At higher latitudes you can only get vitamin D from the sun for a few months of the year. In winter, the sun is too low in the sky, and the UV wavelengths neede

At higher latitudes you can only get vitamin D from the sun for a few months of the year. In winter, the sun is too low in the sky, and the UV wavelengths needed to produce the vitamin D is entirely blocked by the atmosphere(though unfortunately the UV for sunburns is not). This is a method of "storing" summer vitamin D for winter consumption.