Fasted workouts are a controversial topic in the fitness world. To some, the idea of working out without “carbing up” or doing the pre-workout protein shake is unthinkable. Won’t my performance suffer? Won’t my muscles shrink?Won’t my body think I’m in the middle of some horrible famine and go into starvation mode?
To others, fasted workouts are sacred tools, the perfect antidote to modern decrepitude. When I train in a fasted state, I can will my adipocytes to release fatty acids and feel the heat as they burn, hear the barely audible *pop* of muscle satellite cells replicating and proliferating, and see visions of my future physique through my gaping third eye.
Where does the truth lie? Let’s look….
To begin with, the evolutionary argument—the Grok logic—for fasted workouts is extremely appealing and intuitive.
Humans did not evolve with access to 24-7 fast food restaurants, grocery stores containing hundreds of millions of calories, and food supplies so ample that we often throw out half of it before we’re able to eat it. If paleolithic humans wanted to eat, they had to hunt or gather something—both of which require the expenditure of caloric energy—often on an empty stomach. In fact, these “workouts” for hunter-gatherers probably occurred more often than not in a fasted state.
This doesn’t mean that fasted workouts are ideal or optimal for health, performance, and fat loss. It does suggest that humans have the capacity for working out in a fasted state without falling apart or losing all the benefits normally associated with exercise. The question is if fasted workouts offer any special advantages.
Today, I’m going to dig into the literature to explore the most frequent questions and claims about fasted workouts and arrive as close to the truth as we can.
Are Fasted Workouts Good or Bad for Muscle Gain?
Let’s take a look.
One common argument is that since you’re not eating, which already “stresses” the muscles and deprives them of structural substrate, stressing the muscle with exercise causes it to “melt away.” This is overly simplistic, if attractive.
For one, that first bit is wrong. Reasonable durations of fasting don’t cause muscle loss. In fact, you can do a few days of fasting without incurring any significant muscle loss. The ketones generated during the fast have protein-sparing effects, and the fasting-induced spike in growth hormone also spares muscle from breakdown. There was even a study where blocking growth hormone with a GH blocker caused fasting people to lose 50% more muscle than fasters who didn’t get the blocker.
For two, strength training itself is a powerful signal to your body that your muscles are essential tissues vital to your survival. Your body generally tries to avoid burning through essential tissues. Lifting also increases growth hormone. Paired with the fasting-induced GH boost, your muscles will be in good standing.
Okay, so fasted workouts don’t appear to be bad for gains. Are they good?
Fasted training augments the anabolic response—the ability of muscles to take up protein and get bigger and stronger. A 2009 study found that, compared to athletes who lifted weights after breakfast, athletes who lifted weights in the morning before eating had an augmented anabolic response to a post-workout protein-and-carb shake.
Are Fasted Workouts Good for Fat Loss?
This one makes sense, doesn’t it? When you don’t have exogenous calories coming in, and you go for a run or walk or bike ride, your body should burn more body fat since it’s the only energy source available. But does it actually happen?
Well, short term studies find that fasted cardio increases fat oxidation in the body. People who go for a run in a fasted state have a lower respiratory quotient, an indication of greater fat burning versus glucose burning. One study found that a morning fasted cardio session increased 24-hour fat oxidation by 50% in young men.
An increase in 24-hour fat oxidation doesn’t say much about long term fat loss, however.
Another study followed a group of healthy women for four weeks, placing them on a morning fasted cardio routine. Three mornings a week, the subjects would perform 50 minutes of treadmill cardio at 70% of their max heart rate in a fasted state. Both the fasted group and the control group (who performed the same cardio, just not fasted) maintained a daily 500 calorie deficit. What happened?
There were no differences in fat loss between groups. Both groups lost weight and lost body fat, but fasted morning cardio did not accelerate the loss. A recent analysis of the available research came to the same conclusion: no difference in fat loss or weight loss between fasted workouts and fed workouts.
I’d like to see a similar four-week study done with men, who in my experience and from reading the fasting literature tend to have a more favorable response to extremes in caloric restriction.
This isn’t a perfect fasted workout study, but it’s better than nothing. A group of triathletes was placed on a “sleep-low” program: instead of eating a ton of carbs after their afternoon workouts, they ate none at all. They depleted their glycogen with the workout, ate a very low-carb dinner, and went to sleep. Then they woke up and did low-intensity cardio in a fasted state, which is the equivalent of a normal person going for a walk. The study was interested in performance, not fat loss, but the group who did their cardio in a glycogen-depleted, fasted state lost more fat than the control group.
An old bodybuilding classic for shedding fat is the fasted morning walk. Wake up, consume no calories, and go for a brisk 20-30 minute walk. In those who are already pretty lean but want to get very lean (like bodybuilders preparing for competition), fasted low-level cardio can be very effective. This is the hardest body comp transition—from lean to very lean. Lean is what the body “wants,” and going lower requires getting over the natural tendency to hold on to diminished body fat stores. A fasted walk, jog, or cycling session performed in the aerobic zone almost forces the body fat to release into circulation. Insulin is low. Sensitivity is high. The stage is perfect, in theory.
