A lack of Vitamin D, which is generated by the body through exposure to sunshine, can cause the bone disease rickets. It may also lead to the brittle bone disease osteoporosis in later life.

Researchers from the Institute of Child Health, London, carried out blood tests on more than 600 Asian chidren aged two years.

They found that one fifth of the youngsters had low concentrations of Vitamin D. Many also showed signs of iron deficiency anaemia.

They recommend that children with low haemoglobin concentrations should be given vitamin D supplements and screened for rickets.

Rickets has been recognised as a problem in children of Asian immigrants since the 1960s.

Study co-author Dr Margaret Lawson, senior lecturer in paediatric nutrition, Childhood Nutrition Research Centre, Institute of Child Health, London, said the problem could be due to several factors.

Dr Lawson said Asian children tended to be weaned on to cow's milk, and to drink a lot of it so that they were less likely to take other forms of food.

Cow's milk is low in both Vitamin D and iron, and so Asian children tend to get less nourishment than their white counterparts, who tend to be weaned on to fortified infant formula.

Dr Lawson said the presence of chipati bread in the diet could also help to explain why children of Pakistani origin suffered from the worst Vitamin D and iron deficiency of all.

Chipati flour

Cow's milk is low in Vitamin D

Chipati flour interferes with the digestion process, making it more difficult for the body to absorb nutrients. Also, unlike most Western bread, it is not fortified with vitamins.

Another factor was the low use of Department of Health Vitamin A and D supplements among Asian cultures.

In addition it is believed that Asian children are at a disadvantage because their skin cannot manufacture Vitaimin D from sunlight as readily as paler Caucasian skin.

It was thought that this was due to the greater pigmentation of Asian skin, but scientists now believe it may have a genetic cause, as Afro-Caribbean children are more easily able to manufacture their own Vitamin D.

Dr Lawson said: "This is quite a problem. A deficiency of iron early in life can make you rather slow to develop, and in older children and adults it can cause tiredness and make it difficult to concentrate.

"A lack of Vitamin D means that calcium is not being laid down in the bones quite as well as it should be, and this could explain the prevalence of rickets among Asian teenage girls.