My self observations (autopsy). I’ve been relatively thin all my life. I ate SAD (and drank beer) for pretty much 70 years without serious detrimental effects until my 60s when I started slowly gaining weight. So my guess is that by then my metabolism was beginning to get insulin resistant. This reversed completely when I started keto at the age of 71 four and a half years ago. I’m back to about exactly my weight and comp when I was 18. And I’ll take it!
All those years I preferred cold to hot. Sure, when young I could sun bathe on the beach - for a couple of hours. I spent most of my childhood in Savannah, Ga. But even then I preferred the air-conditioned lounge afterwards. Heat literally makes me melt. Did so all my life and continues to do so. I lived for a decade at Lake Laberge Yukon (the 70s, when the big climate threat was the coming Ice Age!). Summers were moderately warm and short, winters were very cold and long. I thrived in that environment. In fact, I looked forward to the coming Ice Age with eager anticipation.
So I think there are some of us who are genetically preconditioned to being heat radiators, or ‘wasting energy’, as Bikman puts it so succinctly. Maybe my predilection for cold is an indication that I gravitate towards the best place to be - for me.