I learned fairly recently that it's more impotant to be "fat adapted" than in ketosis, though you may need to be in ketosis, or at least eating a very low levels of carbs, to get there to start with. We need to be eating few enough carbs for long enough (times & levels depend on the individual) for our body to make a chemical change to enable this to happen, and we can then use both carbs/glucose and fats for fuel, but we don't actually need to be in ketosis at all for this to work. Because it's easier for our bodies to preferentially use carbs as fuel, on a high carb diet this is what happens and it would appear that we then "lose" our natural ability to also use any fats either dietary or from our stored body fats for fuel as we will tend to get enough from our carby diets.. But once we're (back being) fat adapted it's much harder to lose this ability than it is to stay in ketosis if we might eat (a few!!) more carbs occasionally. Apparently being in ketosis and being fat adapted can be mutually exclusive.
(This is my idiots understanding/guide, and there are various blog posts which will explain the process in more technical detail. I've posted links elsewhere, I'll see if i can find this post again...))
Robbity