Most people either underestimate or completely skip this layer and only a few realize its significance. It wicks away moisture (yes, your body also likes to sweat in winter) into the next layer where it evaporates better. All base layer items is to be worn on naked torso and should not be bulky, as bulkiness lowers its effectiveness. When choosing the piece for your base layer, search for the right materials:
Synthetic fabrics: This type of material is excellent in wicking away moisture, and is commonly used by professional athletes. The problem comes with frequent use – when worn (and sweat into) over longer periods of time, it tends to smell bad and requires frequent washing. However, technology and development alowed advanced synthetic fibres to lose this handicap (and also gain in price).
Merino wool: Another popular base layer material. It has outstanding moisture absorption properties and keeps you warm. Cheap merino clothes can feel itchy on your skin, so best avoid those. High-quality merino is comfortable, but gets a bit pricey. It gets no unpleasant odour, so you can wear it over the course of a few days and hop on the tube right after without causing biohazard alarm.
Silk: Becoming less popular, but still works its way. The problem with silk is its demanding maintenance, which should not be neglected. Price is another minus, due to which synthetic fibres and merino wool are more widespread.
Cotton: Tends to be fairly popular, but does not have the best properties. It absorbs moisture very well, but does not wick it away very well, which is a problem in below zero temperatures. Nobody wants a leaking piece of clothing on in cold winter. Put simply, cotton is terrible for your base layer, so best avoid it completely.