Women infected with HIV who exclusively breastfeed their babies reduce the risk of transmitting the virus, researchers in South Africa have found.

Infants who were given solids in addition to breast milk were almost 11 times more likely to become infected compared with those who had only breast milk, the team reports in Saturday's issue of the medical journal The Lancet.

Infants who received formula milk or animal milk in addition to breast milk were nearly twice as likely to be infected.

In comparison, the risk of transmission to infants fed only breast milk was four per cent.

"The question of whether or not to breastfeed is not a straightforward one," said study author Prof. Hoosen Coovadia from the Africa Centre.

"We know that breastfeeding carries with it a risk of transmitting HIV infection from mother to child, but breastfeeding remains a key intervention to reduce mortality."

In the developed world, antiretroviral therapies, exclusive feeding of formula and excellent health-care systems have helped reduce the transmission of HIV from mother to child from about 25 per cent to less than two per cent.

But in many poor parts of Africa, formula or animal milk is expensive and cannot act as a total substitute for breastfeeding.

"Based on our findings and evidence of being able to successfully support exclusive breastfeeding in HIV-infected women, and recent data from other parts of Africa, we believe that current guidelines on infant feeding warrant revising," said Coovadia.

Solid foods may damage stomach lining

As for why breast milk may be protective, the researchersspeculated that the mucous membrane within the intestines may act as a barrier to HIV infection, and that breast milk may reinforce this lining.

On the other hand, previous studies have suggested the larger, more complex proteins found in solid foods may lead to more damage of the lining of the stomach, allowingthe virus to pass through.

Resources to prevent HIV infection in children should be invested to improve maternal and child health in general, a journal commentary said.

"Investment in promoting, protecting, and supporting exclusive breastfeeding to six months has the greatest potential to improve HIV-free child survival in settings with both high and low HIV prevalence," wrote Wendy Holmes of the Macfarlane Burnet Institute for Medical Research and Public Health in Melbourne, Australia, and Felicity Savage of the Centre for International Health and Development in London, U.K.