It usually takes three to four days for your body to go into ketosis because you have to use up your body's stores of glucose, i.e., sugar first, Keatley says. Any major diet change can give you some, uh, issues, and Keatley says he often sees patients who complain of IBS-like symptoms and feeling wiped out at the beginning of the diet. (The tiredness happens because you have less access to carbs, which give you quick energy, he explains.)
You say keto is a highly controversial topic. For those of us following a keto diet, there is no controversy whatsoever because the diet proves itself efficacious very quickly. I think the real controversy comes in because the ADA has been recommending dangerous levels of carbs for decades now, and they would lose face if they had to change their recommendation and admit they’ve been wrong for so long. You say there are not many studies on the keto diet, and I disagree with you. You’ll find the evidence if you look for it. Eric Westman, Steve Phinney, Jeff Volek and many other researchers have written volumes about this.
The remaining calories in the keto diet come from protein — about 1 gram (g) per kilogram of body weight, so a 140-pound woman would need about 64 g of protein total. As for carbs: “Every body is different, but most people maintain ketosis with between 20 and 50 g of net carbs per day,” says Mattinson. Total carbohydrates minus fiber equals net carbs, she explains.
Keto decreases inflammation and improves brain function — so much so that Alzheimer’s patients who switch to a keto diet actually begin to recover their brain function, which up until now was unheard of.[20][21][22][23][24] So there’s that. (Dale Bredesen and Mark Hyman have discussed these on the Bulletproof Radio podcast. Check out Dale’s episode here, and Mark’s here.)  
The Ketogenic Diet is a low carbohydrate diet, consisting initially of less than 20 carbohydrates per day. Not per meal, yes, you heard me correctly, per day. It is not for the faint of heart and yes I am writing from experience. Of course I have tried it! Hasn’t everybody in America at some point who has wanted to lose weight? Does it work you ask? Of course it does! The problem is how long can you keep it up?

Flexibility. A flexible plan doesn't forbid certain foods or food groups, but instead includes a variety of foods from all the major food groups. A healthy diet includes vegetables and fruits, whole grains, low-fat dairy products, lean protein sources, and nuts and seeds. A flexible plan allows an occasional, reasonable indulgence if you like. It should feature foods you can find in your local grocery store and that you enjoy eating. However, the plan should limit alcohol, sugary drinks and high-sugar sweets because the calories in them don't provide enough nutrients.
A ketogenic diet helps control blood sugar levels. It is excellent for managing type 2 diabetes, sometimes even leading to complete reversal of the disease. This has been proven in studies. It makes perfect sense since keto lowers blood-sugar levels, reduces the need of medications and reduces the potentially negative impact of high insulin levels.