Fasted Cardio—Good Idea or Waste of Time?

It sounds perfect: Work out on an empty stomach to maximize your fat burning potential.

The New York Times even jumped on board, suggesting that the best thing to eat before a workout is nothing .

Digging into the research on fasted cardio reveals mostly empty promises. Although it is true that fat burning increases when you exercise in a fasted state, it doesn’t generally appear to pay off in terms of greater fat loss.

For example, a 2013 study compared the effect of 6 weeks of interval training on a bike in either the fasted or fed state in overweight young women and found that subjects lost the exact same amount of body fat regardless of which group they were in. The women in both groups decreased fat mass by 0.6 kg and increased lean mass by 0.6 kg.

A second study that combined a reduced calorie, higher protein diet with 60 minutes of exercise in either a fed or fasted state found no significant difference in fat loss after four weeks. The authors explain that fasted cardio is a nice idea that doesn’t pan out because it ignores the dynamic nature of the human body.

Why Doesn’t Fasted Cardio Lead To More Fat Loss?

Even though you burn more fat during fasted exercise, calorie expenditure will be lower and the body will compensate by reducing the use of fat for fuel later during the recovery period. Basically, you have to look at how the body acts over a longer 24-hour period to get an idea of what might happen over the long-term, which is what really matters when it comes to losing body fat.

In one study, subjects did a 36-minute cardio workout fasted or after having breakfast. Although fat burning, as measured by respiratory exchange ratio, was greater during the workout in the fasted condition, things evened out by the 12-hour mark after training. Fat burning was noticeably higher in the “fed” group that had breakfast than in the fasted group at 12 hours and the difference remained significant after 24 hours.

Another factor is how eating pre-workout affects the amount of calories burned.

When trainees eat pre-workout they consistently burn more calories during the post-exercise recovery period in both moderate- and high-intensity ranges. Scientists explain this as the body undergoing metabolic adaptation whereby it senses the combination of exercise coupled with lack of food as a “threat,” reducing total energy expenditure.

Lack of food intake may also lead to lower quality workouts.

This isn’t a problem for your average person getting on the treadmill and walking for an hour, but it’s not a great idea for athletes or people who actually want to accomplish something with their training. One solution is to consume caffeine pre-workout, which may partially help offset the dip in exercise quality when training on an empty stomach.

Another negative is how it affects cortisol and the stress response.

Cortisol is a hormone that releases energy stores when you haven't eaten recently. It is highest in the morning due to lack of food overnight. Normally, having breakfast helps lower cortisol. If you skip breakfast, cortisol will stay elevated and even increase. If you couple it with cardio, which also tends to increase cortisol output, you could end up with sky-high cortisol, which has negative effects on anxiety, fat loss, and health.

High cortisol also degrades muscle mass, breaking down tissue in the effort to keep blood sugar steady. Although it’s unlikely to be evident in the shorter term, especially if a higher protein intake is consumed, you are likely to lose muscle if you make fasted cardio a long-term habit.

Of course, no one says you have to do it indefinitely, which brings us to the question:

Are there short-term benefits to fasted cardio that make it worthwhile?

Research shows there may be metabolic health benefits to fasted exercise.

A recent study found an increase in gene signaling that is associated with better blood sugar regulation and insulin levels in response to a walking protocol that was preceded by an overnight fast. In light of the diabetes epidemic and all the problems people are having with poor insulin sensitivity, this could be a game changer by improving metabolic health.

Another thing to consider is that after exercise, muscles are “hungry” for glucose, insulin sensitivity is maximized, and nutrients are absorbed better. Therefore, if you have the option of eating either before or after, the argument could be made to save your meal for post-exercise, assuming the quality of your workout doesn’t suffer too much.

A second possible benefit is improved metabolic flexibility.

Overweight people lack the enzymes necessary to burn body fat effectively. Known as metabolic flexibility, the body is unable to switch between using fat and glucose for energy. This makes a high-carb intake necessary to sustain blood sugar and energy levels, which has negative metabolic consequences and often impedes the ability to produce lasting fast loss. Training fasted requires the body to undergo adaptations that lead to fat burning for greater metabolic flexibility.

Of course, exercise is a known catalyst for helping overweight people become metabolically flexible and fasting beforehand isn’t necessary to get the benefit. You’ll remember that eating pre-workout leads the body to burn more total calories, which is what we really care about when it comes to fat loss.

What about using fasted cardio when cutting?

One situation when fasted cardio might be worth the trouble is if you are already fairly lean. After all, fasted cardio is practically the holy grail for fitness competitors who are trying to get super lean, so you’d hope there was some validity to it.

It is important to understand the three steps the body undergoes when reducing body fat:

1. Mobilization of fatty acids,
2. Transportation of fatty acids, and
3. Oxidation (or burning) of fatty acids

Mobilization is the process by which fat is released from fat cells. If insulin is high, you can’t get the fat out of the cells, and it won’t be burned.

Transportation is the movement of fatty acids within the blood stream so that they can be burned.

Oxidation is the actual burning of fat and it only occurs after fatty acids are mobilized and transported to target tissues such as muscle, liver, and heart.

In very lean individuals, the body experiences adaptations to help it hold on to fat stores, particularly in the abdominal and upper thigh area. Blood flow to these regions is often reduced, decreasing mobilization and transportation of fatty acids. In this situation, fasted cardio might be worthwhile since it has been shown to increase blood flow to the abdominal and upper leg area, which could result in a favorable decrease in these “stubborn” fat stores.

The Bottom Line: Fasted cardio is a tool that can be used in specific situations for a select period of time. It may be worth it in the following cases:

You are already fairly lean and are trying to lose “stubborn” body fat. Use fasted training as a time to buckle down and give it all you’ve got. Don’t turn it into a long-term habit, because this will only lead to poor quality workouts, elevated cortisol, and muscle loss.

You’ve already established an exercise habit and want to mix things up. Trying something new is one of the best ways to kickstart fat loss results. Think of fasted training as a short-term fix (2 to 3 weeks) that may have metabolic benefits.

You use it selectively to boost insulin sensitivity and get more out of a lower calorie intake (due to increased absorption of nutrients). For example, you could train fasted on a periodized plan for a few weeks and then go back to eating pre-workout. Or you could do cardio or sprint workouts fasted, while eating prior to weight workouts. Instead of adopting fasted exercise as a long-term habit, use it when you need it.

Avoid training on an empty stomach in these situations:

Your main goal is to increase muscle mass. If you’re priority is muscle, you shouldn’t be doing cardio to begin with because it interferes with muscle building pathways.

You’re an athlete training for peak performance. Athletes need to take every advantage for optimal nutrition and recovery in order to elicit the greatest training adaptations.

Stress and high cortisol are a concern for you. Skipping meals and cardio are two of the worst things you can do if stress and high cortisol are an issue. Pairing them together is a disaster waiting to happen. Play it safe by favoring weight training and intelligent pre-workout nutrition.