Hi Dutchie–The first part of what you said was part of my point, but I may not have explained myself well. It’s not the overfeeding, per se, that increases BMR, but the resulting weight gain. I think most people will gain a combination of fat and lean tissue from overfeeding, and these are the two greatest factors in explaining an individual’s BMR (from the paper I looked at–http://ajcn.nutrition.org/content/82/5/941.full).
Even if we just gain fat from overfeeding, that fat alone will increase our metabolism. However, our fat-free mass (mainly muscle, I suppose) is five times more metabolically active, so increasing muscle mass will have an even greater effective on BMR.
Of course, it’s much harder to build muscle than accumulate fat. An increase of 2-3 pounds of muscle in one month would be a great achievement, while gaining 10-15 pounds of fat isn’t really all that difficult. So the “easiest” way to increase one’s metabolism is just to eat until you get fat.
Not that that’s a good thing! I’m not convinced that metabolic rate is all that significant for our well-being, except in extreme cases of hormone deficiencies or starvation. But these situations can easily be reversed by hormone replacement or eating. Normal dieting does lower metabolism, but that can be explained simply by the fact that thinner people have slower metabolisms than bigger people! That’s the opposite of what many people expect, but that’s confirmed by research on BMR.
Dieting that burns muscle will lower metabolism even more than just fat loss, so of course that should be avoided. Resistance training while dieting can help maintain lean mass, and adequate protein is also important.