The major cause of Vitamin D deficiency is lack of sunlight. In colder countries in the winter, it's very common to have too little Vitamin D, and with a slight deficiency the risks are quite mild. Many people have no idea that their Vitamin D is low, but may only be aware of feeling rather low-spirited and lacking in energy, or have some general pains and discomfort with no obvious cause.

With more sever deficiency, there is a risk of a number of serious problems. These include depression (especially seasonal affective disorder-related depression) and severe bone conditions. Osteoparosis is linked to lack of Vitamin D and in recent years, doctors in the UK have seen a return of rickets, a childhood disease that can lead to bones not forming properly. This illness had almost disappeared but is now being seen again, and the chief cause is believed to be the very indoor, restricted lives led by many children in cites today, as well as (often) a poor diet.

You can learn more about the risks here.