Many people swear by fasted cardio. In theory, working out in a fasted state should burn more fat. With no fuel for the body to burn, it would have to turn to fat for energy. This would enhance fat loss and body composition. It makes logical sense and sounds pretty good, but does fasted cardio really work?
What is Fasted Cardio?
Fasted cardio is cardiovascular training in a fasted state; meaning the body has digested all food and either used or stored away all macronutrients from your last meal. A fasted state results in low insulin levels, and is generally around 4-8 hours after your last meal for most people, depending on your metabolism. For this reason, most people choose to perform fasted cardio after sleeping through the night, usually early in the morning.
How Fasted Cardio is Supposed to Work
In his 1999 book "Body For Life," bodybuilder Bill Phillips discussed the idea of fasted cardio as the ultimate method for enhanced fat loss and body composition. He is generally credited with introducing the idea of fasted cardio.
With the body in a fasted state, all food has been digested and either used or stored away. The best example of this is when you wake up in the morning. Insulin levels are low, and the body is using fat as it's primary energy source.
According to proponents of fasted cardio, exercising in a fasted state will force the body to use fat as an energy source, as no carbohydrate will be available in the bloodstream.
Benefits of Fasted Cardio
According to research, there are certain benefits to training in a fasted state. Whether it's a good idea for you or not really depends on what you're trying to achieve.
Improved Endurance Exercise Performance
One Study found that 6 weeks of regular cardiovascular training in a fasted state improved Metabolic Flexibility in subjects. This means that performing fasted cardio resulted in the body improving it's ability to use fat as fuel during exercise.
The study also showed that training in a fasted state prevented a drop off in blood glucose levels during exercise. These adaptations would likely result in improved performance in endurance exercise.
Does Fasted Cardio Boost Fat Loss?
While the above study was encouraging, it's not so clear cut.
A huge part of the fat being used as energy is coming from intramuscular fat deposits. This fat, stored within our muscle tissue, has little effect on how we look. It's not the type of body fat most of us are trying to get rid of.
Another study from 2014 showed no difference in body composition or fat loss between a fasted cardio group and a group eating a meal before exercise.
So, do you burn more fat with fasted cardio? Maybe, but a huge percentage of it isn't the fat you're interested in getting rid of.
Problems With Fasted Cardio
There are several reasons why fasted cardio doesn't work the way we'd like it to. While it seems like a logically sound method for burning extra fat, the human body is very complex. It doesn't always work the way we'd like it to.
All 24 Hours Count, Not Just Your Workout
Even if a fasted cardio workout did burn more fat, it only represents a small portion of your day. All 24 hours count, not just the hour or two you spend in the gym.
Thermic Effect of Food and Exercise ("Afterburn")
Taking in food before a workout has been shown to increase the Thermic effect of exercise after the workout. This means that fueling before you exercise increases the amount of calories (including fat) you'll burn throughout the day. (Also known as the "afterburn" effect of exercise.)
Muscle Loss With Fasted Cardio
Training for one hour in a fasted state has been shown to more than double muscle loss compared to training while fueled. Approximately 10% of calorie expenditure can come from breaking down muscle protein (muscle gluconeogenesis, if you like fancy terms).
If you care at all about gaining or maintaining muscle mass, fasted cardio is a poor method, as you are literally forcing your body to burn away muscle tissue.
Exercise Intensity Suffers
Performing high intensity interval training (HIIT) has been shown to help you speed up fat loss when compared to steady state cardio. Studies have shown that HIIT can improve fat loss by 28% compared to steady state training.
Unfortunately, performing high intensity training in a fasted state is never going to be optimal.
With the body in a hypoglycemic state (low blood sugar), and low on fuel, exercise intensity will suffer. Part of the benefit of high intensity training is the post-exercise increase in metabolic rate, where you'll be burning more calories all day long. You sacrifice part of this benefit when you train on empty.
Trading solid HIIT sessions for the sake of using fasted cardio just isn't optimal. Attempting to perform high intensity workouts without fuel is a poor choice, as you'll be stressing your body out and can impair your recovery abilities as well.
What Works Better Than Fasted Cardio?
When it comes to burning fat, people have cardio tunnel vision. Cardio has become synonymous with fat burning, and many people mistakenly believe that cardio alone will get them leaner and meaner.
But if you focus only on cardio, you're missing out on a huge piece of the puzzle.
Lift Weights to Burn More Fat!
Your muscles are the metabolic engine of your body. Metabolism takes place inside your muscle tissue. They are essentially fat burning machines.
More muscle requires more energy. Resistance training can boost your resting metabolic rate, leaving you burning more calories all day, every day.
Variety and Balance are the Best Option
Using some variety is the most effective way to burn fat and get leaner without losing muscle mass in the long run. The following would be a pretty good approach to losing fat and maintaining or gaining muscle.
- 1-2 HIIT workouts per week
- 2-4 steady state cardio sessions per week
- 2-3 Resistance Training sessions per week
- High protein diet, cut out the sugar and limit your carbs, focus on whole foods.
Conclusion: Should You Use Fasted Cardio?
As we said earlier, it really depends on you and what your goals are.
If your goal is improved metabolic flexibility and improved endurance exercise performance, then there's some research to back up using fasted cardio.
If you just want to lose fat or get shredded, research has pretty definitively shown that training in a fasted state does not provide any measurable benefit vs. training in a fueled state.
At the end of the day, if you feel like something is working for you, and you're seeing results and like the way you feel, then go for it!
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