Babies with either high or low levels of manganese in their blood may be slower to hit certain developmental milestones.
Small amounts of manganese (found in food, water, air and soil) in the diet are necessary for normal nervous system function, while high amounts can be harmful.
This study focused on manganese levels in the first few years of life, and whether there might be any effects of relatively low-level environmental exposure on the still-developing brain.
In their study, Dr. Birgit Claus Henn and her colleagues found that at the age of 12 months, children in either the bottom 20% or the top 20% for blood manganese levels had lower scores on a standard test of mental development than those whose manganese levels fell in between.
Manganese is a natural component of rock and soil, and people are routinely exposed to it through air, water and food, including grains, fruits and vegetables. It is also used in industry, mainly in steel production, and heavy occupational exposure to manganese can be toxic to nerve cells - leading to symptoms such as difficulty concentrating and Parkinson-like problems like slowed movement and coordination problems.
For the current study, Henn and her colleagues followed 448 Mexican children from birth to age 3. Every six months, the children were given standard tests of mental development, including measures of vocalization and communication, memory and problem-solving - such as tackling simple goals like reaching a toy.
In general, children in the bottom and top 20% for blood manganese at the age of 12 months scored about three points lower than their peers on the mental development.
According to Henn, that difference is akin to what has been seen when young children's blood levels of lead - which is known to harm early brain development - rise from 10 micrograms per deciliter of blood to 30 mcg/dL.
Henn said that it is still possible that other factors, such as exposures to other environmental toxins, could explain the findings.
For now, she said, there are some steps parents can take to limit young children's exposure to manganese while ensuring that they also get enough of it. Manganese is present in some fertilizers and fungicides, for instance, so parents can try to limit their children's exposure to those products.
Henn also noted that while manganese levels in public drinking water are regulated, there can be high levels in well water. So avoiding that water source may be helpful.
On the other hand, Henn said, it is rare for people to be overexposed to manganese through food. So parents should make sure that their children get the healthful foods - including whole grains and green, leafy vegetables - that contain manganese.