Even relatively low levels of lead in the blood may be linked to an increased risk of gout, a painful form of arthritis, researchers reported Monday.
Researchers have known that heavy lead exposure is linked to the condition - a form of arthritis in which the joints periodically become swollen, red and hot. Gout most often affects the big toe, though it can also strike the feet, ankles, knees, hands and wrists.
But the new findings connect gout risk to low levels of lead in the blood - well within the range currently considered acceptable for adults.
For adults, lead levels of 25 micrograms per deciliter of blood or lower are considered "non-elevated."
In developed countries - where policies like removing lead from gasoline and paint have greatly cut people's exposure - typical lead levels are far below that threshold. In the U.S., the average level is about 3 mcg/dL.
But in this study, gout risk started to climb once adults' lead levels surpassed 1.2 mcg/dL.
That's in line with some past research that has linked relatively low lead levels to other risks, including worsening of kidney disease and death from heart disease.
"All of this suggests there's no such thing as 'safe' or 'acceptable' lead levels," said study leader Dr. Eswar Krishnan, of Stanford University School of Medicine in Palo Alto, California.
The study, reported in the Annals of Internal Medicine, is based on data from a periodic federal health survey. More than 6,100 Americans age 40 and up had their blood lead levels measured, and were interviewed about their health and lifestyle.
Overall, just under five percent said they'd been diagnosed with gout. And the risk went up as people's lead levels rose.
Of the adults with the highest lead levels, about six percent had gout - versus less than two percent of people with the lowest lead levels.
That does not prove the higher lead levels were to blame, Krishnan acknowledged. But he said he and his colleagues controlled for a number of other factors that might explain the connection.
Gout arises when uric acid crystals build up in the joints. The body produces uric acid when it breaks down purines - substances found naturally in the body, but also in certain foods, like organ meats, anchovies, mushrooms and some seafood.
Anything that boosts the body's production of uric acid, or slows the clearance of it, also raises the risk of gout. Obesity, high blood pressure, diabetes and kidney disease are some examples.
But even when Krishnan's team accounted for those factors - plus people's incomes and smoking habits - lead levels remained linked to gout risk.
People in the top 25 percent for lead levels had the highest gout risk: four times higher than people in the bottom 25 percent.
That top group had lead levels ranging from 2.6 to 27 mcg/dL, according to the researchers, but most were well below the acceptable lead threshold. Only three people in the study had lead levels of 25 mcg/dL or higher, while 31 had levels of 10 mcg/dL or higher.
It's not clear why low-level lead would boost gout risk. But Krishnan said the theory is that even small amounts of lead can hinder the kidneys' ability to excrete uric acid.
Dr. Ashwini R. Sehgal, who wrote an accompanying editorial, agreed that the threshold for acceptable lead levels in adults should be revisited.
In an interview, Sehgal said that many people with elevated lead levels are exposed to the metal at work - in industries like mining, construction and battery making.
If you've ever worked with lead, tell your doctor and ask whether a blood test is a good idea, said Sehgal, a professor of community health improvement at Case Western Reserve University in Cleveland, Ohio.
"I don't think that everyone should have their lead levels checked," Sehgal said.
Krishnan agreed. He also noted that while the average blood lead level in the U.S. is 3 mcg/dL, "that would include a large number of people with zero or near-zero levels."
But Krishnan said that if a person has gout and no known risk factors for it, like obesity, doctors could consider lead testing.
Typically, if someone's lead levels are elevated, it would be important to reduce any known exposure to it.
Besides job exposure, Krishnan said that people can inhale lead through cigarette smoke or lead-containing dust; deteriorating lead-based paint in an older home, for example, can contaminate household dust.
Lead can also be in drinking water, especially in older homes with lead-based pipes. And some people are exposed through hobbies, like working on cars, or making glazed pottery or stained glass.
When a person's lead levels are particularly high, a treatment called chelation therapy might be recommended. That involves giving certain drugs that bind to lead and allow it to be excreted in the urine.
But no one knows if that therapy can ease symptoms of gout in people with mildly elevated lead levels
"There are medications that can leach lead out of the blood," Krishnan said. "But they are not approved to treat gout. And they are not without risks."
Chelation therapy can, for instance, damage the kidneys permanently.
So even if someone with gout were found to have relatively elevated lead levels, no specific treatment recommendations can be made, Krishnan said.
Both he and Sehgal said the findings' main importance have to do with bigger-picture public health.
"I think policymakers should consider these data" in any future review of the lead threshold for adults, Krishnan said.
The U.S. Centers for Disease Control and Prevention recently lowered the maximum acceptable lead level for children - from 10 mcg/dL to 5 mcg/dL. The threshold for children is lower because their developing brains are particularly vulnerable to damage from lead.
Sehgal said he thinks lower thresholds for adults are feasible. He noted that in Germany, the standards are 7 mcg/dL for women and 9 mcg/dL for men.