I had intended to turn this in to an article, or just post it on my blog. However, I came up short on content, and thought it would better serve the community here. The forums also provide more exposure, and thus a higher potential for interaction and feedback. I know others (Ryan J., Dave C., etc.) have likely had similar experiences and will hopefully chime in.
In the late fall of 2007 I was roughly 175 pounds and in the range of 18-20% body fat.

This photo is from 3-day loop around Newton Bald in the Smokies. We experienced temperatures in the low teens, and I was toasty in a thin wool shirt and 50g high loft synthetic jacket.
Between the first of 2009 and 2010 I dropped to around 138 pounds and a pretty consistent 6-7% body fat. I’ve since been out in similar conditions to that 2007 trip, and despite a couple layers of down on my torso (probably 3x the warmth of that 50g jacket I had), I was cold.
On a recent trip, we experienced pouring rain and 60-65 degree temps for a couple hours. My wife and dog enjoyed themselves in the cooling rain, a nice break from the southern heat, while I was miserable. Even at a light jogging pace I was covered in chill bumps and trying not to shiver.
The other side of this is thermodynamics. Post meal, when turning in to bed, or during physical activity (especially of the intense variety) I give off heat like a furnace. But, as soon as I stop moving or run out of calories, I chill quickly.

This photo is from a traverse in the Bob Marshall Wilderness Complex of Montana. I was wearing a thin merino wool hoody and compression shorts, as well as full gaiters and snow shoes. But, if we stopped moving for very long, I started to get cold and had to suggest we start moving again.
So what does all this really mean? A tipping point with body fat appears to exist, where going too low requires carrying additional insulation to maintain warmth. Finding that exact point would require study of a more scientific nature, but I’d estimate it to be in the range of 10-12%. Below 10% and you start to need additional external insulation to make up for a lack of internal insulation. Were that the only trade-off, I’d probably say it’s a good one. After all, a one pound down jacket is, in theory, likely to insulate the core far better than a pound of fat would. Unfortunately, low body fat also starts to limit your reserves, thereby limiting your potential trip length. We can only carry so much food into the back country, and it’s never quite enough to cover our daily expenditure. Couple that with little to no on-board reserves and you spell trouble, fast.
chris wallace | outdoorswith.me

