In particular it helps maintain healthy bones due in part to its role in aiding calcium absorption.
It has also been shown to be of particular importance to pregnant women, with sufficient levels cutting the proportion of underweight babies and premature births.
It is fat-soluble, meaning the body can store it.
Despite this, more than half Britain's population is thought to be deficient.
The main problem is that we do not get most of our Vitamin D from our diet; rather our skin manufactures 90 per cent of it with the help of sunlight.
In winter months we are largely covered up, except hands and faces, and in any event the sunlight tends not to be strong enough to trigger the conversion process.
Getting enough Vitamin Dfrom diet alone is difficult, although oily fish (eg mackerel, trout, tuna) is a good source, as is liver, mushrooms and dairy products.
Black and Asian people do not manufacture Vitamin D as readily as white people, as the natural pigment of their skin knocks out much of the UV light needed.
More generally, as people spend more work and leisure time indoors, so their exposure to sunlight and ability to make Vitamin D, falls.
Doctors recommend two to three periods of direct exposure to sunlight per week throughout the summer months. This, they say, is sufficient.