Physiology Friday #252: How a Week-long Fast Affects Performance and Metabolism During Exercise
Strength prevails while aerobic capacity declines under nutrient-deprived conditions.
Greetings!
Welcome to the Physiology Friday newsletter.
Details about the sponsors of this newsletter including FSTFUEL electrolytes, Examine.com, and my book “VO2 Max Essentials” can be found at the end of the post. You can find more products I’m affiliated with on my website.
Imagine yourself living tens of thousands of years ago. Doordash and Uber Eats didn’t exist—humans had to hunt for (and gather) their food, which was hard to find and even harder to catch.
Periods of fasting and feasting characterized prehistoric eating habits, and this meant that food often had to be obtained under conditions of low energy availability. Humans needed to maintain a high level of physical performance and stamina—especially under fasted conditions—if they wanted to obtain their next meal and survive until tomorrow.
It’s for this reason that people today are well adapted (or at least have the capacity) to go long periods of time without eating. Indeed, even the leanest of humans has enough body fat to last several weeks sans a meal.
Ketones play a key role here—providing an alternative energy source for the body and brain to maintain high performance during prolonged periods of fasting or starvation.
Of course, just because we can survive prolonged fasting certainly doesn’t mean it’s optimal for human performance, which occurs in the well-fed state. Most of us have experienced the lethargy that comes with trying to do a workout when we’ve not eaten as much or as frequently as we probably need to. Fasting is the opposite of a performance enhancer—it leads to a loss of lean body mass, glycogen depletion, and other compensatory effects. Physical performance suffers.
Or does it? The metabolic and performance effects of prolonged fasting haven’t really been well characterized in humans, at least using standard exercise tests.
For this reason, researchers involved in a new study set out to characterize how the body responds to a week-long fast, both metabolically and during strength and endurance exercise.1
The results might surprise you.
Fasting affects (some) measure of performance
For the study, 6 men and 6 women (average age of 30) fasted for a total of 7 days—drinking only water but otherwise going about their normal day-to-day routines.
Before and after the fast, they underwent a slew of tests to measure their body composition, their muscle strength, their aerobic capacity, their fuel utilization at rest and during exercise, their resting energy expenditure, and levels of proteins and enzymes involved in metabolism.
The most notable finding was that there was no decline in maximal leg strength after 6 days of fasting. This was despite the participants losing 5.8 kg during the study, 4.6 kg of which was lean mass and 1.7 kg of which was lean mass from the arms and the legs. This corresponds to a 7.5%, an 8%, and a 6% loss of body weight, lean mass, and lean mass in the extremities, respectively.
Bone mass, on the other hand, was unchanged throughout the 7-day fast.
Although strength was maintained during fasting, aerobic fitness wasn’t. Peak VO2 dropped from ~48 at baseline to ~44 after fasting—an average reduction of 13% (absolute) and 7% (relative to body weight). Accompanying the loss of maximal aerobic capacity was a reduction in peak power, which declined by 16%. During the maximal exercise test, the participants also achieved a lower maximal heart rate (about 5 bpm lower compared to pre-fasting) and a lower respiratory exchange ratio—indicating the participants’ carbohydrate oxidation capacity was lower at maximal exercise. Surprisingly, perceived exertion during the maximal exercise test wasn’t any higher after fasting compared to before, suggesting effort might not be affected by prolonged fasting.
The drastic reduction in maximal aerobic capacity was attributed to one primary metabolomic change: A 13-fold increase in an enzyme known as pyruvate dehydrogenase kinase 4 (PDK4). This enzyme inhibits (blocks the activity of) another enzyme called pyruvate dehydrogenase (PDH), which plays a major role in our muscles’ ability to use carbohydrates as a fuel source during exercise.
Higher PDK4 levels mean less PDH activity and a lower capacity to burn carbs.
Another contributing factor to the lower maximal aerobic capacity—though less likely given the researcher’s interpretation—was a massive reduction in muscle glycogen content during fasting, which fell by 50%.
