Hi, and welcome to the forums.
I see from your other post that you are a teenager. I’m wondering what you’re hoping to accomplish with such a long fast. You are already very thin, so hopefully it’s not about weight loss.
I often wish I could go back and take care of my body from such an early point. You have a unique opportunity to optimize your health for life, while your body is still building itself, and it seems to be important to you now, which is wonderful.
I am not a doctor, but the needs of a young body are different. Unfortunately, teen years are also a time when eating disorders that can affect your long term health are more likely to form. I think you would be much smarter to focus on putting the right stuff in, rather than keeping nutrients out.
I’m older, a small person of healthy weight for my size but without a lot of extra padding. I’ve done a lot of dumb s*** to my body over time, and I understand the emotional temptation to try a long fast, but I don’t think it would be wise for my body, so I try to create other physical and spiritual practices that feel cleansing and renewing. I definitely wouldn’t consider a three week fast if I was already ‘bone thin’, even one week doesn’t seem appropriate.
A ketogenic diet can lead to lower appetite, so yes, that’s normal. Whether to go with it or avoid calorie deficit is up for debate, but a calorie deficit can lead to metabolic slowdown, which most people agree is not a good thing.
Please be kind to yourself! I’m hoping you feel comfortable to share some of your ideas and personal health journey here. There is a lot of good info here … And some pretty nice people, too. 