On Tuesday I tested out how my training was impacted by fasting. Overall I lost about 20% of my power output over 30 hours of fasting. Over this period I lost about 2 kg of body mass, which I assume was mostly due to emptying my digestive system and some loss in water due to losses of salt and glycogen. Both this weight and power were quickly restored after resuming eating.
Numbers from the fast:
7 hours before fast begins: 90 minutes moderate exercise, approx 3600 kJ
5 hours before fast begins: Weight 67.7 kg
0 hours (final meal before fast): Consumed 1 bowl (~200 g) oats + 30 g pea protein in water, approx 3500 kJ
5 hours into fast: Light exercise (30 mins indoor cycling) approx 1000 kJ
16 hours into fast: Hard exercise (30 minutes indoor rowing) approx 1700 kJ. Able to sustain 93% of maximum steady state power output.
18 hours into fast: Weight 66.4 kg
28 hours into fast: Weight 65.7 kg
29 hours into fast: Hard exercise (30 minutes indoor rowing) approx 1600 kJ. Able to sustain 81% of maximum steady state power output.
2 days after fast: (normal diet resumed) Weight 67.9 kg
Energy expended over fast roughly approximates to 9000 kJ, i.e. 110% of glycogen stores, or equivalent to 250 g fat.
Things I consumed during the fast:
2 double espresso (coffee with no sugar or milk)
about 1 g salt
about 4 L of water
Things I felt during the fast:
Hungry (for periods, but not consistently. Hunger mostly peaked in the first few hours, at approximately 10 hours in).
Foggy (felt a little more distant than usual after the afternoon coffee wore off).
Unmotivated to exercise vigorously (wanted to quit while performing hard rowing efforts earlier and more intensely than usual).
This was a spontaneous experiment. I happened to have a very early breakfast on Tuesday and missed lunch due to being needed in the lab, so decided to use the inconvenience as an opportunity to test performance impacts of fasting. The result was a small loss of power, and a moderate loss of motivation to exert that power.
I feel, but have no first hand evidence, that long distance running has left me pretty well physiologically adapted to having a large energy reserve and being able to utilise it (e.g. running a marathon uses up approx 10,000 kJ, or about the same as the entire fasting period). I also think that mentally, knowing that I am capable of going on long runs before breakfast, and knowing what it feels like to use up my glycogen stores (known as “hitting the wall” in running or “bonking” in cycling), together give me confidence to undertake fasting with exercise safely.
Given many people in the developed world struggle with obesity, I would recommend experimenting with fasting. Simply re-calibrating one’s sense of hunger (and knowing it is possible to delay the need to eat by at least several hours) could help with generally reducing calories consumed and therefore weight loss. Obviously some people struggle with disordered eating, which can be more harmful than obesity. Online communities around weight loss can drift from being supportive about achieving a healthy weights to creating unhealthy expectations about weight loss, particularly rapid weight loss.
Rafael de Cabo and Mark P. Mattson published a review on Intermittent Fasting in the last week of December 2019, which longevity specialist Peter Attia commented on in his email today. The science is currently inconclusive, but there seems to be a growing body of evidence that the human body is capable of going extended periods without food not only without detrimental effects, but with benefits to overall health.
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