Environmental Research Section A 82, 81}90 (2000)
Article ID enrs.1999.4007, available online at http://www.idealibrary.com on
Maternal Bone Lead Contribution to Blood Lead
during and after Pregnancy1
Stephen J. Rothenberg,* - ? Fuad Khan,* A Mario Manalo,* - Jian Jiang,- Rosa Cuellar,- Sergio Reyes,-
Susana Acosta,- Maritza Jauregui,- Maria Diaz,- Margarita Sanchez,-
Andrew C. Todd,A and Calvin Johnson* -
*Environmental Research Center and -Department of Anesthesiology, Drew University of Medicine and Science, 1621 East 120th Street,
Los Angeles, California 90059; ?Instituto Nacional de Salud Publica, Cuernavaca, MeH xico; and
H
ADepartment of Community and Preventive Medicine, Mount Sinai School of Medicine, New York, New York
Received February 25, 1999
inBuence blood lead levels in this sample. 2000
Academic Press
We examined bone lead contribution to blood lead
Key Words : blood lead; bone lead; pregnancy; cal-
in a group of 311 immigrant women, 99% from Latin
cium; Latina.
America, during the third trimester of pregnancy
and 1 to 2 months after delivery. We measured in
vivo tibia and calcaneus (heel) bone lead concentra-
tion in the postdelivery period with K shell X-ray
Buorescence. Prenatal and postnatal geometric
INTRODUCTION
mean (range) blood lead level was 2.2 g/dL (0.4 to
38.7) and 2.8 g/dL (0.4 to 25.4), reBecting low cur-
Fetal development is altered by toxic exposure of
rent exposure. Postnatal blood lead level was signif-
the mother during pregnancy. Controlling such ex-
icantly higher than prenatal (P < 0.0001). Mean
posure before and during pregnancy can protect the
(range) tibia and calcaneus lead concentration was
fetus from teratogenic effects. Bone is one of the
6.7 g/g ( 33.7 to 62.2) and 8.4 g/g ( 30.1 to 66.4),
reservoirs for lead in the body. It has been suggested
reBecting varying but elevated past lead exposure.
that bone stores of lead might be readily mobilized
Mean calcaneus lead concentration was signiA-
into circulation during pregnancy and lactation
cantly higher than mean tibia lead concentration
(P 0.055). Variance-weighted multiple regression (Manton, 1985; Silbergeld 1991). There is indirect
and structural equation models showed that both
evidence that bone lead stores reenter circulation
calcaneus and tibia lead were directly associated
and raise blood lead levels outside of pregnancy
with prenatal blood lead but only calcaneus lead
(Gulson et al., 1995), raise circulating maternal
was associated with postnatal blood lead. Increas-
blood lead levels during pregnancy, raise blood lead
ing natural log years in the United States indepen-
levels of the umbilical cord at term, and raise post-
dently predicted decreasing calcaneus and third-
partum blood lead of the mother (Manton, 1985;
trimester blood lead. The data suggest that while
Rothenberg et al., 1994; Rothenberg et al., 1996;
some exogenous lead sources and modulators of
Gulson et al.,1998). Indirect evidence of the contri-
blood lead level, such as use of lead-glazed pottery
bution of bone lead to blood lead level in postpartum
and calcium in the diet, control lead exposure dur-
lactating women has recently been published (Her-
ing and after pregnancy, endogenous lead sources
from past exposure before immigration continue to nandez-Avila et al., 1996; Hu et al., 1996). Other
work using blood lead isotope ratios strongly impli-
cates bone lead as the source for more than 30% of
1
This work was supported by a cooperative agreement for Sub- blood lead in a small group of pregnant women (Gul-
stance-Speci7c Research Program Grant number U50/ATU398948
son et al., 1997). As lead in bone has a half-life of the
with the Agency for Toxic Substances and Disease Registry, CDC
order of years to decades (Steenhout, 1982; Chris-
and the Minority Health Professions Foundation but does not
tofferson et al., 1987), its effects on health may far
necessarily represent the views of those organizations. This pro-
outlast the end of signi7cant exogenous lead expo-
ject was approved by the King/Drew Medical Center Institutional
Review Board. sure.
81
0013-9351/00 $35.00
Copyright 2000 by Academic Press
All rights of reproduction in any form reserved.
