(See Gulson et al., p. 667)

This issue of Environmental Health Perspectives includes the latest research installment in a series of recent findings by Gulson and co-workers on the quantitative and temporal aspects of human bone lead releases during pregnancy and lactation. Using the technique of stable lead isotope ratio analysis, Gulson et al. describe bone lead releases to breast milk and the relationship of such releases to both early infant lead exposures and overall lead biokinetics in the early infant. Comparative assessment is also made of any external lead contributions to infant lead intakes that would arise from either breast milk containing maternal diet lead or direct intake of external lead by infants ingesting lead in infant formula. The authors also carried out a critical appraisal of the existing literature on lead in breast milk, and these earlier data were placed in context with the present results.

Gulson et al. find that the majority of lead ingested by nursing infants arises from breast milk lead and that a significant fraction of breast milk lead arises from continuing bone lead release, although the rate of release over time may decline. Similarly, formula-fed infants show blood lead isotopic ratios typical of lead in the formula, but with a high level of variability to be expected for lead in commercially available formula from diverse sources. Blood lead concentrations in infants receiving breast milk as the nutrition source show clear linkages of breast milk lead isotopic ratios with both infant blood and urine, indicating ready lead bioavailability in the infants. For mothers, diet lead and breast milk lead levels were correlated as to both concentration and isotopic ratios.

Overall, ratios of breast milk lead levels to maternal blood lead concentrations approximated those of plasma. In these immigrant subjects, blood lead levels at the time of lactation were low, producing quite low breast milk levels, less than 1 ppb on average. However, it is probably the case that where maternal blood lead levels are significantly elevated from whatever mix of bone and diet lead during the nursing period, breast milk lead rises proportionately, akin to plasma increases. Gulson et al. produce data in their analysis of all breast milk lead studies to show that plotting of the more reliable earlier data sets along with their results yields a linear increase when the ratio of breast milk lead to blood lead is plotted against blood lead. That is, as blood lead rises, the relative lead amount seen in breast milk compared to blood lead increases. This would make breast milk an especially toxic source of lead for the early infant in those cases where mothers with nursing infants had high, chronic lead exposures.

The current study report and earlier research papers by these investigators demonstrate the complex biokinetic interplay between endogenous lead released from bone and exogenous lead entering blood and other body compartments from (mainly) ingestion for people with diverse exposure histories. Through extended funding from the NIEHS, the Australian team of Gulson and co-workers has been productively examining the important issue of the role of accumulated bone lead stores in human lead exposure and the specifics of its potential for release under diverse external and metabolic circumstances over the last 6 or 7 years. Their earlier research directions have included the isotopic "fingerprinting" of environmental and tissue lead (1-3), the release of bone lead when one alters his or her exposure status (4), additional releases of lead during pregnancy (5), continued release of bone lead during lactation (6), and biokinetic studies of lead in mother/infant pairs (7).

Bone lead is now recognized as being in equilibrium with lead in blood (8,9), replacing the traditional, albeit never physiologically plausible, view that lead lodged in bone was metabolically and toxicologically inert. Release of bone lead in humans during either shifts in (near) steady-state equilibria with blood lead owing to external disturbances such as altered lead intake/uptake rates, or in response to various physiological stimuli and stressors such as pregnancy, has long been suspected or suggested in the toxicological and occupational health literature. This current view has been slowly developed from information based on 1) isolated case reports of latent lead exposures via elevated blood lead in individuals exposed in earlier years but not at testing (10); 2) national epidemiological data conveying the inference that postmenopausal women as a group show an increase in blood lead that is explicable in terms of lead releases during osteoporotic bone demineralization (11); 3) occupational exposures to lead in individuals whose blood lead and other lead biomarkers were monitored during retirement (12-14), transitory changes in workplace contact such as strike actions (15), or as a function of exposure duration (15,16); 4) human data indicating increases in blood lead during pregnancy (17) and modeling estimates of lead's entry to, and exit from, bone (18-20).

Clear empirical (epidemiological) documentation of the phenomenon of bone lead remobilization in pregnant and lactating women and its quantitative biokinetic characterization have until recently evaded easy study (21). This was so despite experimental animal evidence suggesting the phenomenon [e.g., Keller and Doherty (22)]. By contrast, characterization of some of the temporal and quantitative features of the reverse process, bone lead accumulation, has advanced farther. The latter process was favored for study through its reliance on available autopsy tissue stratified as to age (23-25), cortical versus trabecular bone (26), and use of in vivo relatively static measures of lead accumulation in bone using either K- (27) or L- (28) X-ray fluorescence analysis.

One impediment to the study of bone lead mobilization, until recently, has been traditional reliance on conventional total mass analysis methods for lead quantitation. For example, incremental changes in total blood lead concentration during pregnancy and afterward may not be readily detectable at lower absolute values without problems of precision and accuracy. More problematic in using total mass methods is the need for correct apportionment of the source of any such incremental rises in blood lead to either external lead uptake or bone lead release to blood in pregnancy (and lactation). Appropriate study control of lead intakes via duplicate diet lead determinations in pregnancy versus prepregnancy have not been done other than in the Gulson et al. research series.

The extent to which maternal lead absorption through the gut or lung changes during pregnancy is largely unknown. One can conclude from the overall information base in the scientific literature, including recent information from Gulson et al. (5), that the overall circulating calcium requirement in pregnancy increases owing to fetal growth requirements; the rate of calcium intake and/or uptake must thereby increase; dietary (exogenous) calcium availability is a determinant in bone calcium (and lead) release such that more calcium in the diet is linked to less bone lead release (5); and amounts of dietary lead absorbed would increase as calcium uptake increases, because enteric uptake mechanisms for calcium also favor uptake of lead (29-32).

