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Children breastfeeding after their first birthday should take vitamin D supplements, says a study – even if they are also eating solid foods.

According to Dr Jonathon Maguire, a paediatrician and researcher at Canada's St. Michael's Hospital, while breast milk contains many of the required nutrients for supporting growth, it does not provide adequate amounts of vitamin D. This may be more important for children from northern countries, where there is less exposure to the sun's ultraviolet rays, which the body uses to produce vitamin D, Maguire said in a paper published in the American Journal of Public Health.

However, exclusive breastfeeding in the first year of life without vitamin D supplements is a known risk factor for rickets, a disease that leads to softening and weakening of bones. Less is known about the relationship between the total length of time a child is breastfed and vitamin D. An increasing number of children are breastfed after their first birthday in addition to receiving solid foods and the World Health Organization recommends breastfeeding to age two and beyond, as mutually desired by the mother and child.

This study found the risk of being vitamin D deficient rose six percent every month a child was breastfed after one year, reaching a 16 percent chance of being vitamin D deficient at age two and a 29 percent chance by age three. The researchers measured vitamin D levels in the blood of 2,500 children aged one to five who were participating in TARGet Kids!, a program which follows children from birth with the aim of understanding and preventing common nutrition problems in the early years to minimise their impact on health and disease later in life.