Breast milk is nature’s way of delivering the right balance of essential nutrients to newborns. Breastfeeding is linked to lowered risk of obesity, diabetes, asthma and other conditions.

But the fat-rich fluid mothers produce can also harbor chemicals such as dioxin, which animal studies show to be associated with immune suppression, genital malformation and effects on behavior.

That makes some breastfeeding mothers worry about whether they are helping or harming their babies.

A research review published in the journal Breastfeeding Medicine shows that despite small potential negative effects from toxins, breast milk is still the food of choice for most newborns.

As an example, the researchers— from a Maryland environmental consulting firm, the University of Maryland, the Penn State College of Medicine and the National Institutes of Health— looked at studies on dioxin, a byproduct of industrial and waste-burning processes.

Breastfed babies tended to have higher levels of the chemical in their bodies than formula-fed babies, but over time those differences go away.

Subtle effects, such as higher levels of thyroid hormones and lower blood platelet counts have been reported for breast-fed babies. But values generally were still within normal ranges, and did not negatively affect growth or development.

In any event, cutting off breast milk doesn’t cut off babies’ exposure to toxins, since tap water used to mix formula might also contain chemicals.

Additional research will undoubtedly shed more light on the subject. But for now, mothers who think breast milk is best should rest easy.