Study Finds Lead May Be Culprit in ADHD

/ Author:  / Reviewed by: Joseph V. Madia, MD

Attention deficit hyperactivity disorder (ADHD) is among the costliest of behavioral disorders. Its combination of inattention, impulsivity and hyperactivity leads to accidental injuries, school failure, substance abuse, antisocial behavior and more.
Despite nearly a century of study, the disorder's roots remain a mystery.

Much of modern ADHD research has focused on heritability of the condition; evidence suggests genes may account for as much as 70 percent of hyperactivity and inattention in children. But that leaves 30 percent unexplained, so recently the focus has shifted to the environment. What is it that triggers an underlying susceptibility and changes it into a full-blown disorder?

New research suggests the culprit may be an old villain: lead. That finding may also explain the causal pathway from exposure to disability.

Lead has long been known to be a neurotoxin. Government regulation drastically reduced environmental lead a generation ago, but regulating automobile fuel and paint didn't entirely eliminate lead from the environment. It's found in trace amounts in everything from children's costume jewelry and imported candies to soil and drinking water. Every person living in the U.S. today is exposed to low levels of the metal, and nearly all children have measureable levels of lead in their bodies. According to psychological scientist Joel Nigg, of the Oregon Health & Science University, this universal low-level exposure makes lead an ideal candidate for the disorder's trigger.

The connection between lead and ADHD was just a theory until quite recently. Two recent studies now provide strong evidence of the possible link.

The first study compared children formally diagnosed with ADHD to control participants and found the children with the disorder had slightly higher levels of lead in their blood. This study showed a link only between blood lead and hyperactivity/impulsivity symptoms, not inattention. The second study, however, showed a robust link between blood lead and both parent and teacher ratings of ADHD symptoms, including both hyperactivity and attention problems. In both studies, the connection was independent of IQ, family income, race or maternal smoking during pregnancy.

Nigg offers a causal model for the disabling symptoms associated with ADHD: Lead attaches to sites in the brain's striatum and frontal cortex, where it acts on the genes in these regions, causing them to turn on or remain inactive. Gene activity shapes the development and activity of these brain regions. By disrupting brain activity, the toxin in turn alters psychological processes supported by these neurons, notably cognitive control. Finally, diminished cognitive control contributes to hyperactivity and lack of vigilance.

Nigg describes his new data and his explanatory model in the February issue of Current Directions in Psychological Science.

Catherine Allen-West
[email protected]

Reviewed by: 
Review Date: 
September 22, 2010
Last Updated:
September 22, 2010