Following reports that too much sunscreen led to one little girl being at risk of rickets, it seems we are confused about Vitamin D – which is needed for healthy bones because it helps to absorb calcium from the digestive system. Rather cleverly we make it in our skin but less cleverly we may not realise when we're running short of it.

What is it?

Vitamin D is a fat soluble vitamin. There are two main types - ergocalciferol and cholecalciferol - the latter is made in the skin when it is exposed to UVB rays from the sun.

Vitamin D not only regulates calcium uptake by the body but also phosphate concentrations in the body - together these are responsible for healthy bones. So, if you don't get enough Vitamin D you can get rickets if you are a child (bowing of the legs, pigeon chest and other misshapen bones) or thinning of the bones (osteomalacia) as an adult. Adults may also get bone and muscle pain.

Where do you get it from?

Not many foods contain Vitamin D. We need about 600iu (international units) a day, on average, after the age of one. The Office of Dietary Supplements in America (a government funded body) says that fish such as salmon, tuna and mackerel and cod liver oil are good sources. A 3oz portion of salmon, for instance, has 799iu. Some foods are fortified with vitamin D and will have about 100iu or more per serving, eg orange juice or milk. Surveys in America have found that people got only about 204iu to 288iu a day from food.

Some people get at least some of their requirements from the sun – when UVB light interacts with a previtamin to make Vitamin D. But clouds, the amount of melanin in your skin, how much skin you have exposed to the sun, and the amount of sunscreen you use will all affect how much you make. The Office of Dietary Supplements estimates that you would make enough Vitamin D by having between five to 30 minutes exposure to the sun twice a week, between 10am and 3pm, to the face, arms, and legs or back – without sunscreen. If you are in fiercely hot climates you may need to get your sun outside of the hottest times. Babies should never be put in the sun – even with sunscreen.

If you are pregnant or breastfeeding you may want to take a supplement because there is some evidence that babies who are breastfed by mothers who haven't had Vitamin D supplements have an increased risk of developing Vitamin D deficiency if they are exclusively breastfed. But the evidence is not that clear and Nice says pregnant and breastfeeding women can choose to take up to 10 micrograms a day. You may choose to give your baby supplements if they are exclusively breastfed but should discuss with your doctor or health visitor first.

Who is at risk from not getting enough?

Anyone with limited exposure to the sun. Older people in homes, those who cover their bodies for religious reasons, people with darker skin – because they find it harder to make vitamin D from UVB rays. Anyone who has a problem absorbing fat, because Vitamin D needs fat to be absorbed from the gut. It is not known how much sunscreen restricts the production of Vitamin D in the skin.

What happens if you don't get enough?

Without enough Vitamin D, your bones can get thinner, softer and weaker. They lose mineral content, causing pain and increase the risk of breaking a bone. In children the lack of Vitamin D causes rickets.

Having enough Vitamin D may have a role in preventing bowel, breast and prostate cancers as well as diabetes (both type 1 and type 2) and multiple sclerosis. The evidence is way off being conclusive. And you should not take more than the recommended amount of Vitamin D.

When should I see my doctor?

If you have bone pain or muscle weakness or are exclusively breastfeeding your baby for more than a couple of months. Always ask your doctor before taking a vitamin supplement. Vitamin D supplements can interfere with medicines and some medicines can reduce Vitamin D absorption (eg ones which reduce the amount of fat you absorb).

• This article was amended on 31 January 2011. The original referred to the amount of melatonin in skin. This has been corrected.