Soy milk linked to ADHD

Researchers at the University of California, Irvine have discovered that a mineral found in high levels in soy milk appears to be linked to behavioural problems.

There has been much speculation on whether soy-based infant formula leads to attention deficit hyperactivity disorder (ADHD) but so far little evidence to back up the concerns.The new study on rats, one of the first scientific inquiries into soy milk and ADHD, indicates that the mineral manganese may cause behavioural problems if consumed in high doses. The study appears in the journal NeuroToxicology​.

Francis Crinella, professor of paediatrics, and his colleagues at UCI and UC Davis found that giving rats increasing levels of manganese during infancy resulted in behavioural changes at higher doses. The researchers also found that manganese exposure resulted in lowered levels of the neurotransmitter dopamine, which plays a key role in inhibiting behaviour seen in cases of ADHD.

"Manganese is a mineral that's essential for life. But past studies on manganese miners have shown that it causes a number of behavioural problems,"​ Crinella said. "Soy milk formula contains about 80 times the levels of manganese found in breast milk, posing the risk that infants could receive too much manganese in the first weeks of life. While we've shown that behavioural problems can result from manganese exposure, we don't know if these problems are permanent, or result in ADHD among humans."​

Crinella and his colleagues found that at lower doses, manganese did not result in any significant changes in behaviour in the infant rats. However, at the experiment's highest doses of manganese, researchers saw that the rats were much more inconsistent at completing tasks than they were at lower doses.

In addition, the researchers found significant decreases in dopamine with higher doses of manganese. Previous research had shown that dopamine decreases occurred in areas of the brain that are critical for performing problem-solving tasks. These areas of the brain co-ordinate what is called the brain's "executive function" and are known to be deficient in ADHD.

"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 behavioural problems, including ADHD,"​ Crinella said. "Although soy milk by itself is not harmful, manganese can be removed through a laborious and expensive process. Only more scientific research will determine whether or not removing manganese would provide any prevention of ADHD in infants."​

Manganese is in the Earth's crust and is found in nearly all cereals and grains, including soy. The mineral is important for enabling cells to obtain energy. High doses of industrial exposure have been known to produce a syndrome called "manganism", marked by tremors similar to Parkinson's disease and spasmodic, often violent, behaviour.

Crinella and his team are now working on simulating human doses of soy milk formula to test whether they have any connection to behavioural problems.