82 ROTHENBERG ET AL.
This study examines the contribution of postpar- (95.1% of total subjects returning for postnatal bone
tum bone lead concentration in cortical and trabecular lead measurements). Of recruited immigrant pa-
bone measured by in vivo K shell X-ray 8uorescence tients 1441 (68.6%) delivered at KDMC and 312
to both contemporaneous and third trimester blood (14.9%) returned within 2 to 10 weeks for their 7rst
lead levels in a sample of 311 immigrant, predomi- postpartum visit. All but 3 of these patients identi-
nantly Latina women in South Central Los Angeles. 7ed themselves as Latina.
We administered a screening questionnaire and
drew a venous blood sample for blood lead analysis
MATERIALS AND METHODS
at recruitment. At the postpartum visit [mean (SE)
Subjects
number of days after delivery, 60.2 (2.1)] we admin-
We recruited subjects attending the outpatient istered a risk questionnaire, drew blood for blood
prenatal clinics at the King/Drew Medical Center lead analysis and measured cortical (tibia) and tra-
(KDMC)2 in South Central Los Angeles. South Cen- becular (calcaneus) bone lead concentration. Bone
tral Los Angeles is one of the most economically lead was not measured during pregnancy to avoid
depressed areas in the state of California. KDMC is exposure of the fetus to ionizing radiation.
a county hospital serving primarily low-income in-
ner-city minorities who lack the economic means to
Blood Lead Measurements
purchase private medical care or medical insurance.
The Institutional Review Board (IRB) reviewed and Blood samples were drawn into blue-top (trace
approved all procedures. metal-free) Becton}Dickinson Vacutainers with
We contacted subjects in the third trimester heparin after thorough cleaning of the venipuncture
[mean (SE) number of days before delivery, 71.2 site. Blood samples were analyzed with a Perkin}
(1.5)] of pregnancy from June 1995 through July Elmer 4100ZL Zeeman atomic absorption spectrom-
1998. We distributed educational materials to sub- eter with graphite furnace, correcting for back-
jects and counseled them on the risks of lead expo- ground interference. Each sample was analyzed in
sure and on how to reduce their exposure. Subjects duplicate and the means of the duplicates were used
with greater than 10 g/dL blood lead level (N"44) as data. Our laboratory has participated successfully
were referred to the Los Angeles County Lead Pre- in both the Centers for Disease Control and Preven-
vention treatment program for follow-up. The 2209 tion (now Wisconsin State Laboratory of Hygiene
subjects who agreed to participate by giving an IRB- Pro7ciency Testing program) and the College of
approved informed consent represented 78% of total American Pathology blood lead quality assurance
available patients at the prenatal clinics. programs for 4 consecutive years without any out-of-
Previously published data from this cohort bounds measurements.
(Rothenberg et al., 1999) showed that immigrant Accuracy of blood lead determinations was as-
subjects were signi7cantly older and had higher pre- sessed using blood lead values published by the ref-
natal blood lead level than nonimmigrant subjects. erence laboratories of the CDC quality assurance
The same study also showed that predictors of pre- program during the period of data analysis. We
natal blood lead level of immigrant and nonimmig- grouped the reference values into three ranges:
rant subjects were signi7cantly different. Lead level 1}10 g/dL (low), 11}20 g/dL (medium), and 20}38
of immigrant women was signi7cantly associated (high) g/dL. We calculated mean and standard er-
with duration of residence in the United States, age, ror of unsigned deviations of our analyses from the
secular trend, seasonal trend, pica, coffee drinking, target values for each of these ranges. Mean (SE)
dietary calcium, and use of folk remedies. Age, secu- unsigned deviations were 0.4 (0.1) g/dL for low, 0.6
lar and seasonal trends, and folk remedy use were (0.1) g/dL for medium, and 1.2 (0.2) g/dL for high
also signi7cantly associated with blood lead level in target ranges.
nonimmigrant women but cigarette smoking was Precision of blood lead determination was as-
the only other signi7cant variable in this group. This sessed using the unsigned difference of the duplicate
paper reports results only of immigrant subjects analysis for each sample processed during the data
analysis period. Mean (SE) unsigned difference be-
tween duplicate values for all samples was 0.2
2
Abbreviations used: SEM, structural equation modeling;
(0.007) g/dL. We performed similar calculations on
gpb/gbm, micrograms of lead per gram of bone mineral; gpb/gplaster,
samples whose mean values ranged between 2.0 and
micrograms of lead per gram of plaster in phantoms; PbB, blood
3.0 g/dL inclusive. This range encompassed the
lead level; PbBone, bone lead concentration; KDMC, King/Drew
Medical Center.
