The magic of 16:8 vs 15:9/other variations?

26F/5’5/190lbs (~86kg). Hello! I am wondering if anyone can provide any research or previous threads that demonstrates how results and changes to our body vary with the length of fasts. To clarify the question, Does reaching 16 hours have tangible benefits over say, 15 hrs? Is it as simple as a linear equation, as in, every extra hour in a fast provides the same rate of benefits? Or do certain benefits only kick in after specific intervals of time? Is there a point in time we reach where we begin to get diminished returns? Part of my curiosity was because I found that I’ve been naturally fasting about 15hrs a day for the last several years, just because I don’t usually eat until 11ish and almost always finish dinner by 8. So I’m just curious how much of a difference in benefits I’ll get from waiting for that 16hour mark. (Of course that’s not the only variable because I am now also focusing on my 500kcal caloric deficit during my feeding hours). I also just want to understand and learn more about the mechanisms that make this successful:) Please feel free to redirect me to another post or research if this topic has already been well outlined before! Thank you!

Answer

It’s not magic, but it is non-linear. The most noticeable threshold is when you use up your glycogen and shift into ketosis. Apps will put a specific hour on that, but it depends on how much carbohydrate you’ve been eating and how much energy you’ve been consuming, on top of individual variation.

Changes to hormone levels and the corresponding effects (e.g., hunger signal regulation from Insulin/Leptin/Ghrelin regulation) are very personal, as would be the effect on insulin resistance, for example. For weight loss, those are going to be the important factors, and you basically need to self-experiment to see what works for your body with its unique history and gut biome, the foods you eat, your activity level and timing, sleep cycles, and whatever other variables there might be.

For fitness/hypertrophy, it’s a similar question with growth hormone (somatotropin) production, but IIRC that doesn’t really kick in until the 36-48-hour range.

Autophagy is much harder to measure or notice, and there are different kinds; macro-, micro-, lipo-, mito-, etc., which occur on different timescales and triggers. For that aspect, I think the benefits start around 18 hours, but don’t really become significant until after 24.