The "sunshine vitamin", vitamin D, is increasingly seen as vital to health, yet many people are not getting enough, US researchers have found.
Analysing data from government health surveys, researchers from the University of Colorado School of Medicine found three out of four Americans had "insufficient" levels of vitamin D, up from about one out of two people 20 years ago.
A simple blood test can determine whether someone has what most researchers believe is an ideal level of vitamin D of between 30 and 40 nanogrammes per millilitre of blood.
Getting enough is simple. Spend 10 minutes in the sun with legs and arms exposed, or take vitamin supplements.
People's habits have also changed.
They are less active, spend less time outdoors and also protect their skin from the sun because of the risk of skin cancer.
Slathering on sunscreen and wearing clothing and hats to protect skin from the sun's rays halts production of vitamin D in the skin.
Traditionally, vitamin D has been seen as vital to bone health and to prevent rickets, but recent discoveries show many types of cells in the body contain vitamin D receptors, which indicates they use vitamin D in some way.
Food can boost vitamin D levels, but only a few foods contain significant amounts. Those include some types of fish - salmon, sardines, tuna and mackerel - and most dairy products that are enriched with the vitamin. - Reuters