mean, geometric mean, and median values for the
BONE LEAD AND BLOOD LEAD DURING PREGNANCY 83
group. Mean (SE) unsigned difference between du- We assessed pregnancy and lactaction history by
plicate values for this restricted range of samples asking for the number of prior pregnancies, number
was 0.2 (0.01) g/dL. of known abortions, number of still births, if the
Blood samples were analyzed within 24 h of collec- subject ever nursed, and if so how many children
tion over the course of the study. Inspection of qual- were nursed for how many months. Variables tested
ity assurance and internal control data indicated no in models were the dichotomous   ever nursed  , total
systematic trend over the study period. Coefficients number of children nursed, nursed in the immedi-
of variation calculated from samples on a weakly ately past pregnancy (yes/no, and number of months
basis showed mean values that varied around 3.2% up to testing), total months of nursing, plus the
across the study period. number of pregnancies, abortions, and still births.
Questionnaires were presented as structured in-
terviews by trained bilingual interviewers in the
language of the subject s choice (Spanish or English).
Questionnaires
Duplicate entry of the responses into the computer
We used screening and risk-factor questionnaires data base minimized transfer error.
to gather basic socioeconomic and demographic in-
formation, medical and reproductive history, history
Bone Lead Measurements
of lead exposure, and dietary habits. Risk factors
included work and hobby histories, use of leaded We measured bone lead concentration at mid-tibia
paint and ceramic ware, pica, cigarette, alcohol, and (cortical bone) and mid-calcaneus (trabecular bone)
109
other drug use. Use of low-temperature ceramic with a Cd K-shell X-ray 8uorescence (KXRF) sys-
ware and folk remedy use was assessed by asking tem described previously (Todd et al., 1992, 1993).
about current use and coding the response as The technique uses photons emitted from the decay
109
a dichotomous variable. Dietary information of Cd (88 keV) to excite lead atoms within the bone
centered on consumption of high-calcium foods, oils, matrix, causing them to emit characteristic X rays.
and other fats. Physical activity levels were also The amount of coherent scattering of the 88-keV
109
assessed. As part of this assessment, we asked the Cd photons is proportional to the amount of bone
subject how many hours/day in the past week were mineral. By normalizing the lead X-ray signal to the
spent in bed and in light, moderate, and heavy phys- coherent signal we can specify the measurment in
ical activity (all de7ned by example), restricting the micrograms of lead per gram of bone mineral
sum of total hours to 24. Number of hours for each ( gpb/gbm). The technique allows estimation of the
activity was used in model testing. This question- error of measurement, which is equivalent to one
naire was used in a previously published study standard deviation.
(Rothenberg et al., 1999). Prior to measurement, the subject removed all
Questions regarding diet were based on frequency metal and plastic objects. We washed the skin above
of consumption over the past month. We coded die- each bone site and then thoroughly cleaned it with
tary variables with a three point scale: never to less 4% glacial acetic acid to remove any super7cial con-
than once per week, one to two times per week, and tamination. The collimated beam of photons from
109
greater than two times per week. We later recoded the Cd source was directed orthogonally to the
these variables dichotomously into use less than bone surface at each site. We measured each bone
once per week (coded zero) and at least once a week site for 30 min. Subjects were comfortably seated
(coded one) categories, because few subjects reported with the leg restrained.
at the intermediate level. Most subjects either did Nonlinear least squares 7tting (Marquardt algo-
not use these dietary items at all or tended to use rithm) of the spectra provided unbiased estimates of
them daily. Dietary variables related to calcium lead concentration and of error of measurement. The
2
(milk, yogurt, ice cream, and cheese) were similarly algorithm provides a goodness of 7t statistic.
combined. No subject used calcium supplements. We calibrated the system every 2 weeks or after
Years of education was a count variable, while work any change to the system (e.g., detector warm-up,
history, hobby, medication, disease, and drug use change of spectroscopy settings) with a graded series
variables were dichotomous yes/no. Pregnancy and of plaster of paris tibia phantoms doped with known
lactation history variables were coded both as counts amounts of lead. A complete line of phantom
(number of pregnancies, deliveries, number of chil- measurements consisted of measuring the phan-
dren breast fed) and as dichotomous variables (pre- toms, ranging from a nominal zero g /g to
pb plaster
vious, coded one; no previous, coded zero). 220 g /g , a total of 20 times. Three quality
pb plaster
84 ROTHENBERG ET AL.
control phantoms were run during every subject TABLE 1
Variables Tested in Bone Lead Modeling
measurement day to detect out-of-control-range
peak ratios. Calibration lines were used for subjects
Age*
measured from the start of the collection of the line
Years resident in United States*
until the start of the next line.
