I want to know which animal models these studies use and whether those animals actually are analogous to humans.
From my own experience (aside from the MCT oil debate): Over 20 years ago, I increased my fat intake and decreased my intake of sugar and processed foods. Doing so benefitted my brain health. Over the years I’ve fallen off that, but in the last few years, since “the keto diet” has become a thing, I’ve tweaked my diet some more. My partner went on the keto diet last year to lose weight and gain more energy and, because I’m now living with someone who eats the way that works best for my brain, I’m more diligent with it.
I always feel better when my consumption of healthy fats is high, and my sugar intake is low. I focus better. I think more clearly.
That said, your mileage may vary. Do I think that MCT oil and the keto diet are for everyone? No. I don’t believe in a one-size-fits-all approach to health. Some people do feel better on an all-meat diet or on a raw, vegan diet. Some nutritional facts are universally accepted — such as sugar being an immune-suppressant, vitamin D being good for brain health and the immune system, the benefits of probiotics, etc. The approach doesn’t need to be the same for everyone, though. (Aside from the fact that B-12, collagen & other nutrients can only be obtained from animal products.)
I also have sensitivities to yeast, which is a separate thing entirely.