Lack of skeletal lead accumulation during calcium citrate supplementation.

The deleterious biological effects of lead ingestion are well established. There has been a growing concern that ingestion of certain oyster shell-containing calcium supplements and antacids might contribute to an increased risk of lead poisoning in individuals who consume these products [1]. Consumers of calcium supplements and calcium antacids, especially pregnant and lactating women, their unborn fetuses, infants, and small children would thus be exposed to a serious and documented risk of reproductive and developmental harm. Because calcium supplements are invariably prescribed in the prevention and treatment of osteoporosis, elderly osteoporotic individuals may also be at increased risk for lead toxicity. Because the skeleton represents the major repository of lead in the human body, containing 90% or more of total lead content [2], we reasoned that measurement of skeletal lead concentration in osteoporotic individuals before and after calcium supplementation would disclose whether there was significant lead accumulation from administration of calcium supplements. The patient population was composed of 11 women and three men with a mean age of 60 years (range 38–76). All patients had osteoporosis as evidenced by reduced vertebral bone mass, histologic evidence of osteoporosis, and at least one vertebral fracture. All patients and controls were involved in research protocols approved by the Institutional Review Board on the Use of Human Subjects. A transcortical iliac crest bone biopsy was obtained from each patient after obtaining informed consent and before beginning calcium citrate therapy (400 mg of calcium twice daily as Citracal, Mission Pharmacal). A second bone biopsy was procured on the opposite side after completing, on average, 4.75 years of calcium therapy (range 1–9 years). Four healthy men (mean age 35 years, range 20–53) who participated in a different protocol without calcium supplementation and who also underwent transcortical iliac crest bone biopsy 12 weeks apart served as methodology controls. Bone biopsies were dehydrated and embedded in methylmethacrylate for sectioning and histologic analysis. Upon completion of bone histology, a 100-mm longitudinal section was cut from each biopsy with a low-speed wafering saw. These sections were then ashed overnight in tared platinum crucibles at 600 °C and the ash weight of bone (representing the mineral phase) then determined. Ashed bone specimens (5–20 mg ash weight) were dissolved in 1 mL of 5% nitric acid and diluted to 2 mL with water from a MiiliQ (Millipore) water purification system. In addition, sections of methylmethacrylate that contained no bone specimen were also removed and subjected to the same procedure to ensure that the embedding material contained no significant amounts of lead. Bone lead was quantified via electrothermal atomic absorption spectrometry. The analytical equipment was a Varian SpectrAA 20 equipped with a graphite tube assembly (GTA 96) electrothermal oven (Varian Instruments). A calibration curve was prepared over the range of 0.2–1 ng of elemental lead in 2.5% nitric acid with a total injection volume of 25 mL. The sample was dried at 100 °C (ramp 5 s, hold 10 s), ashed at 400 °C (ramp 10 s, hold 8 s), and atomized at 2000 °C (5 s). The absorbance was measured at 283.3 nm. A deuterium background correction was used because the above-mentioned spectrophotometer was not equipped with Zeeman correction capabilities. Samples (25 mL) were assayed directly without further additions. This methodology gave a detection limit of 0.05 ng of lead when processed and assayed as described above. The intraassay CV was 3.9% when determined on three specimens assayed 15 times each (range 1.6–6.5%). The interassay CV was 4.5% when determined for three different concentrations on five different days (range 2.5–5.6%). Assessment of method accuracy yielded an average recovery of 107% (range 97–116%). Preliminary studies in a few specimens demonstrated comparable results for either direct addition or by calibrator additions. Thus, all results were obtained by the method of direct analysis. Table 1 summarizes the results. For all 14 patients, mean bone lead increased nonsignificantly (P 5 0.203) from 3.3 6 1.8 to 4.2 6 1.8 ng/mg bone ash. All mean values were within reported normal values of 1–8 ng/mg bone ash when performed by electrothermal atomic absorption spectrometry [3–5]. Variability of lead in the iliac crest was observed to be minimal on the basis of sampling from contralateral sites 12 weeks apart in the four healthy subjects. It is interesting to note that preand postmean