Prenatal blood lead level*
Postnatal blood lead level*
Smoking
Data Analysis
Alcohol use
Pica
We used Statgraphics Plus (Manugistics, Inc.,
Use of folk remedies*
Rockville, MD), SPSS (SPSS, Inc., Chicago, IL), Amos
Coffee drinking
(SmallWaters Corp., Chicago, IL), and Stata (Stata Other caffeine beverages
Occupational exposure (various)
Corp., East College Station, TX) for data manage-
Hobby exposure (various)
ment and analysis. We transformed blood lead level
Low temperature ceramic ware use
variables into their natural logarithms to reduce the
Currently use*
effects of outliers on subsequent statistical analyses.
Frequency of use
Descriptive statistics and graphics provided quality Years of use
Use to prepare (a) food; (b) sauces; (c) drinks
control for all variables. We removed one subject with
Use to store foods
a blood lead level of 81 g/dL, 5.8 geometric standard
Used in childhood home
deviations above the sample mean.
Diet
Univariate and bivariate analyses of a priori
Frequency of fat intake
selected predictor variables against the various lead Frequency of calcium intake*
Activity
variables with P 0.10 probabilities determined the
Hours in bed*
variables that were made available in multiple re-
Hours in light activity
gression analyses of the lead variables. We construc-
Hours in medium activity
ted several models, including tibia and calcaneus
Hours in heavy activity
lead concentration with and without weighting for Number of city blocks walked
Number of stairs climbed
bone lead error terms. Models were constructed by
Pregnancy of lactation history
forward stepwise and by backward elimination tech-
Number of pregnancies
niques. The two models presented contained the
Number of abortions
same terms using either elimination technique. We
Number of still births
used variance-weighted least squares regression Ever nursed
Number of children nursed
(Stata Corporation, 1997) for modeling bone lead
Total months nursed
concentration, weighting each bone lead measure-
Presently nursing
ment by its respective measurement error. Table 1
shows the variables considered for entry into mod-
*Signi7cant at P 0.10.
els. Most variables tested were those previously
shown to be associated with blood lead levels or bone
lead concentration in the population from which this
sample was derived or in similar groups.
We used exploratory structural equation modeling Subjects returning for bone lead tests were signi7-
(SEM) with maximum likelihood estimation (Ar- cantly older, had spent signi7cantly more years in
buckle, 1997) to determine if dietary calcium made the United States, and were more likely to use folk
independent contributions to blood lead level and remedies than subjects not returning for bone lead
bone lead concentration. We also used SEM to ex- tests, though the differences between the two groups
plore alternate models of the effect of bone lead were small (Table 2). There were no other differ-
concentration on pre- and postnatal blood lead ences between those returning and those not return-
levels. An acceptable model contained all signi7cant ing.
(P 0.10) variables and had an overall nonsigni7- Geometric mean (#/!geometric SD) postnatal
2
blood lead level was 2.8 (#4.9/!1.2) g/dL with
cant (P 0.10).
a range of 0.4 to 25.4 g/dL. The increase in blood
lead from prenatal to postnatal was signi7cant
RESULTS
(P 0.0001). Prenatal and postnatal blood lead
Subjects ranged in age from 15 to 44 years, and (natural log transformed) were signi7cantly corre-
their education level ranged from 0 to 17 years. lated (r"0.76, P 0.0001).
BONE LEAD AND BLOOD LEAD DURING PREGNANCY 85
TABLE 2
Subject Characteristics
Sample returning Sample not returning
N"311 N"1697
Variable Mean SD Mean SD Probability (t test)
Age 27.8 7.5 26.1 6.1 (0.001
Education 8.5 3.5 8.6 3.2 '0.10
*Prenatal blood Pb 2.2 #4.8/!1.0 2.2 #3.1/!1.4 '0.10
*Years in United States 5.9 #14.2/!2.4 5.4 #13.8/!2.1 0.058
Hours in bed 8.6 2.2 8.8 2.0 '0.10
2
% Yes % Yes
Use folk remedies 6.4 4.3 0.078
Adequate calcium 54.5 52.0 '0.10
Use clay pottery 7.0 7.9 '0.10
*Geometric means and standard deviations.
