"chronic stress aside, that decline in BMR mirrors almost perfectly the curves of PUFA consumption rates in the general population. Namely, as the BMR curve has steadily declined over the last 100 years the PUFA consumption rate curve has steadily moved upwards over time. Unless this trend of ever-increasing PUFA consumption is interrupted, I don’t see the decline of BMR flattening (let alone reversing) any time soon." eating massive quantities of highly processed seed oils that are ubiquitous in our food supply because they are much cheaper than "real" food is surely a contributor.
http://haidut.me/?p=2195