From what I understand, keto goes further than just low carb high protein. The goal is to have your body burn fat as its primary energy source (rather than as a secondary behind carbohydrates). So you cut back on carbs almost entirely, and ramp up you intake of fats. I believe it takes a couple weeks or so to get to “ketosis”—where you’re burning fats as your primary energy. Some who do the diet will actually test whether their body’s in ketosis by measuring the amount of ketones in their blood (I assumed it was a finger prick test along the lines of a glucose reading, but the Wikipedia article I pulled up for reference shows an image of a urine-based test.) I’ve heard from people on it that they feel good—more consistent energy levels, rather than ups-and-downs caused by sugars.
From what I understand about intermittent fasting, there’s thinking now that the body can do just fine with longer breaks between meals—and that longer breaks, at least on occasion, may even be beneficial for certain functions of the body (though I don’t think the science has quite decided on that part yet/) It’s interesting to me because it’s in direct conflict with what I’d been told growing up, that if I didn’t eat every x number of hours, I would wreck my metabolism and store more fat (“x” being anywhere from 3 to 6, depending on who was saying it). Well, now science is showing us that such “metabolic meltdown” isn’t true—that the body can regulate itself during longer breaks between meals… and frankly, that makes sense. For now, I’m just going with “don’t sweat the timing of meals so much,” and listening to my body instead of the clock.
I don’t have very strong opinions on either diet. I tend to be a bit leery about fad diets in general—I’d rather just try to make sure I’m eating balanced, varied, nutrient-rich foods—and admittedly, I’m bad at following any diet much more specific than that—but if these work for people, then cool.