Dietary fat and body fat are different things. Keto is associated with the former, but if anything it reduces the latter. I think his claim is that (excess) body fat is what makes you stupid.

I don't know what specifically this is supported by, but your objection is based on misunderstanding the claim.

Edit: I just Googled it and I think he probably means this CMU study: http://www.menshealth.com/weight-loss/how-being-fat-makes-yo...

Yes, body weigh fat, not fat. Your brain actually need fat. But obesity compromises the major organs. Organs the brain depends on. Less effective organs - e.g. heart and circulation - is going to compromise the brain.

https://duckduckgo.com/?ko=-1&q=obesity+effects+on+brain+fun... https://duckduckgo.com/?q=https://duckduckgo.com/?ko=-1&q=ob...

First, it's kinda obvious. That is obesity compromises the organs. That would include your brain. Furthermore, the brain outsources a lot: blood, oxygen, glucose, etc. Obesity compromises the foundation the brain stands on.

That said, here ya go...

https://duckduckgo.com/?ko=-1&q=obesity+effects+on+brain+fun... https://duckduckgo.com/?q=https://duckduckgo.com/?ko=-1&q=ob...