By Natasha Gilbert

The World Health Organization recommends that mothers exclusively breastfeed for the first six months of their infants' lives. The advice, issued in 2001, has since been adopted by national governments, including the British government. But a new review of the existing evidence, published yesterday in the British Medical Journal, contradicts this guidance. Instead, it suggests that breastfeeding for four months is best for babies. Nature unpicks the evidence.

What are the findings of the new study?

Babies exclusively breastfed for six months are at a higher risk of developing anaemia, which has been linked to adverse mental, motor and psychosocial problems. The study also found evidence of a higher incidence of food allergies and a higher risk of developing coeliac disease.

But it also notes the findings of observational studies that babies in the West who were exclusively breastfed for six months were less likely to succumb to infections, such as pneumonia, than those fed for less than six months. Therefore, "exclusive breastfeeding for six months is readily defendable in resource poor countries with high morbidity and mortality from infections", the current study says.

These findings are echoed by advice issued in 2009 by the European Food Safety Authority based on a detailed review of scientific evidence which suggested that mothers should introduce solid foods at between four to six months.

How and why does this differ from the advice from the WHO?

The initial WHO recommendation is largely based on a review3 of evidence available before 2002. The BMJ review, carried out by child-health and nutrition scientists at University College London and the universities of Edinburgh and Birmingham, UK, included several more recent studies. These newer studies found evidence of adverse health effects when the introduction of solid foods is delayed until six months, including mothers' inability to support their infants' energy and iron requirements.

What is the WHO's response?

A spokeswoman for the WHO rejected suggestions that its recommendation is based on outdated evidence. "WHO closely follows new research findings in this area and has a process for periodically re-examining recommendations," she says. This is to ensure its recommendations "are based on the best available evidence and free from conflicts of interest", she adds.

The last major study the WHO considered was a 2009 systematic review by the Cochrane Collaboration, a respected independent organization based in Oxford, UK, that aims to provide accurate healthcare information. The Cochrane review backed sticking with the status quo.

The WHO spokeswoman raised concerns about the independence of the BMJstudy as some of its authors have received funding from the baby-food industry.

The BMJ paper declares that three of the four authors have performed consultancy work or received funding from companies manufacturing infant formula and baby food in the past three years. But one of the authors, Mary Fewtrell, a child nutritionist at University College London, told Nature: "My colleagues and I are independent paediatricians and scientists, funded by universities or hospitals, and we received no funding for doing this review other than our normal salaries. All of us have had links with industry at some point. We are making no comment in our paper about what type of solid foods should be introduced - this could be home-prepared or commercial depending on the mum's choice - the main issue is that the food should be nutritionally adequate and safe."

What do others say?

Baby Milk Action, a breastfeeding advocacy group based in Cambridge, UK, is concerned that companies will use the study to weaken national policies and legislation requiring baby foods to be labelled 'for use from six months'.

It also questions the evidence that introducing solids at four months prevents coeliac disease and allergies.

So what should mothers do?

Fewtrell told Nature, "We are not ourselves giving 'new advice' as has been stated in some papers - that is not our job. However, our own opinion is that currently the balance of data would favour introducing solids alongside continued breastfeeding between 4 and 6 months - when the mother feels her baby is ready."