Martin Berkhan created a very lucrative coaching platform with Intermittent Fasting: https://leangains.com/
Many books have been written on IF for profit:
And countless others.
After about 36 hours I've consistently measured 1.3-6 mmol/L. Between 48-60 hours I've measured around 4.4-4.6. As I understand it, anything above 0.5 is essentially ketosis. I agree with you in that, at least to me, there is a feeling of when you are in ketosis.
Ketosis is the generation of ketone bodies in the absence of glucose, ketoacidosis it the generation of ketone bodies despite the fact that the bloodstream is flush with glucose. Ketoacidosis is clearly a real concern, however as I understand it it's primarily an issue for type-1 diabetics.
This article feels awfully superficial, but I find the final statement, "maybe not being in ketosis isn’t so bad after all—now just cut back on the saturated fats", to be very reasonable.
Some say that fasting activates "autophagy" which has great health benefits, supposedly being great for preventing the remission of cancer: https://en.wikipedia.org/wiki/Autophagy — but note that I could not find any concrete study or proof of it, this being anecdotal evidence from some clinics that advise their patients to do fasting instead of chemotherapy.
I practice fasting for 36 hours, 3 times per week. Monday, Wednesday and Friday I don't eat or drink anything other than water and coffee. You can go shorter, like one meal per day in some days or longer, e.g. 5 days is easy once you get used to it.
As a tip: eating high fat, low carbs meals before fasting helps. Also if you get dizzy or if you get cramps, you probably have an electrolytes imbalance and you need to stop your fast. If that happens to you, make sure to take salt (sodium) and possibly magnesium supplements. Once I started doing that, the dizziness and the cramps stopped. But document yourself first and consulting a doctor and doing some blood work might be good ideas.
This is a pretty good book on the subject: https://www.amazon.com/Complete-Guide-Fasting-Intermittent-A...
For context, I'm a T2 diabetic, whose diabetes has "progressed" to the point that I take insulin. I'd recently had to adjust my dose to 100U / day (I only use a once a day, long-acting insulin at the moment).
Then a buddy of mine mentioned a book on fasting and said that some diabetics had had great results from fasting. I ordered the book, but haven't read it yet. But I did start a sort of psuedo-fasting routine of my own design. Basically, I allow myself a normal sized meal, and then for the next 48 hours I an aggregate total of maybe 1000 calories. A typical meal during that time is a thin soup made of chicken broth, jalapeno peppers, banana peppers, cherry peppers, cactus and onion, with some spices thrown in for flavor. Note that jalapeno peppers, banana peppers, and cherry peppers, and cactus share a common trait: extremely low calorie content, and low carbohydrate content.
Anyway, after doing this for about 2 weeks, I can already see a pronounced difference (for the better) in my blood sugar numbers. And that's even after lowering my insulin dose a bit.
I don't know that the protocol I'm using is anything close to what the experts recommend or not. But it definitely seems to be having a positive impact. I'm really looking forward to seeing how this works out long-term. Especially once I can start lifting weights again (I hurt my arm a few weeks ago doing some bench presses and haven't been able to lift for a while). Resistance training has been shown to help with insulin resistance, so I'm hoping the combination of regular lifting and this dietary approach will yield some good results.
These videos by the channel "What I've Learned" are great:
- Fasting vs. Eating Less: What's the Difference? 
- Longevity & Why I now eat One Meal a Day 
The channel  also has many other videos on the topic of nutrition, with similar emphasis.
Also see the excellent and readable book "The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting" by Dr Jason Fung and Jimmy Moore .
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