This article originally appeared on Tonic.
Working out on an empty stomach is supposed to help you lose fat faster than the same amount of exercise done later in the day, after you’ve had something to eat. You wake up, get some caffeine inside you, then hit the gym. Because you haven’t eaten anything, your body is forced to use its large reservoir of fat to fuel the workout.
On the flip side, critics say that exercise on an empty stomach makes your body more likely to burn protein. Fasted cardio is nothing more than a catabolic menace that will strip away much of the muscle you’ve worked so hard to build. Will fasted cardio help you lose fat faster? Or is it just putting you on the fast track to muscle loss? Let's look at the science.
Most research shows that exercise done on an empty stomach leads to more fat being burned than the same amount of exercise done after a meal. When you wake up in the morning, insulin and blood sugar levels are low. As a result, there are plenty of fatty acids floating around in your bloodstream, just waiting for your muscles to pluck them out and burn them for energy.
In a study carried in the British Journal of Nutrition, researchers got a group of men to go for a morning run, either after eating breakfast or in a fasted state, having not eaten since the night before. Skipping breakfast and running on an empty stomach led to an average of 20 percent more fat being burned. Japanese scientists also report that fasted exercise accelerates fat burning throughout the day. In fact, over a 24-hour period, fasted cardio led to almost twice as much fat being burned compared to a day where the exact same workout was done after lunch.
Exercise in a fasted state also does a better job of activating various fat-burning genes compared to the same amount of exercise done in a fed state. These changes in gene expression mean that your body will find it much easier to break down stored fat and burn it off. So, there you have it: Not only does fasted exercise increase fat burning during a workout, it also boosts the amount of fat burned over a 24-hour period, as well as “switching on” various genes that play a key role in getting you lean, mean and ripped. Case closed, right? Not exactly. Here's why.
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Over time, your body will adjust the rate at which it burns fat and carbs. That’s why we need to look at how fasted cardio affects fat loss over a period of weeks and months. What happens during a workout itself, or even what happens in the 24 hours after that workout is over, doesn't tell the whole story. There are only a handful of studies to look at the long-term effects of fasted cardio on body composition. All show pretty much the same thing: Whether you do your cardio fed or fasted, it isn’t going to make much difference to the amount of fat you lose one way or the other.
When a team of researchers looked at the effect of four weeks of cardio performed in a fed or fasted state, it was only fasted cardio that led to a decrease in body fat percentage. However, the amount of extra fat lost—less than one half of one pound—was very small. The differences between the groups could have been down to the fact that skinfold calipers, which aren’t very accurate, were used to track changes in body composition.
In a follow-up study, 20 young women were assigned to one of two groups: a fasted group that performed 50 minutes of walking or jogging three times a week after an overnight fast, or a non-fasted group that ate breakfast before exercise. After four weeks, there was no significant difference in the amount of fat lost between the two groups. In a similar trial, this time using interval training rather than steady-state cardio, researchers from Canada’s McMaster University looked at the impact of fasted versus fed exercise in a group of overweight and obese women.
After six weeks, both groups lost an average of 1.3 pounds of fat. But there was no difference in the amount of fat lost between the fed and fasted groups. Women who hit the gym before breakfast lost no more fat than women who exercised 60 minutes after eating. In short, the beneficial effect of fasted cardio on fat metabolism both during and after a workout doesn’t translate into a significant increase in the rate of fat loss over time.
However, the story doesn't end there.
While fasted cardio doesn’t affect the total amount of fat you lose, it may increase the amount of fat lost from certain areas of your body. After an overnight fast, there’s an increase in the rate at which abdominal fat stored directly under the skin is being released, most likely due to an improved blood flow to the area. The increase in blood flow makes it easier for the various hormones that trigger the breakdown of stored fat to reach the fat cells in the first place. It also helps transport fat away from the fat cell so that it can be burned off elsewhere.
Doing cardio on an empty stomach won’t help you lose more fat in total. But for someone who is already relatively lean and wants to get ripped, fasted cardio may make it easier to tap into and burn stubborn fat stores, helping you lose a little more fat from the places you really want to lose it from. The bottom line is that there’s very little research to support the idea that fasted exercise is dramatically more effective for fat loss than the same amount of exercise done in the afternoon or evening.
Nor is there any strong evidence to suggest that fasted exercise "burns up" muscle tissue to a greater extent than exercise in a fed state. When it comes to losing fat, the food you choose to eat is a lot more important than what you do in the gym. Cardio doesn’t have much of an impact on your fat loss results, fasted or otherwise. As long as your diet is set up properly, the decision to train in a fasted or a fed state can be based largely on personal preference. When it comes to improving your body composition, there doesn’t seem to be any great advantage or disadvantage to one or the other.