Nutrition studies are mostly very low quality. Nina Teicholtz has a new article out: US Dietary Guidelines: Science is "Subpar," Cannot be Replicated: New paper finds systematic reviews to be of “critically low quality"

If you can't access the above link, here's an unpaywalled version: https://archive.vn/XhXvg

I recommend the book The Big Fat Surprise, by Nina Teicholtz. It's an amazing journey through our nutritional guidelines, how the "science" going into them was a giant tarball of special interests and poorly done research.

I wrote a more involved review of The Big Far Surprise here: #320715

That book really took the scales off my eyes. I no longer believed the standard narrative (low-fat, fruit, vegetables, and fiber is good, meat, eggs, butter, saturated fat is bad). I started reading and listening to carnivore authors, and am now carnivore. Best thing I've ever done for my health.