I'll just chime in here and say that perhaps one or two days a week where you eat "cleansing" foods, perhaps delay eating, could work without stringent calorie reductions.
I was as skinny in my late teens; 5' 8" and 98lbs, so I can relate to the ultraslim perspective. (I actually went on a Jack LaLanne--remember him?--diet to GAIN weight.) I used to pray Make me look like a girl...be careful what you ask for!
Before 5.2 I did a food combining thing; it was great, just too hard to do long term. However...my favorite thing about it was the rule of "eat only fruit (well, black coffee or tea was allowed, or I wouldn't have been able to do it) before noon." Mind you, that means NO yogurt or granola or anything else. I still do that sometimes, especially if I feel a cold threatening.
One of the "problems" I had when very thin (also remembering this is 40+ years ago, when we didn't know much about nutrition/health) was I ate a lot of junk; given I didn't need to worry about my weight, hey ho, chips and ice cream etc! But a bout of hypoglycemia in my early 20's led me to learn more about diet, and over the years I educated myself. So even skinnies can't get away with eating junk, it's not just the scale that shows the effects of poor diet.
So this is my way of suggesting that you change your diet more so than fasting...though as I said above, I think there are ways you can adapt 5.2 to your needs without making yourself ill...or freaking out this forum! Please understand that any reaction is out of concern for your health and well-being; we do want to support you in any healthful endeavor.
Suggestions: Eat only fruit before noon most days a week (maybe exception on a brunch out, for example). For guidance, Google "only fruit before noon" (careful, don't fall down the rabbit hole!). A couple-three days a week, delay your first meal of the day till at least noon, experimenting with going as late as you can. I fast Mon & Thurs, and have only black coffee and plain green tea (and water, of course) till 4pm, when I have a cup of veggie broth; I generally have a "snack" 6pmish (ff greek with fruit this time of year, sometimes a couple of eggs, or a small salad), then dinner 9pm or so. And try eating as healthy as possible. Many of us on the forum have eaten pretty darn healthy but since it didn't result in weight loss (not your issue), we had to branch out from there.
You should only need to "cleanse" if you do drugs or eat a lot of crap. While I'm really into alternative medicine, I do think that most of the cleansing stuff is bunk. Though hot lemon water in the morning isn't a bad thing...