And if for whatever reason you're not eating enough fat, your body will crave sugar.

So basically eat healthy fats. If you don't eat enough healthy fat (e.g. from walnuts), your body will start craving unhealthy fats. And if you don't eat either then your body will crazy sugar as a last resort.

As far as I can tell that's basically the secret to staying healthy, just make sure to get enough walnuts and avocado or whatever, and once you have your RDI of healthy fats then craving unhealthy fats or sugar won't be an issue.

That's in contra to how my system responds - it craves fat when I'm not eating enough of it. Since I gave up eating (refined) sugar, I don't even think about it.

Does that mean you haven't included sugar in your foods yourself, or you've managed to avoid eating any prepared food since you started?

Practically all prepared foods contain sugar, though they might not call it that in the ingredients list. When next you go boy chicken or turkey, read the list of ingredients. I'm talking about the raw forms, not the already cooked ones. It's shocking.

For me, the big change was desserts. I was a fiend for those. I've tried a lot of things to ween myself from eating and binging on sugary treats. I wasn't expecting IF to be the thing that worked. That was totally unexpected.

Whenever your diet consists of carbs and fat nutrients, the fat is going to be immediately stored in your fatty tissues, and any excess carbs that you can't burn quickly will go there also.

I buy that argument with high-fat zero-sugar diet, but is there a reasonable evidence that eating a high-sugar but low-fat diet is healthy?

There are essential fatty acids, there are essential amino acids (protein), but there are no essential carbohydrates.

The "need" for sugar is completely manufactured, literally and figuratively.