Mean (SD) [range] tibia lead concentration was negligible bias among coef7cients and their standard
6.7 (12.5) [!33.7 to 62.2] g /g . Mean (SD) [range] errors, justifying normalization.
pb bm
calcaneus lead concentration was 8.4 (13.2) [!30.1 Both tibia lead and calcaneus lead were positively
to 66.4] g /g and was signi7cantly higher than associated with third trimester blood lead, each with
pb bm
tibia lead concentration (P"0.055); 3.5% of cal- approximately equal effect size, but only calcaneus
caneus and 4.8% of tibia measurements had uncer- lead was associated with postnatal blood lead. In-
tainties greater than 15 g /g . The two bone lead creasing years of residence in the United States was
pb bm
measurements were signi7cantly correlated associated with both decreasing calcaneus lead and
(r"0.244, P 0.001). third trimester blood lead, though the effect size on
Tables 3 and 4 show the variance-weighted least blood lead was twice the effect size on calcaneus
squares multiple regression models of tibia and cal- lead. Dietary calcium was associated only with third
caneus lead. Increased tibia lead in the sample was trimester blood lead, and use of lead-glazed ceramic
related to lead-glazed ceramic ware use, use of folk ware was associated only with postnatal blood lead.
remedies, and daily number of hours spent in bed Increased time in bed was associated with increased
during the pregnancy. Increased calcaneus lead was tibia lead but with decreased third trimester blood
related to higher maternal age and fewer years resi- lead.
dent in the United States. (Fig. 1). It is noteworthy that neither pregnancy nor lacta-
Table 5 shows the system of simultaneous equa- tion history was associated with bone lead concen-
tions used to build the structural equation model tration in this data set.
shown in Fig. 2. The coef7cients in Table 5 are based
on original measurement units. The coef7cients
DISCUSSION
shown in Fig. 2 are normalized to allow direct-effect
size comparison among variables without reference Published studies from Mexico City (Hernandez-
to measurement units. A distribution-free, 1000- Avila et al., 1996) and Boston (Hu et al.,1996) have
sample bootstrap estimation (not shown) revealed
TABLE 4
Variance-Weighted Multiple Regression Model of Tibia
TABLE 3
Bone Lead Concentration ( gpb/gbm) (N 311)
Variance-Weighted Multiple Regression Model of Calcaneus
Bone Lead Concentration ( gpb/gbm) (N 311)
Coef. Prob. 95% CI
Coef. Prob. 95% CI
Daily hours in bed 0.51 0.033 0.04 0.99
Use folk remedy 7.18 0.003 11.96 2.40
ln (years in United States) !2.10 0.001 !3.32 !0.89
Use leaded pottery 4.02 0.077 !0.44 8.48
Age (years) 0.14 0.067 !0.01 0.30
Constant 8.75 0.175 !3.89 21.39
Constant 8.11 (0.001 3.70 12.51
86 ROTHENBERG ET AL.
FIG. 2. Structural equation model of bone lead and blood lead.
Each unidirectional arrow represents a directional effect of one
variable (tail) on another variable (head). The numbers beside
each arrow represent the standardized regression weight and, in
parentheses, the z score of the regression weight in the model.
A z score of 1.96 is equivalent to a probability of approximately
P"0.05, assuming a very large N. The curved two-headed arrow
represents a covariance relationship and the number in italics
above it is the correlation coef7cient. Each   e  represents an
2
unobserved error term, 7xed at 1. The model "30.24,
df"24
P"0.18.
FIG. 1. Natural logarithmic relationship between years resi-
dence in the United States and calcaneus lead concentration
adjusted for age in the variance-weighted multiple regression. level of 9.6 g/dL. The mean tibia and patella bone
Size of the circle is directly proportional to the measurement error
lead concentrations were 12.5 and 13.3 gpb/gbm, re-
of calcaneus lead concentration.
spectively. These data suggest moderate past and
continuing lead exposure.
The Boston study examined women 3 to 6 months
examined the relationship between bone lead con- postpartum and found a geometric mean blood lead
centration and blood lead levels in postpartum level of 3.0 g/g. Mean tibia and patella bone lead
women. The Mexico City study measured blood lead concentrations were 4.5 and 5.8 gpb/gbm, respective-
in lactating women 1 month after delivery at a mean ly, These data suggest low past and present lead
exposure.
