Nov. 10, 2000 CHAPEL HILL -- Pregnant women who don't consume enough calcium in their diets or through supplements show greater increases in lead in their bloodstreams than pregnant women with normal calcium levels, according to a new University of North Carolina at Chapel Hill study.
That is because 95 percent of the body's lead is contained in bone tissue, which, in pregnant women getting less calcium turns over, or "demineralizes," more rapidly than bone in other pregnant women and releases locked-up lead into the blood, researchers found.
Lead emerges from bone during the second half of pregnancy and might hurt both mothers and their babies, their work showed. The study also offers new evidence that calcium consumption can minimize bone demineralization in pregnant women.
"Past research has linked lead to many adverse conditions, including nervous system and possible cardiovascular problems," said Dr. Irva Hertz-Picciotto, professor of epidemiology at the UNC-CH School of Public Health and lead author. "In this study, in which lead levels were very low overall, we saw that the same characteristics that predict greater lead at high exposures also predict a greater lead level even at a much lower range. That means there may be virtually no 'natural' background of blood lead for which man-made sources can't be identified."
Those characteristics, or variables, that correspond with more lead include smoking, less education, being black and low calcium intake, she said. Variables associated with a reduced lead level include history of breastfeeding and higher calcium intake.
"We are still trying to determine whether there are health effects from these low levels," Hertz-Picciotto said.
A report on the findings appears in the November issue of the Journal of Epidemiology. Co-authors are Margaret Schramm and Dr. Margaret Watt-Morse of Magee-Women's Hospital at the University of Pittsburgh, Kim Chantala of UNC-CH's Carolina Population Center, Dr. John Anderson of the UNC-CH department of nutrition and Dr. John Osterloh of the University of California at San Francisco's department of laboratory medicine.
The study involved 195 women who entered prenatal care at Magee-Women's Hospital in Pittsburgh between 1992 and 1995. Using a technique called atomic absorption spectrophotometry, researchers analyzed up to five blood samples from each woman for lead during their pregnancies and took extensive patient histories.
"We found not only that calcium intake at the recommended daily allowance level protected somewhat against lead, but also that higher calcium levels corresponded with even less lead," Hertz-Picciotto said. "The protective effect of calcium became stronger as pregnancy progressed."
The blood lead levels of older mothers increased more during pregnancy compared with younger mothers. The researchers suggested that was because older mothers had accumulated more lead in their bones, particularly in the 1960s and 1970s when leaded gasoline was still used in automobiles.
Age and calcium had combined effects. From week 20 to week 40, for example, blood lead levels in pregnant women with low calcium intakes increased 25 percent at age 18, 37 percent at age 23, 65 percent at age 33 and 99 percent at age 43, she said.
Pregnancy and the first four years after menopause are periods when women's bones are most likely to undergo turnover, including demineralization, the scientist said.
The new findings mirror those of a study published in 1994 of about 200 women in Mexico City whose blood lead levels were significantly higher because of pollution and greater exposure on average to lead at work and home.
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