Oct. 8, 2002 -- Laboratory rats fed a diet high in manganese -- a mineral found in soy-based infant formulas -- experienced behavioral problems similar to attention deficient/hyperactivity disorder (ADHD).
Not necessarily, say experts. But manganese, which is about 80 times higher in soy formulas than in breast milk, may warrant concern. Soy-based infants formulas, which have doubled in popularity in the past decade, now hold a 25% share of the baby formula market. Soy-based formulas contain 0.2-0.3 mg of manganese per liter. A liter of breast milk contains only 0.004-0.006 mg and a liter of cow's milk-based formula contains 0.030-0.050 mgs.
Children with ADHD typically have lower levels of the brain chemical dopamine than do their peers. Dopamine transmits nerve messages to regions of the brain controlling attention, social judgment, and problem-solving.
In the study, published in the current issue of Neurotoxicology, researchers observed behavior and measured dopamine levels of rats fed a diet supplemented with high or low doses of manganese.
They found that rats that ate the high-manganese diet showed less goal-oriented behavior, such as obtaining food when hungry, and had lower dopamine levels in a critical problem-solving area of the brain. Rats fed the low-manganese (similar to the amount in breast milk) diet, on the other hand, displayed completely normal behavior.
"Our study shows a definite correlation between high manganese intake and low dopamine levels, but we don't know if soy formulas will have the same effect in babies," says study leader Francis Crinella, PhD, a University of California, Irvine, professor of pediatrics. "Formulas have other ingredients that may offer a protective effect against manganese toxicity. At this point, the only sure-fire thing we can tell parents is to breastfeed if at all possible," he tells WebMD.
Excess amounts of manganese have been linked to ADHD-like behaviors in past studies on humans, and hair samples from kids with learning and attention deficits, tested in the 1970s and 1990s, showed elevated manganese levels.
"It makes sense that manganese could cause attention problems, because ingesting high levels of lead could also produce ADHD behaviors," says noted ADHD specialist Edward Hallowell, MD, a psychiatrist at Harvard Medical School and author of Driven to Distraction, a best-selling book about the disorder.
"Certainly, excess amounts of any sort of metal or element could be associated with ADHD symptoms. While the best evidence indicates that ADHD is related to genes, there is plenty of evidence that it can be acquired environmentally -- diet, food additives; even too much television and video games can produce a consistent symptom profile," he tells WebMD.
There is no official Recommended Daily Allowance for manganese, but the suggested daily "adequate intake" is 0.3 mg for infants aged 0-6 months, and 0.6 mg for those aged 7-12 months. A single 5-ounce serving of Isomil, the most popular soy-based formula, contains 0.025 mg. Most babies consume four to 10 servings per day, which falls well within the suggested intake.
"Manganese is a nutrient essential for human growth and skeletal development. It is widely distributed in nature and required by law to be present in infant formulas," Mary Beth Arensberg, a spokeswoman Ross Products, the manufacturer of Isomil, tells WebMD. "The higher levels of manganese in cow- and soy-based formulas reflect higher levels in the protein sources used to make these formulas. However, these are within the recommended ranges for infant nutrition. Infant formulas, and soy formulas in particular, have a long history of safe use and there are no reports of either manganese deficiency or toxicity unless there is some type of severe medical condition with the child."
Both Crinella and Hallowell maintain that breast milk is the best choice for infants and should be used, whenever possible, over commercial formulas -- an opinion shared by most physicians and the American Academy of Pediatrics. However, the Academy says that soy-based formulas, which are lactose-free, are appropriate for infants with lactose intolerance.