Vitamin D is referred to as the sunshine vitamin, because the best source is the action of sunlight (specifically UVB) on the skin. Vitamin D is essential for maintaining the correct levels of calcium and phosphate in the body, which are vital for bone health. It also ensures good immune health and overall vitamin D status is correlated with lower levels of chronic disease. Daily recommended amounts for children and adults are 200 IU per day, increasing to 400 IU over the age of 50r and 600IU past 70 years.

Problems in obtaining vitamin D can occur when the amount of UVB light is restricted for a number of reasons. Beijing lies at around 39 latitude - but above 40-42, the light quality is insufficient to make vitamin D for around 4 months of the winter. UVB light in Beijing can be significantly blocked by industrial pollution and cloud cover, so if it has been a particularly grey week, it is unlikely you've received your quota from the sun. During the midwinter, we clad ourselves with as many clothes as possible and so only a minute part of our skin actually sees the sun. During the hot summer months, people routinely use sunscreen, or live under the shade of a parasol, which also blocks UV B light. Fortunately vitamin D is fat soluble so it can be stored in the body, and daily doses are not required.

To ensure adequate sun exposure for vitamin D synthesis, aim for 15 minutes of exposure of hands, face and arms or back to the sun without sunscreen at least twice a week. Of course sun protection is very important, so after 15 minutes slap it on - especially in the summer. Darker skin has a higher melanin concentration, so exposure time before sunscreen should be increased.

For those of you who can't bear to expose an extra inch of skin for even a second during the winter, there are some dietary sources of vitamin D. Naturally vitamin D is present in oily fish and animal liver. A portion of tuna in oil will provide around half your recommended daily intake, 100g salmon will provide almost 100%. In many Western countries such as the US and UK, mandatory fortification of milk and margarine helps to ensure intake. In China many milks and soy milks are also fortified with both vitamin D and calcium. If you are unsure - look at the nutrition labels - even if you can't read Chinese, it will say 'D' or 'D3' with the amount in IUs next to it, alternatively ask your friendly shop assistant if your brand, "you wei sheng su D ma?". Many brands advertise their fortifications on the front packaging. Imported breakfast cereal is another source of vitamin D, particularly for expats- usually a 30g serving provides around 1/3 of your recommended intake but check the labeling. The same applies to imported margarines.

If you still feel unable to meet requirements through sunlight or food then consider a supplement. Fish liver oil is one option but if the taste is unbearable and you consume little dairy then it is best to go for a combined calcium and vitamin D supplement such as Caltrate made by Wyeth (US), which is widely available in China. If you are pregnant or breastfeeding then I would definitely recommend a supplement of 400IU per day to protect both you and your baby.

This nutrition-related column is written by Nina Lenton, a qualified dietitian living in Beijing and working at Bayley and Jackson Medical Center. Contact her at nina.lenton@ikang.com

(China Daily 10/31/2007 page14)