Didn’t see it in the article, so to add: the brain works on carbs, which, if you don’t eat enough, the body doesn’t make from fat, but from muscle. So you’re not just losing fat, but also muscle, which is counterproductive to long term health.

https://nutritionfacts.org/blog/muscle-shrinkage-and-bone-loss-on-keto-diets/#:~:text=Ketogenic diets have been found,is the opposite of ergogenic.

Here’s one source, there’s plenty more if you google ‘keto muscle loss’