The geometric mean blood lead level from our
TABLE 5
study group is nearly the same as that of the Boston
Simultaneous Equations for Structural Equation Model
group but mean bone lead concentrations fall mid-
Calcaneus Pb "!1.63* ln (years in United States)#C
way between the Boston and the Mexico city study
Tibia Pb "0.62* (hours in bed)
groups. Our data suggest moderate past lead expo-
#8.80* (use folk remedy)#C
sure but low current exposure, congruent with the
ln (blood Pb pre) "0.009* (calcaneus Pb)#0.008* (tibia Pb)
immigrant status of our study group.
!0.51* (daily Ca)
!0.21* ln (years in United States)
!0.06* (hours in bed)#C
Blood Lead and Bone Lead
ln (Blood Pb post) "0.60* ln (blood Pb pre)
#0.006* (calcaneus Pb)
Prenatal blood lead level is a strong predictor of
#0.35* (lead-glazed pottery)#C
postnatal blood lead level. The signi7cant increase
BONE LEAD AND BLOOD LEAD DURING PREGNANCY 87
in blood lead level between prenatal and postnatal Mexico City study. This could be due to varying
measurements may be explained by changes in he- contributions of different levels of baseline dietary
matological dilution, body weight, and organ size calcium in the two countries, differences in current
after delivery. In the 7rst months after pregnancy exogenous lead exposure, and possibly the difference
these variables return to near prepregnancy values, in trabecular bone measured. Though both cal-
the fetus is no longer a sink for lead in the mother, caneus and patella are trabecular bone, the cal-
and blood lead levels, assuming lead sources are caneus is subject to considerably more physical
constant, are expected to rise. stress than would be expected for patella. The re-
We have previously suggested (Rothenberg et al., peated stress incurred by the calcaneus during walk-
1999) that the immigrant subjects in this study have ing or running might well produce metabolic
higher third trimester blood lead levels than the differences between calcaneus and patella (Felson et
nonimmigrant subjects due to past exposure before al., 1993). As the present study had over three times
immigration. The immigrants, almost all from Latin the number of subjects as the Mexico City study, it is
America (and principally from Mexico), have lived unlikely that low power in the present study led to
much of their lives in environments in which their insigni7cant results.
exposure was higher than it presently is in the The Mexico City study found that only dietary
United States. Thus, most subjects would experience calcium was associated with tibia bone lead. We
a sharp drop in exogenous lead exposure upon found a different pattern of results. Folk remedy use,
immigration. With the short half-life of lead in blood hours in bed, and use of lead-glazed pottery were all
(around 30}36 days), a sustained elevation in blood associated with increased tibia lead. The use of lead-
lead after immigration would require continued ex- containing folk remedies by people of Latin Ameri-
posure to exogenous or endogenous sources of lead. can origin is well documented (Baer et al., 1988;
In the previous study (Rothenberg et al., 1999) we Fernandez et al., 1997; Rothenberg et al., 1999), as is
documented the in8uence of exogenous factors, such the contribution to blood lead of using lead-glazed
as use of folk remedies, pica, and dietary calcium on pottery (Hernandez Avila et al., 1991; Rothenberg et
increased prenatal blood lead level of immigrants. al., 1992, 1996, 1998; Matte et al., 1994; Rojas Lopez
However, we found that the strongest in8uence on et al., 1994; Romieu et al., 1994; Fernandez et al.,
prenatal blood lead came from maternal age and 1997). The number of hours in bed variable, on the
years resident in the United States. We suggested other hand, requires additional comment. Bone
that both factors were surrogates for mobilization of metabolism is altered by physical stress and bone
lead from bones. As lead in bone has a half-life mineral loss increases with inactivity. If time spent
measured in years, lead tends to accumulate in bone in bed re8ects inactivity and lack of bone stress, the
with age (Drasch et al., 1987; Manea-Krichten et al., altered metabolic activity induced with changes
1991; Kosnett et al., 1994; Hu et al., 1996), even with associated with pregnancy might lead to increased
very low daily exposure. Similarly, after the end of accumulation of bone lead. Though the question sol-
elevated lead exposure, lead in bone decreases with iciting number of hours spent in bed directed the
time (Borjesson et al., 1997). The 7ndings of positive subject to respond based on her past week s history,
association of prenatal blood lead level with age and the response could also re8ect long-term activity
negative association of prenatal blood lead level with levels, perhaps even predating the study pregnancy.
natural log years residence in the United States lend
support to the hypothesis that maternal bone lead is
Integrated Structural Equation Model of Blood
an endogenous source of lead in blood.
