Bearhawker

Kind of a "how long is a piece of string" question but a lot depends on your metabolism. If you're "fat adapted" where fat is your primary fuel source there is no reason to suspect that, with no caloric intake, will be substantially burning anything *other* than fat. If you're specifically carb-adapted and not metabolically flexible, then in a complete absence of your primary fuel what your body turns to in that circumstance is less clear.

As impressive as your BMI is it doesn't tell me how much body fat you have. If you are 150 pounds with an impressively low 10% body fat you could be at a BMI of 21 with the appropriate height but that is not important for my next point on burning fat if fat-adapted.

10% of 150 pounds is 15 pounds of fat. if we work with 3600 calories per pound of fat that is 54,000 calories. If we assume you consume 2,000 calories a day when not fasting and have the same burn rate fasting or not, then the 6,000 calories your body would burn while fasting for 3 days leaves you with ~13 pounds of fat, or ~9%.

If your body is *not* accustomed to burning fat and you dive into a 3-day fast you are going to be hungry and likely lethargic and irritable as your body will be in full-on panic mode. If you are fat adapted, and especially if you are extreme low-carb, you might feel hungry on day one which is more than likely a habit reflex rather than actual hunger... but by day three you will not even be hungry and things will be essentially normal regardless of activity level.

I've done it both ways ad can say with absolute certainty that it's way easier fat-adapted. Food is such a minor consideration for me I don't even include it in any of my emergency preparedness planning - it's just extra weight I don't need to carry and in an emergency situation I'm good for weeks before actually *needing* to eat.

I've also been in a situation where I was essentially forced into few days of eating little or nothing due to a dog attack while riding - I ended up with 15 rib fractures and a collapsed lung. Stuck in the hospital with *no* options for healthy eating (IE: carb-free) I ate essentiallt nothing for days. I personally credit that with my recovery... I was riding to work within 4 weeks (would have been 3 but my Dr threatened me with quitting lol) and 2 months after the crash I rode my fat bike around the Cabot Trail in 3 days. It's astounding how quickly you can heal when you are not eating any foods that cause inflammation.