We know little about the effect of diet on health. That’s why so much is written about it

DC's Improbable Science: Truth, falsehood and evidence: investigations of dubious and dishonest science says:-

"The question of what to eat for good health is truly a topic about "which so much has been talked and so little can be said"That was emphasized yet again by an editorial in the Brirish Medical Journal written by my favourite epidemiologist. John Ioannidis. He has been at the forefront of debunking hype. Its title is “Implausible results in human nutrition research” (BMJ, 2013;347:f6698.
Get pdfGet pdf)."

More...