The fasted cardio debate has raged for years. People swear by it or curse it, the research supports it or criticizes it, and trainers/coaches argue that it works and that it doesn’t. But before I weigh in and offer some points for thought, I think it’s really important to define what fasted cardio is- because most people don’t even truly understand what qualifies cardio as fasted.
Most people think fasted cardio is simply training on an “empty stomach,” which they usually think is simply a stomach that “feels empty.” But fasted cardio is cardio performed while in a “fasted” state, and this means more then just an empty stomach feeling. It has to do with how your body processes and absorbs the food you eat.
When you eat food, it gets broken down into various molecules that your cells can use, and these molecules are released into your blood. The body releases insulin as well, and the job of insulin is to shuttle these molecules into cells. Depending on how much food you eat in a meal, your insulin levels can stay elevated anywhere from 3-6 hours after. When your body is digesting and absorbing what you’ve eaten, your body is in a “fed” or “postprandial” state. Once it has finished processing and absorbing the nutrients, insulin levels drop to a “minimum” low (or “baseline” level), and your body enters a “fasted” or “postabsorptive” state. Every day your body moves between these two “fed” and “fasted’ states.
So, knowing this, you know that training done when insulin is elevated and food is being processed is considered “fed” training. Exercise done during periods where insulin is low and food is done being processed is considered fasted.
So what is the benefit of fasted training? Well, the idea is that the higher your insulin levels are (fed state), the less your body is going to use fat for energy. Why burn fat when there’s a surplus of energy readily available via the food that you just ate? This makes sense, but the problem is that fasted cardio is much more complex then just “work out without eating to lose fat.” What kind of cardio? How long? And what about the impact of individual factors which vary from person to person? Are we doing fasted low intensity cardio? Or fasted HIIT? And what is the purpose- performance? Fat loss? Is the individual male or female? What other training are they doing that day? Are they 10% bodyfat or 30% bodyfat? All of these things play a role in whether or not fasted cardio hurts or helps. Rarely in fitness is an answer just a”good or bad, yes or no black and white” answer.
1 The Impact of Fasted Cardio
There is the impact of fasted cardio during the cardio session itself, and then also the impact of cardio after the session is completed. For example, some studies have found that fasted cardio can burn 20% more fat then fed cardio. Why? Once we eat, insulin (which regulates the breakdown of fat) increases in our body. According to some research, higher insulin levels have been shown to suppress fat metabolism. This makes fasted cardio sound GREAT for fat loss. But what about the impact of fasted cardio AFTER the session? Because research has also demonstrated that fasted cardio does NOT increase fat burn in the 24 hour period following the session. Another study indicated that consuming carbs before morning cardio increased the post-exercise “after-burn” effect when compared to training fasted. This would mean if you eat before you do morning cardio you’ll wind up burning more calories throughout the day, not just during the cardio session itself. So it’s important to consider not just the impact of the cardio session during the session itself, but also the impact of the session AFTER it’s completion.
2 Fasted Cardio Sessions Increase Your Risk of Muscle Loss.
This is, imo, one of the strongest arguments AGAINST fasted cardio. In general, when you wake up in the morning, your body is in a catabolic state due to high levels of cortisol — the hormone associated with stress. That means that your body is primed to break down molecules for energy. But if you’re in this state too long, it can lead to degradation of body tissue, including muscle, which isn’t desirable all no matter your goal. The number one priority when trying to lose fat should always be the preservation of muscle. Losing muscle mass has NO benefit, regardless, and will make it harder to continue losing fat since muscle tissue is metabolically responsible for most of the fat you’ll lose. For example, 10 pounds of muscle burns 50 cals a day, even at rest, so if you lose 10 pounds of muscle, your RMR (resting metabolic rate) will lower in response, and you will burn approximately 50 fewer cals a day which adds up to 350 LESS cals per week. Keep doing the math and this adds up over the long run.
There is also the issue of insulin sensitivity. If you have more muscle, your insulin sensitivity will be directed more towards accruing muscle mass. A larger muscle has more insulin receptors, which makes the muscles more insulin sensitive which means you’ll tend to store more of what you eat in the muscles instead of storing it as body fat. This is why people who carry high levels of bodyfat (30%+) usually also have insulin sensitivity issues. They have insufficient muscle tissue in comparison to an abundance of adipose tissue, which makes them less insulin sensitive.
Lastly, when you have more muscle mass, you’re able to lift more weight, train harder and longer, which increases the amount of calories you burn during your workouts.
And research has shown that intense fasted cardio sessions like metabolic conditioning or HIIT, for example, can lead to the breakdown of muscle. This concern about muscle loss becomes even more paramount as you get leaner and the body has less fat reserves to tap into. If you’re 30% bf then fasted cardio can make a lot of sense. And be beneficial. When you’re super lean however there’s a really likely chance you’ll just elevate cortisol beyond what the body can handle and wind up wasting muscle. When I’m prepping athletes, particularly if they’re natural, I actually pull back on intense cardio as they progressively lose fat because all it does is heighten the chance we will lose precious muscle tissue.
3 HIIT vs LISS
I’ve posted before about my two favorite forms of cardio being HIIT or LISS. Of the two, the “safe” option for fasted cardio is LISS. MISS (moderate intensity/steady state cardio – the type that most people do when trying to lose fat) increases cortisol levels the most. The activity intense enough to stimulate the release of cortisol, and long enough to elevate it significantly. But LISS, for example in the form of maybe taking a walk, isn’t intense enough to stimulate much cortisol release. In fact it actually has the potential to lower morning cortisol levels by having a relaxing effect. So if I do suggest fasted cardio I usually encourage longer, low-intensity work (a relaxing pace at which you can sustain a conversation) for 45-60 minutes. This style of cardio can increase overall energy expenditure without the negative hormonal impact that other forms of cardio can have. It also has a VERY low risk of muscle wasting, which is a primary concern of mine where my clients are concerned.
The bottom line is that there’s no absolute. But before you start adding fasted cardio ask yourself if it’s right for you given your goals and your individual factors. And if you are truly interested in improving athletic performance or losing fat, make sure you’re asking the right question- which is not are you eating before workouts, but what and how are you eating throughout the whole day? Check in on your diet and reduce your calories or make some healthier dietary changes overall. If you’re dieting properly, and training right, chances are something like whether or not your cardio is fasted is going to have a negligible impact, and the risks you may run in doing it simply might not be worth it.