If you’re not familiar with base layers, the term referrers to the layer of clothing worn closest to the skin. The primary function of base layers is to ‘wick’, or draw off, moisture from the skin, so that you stay cool and dry in all weather conditions!
An effective base layer is important in both hot and cold conditions, although in the latter it should also have insulatory properties to maintain the core body temperature without causing overheating. If you are active all year round, you may need to invest in a base layer with wicking properties in the summer and another with increased warmth in the winter.
Materials such as cotton or polyester absorb moisture from the skin and, as such, become damper and damper over time, eventually leaving the wearer cold and wet. A typical base layer is therefore constructed from synthetic materials, such as polypropylene, or natural, sustainable materials, such as bamboo fibre or merino wool, which have better moisture wicking properties and breathability.
Synthetic materials have a reputation of trapping odour after just a few hours of intensive use. This is not, necessarily, a problem, but can become unpleasant if you engage in longer periods of activity. Merino wool is softer and warmer than synthetic material, but are less durable and, despite the wool fibres allowing perspiration to evaporate, do not wick as well. They are also significantly more expensive. Bamboo fibre is often combined with other materials, such as cotton or Lycra, to produce a base layer that is similar in performance to a merino wool base layer, but softer, more comfortable and less expensive.
So-called 'compression' base layers are available in the form of long or short-sleeved tops, leggings, shorts and underwear, but the idea is that, as well as regulating body temperature, they hug the figure to promote muscle efficiency and recovery, even in extremes of temperature. Compressing the muscles reduces the build-up of lactic acid, which operates as a temporary energy source, but also causes a burning sensation, during exercise. The nature of these garments means that they must be comfortable, so they a typically constructed from extremely fine fabric, with flatlock stitching for less bulk and less chance of abrasion against the skin.