Changes also occur in body and hemovascular indices. For example, an increase in overall blood volume to accommodate the placental vascular bed and the fetal growth rate has long been known. Hytten (33) has produced estimates of an approximate change of 50% in plasma volume from conception to the final trimester of pregnancy, in tandem with only an 18% increase in erythrocyte mass. Birth weights and parity status are additional factors that affect individual circulation indices during pregnancy (33).

Recent advances in this area of research represent a textbook illustration of productive collaborations through interdisciplinary expertise and applications to a multifaceted problem. The tools and approaches of geology, geochemistry, and biogeochemistry have contributed significantly to assessment of the biokinetics of skeletal lead mobilization and its potential role in producing fetal and nursing infant lead exposures and poisoning risks. In the 1980s, Manton (34) posited the notion that historical body lead deposited in bone years ago could be released through mobilization and provided evidence for this in a single subject using stable lead isotope mass spectrometric quantitation. Rabinowitz (35) developed the notion further in terms of more systematic approaches. The stable isotopic lead ratio approach is relatively complex conceptually and computationally, is laborious and demanding in its requirements for clean-room methodology to avoid even ultratrace contamination by lead of external isotopic ratio origin, and is quite costly on a unit-sample basis. It remains, nevertheless, the best methodology to address this topic.

Successful application of the stable lead isotope technique to study of bone lead remobilization also requires that the isotopic composition of lead in bone be both distinct from external isotopic mixes and that release be sufficient to permit mass spectral quantitation of bone lead releases within established analytical statistical bounds for mass spectral methodologies. In many cases, the required isotopic distinctions do not exist. Epidemiology is still determined by opportunity, however (36). Gulson and co-workers were able to exploit this mass spectral analysis technique within a multidisciplinary program to study a recruited population of European immigrants to Australia whose bone lead isotopic ratios, including that of ²⁰⁶Pb/²⁰⁴Pb, were known to arise from geochemical lead sources, different from lead sources, and therefore body lead burdens, of individuals who spent their lives in Australia. The various types of studies carried out by Gulson et al. were cited earlier. In summary, Gulson et al. found that bone lead releases in adults and children entering Australia are sufficiently significant that they can account for up to 70% of total blood lead content, with a persistence over many months (4). This bone lead release rate in women of childbearing age is accelerated during pregnancy in these subjects (5). Accelerated bone lead release continues into lactation, and lactation release can in fact exceed that occurring in pregnancy (6).

The series of bone lead studies by Gulson and co-workers has a number of important implications for public health and lead regulatory policy. I noted some of these earlier (21). First, bone lead releases in the Gulson et al. population are significant, in fractional terms, despite the fact that the total blood lead concentration was low in these women. Many women in the Gulson immigrant group have blood lead values that would be viewed as largely nontoxic. Gulson et al. demonstrated that, in pregnancy, bone lead is released into maternal circulation, enters fetal circulation, and is lodged in the fetus. Similar accelerated bone lead releases during lactation extend the developmental lead exposures to early infancy via breast milk lead intake.

Total lead transfer to the fetus can be significant, even when a static measure such as blood lead appears low. An easier estimate for the nursing infant milk lead uptake is possible with estimated ingested breast milk volumes. Therefore, lead transfers to the fetus from bone, which is followed by additional circulating bone lead entering the nursing infant.

A second issue of concern revolves around the question of whether mobilized bone lead may in fact exhibit biokinetic features that result in a higher percentage of total blood lead being present in the plasma/serum fraction. This is the fraction of whole blood lead that comprises the deliverable lead dose to tissues. Cake et al. (37) employed serum lead analysis to argue that, where circulating lead was expected to mainly represent bone lead rather than current exogenous lead uptake, the fraction of total blood lead in serum is higher than reported for serum/plasma in ongoing, i.e., external exposures. A later Swedish study (38) and a revisitation of the earlier paper (37) by some of the original authors (39) suggested that may not be the case. However, the two studies differed so widely as to methodology, design, and subject exposure profiles that the matter remains an open question.

High-risk lead exposure populations still exist in the United States in the form of inner-city, poor women of childbearing age, especially those in densely populated older cities, and other women having localized high lead exposures in communities affected by lead-emitting industrial operations or by hazardous waste sites. The collective message from the Gulson et al. studies is that life-long lead exposures of this demographically and socioeconomically diverse risk group must be considered in determination of long-term risk assessment approaches and risk management policies and initiatives. Lead regulatory and public health policy initiatives must particularly encompass a transgenerational time frame for dealing with lead exposure and lead toxicity or toxicity risk. Both historical, endogenous and current exogenous lead play exposure roles.

The focus of the Gulson studies has been on the fetus and the nursing infant. It is critical to point out, however, that maternal blood lead increases from bone lead during pregnancy and lactation obviously have a potential impact as well on the mother in terms of adverse health effects. Such toxicological roles for bone lead releases in incremental maternal blood lead increases and associated toxicity risks were recognized and appropriately emphasized by Silbergeld (40). In some cases, released lead can affect the health of the fetus indirectly by affecting maternal health. Lead is vasoactive, causing elevations in both systolic and diastolic blood pressure [see National Research Council (9) and citations therein]. Elevations in maternal blood pressure during pregnancy is an effect of concern for the mother but is also a known risk factor for adverse pregnancy outcome, particularly in the form of retarded fetal growth that is itself a risk factor for adverse developmental effects (41).

Paul Mushak
PB Associates
Durham, North Carolina
and Albert Einstein College of Medicine of Yeshiva University Bronx, New York