D-Days?

Vitamin D can be manufactured under the skin by exposure to UV light. People living nearer the poles (especially if they have darker complexions), are likely to have to take in more of this material, in the winter (when sunlight in weaker and clothing heavier), via their diets, if they want healthy muscles and bones. In deed, it has now been suggested that this vitamin has a beneficial effect, preventing rheumatoid arthritis, meaning that people should be encouraged to take supplements, especially when days are short (https://www.theguardian.com/lifeandstyle/2017/nov/21/vitamin-d-may-help-prevent-rheumatoid-arthritis-suggests-study). Some people have even suggested that vitamin D should be added to some common foods (as iodine is added to table salt).