Within just a couple days of cutting out carbs and raising fats, ketone concentrations in the blood rise and the brain will begin using them for energy preferentially. This initial keto-adaptation process usually takes about four weeks to complete, at which point you'll reach peak fat-burning adaptations. But for every lifter who ends up loving the ketogenic diet, you'll find another who had a miserable experience and bailed after just a few days. This is a shame, because they probably could have felt great if they had simply had a better plan—or a plan at all.