Lead and Bone Lead
The multiple regression model for calcaneus lead
concentration shows the pattern that one would ob- Single-equation regression models typically de7ne
serve if lead in bone increases with age and de- a single dependent variable and one or more inde-
creases with time after the end of a period of pendent variables. With SEM the investigator uses
elevated exposure. The investigation of postpartum a system of simultaneous equations in which the
mothers in Mexico City showed a similar in8uence of dependent variable of one equation can appear as an
time from the start of high exposure (number of independent variable of another. A system of equa-
years resident in Mexico City) on lead in the patella tions allows testing hypotheses of effect direction
(trabecular bone) (Hernandez-Avila et al., 1996). The that is not possible in single-equation models. Sev-
lack of effect of dietary calcium on calcaneus lead in eral recent publications address these and other
the present study, however, contrasts with the sig- issues relevant to SEM and related techniques
ni7cant effect of calcium on patella lead in the (JoK reskog et al., 1989; Bentler et al., 1996).
88 ROTHENBERG ET AL.
We used SEM analysis to quantify directional re- us, we performed all further modeling with the non-
lationships between blood lead and bone lead, ac- directional covariance relationship.
counting for other explanatory variables that might The most interesting relationships among vari-
affect one or the other lead variable or both simulta- ables described by SEM are those in which one
neously. We used bone lead measurements 2 months variable is signi7cantly associated with two other
after pregnancy as a surrogate for bone lead during signi7cantly associated variables. These relation-
pregnancy. The long half-life of lead in bone and the ships can be found in Fig. 2 by looking for variables
limited precision of the bone lead measurement whose relationships form triangles. We will discuss
makes it unlikely that there were measurable cha- two of these relationships in the integrated model:
nges in bone lead between the third trimester and those involving years in United States, calcaneus
the postpartum period. lead, and prenatal blood lead and those involving
SEM gives the size, direction, and signi7cance of calcaneus lead and prenatal and postnatal blood
2
proposed linkages among variables. The model lead.
goodness of 7t statistic evaluates how well the pro- We hypothesize that many immigrants in the
2
posed model 7ts the data set (i.e., if the model is study experienced a step decrease in environmental
signi7cant, then the proposed model does not 7t lead exposure upon moving to the United States. If
the data, no matter how signi7cant each of the this were so, immigration was clearly not accom-
coef7cients describing linkages among variables panied by a step decrease in blood lead. Instead,
may be). prenatal blood lead level decreased as a function of
In developing the model shown in Fig. 2, we log years in the United States. Calcaneus bone lead
started with one strong assumption: blood lead mea- in the variance-weighted multiple regression model
sured earlier in time controls blood lead measured decreased with the natural logarithm of years in the
later in time and not the reverse. With this direc- United States. Thus, calcaneus lead contribution to
tional relationship 7xed, we tested models in which blood lead level might explain the effect of years
bone lead contributed only to postnatal blood lead, residence on blood lead. In the SEM we observe
only to prenatal blood lead, or to both. Although a signi7cant path from the years variable to cal-
coefficients describing the relationship between caneus lead and thereafter to third trimester blood
bone lead and postnatal blood lead alone were con- lead, as predicted. However, we see that the original
sistently larger and more signi7cant than coeffi- relationship between years of residence and third
cients between bone lead and prenatal blood lead trimester blood lead is still signi7cant, despite hav-
alone or both prenatal and postnatal blood lead, the ing accounted for the contribution of calcaneus lead
2
model in the 7rst scenario was highly signi7cant to blood lead. Removing any leg of this triangular
2
(P 0.001), indicating a poor 7t (signi7cant differ- relationship results in signi7cant model s, without
ence) of model to data. substantial change in the remaining coefficients.
We modeled the relationship between calcaneus This pattern of results suggests that not all of the
and tibia lead concentration as a nondirectional effect of years residence on third trimester blood
covariance term. The shorter half-life of calcaneus lead can be due to the effect of decreasing bone lead
lead suggests that calcaneus lead may be more labile after immigration. Some signi7cant part of the per-
than tibia lead. Thus, calcaneus lead might be more sistence and slow decrease in prenatal blood lead
easily liberated from the bone and so contribute to level after immigration must be due to one or more
tibia lead. Uptake of lead by calcaneus from unobserved variables, perhaps acculturation that
exogenous and endogenous sources may be more slowly reduces exposure to culturally determined
ef7cient than uptake by tibia. We tested alternate lead sources.
