A new study shows why breastfeeding is generally safe even when mothers are infected with the hepatitis C virus (HCV).

The reason is that human breast milk inactivates hepatitis C virus (HCV) infectivity by disrupting its envelope, researchers from Germany have found.

“This study provides a novel mechanism for the protective properties of human mother’s milk against HCV,” Dr. Eike Steinmann from the TWINCORE Center for Experimental and Clinical Infection Research in Hannover told Reuters Health by email. “A new finding is that lipases in human milk generate free fatty acids that damage the viral envelope and render them non-infectious.”

In an editorial published with the paper online September 24 in The Journal of Infectious Diseases, Dr. Ravi Jhaveri from the University of North Carolina in Chapel Hill says “the results provide a plausible explanation for why breastfeeding is not a risk factor for HCV transmission. This is reassuring for us as practitioners when we counsel our HCV patients that it is safe for them to breastfeed.”

Using breast milk from healthy HCV-negative women, the research team found that even short preincubation periods of HCV in the milk brought consistent reductions of HCV infectivity by 2 to 3 orders of magnitude.

The breast milk inactivated HCV infectivity independent of the viral genotype, and antiviral activity was concentration dependent. Concentrations between 4% and 6% milk were sufficient to reduce HCV infectivity, whereas higher dilutions abolished the antiviral effect.

The antiviral activity was specific to human milk. It was not found in milk from horses, cows, or commercial infant formula.

The anti-viral activity was not destroyed by heat treatment, the authors reported.

In a series of experiments, the researchers showed that lipases in human milk generated fatty acids that disrupted the viral envelope, resulting in the loss of viral infectivity.

“Similar processes concerning the release of free fatty acids take place upon digestion of human breast milk by the infant,” the investigators note. “Therefore, milk digestion products, like free fatty acids, released in the stomach might be able to inactivate residual viral particles which otherwise could be transmitted upon breastfeeding.”

Human breast milk also had significant antiviral effects against other enveloped viruses (influenza, herpes simplex, and vesicular stomatitis virus) but no pronounced effect on non-enveloped viruses (murine norovirus, rotavirus).

“As there are far more enveloped viruses known than tested in this study, further investigations are necessary,” Dr. Steinmann said.

“Human breast milk efficiently inactivates HCV in vitro and neither the Centers for Disease Control nor the American Association for the Study of Liver Diseases argues against breastfeeding from HCV infected women unless they have cracked or bleeding nipples,” Dr. Steinmann concluded.

Dr. Jhaveri’s editorial concludes, “After reading this article, when we clinicians next encounter an HCV infected patient that just delivered a healthy infant and wants to breastfeed, we have yet another reason to say ‘Breast is Best.'”