I’m a huge fan of fasted training. It feels right, it feels “Primal.” And it jibes with my sense of how life was back in the hunting and gathering days: if you wanted to eat, you had to go hunt, and you had to hunt on an empty stomach (because you didn’t have much food laying around, let alone a refrigerator full of it). This is the natural state of animal life in the wild—get hungry, perform physical tasks to obtain food, eat—and it always made intuitive sense that following that pattern when working out as a modern human would confer special benefits. Our big disconnect nowadays is that food is separate from physical labor. You no longer earn your meal on a visceral, physical level. There are social benefits to this new setup, but there are also metabolic, health, and fitness consequences.
Fasted training could be a way to correct that disconnect and restore the ancient relationship between food and movement. It’s plausible. But what does the research say?
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Benefits of Fasted Training
There is actually a decent body of evidence suggesting multiple benefits to fasted training.
Improved insulin sensitivity
Stronger anabolic response to weight lifting
Improved capacity to perform without food
More fat burning
Improved Insulin Sensitivity
Intermittent fasting improves insulin sensitivity, as I’ve mentioned before. And exercise is perhaps the most reliable way to increase insulin sensitivity, even absent any changes to your diet. When you combine the two, the effect is even greater. By the end of one study, subjects who fasted while training had lower body weights (the only group not to gain weight), better body-wide glucose tolerance, and enhanced insulin sensitivity compared to the subjects who ate normally and exercised.1
Better Exercise Recovery
Three weeks of overnight-fasted endurance cycling (with caloric restriction to boot) improved post-workout recovery, maintained lean mass, lowered fat mass, and maintained performance. There was unfortunately no control group, but this study does show that fasting doesn’t hurt (and it may help).2
Another study suggests that fasted endurance training quickly re-activates the muscle protein translation that’s otherwise negated in athletes who had eaten carbohydrates before training. This could be the reason why excessive endurance training is normally linked to muscle loss (or at least a lack of muscle gain)—the average endurance runner is doing so in a fed state.3
Stronger Anabolic Response to Weight Lifting
According to an older study, subjects who lifted weights in a fasted state enjoyed a greater “intramyocellular anabolic response” to the post-workout meal. Levels of p70s6 kinase—a muscle protein synthesis signaling mechanism that acts like an “indicator” of muscle growth—one hour after a fasted workout doubled compared to one hour after a fed workout (in the same group).4 In other words, fasting boosted the best short-term correlate of post-workout muscle growth we have.5 Obviously, a single workout resulting in a single instance of doubled P70s6 kinase isn’t going to mean much in terms of muscle growth, but if you do so consistently over a period of months, and make sure to eat plenty of protein after your fasted training session, you should see appreciable gains. At the very least, fasted strength training won’t cause you to lose muscle.
Improved Capacity to Perform Without Food
Just like your muscles adapt to imposed stressors by getting stronger, your body adapts to fasted training by learning how to perform under conditions of low food and energy availability. It’s the classic “train low, race high” idea that I’ve discussed before, usually in reference to carbohydrate intake. It’s the specific adaptation to imposed demand (SAID) principle, only in this case the “imposed demand” is a low-glycogen, low-food environment.
A recent study exemplifies this phenomenon, pitting a group of untrained, carb-fed cyclists against a group of untrained, overnight-fasted cyclists and comparing both groups’ muscle glycogen content and V02 max.6 What happened? The fasted group improved their V02 max by nearly 10%, while the fed group improved V02 max by just 2.5%. Loading up on carbs before a training session impaired their ability to adapt to that training; training in a fasted state improved their ability to perform without food.
What do you notice? Fasting is not about acute performance boosts. Improved performance during a given training session isn’t really the point of fasted training. The point of fasted training is to maintain performance while enjoying the metabolic benefits, like improved recovery, higher glycogen stores, better insulin sensitivity, and improved muscle response to exercise. This sets you up for better performance down the line because you’ve gotten so good at performing in a depleted state.
More Fat Burning
Without carbohydrates available (or any exogenous energy source), your only recourse for energy when training in a fasted state is to burn through existing glycogen and body fat stores. This already happens simply by foregoing food without any exercise. Introducing exercise causes this effect to skyrocket. People who go for a run in a fasted state have a lower respiratory quotient, an indication of greater fat burning versus glucose burning.7 One study found that a morning fasted cardio session increased 24-hour fat oxidation by 50% in young men.8
Not all research finds this increased fat burning effect causes increased actual fat loss. For instance, one recent paper claimed to find no difference in the effect of fed and fasted training on waist size in postmenopausal women, but I actually think it deserves a closer look.9
The fed group started with an average waist circumference of 100.4 centimeters and finished the 12 week study at 99.1 cm for a loss of 1.3 cm. The fasted group started with an average WC of 93.1 cm and ended at 92.2 cm for a loss of 0.9 cm. 1.3 is a little more than 0.9 but not enough to indicate significance.
