Superfund Research Program
Studies Determine Factors That Contribute to Bone Lead Accumulation and Release
Lead is present in most of the Superfund sites around the country, and it has a high potential for gaining entry into the body through contamination of dirt, water, food and air. There is general agreement that once the level of lead is elevated in an individual's blood from such exposure, the individual is at risk for damaging effects to the nervous, cardiovascular, and kidney systems. When external exposure is lessened, blood lead levels decline. However, lead does not disappear from the body - it is deposited in the bone and remains in the body for years. Although bone appears to be a static tissue, it actually undergoes heightened turnover during key periods of life, such as pregnancy, lactation, and the osteoporosis associated with aging. Therefore, bone stores of lead may become mobilized and serve as an internal reservoir of lead exposure, which implies that lead based toxicity may occur in individuals many years after environmental lead exposures have declined. As scientists learn more about the disposition of lead in the human body, it has become evident that the factors contributing to bone lead accumulation and release need to be closely examined.
Recognizing the importance of elucidating the factors that affect the accumulation and release of lead in the bone, scientists at the Harvard School of Public Health have been conducting two on-going epidemiological studies that address questions surrounding the uptake and release of lead in bone tissue. The studies are employing a newly-developed instrument, K-X-ray fluorescence, to make safe, accurate and noninvasive measurements of bone lead levels in the subjects. By reviewing the histories of the subjects and comparing them with measured bone levels, the factors that contribute the most to accumulated lead burden are being identified. In addition, by comparing bone lead levels with health outcomes, it is being determined whether bone lead, and therefore accumulated lead exposure, serves as a risk factor for adverse health effects.
One of the epidemiology studies focuses on bone lead in women giving birth in Mexico City. Lead exposure is higher in Mexico City than in cities in the United States, therefore, this population-based study provides for unique opportunities to study lead's behavior. Scientists found that low dietary intake of calcium enhances deposition of lead in bone. Another finding of this study is that elevated levels of lead in bone are a risk factor for lower birth weight and persistently elevated blood lead levels in women who are breast-feeding.
In the second study, bone lead is being investigated in middle-aged to elderly men and female nurses living in the Boston area. These subjects have had the "usual" environmental exposures to lead that can be expected for members of the general community. One recent finding is that a low intake of dietary vitamin D enhances deposition of lead in bone. In assessing the source of circulating blood lead in these subjects, bone lead appears to be the major contributor to circulating lead, because external exposures to lead have waned significantly in recent years. Additionally, researchers found that the contribution of bone lead to circulating lead is enhanced in women with higher levels of bone resorption. An important health effects finding of the study is that bone lead is a risk factor for the development of hypertension in both the men and women, carrying an influence that is as important as obesity and the hereditary factor of family history of hypertension.
The significance of these recent findings are that (1) dietary factors, such as calcium and vitamin D consumption, may be able to protect against lead accumulation, and (2) factors that enhance bone turnover may increase mobilization of lead from bone into circulation. The latter finding suggests that prevention of bone loss in post-menopausal women may not only be good for preventing bone fractures but may also protect women from a "rebound" effect of stored lead. The influence of bone lead on hypertension is a sobering indicator that lead may be more of health problem than previously thought by contributing to long-term chronic diseases.
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To learn more about this research, please refer to the following sources:
- Payton M, Riggs KM, Spiro, III A, Weiss ST, Hu H. 1998. Relations of bone and blood lead to cognitive function: The VA Normative Aging Study. Neurotoxicol Teratol 20:19-27. PMID:9511166
- Gonzalez-Cossio T, Peterson KE, Sanin L, Fishbein E, Palazuelos E, Aro A, Hernandez-Avila M, Hu H. 1997. Decrease in birth weight in relation to maternal bone-lead burden. Pediatrics 100(5):856-862. doi:10.1542/peds.100.5.856 PMID:9346987
- Hernandez-Avila M, Gonzalez-Cossio T, Palazuelos E, Romieu I, Aro A, Fishbein E, Hu H, Peterson KE. 1996. Dietary and environmental determinants of blood and bone lead levels in lactating postpartum women living in Mexico City. Environ Health Perspect 104(10):1076-1082. PMID:8930549
- Hu H, Aro A, Payton M, Korrick SA, Sparrow D, Weiss ST, Rotnitzky A. 1996. The relationship of blood and bone lead to hypertension among middle-aged to elderly men. JAMA - Journal of the American Medical Association 275:1171-1176.
- Hu H, Payton M, Korrick SA, Sparrow D, Weiss ST, Aro A, Rotnitzky A. 1996. Determinants of bone and blood lead levels among community-exposed middle-aged to elderly men: The Normative Aging Study. Am J Epidemiol 144:749-759. PMID:8857824
- Hu H, Aro A, Rotnitzky A. 1995. Bone lead measured by X-ray fluorescence: epidemiologic methods. Environ Health Perspect 103(supp.1):105-110. PMID:7621788
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