Lead linked to aging in older brains
Updated: 2008-01-28 06:53
Lead in the blood, by contrast, reflects recent exposure. Virtually all Americans have lead in their blood, but the amounts are far lower today than in the past.
The big reason for the drop: the phasing out of lead in gasoline from 1976 to 1991. Because of that and accompanying measures, the average lead level in the blood of American adults fell 30 percent by 1980 and about 80 percent by 1990.
That's a major success story for environmentalists. But work by Schwartz and Dr. Howard Hu of the University of Michigan suggests that the long-term effects of the high-lead era are still being felt.
In 2006, Schwartz and his colleagues published a study of about 1,000 Baltimore residents. They were ages 50 to 70, old enough to have absorbed plenty of lead before it disappeared from gasoline. They probably got their peak doses in the 1960s and 1970s, Schwartz said, mostly by inhaling air pollution from vehicle exhaust and from other sources in the environment.
The researchers estimated each person's lifetime dose by scanning their shinbones for lead. Then they gave each one a battery of mental ability tests.
In brief, the scientists found that the higher the lifetime lead dose, the poorer the performance across a wide variety of mental functions, like verbal and visual memory and language ability. From low to high dose, the difference in mental functioning was about the equivalent of aging by two to six years.
"We think that's a large effect," Schwartz said.
Hu and his colleagues took a slightly different approach in a 2004 study of 466 men with an average age of 67. Those men took a mental-ability test twice, about four years apart on average. Those with the highest bone lead levels showed more decline between exams than those with smaller levels, with the effect of the lead equal to about five years of aging.
Nobody is claiming that lead is the sole cause of age-related mental decline, but it appears to be one of several factors involved, Hu stressed.
If so, it would join such possible influences as high blood pressure, diabetes, stroke, emotional stress and maybe education level, said Bradley Wise of the National Institute on Aging. Nobody knows exactly what causes mental decline with age, he said.
Although the studies by Hu and Schwartz suggest lead is involved, Wise and others say they don't prove the link.
"I think many things impact how we age, but I think right now it's maybe premature to be giving lead a huge role in our age-related cognitive decline," said Dr. Margit L. Bleecker, director of the Center for Occupational and Environmental Neurology in Baltimore. Still, she called the lead hypothesis "a very interesting idea" deserving more study.
Others were more impressed.
"The new evidence from these studies should concern people" said epidemiologist Andrew Rowland of the University of New Mexico. "These two research groups are finding adverse effects on the aging brain at low levels of lead exposure. More work needs to be done, but these studies are raising important questions."
In any case, scientists still face some basic mysteries about the delayed effects of lead. For example, when does it actually harm the brain? Does a high level in the shinbone merely identify those who were the most harmed by chronic exposure decades ago? Or does lead in the bone continue to do its dirty work over a lifetime, leaching into the bloodstream and continuously hammering the brain?
"I think that both things are happening," Schwartz said, though he suspects most of the damage occurred in the past, during years of higher exposure. Hu's suspicions are similar.
Just how lead impairs brainpower is still a mystery. And so is the question of whether anything can be done to help people who have absorbed a lot of lead over a lifetime.
A medical procedure called chelation can remove lead from the body, but it wouldn't help in this case, said experts, who had few suggestions.
For younger people, prevention is a clearer strategy, Hu said. He called for tougher federal standards on lead exposure in the workplace.
And plenty of low-income neighborhoods could use a strong effort to remove lead from old houses, many of which still have lead paint, Rowland said. "It's there on the walls, it's on the radiators, it's underneath the top layers of paint. In places where the paint is crumbling, there's still exposure going on," he said.
Yet another question: Who really has to worry about long-ago lead affecting their brainpower? What about people born after the high lead levels of the 1970s were history?
Schwartz noted that most Americans younger than 30 have gotten much less lead from the environment than the men in his study did. And Hu hopes that the lead effect will peter out in the future.
However, Hu points out that there's still lead in the environment, and exposure remains especially high in many developing countries. And citing evidence that lead can cross the placenta, he says women who grew up in the 1970s might dose their fetuses with the metal.
"Kids who grew up in the 21st century have a lot less to worry about" than their elders, Hu said. But "it's hard for me to be totally optimistic the current generation is completely scot-free."
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