Few multivitamin products for children supply the recommended dose of 400 IU a day of vitamin D, suggest the results of a survey of 91 different products.
Only one multivitamin was suitable for use from birth, supplying 200 IU/day of vitamin D, while for children over 6 months, only between a quarter and a third (25-36%) of the available products supplied at least 400 IU/day.
Some of the products giving a dose range would only supply the recommended vitamin D level at the highest dose.
The vitamin D/healthy bones products supplied between 50 and 1000 IU of vitamin D a day. Six were suitable for use from birth, five of which contained 340-400 IU of vitamin D.
The vitamin D content of these types of supplement was typically higher than that of multivitamins: nearly two thirds (57-67%) contained at least 400 IU/day. But one product labelled as 'for bones and relaxation' contained only 50 IU/day of vitamin D.
The researchers point out that their survey was limited to supplements sold by UK retailers, and many other supplements are available from online retailers, including imported products and fortified food products.
But because multivitamins are classed as food products, under European Union regulations, the permissible vitamin D content can range from 20% below, to 50% above, the amount stated on the label, they say.
"There is a wide range of both multivitamins and vitamin D supplements available for children in the UK, yet most of these do not provide the recommended 400 IU/day," they write.
To obtain this, children would either have to take over the recommended dose, which may increase the risk of toxicity from the other components, or they would have to take a combination of vitamin D and multivitamins, which is more expensive, explain the researchers.
Parents and caregivers need to check that the multivitamins they buy for children contain at least 400 IU/day, they advise.