To get to 85% fat, you can either shove the fat in, or reduce carb/protein, or a combination of the 2.
The medical ketogenic diet in the studies is generally fueled by crap oils, because of the nutrition establishment's fear of gasp horror saturated fat. As it was first developed, it restricts carb and protein and water, to get the concentration of blood ketones high. But recent research suggests something like a modified Atkins works just as well.
The point of nutritional ketosis, imo, is less about achieving an arbitrary fat % than it is about overcoming a glucose-dependent (in whole or in part) metabolism. IOW, it's about eating a diet sufficiently low in insulinogenic foods.
You can have 85% fat in a diet, and still eat 300 g protein. You're not going to get into nutritional ketosis, that is simply too much protein. Protein stimulates the release of insulin, some say moreso than carbs. And some portion of it will convert to glucose, which will exacerbate the insulin problem and even moreso be enough to keep you out of nutritional ketosis.
But if you construct an 85% fat diet with say 50 g protein (or heck, if you're male and athletic and work out like Attia, 125 g protein), then you have a chance at keeping insulin nice and low.
This is where, again, the state at which nutritional ketosis occurs must be highly individual. Someone may have a 75% fat diet, but eat such little food that they only eat 50 g protein and 10 g carbs, and be in nutritional ketosis because the insulinogenicity of their diet is so low.
Bernstein's approach is to figure out what protein you're eating now, and if necessary cut 1/3 of a meal's worth of protein and see what happens--this is for weight-loss, so we'd assume (as most LC'ers do) that weight loss and blood insulin levels are inversely correlated.
Or you can go from the other direction, and use Rosedale's calculus to figure out the eentsy weentsy bit of protein (imo!) you need, and see how much more you need to just make it so you don't have side effects like dreaming of steak and stuff.
But I can't see how nutritional ketosis will occur with an arbitrary percentage of fat, absent a low-enough and highly indivdualized bright-ish line limit on carbs and protein, which are the only dietary means we have to attempt to control release of insulin.
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