Lerner posted a link to this study in comments on another post: Cold-Activated Brown Adipose Tissue in Healthy Men. A small part of the study where they looked at BAT activity for the three subjects with the most BAT under neutral thermal conditions caught my eye, and I think this is central to some of the confusion Kaptain Konfusion has introduced into this whole discussion. They tested all subjects lying on their backs in a chamber, first for one hour in thermoneutral temp, next for 2 hours at 17C. For three subjects they retested in thermoneutral temps for the entire time.
There was no detectable brown-adipose-tissue activity in the three subjects who were retested under thermoneutral conditions. Brown-adipose-tissue activity was recorded in Subject 2 at 856.5 kBq during exposure to cold and 0 kBq in thermoneutral conditions, in Subject 3 at 587.2 kBq during cold exposure and 0 kBq in thermoneutral conditions, and in Subject 4 at 250.0 kBq during cold exposure and 0 kBq thermoneutral conditions.
What this clearly shows is that just having BAT doesn't mean it's active when the thermal energy it is capable of generating is not required. When you look at where humans have BAT, it seems mostly to keep our heads and neck warm. BAT is active in infants -- very small humans -- and generally less active in adults. Not surprisingly, BAT is a far more important factor for small animals, like rodents, than large ones. So in adults we still have to maintain a body temperature often radically differing from our environment, and one can come to the reasonable conclusion that thermogenesis -- heat creation -- comes from processes other than uncoupling in BAT.
Humans living in colder climates tend to retain more BAT/BAT function than those living in warmer climates. While clearly we can use BAT, we also make use of WAT for insulation for body temperature regulation. I mentioned this before, but say it again: cold-water mammals are fatty fat blubbery fatties! However, this 1980 paper cites studies where Alaskan Eskimos had similar subQ fat to whites. Could perhaps our northern cousins have learned the fine art of exogenous insulation? Methinks so. Incidentally that paper claims mostly that the higher basal metabolism is from the high protein and fat content in the diet. This would actually be the high protein content, if true. In any case, whatever the adaptations, the Inuit are cold-adapted from living in that climate all the time, not from jumping in the ice water from time to time!
So this whole cold exposure therapy for adaptation, etc. seems a bit confused. If the goal is to increase BAT, I ask "to what end?". Even if you sit in an ice bath for an hour a day, every day, there's those other 23 hours in the day when you'll probably be in ambient temperatures. I get if you purposely turn down the thermostat in the winter and crank the AC in the summer time and spend almost no time outside when the air temp creeps above, say, 50F, you're going to use that BAT. But I also think that is when your body may start thinking about upping the subQ fat to adapt you to your chronically cold environment. If you're not, then it's really the time you spend in the cold and the calorie-intensive thermogenesis in response that's the issue. I was happy to see Ray Cronise suggest good old fashioned swimming in 80 degree water over the short periods lying in very cold water. He also warned to be mindful of the hunger that might result in compensatory overeating some 2-4 hours afterwards. I say might, because this is something that I have never experienced myself. I always used to lose weight in the summers when I was younger because I went to the pool and swam and dove as much as I could and just never felt particularly hungry afterwards. If anything I like to eat lighter in the summer and my lengthy but moderate "cold therapy" (who knew?) didn't have me itching for a lot of food. Chronic cold makes me hungry, which is the opposite of what some of the Khillin' Krusers are claiming. Eh? On that note it's important to listen to your own body, right?
In any case, this study showed that your BAT is only going to rev up while under thermal stress, so just having more BAT isn't going to do much for you unless you have a use for it. We see that Subject 2 had more than 3X the energy output from their BAT vs. Subject 4, and yet in thermoneutral conditions, didn't manage to produce squat all the same. The "cultivating" of BAT may well have some life-improving qualities such as better cold tolerance, so I'm not saying it's a futile endeavor, however it's not going to be a calorie-burning bonus over WAT if you don't need it to be. Still, it does burn off calories during the cold exposure, and for some, an hour in the cold is preferable to exercise.
So there's cold adapted and then there's cold adapted. If you improve your tolerance of cold with intermittent cold exposure that "pumps up" your brown fat, a better term for this might be cold conditioned. Because to me, cold adapted really applies to folks living 24/7/365 in colder climates. That's adaptation (and favors a layer of fat for insulation if anything), so you won't see many shredded humans living by traditional means in arctic climates.