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I’ve done fasted morning cardio, and I understand why it’s a thing. It feels like such a virtuous act: You’re making yourself suffer, and it’s hard, but it’s also not impossible. You finish your morning jog with an empty stomach, then reward yourself with, hopefully, a nice breakfast. But is there a point to the suffering?
Every now and then a study pops up and points to fasted exercise, or lately, morning exercise as a fat-burning secret. The most recent one to hit the headlines was done in mice, and found more signs of fat-burning in samples taken from mice after they had been made to run on a treadmill in the “morning,” versus mice who had their exercise bout in the “evening.” The study found fasting didn’t make a difference for the mice, only time of day.
(Fun fact about rodent studies on circadian rhythms: mice are active at night and sleep during the day, so the results are always reversed when we describe how they might apply to humans. The “morning” exercise was done in what, to us, would be the evening.)
For humans, exercising in the morning and exercising on an empty stomach often go together, and fasted morning cardio is an increasingly popular weight-loss recommendation. But is it actually any more effective? Personally, these days I like to sleep in, and I’m not going back to that morning exercise routine just because of a few extra metabolites in a few rodents’ fat deposits—but let’s talk about whether this and other supposedly fat-burning forms of exercise actually mean anything in the big picture.
Fat burning during exercise doesn’t matter as much as you think it does
If you want to lose weight, fat burning is good, right? But just because you are burning fat does not mean you are losing fat overall. I like to think of fat as a wallet: You can take some energy out of there and “spend” it, but later in the day you might end up putting more money (or energy) back in. What matters is the number you have in your bank account at the end of the day, or the week, or the month.
In the same way, it really doesn’t matter what fuel you are burning during your exercise. Whether you burn fat from low-intensity fasted morning cardio or carbs from HIIT in the evening after chugging a Gatorade, your body will settle the balance one way or another. Burn more calories than you eat, and you’ll have to lose that energy from somewhere—hopefully fat. Eat more than you burn, and it’s the opposite.
Scientific studies that look at whether fasted cardio causes overall fat loss tend not to find a significant difference between training with and training without food. This study, for example, found no difference between groups of people who showed up fasted and were given a shake before or after their morning run.
Looking at the bigger picture, this review concludes that, “no conclusive evidence exists on the superiority of fasted versus fed cardio to improve body composition” and cautions that doing too much fasted cardio—say, over an hour—may actually cause the body to eat into muscle stores for fuel. So if you got really into fasted cardio, the strategy could backfire.
Fasted morning cardio has downsides
If you train early and don’t always have time to eat, that’s probably okay—you’re not going to kill your gains. But if you’re able to eat first, chances are you’ll have a better workout.
Low intensity cardio (zone 2 on a five-zone scale) tends to use mainly fat for energy, so you may not even notice a difference doing it with or without a good pre-workout meal. Even if it’s tough to get through an easy morning jog the first time you try it fasted, your body can adapt a bit over time. Plenty of folks have a habit of going for a run before breakfast, and that’s fine—maybe not optimal, but fine.
But once you’re doing higher intensity work, you’ll probably miss those carbs. Having glycogen in your muscles or the products of carb breakdown in your veins (aka blood sugar) will make you feel less fatigued and will enable you to work harder. Ultimately, if you’ll get a better workout with food than without, why wouldn’t you give yourself enough fuel for that workout?