption will lead to weight loss, no matter when in the day you workout.

If you hate going to the gym on an empty stomach, fasted cardio may not be the best way to motivate yourself to maintain a healthy habit.

Nevertheless, the trivial benefits of fasted cardio are part of why many people believe morning workouts are superior to afternoon and evening ones. But experts like Ben Brown, a certified personal trainer, nutritionist, and founder of the coaching platform BSL Nutrition, believe that making time for regular exercise is what matters most.

“If a client’s only time to exercise is first thing in the morning, which it is for many parents, then I’d strongly encourage them to workout then,” Brown says. As for whether or not you eat a banana or scarf some cereal beforehand, people should experiment and figure out what works best for them. “If they happen to feel best doing their cardio prior to eating their first meal, then that is what I’d suggest,” he says.

To put it simply, if you hate going to the gym on an empty stomach, fasted cardio may not be the best way to motivate yourself to maintain a healthy habit. But if a good run or a HIIT workout before your kids wake up makes your eggs and coffee taste a little better, that could make it easier for you to keep doing it.

If the question of whether to eat or not to eat before exercise is a matter of preference, then the best way to answer would be by trying the same morning workout both ways. So I woke up at 6:30 a.m. twice in the same week to figure it out on behalf of busy (and potentially hungry) dads everywhere.

My Attempt At Fasted Cardio

When I woke up to my alarm, I wasn’t as hungry as I was tired. But for the sake of science, I got up, dressed, and hydrated before stretching and going on a one-mile jog. Now, I’m not a runner, but I appreciate the convenience of jogging. And unlike other exercises I’ve built up a tolerance to like hot yoga, I’m so bad at running that it doesn’t take more than a 12-minute mile for me to feel the burn every time. It’s the perfect mini-workout when I’m strapped for time, or a great warmup for when I’m looking to test a fitness premise.

By the time I got home, I had to account for a 15-minute dog walk before my pup would tolerate a 30-minute HIIT session. By the time I fed her breakfast, I was hungry for my own, but powered through with a HIIT video from CrossFit coach turned fitness influencer, Anna Engelschall, that has more than 10 million views.

I was so hungry that it almost didn’t matter if working out on an empty stomach burned more calories. I absolutely canceled them out.

As much as I dabble in HIIT and am confident in my ability to handle most forms of physical activity for 30 minutes, this workout would have kicked my ass whether I’d eaten breakfast or not. After it was over, I inhaled an English muffin with peanut butter that I could barely let cool off from the toaster oven, and washed it down with a banana, water, and coffee. But by the time I showered and tried to start work for the day around 8:30 a.m., I felt almost as if I hadn’t eaten anything at all.

For the next two days, I was so hungry that it almost didn’t matter if working out on an empty stomach burned more calories. I absolutely canceled them out with a burger and cookies that I felt entitled to 10 hours later. Although intense exercise can blunt hunger initially for some people, Brown explains, “there is often a secondary rise in hunger later on and in the next days that contributes to increased calorie intake, hunger, cravings, and even a reduction in energy output.”

Although hunger spikes can occur following exercise whether you do it fasted or not, research suggests that fasting in general can increase cortisol levels, a stress hormone that can increase junk food cravings. Inconveniently, moderate- and high-intensity exercise has been found to boost cortisol as well.

Cortisol spikes are a normal response to stress, including exercise. But when people have issues with managing their blood sugar due to diabetes or hypoglycemia, fasting on top of that can be precarious, Brown warns.

Likewise, there’s a big difference between working out after fasting overnight and after fasting for 24 hours, which is becoming more on trend. Prolonged fasting prior to exercise or prolonged workouts following fasting can increase the likelihood of becoming dizzy, nauseous, and developing a low blood sugar headache.

“It is extremely important that people learn to pay attention to their body signals to determine if and what they should be eating around their exercise windows,” Brown says. And if I was listening to my body, it was telling me that I was an idiot for pushing myself so hard without eating anything.

Perhaps I was more of an exercise-after-eating kind of person… or so I thought.

The Post-Breakfast Workout

After my body had mostly recovered from the surprisingly difficult fasted cardio HIIT session, I woke up to my alarm, reluctant to do it all over again. As someone who isn’t hungry first thing in the morning, forcing myself to eat a banana before working out immediately felt unnatural. And as I stretched, even a banana felt like too much to carry around on my run. Unfortunately, the run was abbreviated by the urge to go to the bathroom.

Regardless of the abbreviated run, my morning was taking much longer than usual. By the time I walked my dog and ate another English muffin with peanut butter, for the purposes of consistency, it was almost 7:30 a.m.

Whether or not you eat before working out really comes down to intuition and listening to your body.

This was entirely too much food to have before a HIIT workout and I regretted it quickly. After 10 minutes, I was on the ground trying not to throw up, and it took me another 45 minutes to finish the workout because of all the nausea breaks I needed. Digestion “can take a back seat to the metabolic demands of the exercise,” Brown explains, particularly when the exercise is as intense as mine was. “This can cause nausea, vomiting, and stomach pain if too much food is consumed too close to the training window.”

As thrilled as I was to avoid the hunger spike I’d felt after doing the same workout in a fasted cardio state, I felt sick to my stomach for a good hour after my breakfast-fueled workout and depleted of energy throughout the day.

My Gut Feeling

After a day of recovery, rehydration, and reflection, I had to agree with Brown’s initial take: Whether or not you eat before working out really comes down to intuition and listening to your body. Forcing myself to eat in the name of field research was the opposite of that.

To my surprise, I preferred fasted cardio to non-fasted, but not because it burned more calories or felt like a better workout. After accounting for the time it took to eat and deal with nausea and bathroom detours, rolling out of bed to exercise on an empty stomach was more efficient and conducive for parenting. And you can learn from my mistakes by swapping out an English muffin with peanut-butter post-workout with something lower in sugar and higher in protein, like eggs Greek yogurt, or oatmeal, in order to curb the ravenous hunger afterward.

As for fasted cardio in the future, I will stick to a lighter routine like a short jog followed by yoga or Pilates. And if I were to go the unfasted route again, I would give myself more time to digest — at least an hour or two, Brown advises. My bad.

And for a 30-minute or more HIIT session, I would likely save that for the end of the day or for a weekend when I have nothing to do. That way, afterward I can just lay there, snack, and quietly groan as if I just ran a marathon.

Given that parents have less time to experiment with eating and working out, “worrying about fasted versus fed cardio is largely a waste of time,” says Brown, who is a father of three. So, no matter when and how you decide to fuel your body pre- or post-workout, don’t run yourself into the ground figuring out the right balance. I will happily and hungrily do that for you.