Icebreaker Merino Wool Underwear
 

This New Zealand company's products are great for travelers. Lightweight, warm in cold weather, cool in warm weather. Used the bodywear long-sleeve shirt together with a long-sleeve sweater all over Austria, Switzerland, Italy down to Sicily and it worked in all the climates. And even after wearing three, four days in a row they don't smell bad.
As the guy who told me about this product said, spend a bit on the clothes right next to your skin and you won't regret it.

I'll have to second that...

I have the Ice breaker Pants, top, and jumper which are just the berries...

I fully agree: one of the best items we take for travelling. We both have the sweater-type, black with a short zipper at the collar. Wouldn't leave without them even for a weekend. We tried a few of the synthetic so-called "made for Antarctica" pullovers, and geeeeeee do they get smelly after even half a day and even in cold weather !
The Icebreaker is solid (we've had ours for 4 years and no sign of wear), can be washed at 30 or 40 degrees C or by hand of course, is thin enough to wear underneath anything, looks stylish, and --most importantly-- is very warm.
Not cheap, but none of these types of "underwear" are. Go for the natural fabrics, and keep smiling ;-)

Trui

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Iris and Trui
2 belgian women, often travelling on motorbikes (now on DR650SE's)
2nd overland from home to Northern India and back, April-October 2002