The bigger picture

Vitamin D helps to regulate the amount of calcium and phosphate in the body, which is essential for healthy bones. Unlike other nutrients, there are few dietary sources of vitamin D, and the main way in which it is obtained is from sunlight. Ultraviolet B rays from the sun convert a form of cholesterol into vitamin D3, which is carried to the liver and then the kidneys, where it is transformed into a form of vitamin D that the body can use.

But for around half of the year in the UK, there isn’t the right wavelength of sunlight for the skin to start the process of synthesising vitamin D, and as it can only be stored by the body for one to two months, many people become deficient. National surveys suggest that around a fifth of adults and anything from eight to 24 per cent of children may fall into this category.

A deficiency of vitamin D can be asymptomatic or very vague in nature, producing only tiredness and general aches and pains, for example. But a significant shortage of the nutrient in adults can lead to osteomalacia, a serious condition in which the bones become soft and painful, and muscles become weak as a result.