models in which we speci7ed each directional rela- Calcaneus lead concentration independently ac-
tionship between calcaneus and tibia lead conentra- counts for some variance in prenatal and postnatal
tion. The two directional models and the covariance blood lead level, despite the strong direct association
model gave equivalent results, both in terms of size between the two blood lead measures. However,
and signi7cance of relationships among other vari- tibia lead concentration is signi7cantly associated
2
ables, as well as in the size of the model . Failing to only with prenatal blood lead level. The Mexico City
specify any relationship between the two bone lead study (Hernandez-Avila et al., 1996) found a signi7-
2
measures resulted in a model with a signi7cant cant association between their measure of trabecu-
(P"0.003). As there was no clear advantage gained lar bone lead (patella) and postnatal blood lead also,
by selecting a directional relationship between the without a signi7cant association between tibia lead
two bone lead measures and little literature to guide and blood lead. To directly compare results between
BONE LEAD AND BLOOD LEAD DURING PREGNANCY 89
the two studies, we provisionally constructed a mul- correlated. Despite accounting for that covariance in
tiple regression model for nontransformed postnatal the test SEM models, the collinearity between the
blood lead (the variable used in the Mexico City two variables might have been responsible for the
study) with calcaneus lead and lead-glazed ceramic poor model 7t. It is likely that a bone lead model
ware use as independent variables. We obtained of nonimmigrants, in which time resident in the
a calcaneus coef7cient of 0.071, compared to their United States is not a factor, will contain a signi7-
patella coef7cient of 0.06. The coef7cients of trabecu- cant age term.
lar lead on postnatal blood lead in the two studies
are the same.
When we simultaneously account for the in8uence
CONCLUSIONS
of prenatal blood lead on postnatal blood lead the
relationship between calcaneus lead concentration
Bone lead is a signi7cant source of lead in preg-
and postnatal blood lead level remains. The different
nant immigrant women and remains so in the early
patterns of effect of tibia and calcaneus lead on pre-
postnatal period. As long-term lead exposure elev-
and postnatal blood lead might be due to the relative
ates bone lead, and lead exposure before immigra-
lability of the two bone lead sources. During the last
tion remains beyond control of national health
trimester of pregnancy the fetus experiences rapid
authorities, the bone lead contribution to circulating
skeleton growth. The high calcium demand on the
lead may be dif7cult to control. As long as population
mother may serve to mobilize lead from both cortical
lead exposure remains unregulated in other parts of
and trabecular bones to a similar degree. The effect
the world, endogenous stores of lead will continue to
of dietary calcium on prenatal blood lead level but
in8uence circulating blood lead levels during and
not postnatal blood lead level also suggests that high
after pregnancy in women of all nations. It is clear,
calcium demand in the absence of adequate dietary
however, that other sources, both observed in this
calcium may augment release of blood from bone
study and not observed, play an important role in
during the last trimester of pregnancy. We found no
continuing lead exposure during and after preg-
effect of nursing history, nor of presence or absence
nancy. Dietary calcium and use of lead-glazed ce-
of nursing after the studied pregnancy, on either
ramic ware in8uence prenatal and postnatal blood
bone lead or blood lead. Although precision of bone
lead level, respectively, and these modulators of
lead measurement, discussed above, may have
blood lead level are amenable to control. Further
played a role in our failure to 7nd signi7cant effects
study of the relationship between years of residency
of pregnancy and lactation history on bone lead con-
in the United States and prenatal blood lead level
centration, it is possible that there was signi7cantly
might reveal other controllable factors contributing
more calcium stress during pregnancy than during
to maternal lead exposure during pregnancy.
lactation in our sample. If calcium demand and sub-
sequent lead mobilization from bone was less after
pregnancy than during pregnancy in our sample, the
more easily mobilized trabecular bone lead might be
ACKNOWLEDGMENTS
the only measurable bone lead effect on postnatal
blood lead level. We thank Thomas Yoshikawa, MD; Teiichiro Fukushima, MD;
Tom Carter; Vincent de Ciutiis, MD; the staff of the prenatal
Although other authors have found increasing age
clinics and delivery service at the King/Drew Medical Center; and
associated with increasing bone lead, as did we in
the women who served as subjects, all of whom contributed to the
the variance-weighted regressions above, we could
success of this project.
not place age into the SEM model. Every alternate
model with age associated with either or both of the
bone lead or the blood lead variables had a signi7-
2
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