Bone lead measured by X-ray fluorescence: epidemiologic methods

Bone Lead Levels and Risk of Incident Primary Open-Angle Glaucoma: The VA Normative Aging Study

BACKGROUND

Oxidative stress may play an important role in the etiology of primary open-angle glaucoma (POAG). The association between risk of POAG and lead exposure, which is an environmental source of oxidative stress, has not been fully investigated yet.

OBJECTIVE

Our objective was to determine the association between bone lead—a biomarker of cumulative lead dose (tibia lead) or an endogenous source of stored lead (patella lead)—and incident POAG.

METHODS

We examined a prospective cohort of 634 POAG-free men [mean baseline age=66.8 y of age (SD=6.7)] from the Normative Aging Study (NAS) who had tibia and patella K X-ray fluorescence lead measurements between 1 January 1991 and 31 December 1999. They also had standard ocular evaluations by NAS optometrists until 31 December 2014. POAG cases were identified by consistent reports of enlarged or asymmetric cup-to-disc ratio together with visual field defect or existence of disc hemorrhage. We used Cox proportional hazards regressions to estimate hazard ratios (HRs) of incident POAG and adjusted survival curves to examine changes in the risk of POAG during follow-up according to bone lead quartiles.

RESULTS

We identified 44 incident cases of POAG by the end of follow-up (incidence rate=74 per 10,000 person-years; median follow-up=10.6 y). In fully adjusted models, 10-fold increases in patella lead and tibia lead were associated with HRs of 5.06 (95% CI: 1.61, 15.88, p=0.005) and 3.07 (95% CI: 0.94, 10.0, p=0.06), respectively. The HRs comparing participants in the third and fourth quartiles with the lowest quartile were 3.41 (95% CI: 1.34, 8.66) and 3.24 (95% CI: 1.22, 8.62) for patella lead (p-for-trend=0.01), and 3.84 (95% CI: 1.54, 9.55) and 2.61 (95% CI: 0.95, 7.21) for tibia lead (p-for-trend=0.02).

CONCLUSIONS

Our study provides longitudinal evidence that bone lead may be an important risk factor for POAG in the U.S. population. https://doi.org/10.1289/EHP3442.

Dietary calcium supplements to lower blood lead levels in lactating women: a randomized placebo-controlled trial

BACKGROUND

Pregnancy and breastfeeding mobilize lead stored in bone, which may be a hazard for the fetus and infant. We tested the hypothesis that in lactating women a dietary calcium supplement will lower blood lead levels.

METHODS

Between 1994 and 1995 we conducted a randomized trial among women in Mexico City. Lactating women (N = 617; mean age = 24 years; mean blood lead level = 8.5 ug/dL) were randomly assigned to receive either calcium carbonate (1200 mg of elemental calcium daily) or placebo in a double-blind trial. Blood samples were obtained at baseline, and 3 and 6 months after the trial began. Blood lead was determined by graphite furnace atomic absorption spectroscopy. Bone lead was measured at baseline with a 109cd K x-ray fluorescence instrument. The primary endpoint was change in maternal blood lead level, which was analyzed in relation to supplement use and other covariates by multivariate generalized linear models for longitudinal observations.

RESULTS

An intention-to-treat analysis showed that women randomized to the calcium supplements experienced a small decline in blood lead levels (overall reduction of 0.29 ug/dL; 95% confidence interval = -0.85 to 0.26). The effect was more apparent among women who were compliant with supplement use and had high bone lead levels (patella bone lead > or =5 microg/gm bone). Among this subgroup, supplement use was associated with an estimated reduction in mean blood lead of 1.16 ug/dL (95% confidence interval = -2.08 to -0.23), an overall reduction of 16.4%.

CONCLUSIONS

Among lactating women with relatively high lead burden, calcium supplementation was associated with a modest reduction in blood lead levels.

Season modifies the relationship between bone and blood lead levels: the Normative Aging Study

Bone serves as a repository for 75% and 90-95% of lead in children and adults, respectively. Bone lead mobilization heightens during times of increased bone turnover, such as pregnancy, lactation, hyperthyroidism, and the rapid growth of childhood. Blood lead levels show seasonal periodicity. Children demonstrate peak blood lead levels in mid-summer and a secondary peak in late winter. Pregnant women demonstrate the highest mean blood lead levels in winter (January-March) and the lowest in summer (July-September). This fluctuation in blood lead levels may be related to seasonal patterns of environmental exposures, but it may also be partially related to the increased mobilization of bone lead stores during the winter months. We performed bone lead measurements using a K-x-ray fluorescent instrument to determine micrograms of lead per gram of bone mineral (parts per million) in middle-aged and elderly men who participated in the Normative Aging Study. We obtained measurements of blood and bone lead during the high sun exposure months of May-August (summer; n = 290); the intermediate sun exposure months of March, April, September, and October (spring/fall; n = 283); and the low sun exposure months of November-February (winter; n = 191). Mean blood lead concentrations were 5.8 microg/dl, 6.1 microg/dl, and 6.6 microg/dl for the summer, spring/fall, and winter, respectively. Mean patella (trabecular bone) lead concentrations were 34.3 microg/gm, 29.7 microg/gm, and 29.0 microg/gm for the summer, spring/fall, and winter time periods, respectively. In multivariate regression models, adjusted for age, smoking, alcohol ingestion, and dietary intake of iron and vitamin C, the authors found a strong interaction between season and bone lead level--with bone lead levels exerting an almost 2-fold greater influence on blood levels during the winter months than the summer months. The authors concluded that elevated blood lead levels in winter may be related to increased mobilization of endogenous bone lead stores, potentially from decreased exposure to sunlight, lower levels of activated vitamin D, and enhanced bone resorption.

Levels of lead in blood and bone of women giving birth in a Boston hospital

Blood lead levels declined among Americans aged 1-74 y between the years 1976 and 1991 (National Health and Examination Survey [NHANES III]). In 1990, umbilical cord blood lead levels were surveyed among 223 women who gave birth in a Boston hospital obstetrical service. In a survey conducted 10 y earlier, women had a mean umbilical cord blood lead level of 6.56 micrograms/dl (standard deviation = 3.19). In another subgroup of 41 women who were 1-4 wk postpartum, bone lead levels were surveyed with a K-x-ray fluorescence instrument and by analysis of venous blood lead levels. In 1990, umbilical cord blood lead levels were very low (i.e., mean and median of 1.19 [standard deviation = 1.32] and 1.0 micrograms/dl, respectively. In the subgroup of postpartum women, lead levels in cortical bone (i.e., tibia) and trabecular bone (i.e., patella) were also low (i.e., tibia mean and median of 4.5 [standard deviation = 4.0] and 4 micrograms/g bone mineral, respectively); patella mean and median of 5.8 [standard deviation = 4.5] and 5 micrograms/g, respectively). Maternal age was the only factor associated (i.e., borderline [p=.076]) with bone lead. Umbilical cord blood lead levels among women served by this Boston hospital from 1980 to 1990 declined dramatically, thus paralleling the National Health and Examination Survey. Bone lead levels were also low, but lead remains a reproductive hazard to women in select high-risk groups.