Vol. 8, No. 4, 2002 Page 7
Millions of infants drink soy-based formula, which can contain as much as 80 times the amount of manganese as breast milk. A new study indicates that this elevated exposure to manganese during early development may increase children's risk for hyperactivity or other behavior problems.
Francis Crinella and colleagues administered differing levels of manganese to 32 newborn rats, later evaluating their performance on two problem-solving tests. While group differences did not reach significance, the researchers found that at the highest doses of manganese, the rats were far more inconsistent at completing tasks than they were at lower doses.
Post mortem studies of the rats' brains revealed that rats with the highest level of manganese exposure had only 40% of the normal level of the neurotransmitter dopamine in striatal structures of the brain. Previous research has shown that these structures are critical for problem solving, and that the nigrostriatal system is involved in "executive function" tasks. Defects in executive function (planning, impulse control, cognitive flexibility, goal-oriented behavior, and similar skills) are considered a core feature of attention deficit hyperactivity disorder (ADHD), and brain imaging studies frequently reveal abnormalities of these structures in subjects with ADHD. ADHD is also linked to impaired functioning of the dopamine system.
Crinella et al. note that elevated manganese levels are frequently detected in the hair of children with attention and learning deficits, a finding their group recently replicated in children with ADHD. They also point out that manganese toxicity is known to cause a clinical disorder called "manganism," which results in Parkinson's-like movement problems and aberrant, sometimes violent behavior. In addition, they cite research showing that rats fed diets high in manganese but low in calcium exhibit increased aggressive behavior.
"In demonstrating a significant relationship between neonatal manganese exposure levels and striatal dopamine levels [in rats]," the researchers say, "we lend further support to the findings of others who have shown that neurotoxic damage from manganese is characterized by selective injuries to dopaminergic brain networks." They say their findings raise concerns about levels of manganese in soy infant formulas, particularly since manganese enters the neonatal brain at a much higher rate than it enters the adult brain, and the bodies of newborns cannot efficiently remove excess manganese.
"While this study shows a definite correlation between high manganese and lower dopamine levels, we still need to see whether high manganese doses result in permanent behavioral problems, including ADHD," Crinella says. Noting that there is no evidence that soy milk itself is harmful, and that manganese can be removed from soy formulas (although the process is expensive), he says, "only more scientific research will determine whether or not removing manganese would provide any prevention of ADHD in infants."
"Effects of neonatal dietary manganese exposure on brain dopamine levels and neurocognitive functions," Trinh T. Tran, Winyoo Chowanadisai, Bo Lonnerdal, Louis Le, Michael Parker, Aleksandra Chicz-Demet, and Francis M. Crinella, NeuroToxicology, Vol. 145, 2002, 1-7. Address: Francis M. Crinella, fmcrinel@uci.edu.