Technically, the authors are correct. There was no significant difference. However, consider the starting points. Losing a centimeter of waist circumference is easier at 39 inches (the fed group) than it is starting at 36 inches (the fasted group); when you have more to lose, the loss comes more easily. That the fasted group lost about the same amount of waist despite having less to lose is actually a testament, in my opinion, to the effect of fasted training.
Fasted Training: An Individual Decision
Not everyone should use fasted training. Not everyone should avoid fasted training.
Women might exercise caution with fasted training. I’ve spoken at length about how women often have a harder time with fasting, calorie restriction, and other forms of metabolic stressors. They still benefit from fasting, but often find that shorter fasting periods or less intense fasted training sessions work better.
Low-carbers or keto dieters might be especially well-suited for fasted training. After all, there is one glaring weakness in the fasted training literature: I’ve never seen a study placing low-carbers or keto dieters on fasted training regimens. It simply hasn’t been done in an official, formal context—but I have every reason to suspect that the pairing would be synergistic.
For one, keto/low-carb and fasting support and bolster one another. Fasting already promotes fat-adaptation by up-regulating fat-burning mitochondria, spurring the creation of new mitochondria, and reducing your reliance on sugar. Fat-adaptation makes going low-carb easier, because you’re really good at burning body fat and don’t get so many sugar cravings. Going low-carb makes you even better at burning fat and builds even more mitochondria, which is a prerequisite for fasting for extended periods of time. It’s a beautiful cycle.
Fasting feels more “natural” when you’re already fat adapted. It’s easier, it feels congruent with your physiology. Now, who do you think is going to get more out of training in a fasted state—the sugar-burner who’s miserable because they can’t access stored body fat very well, or the fat-burning beast who’s been living off stored body fat for years?
Two, the closest study we have to this came out in 2016.10 It was in elite endurance athletes who followed a “sleep low” regimen: they ate normal amounts of carbs throughout the day up until a high-intensity glycogen-depleting training session, after which they ate zero carbs, went to sleep, woke up, did a low intensity session, and ate carbs only after the morning session. They were sleeping and training in a low-glycogen state—very similar to a low-carb fasted state—and saw huge improvements.
The sleep-low group greatly improved their submaximal efficiency (power output per calorie burned when pedaling at moderate intensity) compared to the control group. This is a crucial biomarker for endurance athletes. If your submaximal efficiency is high, you get more power out of each stride/pedal/stroke with less energy required. Your “easy pace” is faster than the other runners/swimmers/cyclists/etc.
The sleep-low group’s supramaximal capacity also saw a major boost. They were able to cycle at 150% of their Vo2max for around 12-20% longer than before. There was very little improvement in the control group.
The sleep-low group improved their 10k times by 3-5%. The control group improved by 0.10%.
Despite being fairly lean already, the sleep-low group lost more body fat.
Also consider that we’re talking about the average of a group of individuals. Some individuals in the fasted group lost an incredible number of inches off their waists. Others didn’t. Those individuals who did may have left the study resolving to stick with the fasted training protocol afterwards. That goes for every study imaginable. Every study discusses the average effect. Comprising those averages are individual data points which range all over the place. You are an individual, not an average of the group. A certain protocol—like fasted training—may have unclear or middling effects in the group average, but work incredibly well for the data point of you. The good news is that you get to determine whether a given protocol works for you or not. No one else.
Since we know that fasted training isn’t going to atrophy your muscles or kill you, I suggest you give it a shot. Some people, both anecdotally and clinically, do quite well.
Fasted training isn’t necessary. It simply is another option. If you’re fat-adapted, if you have good metabolic flexibility, if skipping a meal or two is no big deal, then you’re probably someone who could tolerate and perhaps benefit from fasted training.
The easiest and probably most effective thing to try is the morning fasted walk, the old bodybuilder’s classic fat loss standby. Get up and go for a brisk walk before breakfast. That’s it. That’s the training.
If you’re feeling good, extend it into a long hike, or a bike ride, or a session on the rower. The vast majority of my low level aerobic activity is done in a fasted state. Wouldn’t have it any other way.
And if that feels good, consider throwing in some weights or sprints.
Do you engage in fasted training? What do you get out of it? Or do you prefer training in a fed state? Let me know down below, and thanks for reading everyone!
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.