How Intermittent Fasting Boosts Exercise!

So what happens if you add exercise to the whole intermittent fasting scenario? While people quickly assume exercising on an empty stomach will hinder performance and speed up exhaustion, studies consistently show quite the opposite.

Melanie Avalon Exercise

This pic was before my fasting days…but whatever. I used to bike around Sanibel Island 25 miles every day in the summer. Good times. But this picture is actually in Disney’s Animal Kingdom. Which is also magical! ¬†Sorry this so ADD. #Caffeine


When I think of my pre-fasting days, I think of daytime lethargy and equating an hour long workout to¬†eons. Perhaps the biggest difference in my life from intermittent fasting, is that the idea of “running out of energy” just really isn’t¬†a thing¬†anymore. I simply know I can keep going, no questions asked. When fasting, you don’t tire as easily. (I literally don’t remember the last time I took a nap – although I kind of miss those!)

There’s a reason for this magical endurance.

“Typical” non-fasted exercise is fueled by glucose (sugar from carbohydrates) and glycogen (carbohydrates stored in the muscles and liver.) The use of glucose and glycogen for fuel during exercise¬†inhibits¬†fatty acid oxidation. This not only blunts fat burning in general, but also puts a proverbial ceiling on endurance, since glycogen stores are¬†quite¬†limited compared to fat. You can deplete glycogen stores in an intense exercise session, while¬†the average person has enough stored body fat to walk over 1,000 miles!¬†No big deal. (1)

When your body is accustomed to relying primarily on carbohydrates from your relatively-recent last meal, you ‚Äúhit the wall‚ÄĚ when you use up that stored glycogen. In fact, this typically occurs¬†before¬†complete glycogen depletion, as the brain¬†anticipates running out of glycogen, and slows down physical activity in preparation. (So you get tired before you even ‚Äúrun out‚ÄĚ of energy.) Studies suggest that simply¬†thinking¬†you’re going to “hit the wall” may make you “hit the wall.” (2) As one researcher notes, “An apparent paradox of long-distance running is that even the leanest athletes store enough fat to power back-to-back marathons, yet small carbohydrate reservoirs can nevertheless catastrophically limit performance in endurance exercise.” (3)

But with fasted exercise, there is no wall!¬†Fasted training up-regulates the body‚Äôs ability to burn fat for fuel, resulting in much more sustainable exercise: the longer you go, the more energy you free up to use!¬†One trial found that rats on a 24 hour fast could run longer than rats in the fed state. Interestingly, the fasted rats could run longer even when blood sugar dropped¬†below¬†that of the fed rats – the proverbial “wall” was gone. While the carb-fueled rats “ran out of energy” when they started running out of carbs, the fasted rats easily carried on! The study concluded that fasting increases the use of fats as fuel during exercise, while the corresponding ‚Äúglycogen sparing‚ÄĚ effect increases endurance. (4)


Studies consistently support the body’s adaptation to fat as fuel during fasted training, as discussed above.

A 6 week diet-controlled study comparing fasted versus fed exercise, found fasted exercise instigates metabolic adjustments within muscles for fat adaptation. Fasted exercise encourages the muscles to utilize intramyocellular lipids (fat stored in the muscle) for energy, and increases the oxidative capacity of muscle. In other words, fasted training encourages your muscles to use more fat, rather than carbs, as fuel. This prevents the drops in blood sugar which happen during carbohydrate-fueled exercise, when the muscles use more glucose. The study concluded that ‚Äúregular fasted training is a useful strategy to stimulate physiological adaptations in muscle that may eventually contribute to improve endurance exercise performance.‚ÄĚ (5)

Another study comparing an overnight fast to a longer 3.5 day fast prior to exercise found fasted training substantially maintains glucose levels. In the longer fast, the body generated more glucose during the first hour of exercise, while ultimately sparing it, relying more on fatty acids and ketones. Insulin levels also lowered, and performance was maintained.  Like the previously discussed study, fasted exercise supports steady blood sugar levels by sparing glucose, relying on fat instead. (6)



Not only does fasted training utilize more fat for energy, it also may protect against weight gain in a hyper-caloric state (which is pretty cool).¬†One study looked at healthy males eating 30% more calories per day (half of those from fat) for 6 weeks. Some of the men did carb-fueled¬†aerobic training, some did fasted aerobic training, and some didn’t train at all. The ones who didn’t train gained an average of 3lbs after 6 weeks (surprise surprise) and the ones who trained with carbs gained around 1.5 lbs in 6 weeks. But the men who exercised while fasting, even though they ate more calories than they “burned,”¬†gained no weight after 6 weeks! Furthermore, only fasted training increased glucose tolerance and insulin sensitivity. The study concluded that “fasted training is more potent than fed training to facilitate adaptations in muscle and to improve whole-body glucose tolerance and insulin sensitivity during hyper-caloric fat-rich diet.” (7)



But what about muscle you say? Will fasted training make your muscles waste away?


Think about the nature of resistance training for a second. Straining and breaking the muscle forces it to adapt, and ultimately rebuild itself stronger. Stressing the muscle ultimately benefits it. Intermittent fasting is similar in that it stresses the body, and instigates beneficial adaptations.

A 2011 review of 18 studies involving calorie restriction, intermittent fasting, and body composition, found that intermittent fasting patterns favor muscle preservation. While dieting alone averages 25% weight loss from muscle, dieting in intermittent fasting patterns averages only around 10% weight loss from muscle. The study noted that “intermittent restriction regimens may be superior to daily restriction regimens in that they help conserve lean mass at the expense of fat mass.” (8)

But that’s intermittent fasting coupled with a calorie deficit, and without exercise. Since both fasting and exercise individually support favorable gene regulation in regards to muscle adaptations, what happens when you combine the two? The aforementioned review even concluded with this very idea: “As endurance exercise has been shown to…aid in the retention of lean mass, the combination effects of these two interventions on body weight [intermittent fasting and exercise] and body composition could be a fruitful area for future study.”

Studies support the notion. The previously mentioned fasted versus carb-fueled training in a hyper-caloric diet study, for example, found fasted training¬†increased¬†muscle protein content by 28%. (7) Furthermore, a¬†2010 randomized, crossover study analyzed muscle protein uptake in young males after either fed training (a carb breakfast) or fasted training (an empty stomach). It found that fasted training significantly increased P70S6 kinase, a gene which signals muscle growth. The study concluded that fasted training ¬†“may¬†stimulate the intramyocellular anabolic response to ingestion of a carbohydrate/protein/leucine mixture following a heavy resistance training session.” In other words, fasted training may actually prime your muscles for growth! (9)

And on top of all that, remember that fasting promotes autophagy, or the recycling of old and damaged proteins within the body for new growth and repair!  Autophagy is actually required to maintain muscle mass. Oh hey. (10)

With all that said, if you‚Äôre still worried about your muscles while fasting, you can always supplement with BCAAs (amino acids) during the fasting period as a sort of “insurance,” √† la¬†Martin Berkhan at¬†Leangains.

To recap, exercising while fasted ramps up fat burning and provides an abundance of  energy, especially when compared to finicky and controlling carbohydrate stores. Muscles become more adept at using fat as fuel, and glucose is spared. Endurance and stamina increase. Protein is preserved, protecting and supporting existing muscle.

It basically just means exercise in the fasted state is pretty magical.



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