Are Fasted Workouts Good for Performance?
Yes and no.
To answer this question, we must note the distinction between training and competing. You might perform worse in a given workout if you’re fasting. You’ll probably perform better if you’ve eaten. But if you’ve consistently trained in a fasted state, the metabolic and muscle adaptations you’ll acquire will boost performance when you compete in a fed state. And that’s everything, isn’t it? While it’s fun to go hard in a workout, test your PR, and treat your training session like the world championship, the real reason we train is to adapt to the training and get better, fitter, and faster—whether for a legit competition or to simply get healthier. A fasted workout trains you to perform under difficult physiological conditions of low fuel availability, and that comes in handy. You probably wouldn’t enter a race or powerlifting match in a fasted state, but the fasted workouts you did in the months leading up to competition make you more likely to win.
The two are complementary. Train fasted, race fed.
Sprinting performance appears to suffer. In one study, sprinting athletes who had fasted had impaired speed and power thanks to less springiness. In another, fasted sprinting led to slower reaction times. Again—the question is, do the training adaptations you get from sprinting in a fasted state make up for the acute losses in performance?
As for strength training, there isn’t much solid scientific evidence that the fasted state improves or harms performance. One thing I’ve noticed—and have also heard from dozens of anecdotal reports—is that fasted workouts fill me with a special sort of energy. For lack of a better term, it feels more “Primal,” like you’re actually on the razor’s edge of desperation and performance, where your entire being is focused on lifting the weight, sprinting the hill, or spearing the deer that represents the difference between food for a week and total starvation. It’s pretty cool.
Some people report the opposite. Some people seriously lag if they haven’t eaten. They need something in their bellies to have a good workout. This is a subjective thing, and you’ll probably find that it changes from workout to workout. For example, strength workouts and low level aerobic activity (hiking, walking, paddling) go well for me on an empty stomach, while I prefer to have something light to eat before really intense Ultimate Frisbee matches. Figure out what works for yourself.
Implications for Certain Populations and Conditions…
Type 2 Diabetes
Fasted training improves several physiological markers that are especially relevant to people with type 2 diabetes. For one, it improves insulin sensitivity. The basic definition of type 2 diabetes is “extreme insulin resistance”; fasted workouts counter that insulin resistance. It also improves fat burning, another deficiency common in type 2 diabetes.
Keto dieters and fat-adapted folks on low-carb, high-fat programs seem to do better in the fasted state. If you’re already adept at burning your own body fat and training in a low-carbohydrate state, training in the lowest-carbohydrate state—a fasted one—isn’t a big leap.
As I’ve written before, women tend to react more poorly to intermittent fasting, especially fasts exceeding 14 hours. They are simply more sensitive to caloric restriction, seeing as how their biological “programming” prefers they have a steady source of calories in place for growing, feeding, and nursing babies. Whether you have kids or not, that’s what a significant portion of your DNA is geared toward.
That’s not to say fasted training doesn’t work for women. It just might not do anything special compared to fed training. For instance, this study found that whether overweight women did high intensity interval training in a fasted or fed state had no effect on the benefits. Both types of training worked equally well, improving body composition and the ability of the muscles to burn fat.
Other research finds that women can benefit from fasted training, though men may derive unique benefits. In another study, men and women performed fed and fasted endurance training. Both men and women saw better VO2max increases when fasted, but fasted men saw bigger boosts to muscle oxidative capacity. Fasting helped both in this case. It just helped men a little more.
How I Use Fasted Training
These days, most of my workouts are performed in the fasted state. Anything resembling lower level “cardio,” like walking, hiking, standup paddling, and bike rides are all done totally fasted.
Before heavy lifting or HIIT sessions, however, I’ll drink 20 grams of collagen peptides with some ketone salts and often creatine monohydrate. This isn’t to “fuel” me. The collagen provides the raw material my connective tissue (tendons, ligaments, cartilage) needs to adapt to the training stress, the creatine provides the substrate for quick ATP generation for short bursts, and—this is speculative, mostly—the ketones provide brain fuel to prevent “bonking” and act as an epigenetic signal for muscle preservation. This drink doesn’t contain many calories, nor does it provoke a huge insulin response. I’m technically breaking the fast, but I’m retaining most of the benefits.
I always continue the fast after my workouts. Going a few more hours without eating enhances the HGH response, which helps spare muscle burning and augments the adaptive responses. The ability to comfortably fast after a training session is a good sign that you’re fat-adapted. If I were trying to maintain some elite athletic schedule, I’d refill my glycogen stores, but I’m not chasing performance anymore. It just doesn’t make sense to burn through them and eat a bunch of carbs only to go do it again.
I don’t train in a fasted state for magical effects. I’m not expecting any miracles and neither should you. But I do think every healthy human should be able to complete a fasted workout without falling apart or losing more than a step.
I can. How about you? Ever try fasted workouts? How do you use fasting to augment your training?
Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.