Despite the 1978 ban on lead-based paint for residential use, lead poisoning continues to be a serious public health threat, particularly for children because they are most susceptible to its effects.

In a new observational study, a pair of researchers at the San Francisco Veterans Affairs Medical Center and the University of California, San Francisco found that low levels of ascorbic acid (vitamin C) in the blood stream were associated with high blood levels of lead among Americans. The study's findings, which are published in the June 23 issue of the Journal of the American Medical Association (JAMA), also indicate that about half of one percent of all Americans (more than a million people) have elevated levels of lead in their blood.

According to the US Environmental Protection Agency, current studies suggest that the primary sources of lead exposure for most children are deteriorating lead-based paint, lead-contaminated dust, and lead-contaminated residential soil. Prior to 1978, lead was commonly used as a coloring agent and a stabilizer in paint.

"Vitamin C levels are an important independent correlate of blood lead levels among Americans," says Joel Simon, MD, MPH, SFVAMC staff physician and UCSF assistant professor of medicine, epidemiology & biostatistics. "To our knowledge, this report is the first population-based study to establish such an association. If a causal relation is confirmed, increased consumption of ascorbic acid may have public health implications for the prevention of lead toxicity."

The correlation between levels of vitamin C and blood lead levels is supported by the findings of a recent small clinical trial, conducted at the University of Texas Medical Branch in Galveston, that found that 1000mg vitamin C supplements decreased the blood lead levels of heavy smokers. Because smoking decreases the absorption and increases the metabolism of vitamin C, higher dietary vitamin C intake levels are recommended.

According to Simon, there are no acceptable levels of lead in humans. Symptoms of acute lead poisoning in adults include loss of appetite, abdominal pain, renal (kidney) disease, anemia, headache, memory loss, and peripheral neuropathy (pain, numbness, or tingling of the arms and legs). In children, signs of acute poisoning are anemia, abdominal pain and nervous system disorders. At sub-acute levels of lead poisoning, there are often no symptoms, but such levels can cause mental retardation, loss of cognitive function, language deficits, and behavior problems.

As a preventive measure, Simon recommends increasing the amount of fresh fruits and vegetables consumed everyday because of their vitamin C content as well as their many other nutritional benefits. "Humans are one of the very few mammals that do not produce vitamin C on their own," says Simon, "so all of it must be obtained from dietary sources." If people are concerned they are not receiving the proper amount of vitamins in their diets, Simon says a multiple vitamin or a modest dose vitamin C supplement may be taken as an 'insurance policy.'

The Recommended Daily Allowance (RDA) of vitamin C for non-smoking adults is 60 mg and 100 mg for smokers. The RDA for children is 45 mg. However, new dietary reference intakes for vitamin C, such as the estimated average requirement level, are currently being formulated and are likely to be higher. As a point of reference, a medium size orange contains 60 to 80 mg of vitamin C, and one cup of freshly squeezed orange juice has about 120 mg of the vitamin.

According to Simon, there are very few negative effects of too much vitamin C.

However, he says intakes of 1000 mg will result in saturation of the plasma in the blood stream. Larger intakes are probably not indicated and are passed from the body in the stool and in urine. Mega-doses of vitamin C (3000 mg or more) can cause diarrhea and stomach cramps. And while persons with hemochromatosis, a hereditary iron-storage disorder, should not be discouraged from eating fresh fruits and vegetables, they should be cautious about consuming supplements containing more than 500 mg of vitamin C because vitamin C enhances iron absorption and could worsen this condition.

The investigators studied data gathered by the Third National Health and Nutrition Examination Survey, 1988-1994 (NHANES III). The national probability survey of Americans employed a stratified, cluster sampling design. After excluding some participants because of reported histories of lead poisoning, or questionable or missing information, the researchers used statistical methods to calculate their findings. In all, data from more than 19,500 Americans were analyzed.

Co-author of the study is Esther S. Hudes, PhD, MPH, UCSF senior statistician in the department of epidemiology & biostatistics. The study was supported by grants from the US Public Health Service and Hoffmann-La Roche, Inc.