Mothers are often told “breast is best” when it comes to feeding their newborns. But according to a new study, breastfeeding could also expose infants to a number of toxic chemicals.

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Researchers found that for every month a baby is exclusively breastfed, their PFAS concentrations rise by 20-30%.

Published in the journal Environmental Science & Technology, the research reveals that the longer a baby is breastfed, the greater their exposure to a common class of industrial chemicals called perfluorinated alkylate substances (PFASs).

PFASs are chemicals added to clothing, food packaging, lubricants and other products in order to make them resistant to water, grease and stains.

According to study co-author Philippe Grandjean, adjunct professor of environmental health at Harvard T. H. Chan School of Public Health in Boston, MA, and colleagues, PFASs bioaccumulate in food chains and can remain in the body for a long time.

PFASs can often be found in the blood of humans and animals, note the researchers, and previous studies have associated them with immune system impairment, reproductive abnormalities, endocrine disruption and cancer.

Grandjean says it was already known that small amounts of PFASs can be detected in breast milk and that these chemicals can be passed to infants. What is less clear, however, is whether such chemicals accumulate in an infant as breastfeeding duration increases.

To investigate this issue, the team analyzed the blood samples of 81 children who were born in the Faroe Islands between 1997 and 2000.

The blood samples – taken at birth and when the children were aged 11 months, 18 months and 5 years – were assessed for the presence of five PFASs. The team also analyzed blood samples from the children’s mothers, taken at 32 weeks of pregnancy.

Among children who were exclusively breastfed, the researchers found PFAS concentrations rose by around 20-30% for each month of breastfeeding. A lower monthly increase in PFAS concentrations was identified among children who were partially breastfed, according to the team.

The researchers identified one PFAS – perfluorohexanesulfonate – whose concentrations did not increase with breastfeeding duration.

The team found that at time of breastfeeding cessation, the PFAS concentrations of many infants exceeded those of their mothers, though concentrations of all five PFASs started to reduce shortly after.

This study suggests breastfeeding is a key exposure pathway to some PFASs in infants – a worrying finding, according to Grandjean. He says:

There is no reason to discourage breastfeeding, but we are concerned that these pollutants are transferred to the next generation at a very vulnerable age. Unfortunately, the current US legislation does not require any testing of chemical substances like PFASs for their transfer to babies and any related adverse effects.”

While this study highlights a potentially negative implication of breastfeeding, numerous studies have found the practice to be beneficial for infants’ health.

Earlier this year, Medical News Today reported on a study in which researchers found breastfed babies were at lower risk of asthma and allergies, while a more recent study published in JAMA Pediatrics suggests breastfeeding may protect against child leukemia.