Fasting and metabolism
The metabolic changes in response to fasting were what you’d expect. Resting fat oxidation increased—contributing 73% to total energy utilization after fasting compared to 36% before—while resting carbohydrate oxidation decreased—contributing just 19% to total energy utilization after the fast compared to 53% before. However, resting metabolic rate (the total number of calories burned at rest) didn’t change.
This supports the idea that the body ramps up fat oxidation and shuts down carbohydrate oxidation while fasting—it adapts to the fuel sources available. Another fuel source that was in abundance was ketones, levels of which increased from barely detectable before the fast to nearly 4 mM after the fast. Interestingly, blood ketones levels fell slighly during and after exercise, indicating the body was using this “alternative” fuel source.
The participants also experienced an 11 mmHg reduction in systolic blood pressure and a 7 mmHg reduction in diastolic blood pressure during the 7-day study. Their resting lactate levels nincreased from 1.1 to 1.5 mM.
Metabolic changes during exercise were also apparent and included a doubling of maximal fat oxidation during exercise and an increase in the intensity of exercise where maximal fat oxidation occured (60% of peak VO2 vs. 46% before fasting). In support of an increase in fat oxidation was the observation that several proteins and components involved in mitochondrial metabolism were unchanged after fasting, as were proteins involved in lipid transport and metabolism.
A week of fasting presents a dramatic metabolic challenge, but one that the body is well equipped to handle. Based on the results of this study, it does so by responding with a major diminishment in the ability to utilize carbohydrate and a profound upregulation of the ability to burn fat—both at rest and during maximal and submaximal exercise.
That’s no surprise—the body will utilze and adapt to whatever fuel sources are available. During prolonged fasting, that means more fatty acids and ketones and less liver and skeletal muscle glycogen.
These metabolic changes weren’t enough to compromise muscle strength. This is a fascinating finding in my opinion, especially considering the loss of lean mass (and presumably muscle mass) during the study. The participants were just as strong after 7 days of fasting as they were beforehand, indicating a profound ability of the human body to preserve its performance in these conditions. That’s some resilience.
It doesn’t surprise me that aerobic capacity declines whilst strength was maintained—these represent two very distinct physiological demands: One requiring a quick burst of anaerobic metabolism and the other requiring more prolonged yet high-level metabolic output. Fasting might have been compatible with sprinting to kill an animal but less so with persistence hunting.
How can one apply these findings?
I certainly hope that nobody is regularly fasting for 7 days, especially while also trying to train at a high level.
However, it’s promising to see that even without eating for 7 days, we might still be able to maintain high levels of muscle strength. That should give us confidence that we can perform well if we miss our mid-day snack. Strength is resilient.
And while the adaptations to fat metabolism might seem like a good thing (who doesn’t want to become a better fat burner?), these findings are a reminder that it might come with a cost: Downregulated glycolytic capacity and diminished high-intensity exercise performance.
In biology, there are no solutions. Only tradeoffs.
Thanks for reading. See you next Friday.
~Brady~
The VO2 Max Essentials eBook is your comprehensive guide to aerobic fitness, how to improve it, and its importance for health, performance, and longevity. Get your copy today and use code SUBSTACK20 at checkout for a 20% discount. You can also grab the Kindle eBook, paperback, or hardcover version on Amazon.
Examine.com: Examine is the largest database of nutrition and supplement information on the internet.
FSTFUEL combines electrolytes with amino acids to help your body maintain hydration and optimal functioning during exercise or intermittent fasting, so you don't have to choose between fasting and fitness. If you want to try some, the guys at FSTFUEL have agreed to give my audience a 30% discount on their orders. Just use the coupon code BRADY30 at checkout.
Yeah I was surprised at the muscle mass loss as a percentage of total weight loss. I wonder how water weight factors in.
That’s a lot of lean tissue loss. One wonders if there was an equal and opposite feast gain with our Paleolithic ancestors.