Intermittent Fasting is about WHEN you eat, not WHAT you eat.
Diets (LCHF, keto, paleo, Dukan, Atkins etc.) likely neutralize consequences of incorrect eating. They temporarily (as long as you keep dieting) and partially compensate the harm caused by frequent eating. IF just removes the causes resulting in excessive weight – you just stop interfering with you body to perform its normal work of maintaining your health (which it does perfectly fine without interference!).
Sooner or later, most people quit dieting – simply because at some point they had enough of it, and the supplies of your willpower run out before your fat does :-) . By contrast, you can easily eat along with Intermittent Fasting for life – it does not require any willpower, everything goes naturally, and you will not want to change anything.
You may ask whether removing carbohydrates does not make Intermittent Fasting a diet? Just remove bread, spaghetti and potatoes from your menu, and zzzip! – your weight plummets without fasting. Well… you may lose some weight, but will you reach the ideal body mass index? Or you will rather drop a few pounds and stop?
Strictly speaking, removing carbohydrates by itself is not part of the method known as Intermittent Fasting. I read stories and watched videos with people who dropped their weight ONLY due to eating once or twice a day (16:8 or OMAD) – however, these people were not too obese from the very beginning. As for me personally, I removed carbohydrates and the process of losing weight goes by leaps and bounds; but if I did not, I would still lose SOME weight (but I beg your pardon for the lack of desire to experiment with this method). If you allow me to say so, Intermittent Fasting removes the obstacles that were preventing the effect of removing carbohydrates to reveal itself in its entirety. Which means that removing carbohydrates combined with intermittent fasting regime, is as powerful as a nuclear bomb.
Apart from the above, Intermittent Fasting does not involve complicated logistics inherent to diets (whose description takes long paragraphs and require thorough studying). You don't need to count calories, weight your food on a kitchen scale and calculate the ratio of proteins, fats and carbohydrates. You simply remove carbohydrates – that’s it! Two words instead of long explanation.
And now we are talking about the process of losing weight for obese (and VERY obese) people: being technically not part of Intermittent Fasting, elimination of carbohydrates is still necessary. And when you achieve your ideal weight due to IF (which is quite easy, compared to diets), a lot of "usual diet components" will be removed, or their significance will decrease (but of course, if you keep consuming fried potatoes, pizza and ice cream in large quantities, Intermittent Fasting is unlikely to help you a lot). If the ideal weight is achieved due to dieting (which will require titanic efforts and is available rather to those whose weight was not initially huge), any indulgence will immediately result in a turn of 180 degrees. In other words, if you do not want to revert to obesity, the heavy burden of a diet will accompany you to the end of your life.
Fasting provides multiple advantages, important and unique, which are not available in conventional diets. As a rule, a diet makes life complicated and fasting makes it simpler. A diet takes some time and imposes restrictions on your schedule (you must worry about the time and place to eat many times a day), while fasting saves time - it is available always and everywhere, you don't need to perform any actions. A diet costs money, while fasting is not only essentially free, but also leads to significant savings in your grosery expenses (you buy much less food - maybe, 30%-60% of what you used to buy previously, depending on protocol - 1 or 2 meals a day). A diet may or may not be efficient, and fasting is indisputably efficient. There is no other method to reduce the insulin level and the body weight as powerful as this one.
From The Joy of Fasting:
Believe it or not, but after achieving my ideal weight [with Intermittent Fasting], I still do at least one fasting day a week – not out of necessity but simply because I enjoy it. Bonus good news: It means that this lifestyle, and the weight loss that comes with it, is sustainable. It's easy to keep the enjoyable way of life. Major difference from the restrictive diets that make you miserable, isn't it?
Calorie restriction diets decrease the basal metabolic rate
In very simple words, the basal metabolic rate (BMR) is the amount of calories burned by your body at rest (if you have no physical activity during the day). Let's say your BMR is 2000 calories per day. To be in caloric deficit (i.e. to burn more calories than you consume), you decide to start a classic (calorie restriction) diet with daily caloric intake of 1500. After a whilе, your body adapts to new conditions by decreasing its BMR to 1500, so you are not in deficit anymore. To keep losing weight, you keep decreasing your caloric intake, but the body keeps adopting... It's a very primitive explanation (only to illustrate the idea), but you understand that this "ping-pong" can not continue indefinitely. So, you can stop losing weight long before you reached your ideal weight, or your weight will go back up (whether or not the ideal weight was reached). Because of this, people are forced to raise the actual daily calorie burning by sport, becoming attached to the gym under the threat of weight gain. Now you understand the mysterious phrase "without sports it is impossible to lose weight" repeatedly heard from those who are on diets?
The described problem doesn't exist in Intermittent Fasting at all. That's why it's very realistic to reach the ideal weight in a few months (or, maybe, a year or so if you have morbid obesity), and then easlily keep that weight for the rest of your life.
The issue of decreasing metabolism by diets is mentioned by Dr. Fung in this video: