Does Fasted Cardio Help You Lose Fat Faster?
The reason people believe fasted cardio increases fat loss has to do with insulin.
Insulin does more than just shuttle nutrients into cells—it also impairs the breakdown of fatty acids. That is, the higher your insulin levels are, the less your body is going to use fat for energy (both body fat and dietary fat).
This makes sense physiologically. Why burn fat when there’s a surplus of energy readily available via the food we just ate?
Thus, when you eat food, your body basically shuts down its fat-burning mechanisms and lives off the energy provided by the meal, and it also stores a portion of the excess energy as body fat for later use.
As your body processes and absorbs the food, insulin levels decline, which tells the body to start burning body fat for energy as the “fuel” from the meal is running out.
Finally, when the absorption is complete, your body is running almost entirely off its own body fat stores for energy.
Here’s a simple graph that shows this visually:
The rationale for fasted cardio is that by doing your workouts in a fasted state—when your body is running mostly on body fat—you can burn more total body fat than if you did the same workouts in a fed state.
What does the science say?
Fasted Cardio and Fat Burning
There’s little doubt that fasted cardio burns significantly more fat than fed cardio.
In 2016, a group of scientists from Federal University of Rio Grande do Sul in Brazil looked at 27 studies on this topic, and concluded that “… aerobic exercise performed in the fasted state induces higher fat oxidation than exercise performed in the fed state.”
The main reason is simple: it’s very easy for the body to convert carbohydrate into fuel during exercise. Thus, when more of it is available, as is the case after eating a meal containing carbs, your body burns more carbs and less fat for fuel.
This is why a number of studies stretching back to the 70s have shown that when you give people carbs before or during exercise, they burn more carbs and less fat during their workouts.
So, we know that training fasted causes you to burn more fat during your workouts. So far so good.
We also know, though, that the number of calories you burn during a workout is just a fraction of your total daily energy expenditure (TDEE), which is what really determines how much fat you lose.
The real question, then, is does that small increase in fasted fat burning translate into a significant boost in fat loss over the course of the entire day?
That’s what a group of scientists from the University of Padua wanted to find out in a study they conducted in 2011. They split eight men into two groups:
- Group one ate breakfast before their workout.
- Group two ate breakfast after their workout.
For the workouts, everyone ran on a treadmill for 36 minutes at a moderate pace. The researchers measured what percentage of calories the subjects were burning from carbs versus fat before and 12 and 24 hours after their workouts.
As expected, the group that trained fasted burned more fat during the workouts. Later in the day, though, they burned significantly more calories from carbs and less from fat.
The group that trained fed experienced the opposite: they burned more carbs during the workout and more fat during the rest of the day.
When the researchers averaged it all out, both groups burned the same amount of body fat and carbs throughout the day, meaning that fasted cardio offered no clear fat loss advantage.
The body uses whatever fuel it has available for energy, and also prioritizes nutrients in a specific order:
- Blood glucose
- Stored carbohydrate (muscle glycogen)
- Stored body fat
- Stored body protein (muscle).
Therefore, when carbohydrates are readily available, the body prefers to burn them over body fat, and when less glucose is available, it prefers to rely more on dietary fat and body fat stores for its energy needs.
In other words, the body compensates for an increase in fat burning during fasted cardio with a decrease in fat burning during the rest of the day.
One shortcoming of this study is that it only measured fat burning over the course of a single day. Is it possible that the group that did fasted cardio would have lost more fat than the fed group if they kept this up for several weeks?
That’s what scientists from Lehman College wanted to find out in a 2014 study. They split 20, 20-year old women into two groups:
- Group one did their cardio workouts after eating breakfast (fed cardio).
- Group two did their cardio workouts before eating breakfast (fasted cardio).
Both groups ate their last meal at least 12 hours before reporting to the lab to ensure they were in a fully fasted state.
Under the supervision of the researchers, everyone did one hour of jogging on a treadmill three times per week for four weeks. The researchers had both groups follow a diet that kept them in a 500 calorie deficit every day.
At the end of the study, both groups reduced their body fat percentage by about 1%, but there was no significant difference between groups.
Once again, fasted cardio didn’t cause any more total fat loss than regular fed cardio.
Another downside to fasted cardio is that while it does result in more fat burning during exercise, much of the fat isn’t the subcutaneous stuff that wiggles and jiggles when you walk. Instead, about half comes from fat stored in your muscle cells for easy energy.
To make matters worse, the fitter you become, the more your body will tap these intramuscular triglycerides instead of going to body fat stored in other places (like the stuff covering your abs).
Fasted Cardio and Calorie Burning
Although it’s implied in everything we just discussed, let’s address this point directly.
Fasted cardio doesn’t help you burn more calories than fed cardio.
This was proven in a study conducted by scientists at the University of Tsukuba in Japan. The scientists had 12 young male endurance athletes report to the lab and undergo both of the following protocols with one week between each:
- One hour of easy indoor cycling after eating breakfast (fed cardio).
- One hour of the same exercise protocol before eating breakfast (fasted cardio).
The researchers kept all of the subjects locked in the lab for three days so they could carefully measure their food intake, energy expenditure, and fat loss. They found zero difference in 24-hour energy expenditure between the groups.
The bottom line is that fasted cardio—by itself—won’t help you lose fat faster than regular fed cardio. When you combine it with the right supplements, though, then it can help you get rid of “stubborn” fat faster.