Would it be unhealthy to fast until I reach < 15% body fat?

I'm a 1m85 - 92 kg male, I estimate my body fat to be about 20%. I have a muscular body type. For vanity purposes I want to get defined abs, which means I need to be around 14-15% body fat. A quick calculation indicates that this would mean losing 8 kg of fat. My metabolism is quite high, although it probably drops during fasts, but I probably would use 3K kcal a day, which means I would lose 1kg in 2.5 days, accounting for the fact that my appartement is kind of cold. This would mean that I would need to fast for at least 15 days to have any chance at achieving my goal. My previous longest fast was 4 days (goal was 5 but had to stop in an emergency cause my wife got sick and I didn't want my immune system to be weak). Does my plan seem doable? I see a lot of obese / overweight people on this sub who practice long fasts, but I haven't seen people in my exact situation (going from not overweight to low body fat%). Is there a reason for this? I am educated about electrolytes and refeeding syndrome. I am okay with some muscle loss, I am good at building muscles so getting them back shouldn't be hard.

Answer

You can fast to 15%bf. It’s not unhealthy.

The hard part is not fasting until you can see your abs. The hard part comes after. When you re feed. Do you believe in all honesty that you won’t overindulge? You will essentially starve your body to the point you’ll see abs. How do you think your body will react once it gets its first meal???? And second and third and suddenly you get everything back. I’ve been there. I reached 67kg and i am 190cm. And then i ate my way up to 82kg in two months. The hunger was immense. I’m 78kg now and i think that best suits me. There’s a backlash. Are you prepared?

Answer

You can be healthy as a male at around 10% bf, which you will see abs. It can be difficult to maintain though. Below that you start to suffer imo. You can fast all the way to 5% if you wanted to and be ok but you’d need a good plan coming off your fast to bulk back up to a lean healthy bf of say 10-12%

Answer

My plan is to stop prolonged fasting once I reach 20% body fat and switch to alternate-day fasting. I can’t find the source anymore, but it stuck with me all these years. Something about increased muscle loss compared to fat loss once professional fasting animals like penguins and bears dropped below 20% body fat. I think it may have been a fasting documentary. One day, I’ll find it again.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8718030/ shows that after a 10-day Buchinger-style fast, Fat Mass accounted for 40% of the weight lost, while “Lean Soft Tissue” accounted for 60%. Lean Soft Tissue comprised of 44% water, 14% muscle and liver glycogen, and 42% metabolic active lean tissue.

With a relatively short fast with already low body fat percentage, the ratio of muscle-to-fat loss may not prove worth it for you.

https://academic.oup.com/ajcn/article/68/1/1/4666001 states that:

“Administration of as little as 7.5 g carbohydrate to an otherwise starving human halves urinary nitrogen loss (8), demonstrating the exquisite sensitivity of the human body in trying to maintain viability by preserving muscle mass.”

So maybe mix a teaspoon of honey with some tea and sip it during your fast to help cut protein catabolism in half? Might come in handy.

If you decide to go for it, can you follow-up with the results showing your starting muscle mass and ending muscle mass? I know those cheap digital scales aren’t the most accurate, but they’re good enough to show trends over time if it changes. It’d be great data to have for the group.