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Polyunsaturated fats are known to suppress thyroid function, this leading to lowered metabolism and weight gain. A quick search brought up this:

ANTI-THROID EFFECTS OF PUFAS (POLYUNSATURATED FATS) AND HERBS January 2012 Trakya University Journal of Natural Sciences 13(2)

Authors: Başar Altınterim Malatya Turgut Özal University

According to Ray Peat, any amount of PUFA is toxic, but especially so in quantities of more than a gram a day. And most of us today get tons more than 1g. One of the reasons is the desaturation of cardiolipin in mitochondria, leading to instability and dysfunction.

This is consistent with the elbow of the increase in obesity being about 1980. Canola oil was invented in the 1970's and really hit its stride in the US in the 1980's.