Body weight-specific zinc compartmental masses in girls significantly exceed those reported in adults: a stable isotope study using a kinetic model

Maintaining optimal zinc status is important for normal growth and development in children, but minimal data are available regarding zinc metabolism in this age group. Our objectives were to utilize stable isotope-based compartmental modeling techniques to investigate zinc metabolism in healthy children; to expand a current stable isotope-based model to include red blood cell data; and to compare kinetic parameters in children with those previously reported in adults. Seven healthy girls, age 9.94 +/- 0.79 y, received 1.1 mg of a (67)zinc-enriched tracer orally and 0.5 mg of a (70)zinc-enriched tracer intravenously. Blood, urine and fecal samples were collected for 6 d. Stable isotope enrichments were measured by thermal ionization magnetic sector mass spectrometry. A six-compartment model based on a model previously reported in adults was used; the model excluded red blood cell data. Body weight-corrected masses of the body zinc compartments derived using this model were significantly greater in children than those reported in adults. Modification of the model to include a red blood cell compartment increased the total identifiable zinc mass of the nongastrointestinal compartments by approximately 2.5%. We conclude that compartmental modeling can be used to describe zinc kinetics in children, and that the body weight-corrected zinc pool masses are significantly greater in children than in adults.

Exchangeable magnesium pool masses reflect the magnesium status of rats

A sensitive and valid marker to assess magnesium (Mg) status in humans is not available. The kinetically determined exchangeable pool masses have been used for other minerals, such as zinc and selenium, as markers of whole-body mineral status. To evaluate the validity of this relationship for Mg, we measured the exchangeable pools of Mg in rats over a range of magnesium dietary intakes. Rats weighing approximately 170 g were fed a control diet (500 mg Mg/kg), a marginally Mg-deficient diet (200 mg/kg) or a severely Mg-deficient diet (60 mg Mg/kg) for 2 wk. Subsequently, rats were administered an intravenous injection of (25)Mg, and the plasma (25)Mg disappearance curve was followed for the next 7 d. The following two methods were employed to analyze the exchangeable pools of Mg: 1) formal compartmental modeling and 2) a simplified determination of the total mass of the rapidly exchangeable Mg pool (EMgP). The mass of the three exchangeable pools (two extracellular pools and one intracellular pool) determined by compartmental analysis decreased in proportion to dietary Mg intake. EMgP, the combined pools of Mg that exchange with the plasma Mg within 48 h, decreased significantly as dietary Mg was lowered. It was positively correlated with conventional markers of Mg status (total Mg in plasma, erythrocyte and tibia Mg levels). Compartmental analysis assesses Mg exchangeable pools more accurately, but determination of EMgP is simpler and faster. Our findings demonstrate that the exchangeable pools of Mg constitute a good marker of Mg status in rats.

Prenatal iron supplements impair zinc absorption in pregnant Peruvian women

Prenatal iron supplements may adversely influence zinc absorption during pregnancy. To examine the impact of prenatal iron supplements on supplemental zinc absorption, fractional zinc absorption was measured in 47 pregnant Peruvian women during the third trimester of pregnancy (33 +/- 1 wk gestation). Of these 47 women, 30 received daily prenatal supplements from wk 10-24 of pregnancy until delivery. Supplements contained 60 mg of Fe and 250 microg of folate without [iron group (Fe), n = 16] or with [iron and zinc supplemented group (Fe + Zn), n = 14] 15 mg of Zn. The remaining 17 women [unsupplemented control group (C)] received no prenatal supplementation. Zinc concentrations were measured in plasma, urine and cord blood and percentage zinc absorption was determined following dosing with oral ((67)Zn) and intravenous ((70)Zn) stable zinc isotopes. Percentage zinc absorption was significantly lower than controls in fasting women receiving iron- containing prenatal supplements (20.5 +/- 6.4 vs. 20.2 +/- 4.6 vs. 47.0 +/- 12.6%, Fe, Fe + Zn and C groups, respectively, P: < 0.0001, n = 40). Plasma zinc concentrations were also significantly lower in the Fe group compared to the C group (8.2 +/- 2.2 vs. 9.2 +/- 2.2 vs. 10.9 +/- 1. 8 micromol/L, Fe, Fe + Zn and C groups, respectively, P: = 0.002), and cord zinc concentrations were significantly related to maternal plasma Zn levels (y = 6.383 + 0.555x, r = 0.486, P: = 0.002). The inclusion of zinc in prenatal supplements may reduce the potential for iron supplements to adversely influence zinc status in populations at risk for deficiency of both these nutrients.

Calcium absorption, bone mass accumulation, and kinetics increase during early pubertal development in girls

To evaluate the changes in calcium and bone mineral metabolism associated with early pubertal development, we performed longitudinal measurements of calcium absorption, calcium kinetics, bone mineral content, and hormonal markers related to puberty in a multiethnic group of girls beginning when they were 7 or 8 yr old. Girls were Tanner stage 1 (breast) at the start of the study. They were placed on a 1200 mg/day dietary calcium intake and studied at approximately 6-month intervals until they reached Tanner stage 2 (breast). Results at that time point (PUB) were compared to values obtained approximately 1 yr earlier (LatePRE) and those 1 yr before that (EarlyPRE). We found an increase in calcium absorption comparing PUB to LatePRE (n = 34; 36.6 +/- 8.7% vs. 30.7 +/- 9.9%; P = 0.002). Using whole body, dual energy, x-ray absorptiometry scanning, we found an increase in calcium gain during the LatePRE to PUB period compared with that during the EarlyPRE to LatePRE period (135 +/- 53 vs. 110 +/- 45 mg/day; P = 0.04). Calcium kinetic studies showed a significant increase in the bone calcium deposition rate (Vo+) during the PUB compared to the LatePRE period. Hormonal and biochemical markers of bone development were also significantly increased at PUB compared to LatePRE. Hormonal activity, as evidenced by the unstimulated LH level, was significantly correlated with calcium gain between the LatePRE and PUB studies and the bone calcium deposition rate in the PUB study. These data demonstrate, using multiple independent methods, an increase in calcium utilization associated with the earliest physical signs of puberty.

The reference child and adolescent models of body composition. A contemporary comparison

Changes in the relative proportions of bone, muscle, water, visceral tissues, and body fat occur during growth. In the 1980s, reference models of body composition for children and adolescents were constructed by adjusting data on total body water (TBW), total body potassium (TBK), and regional bone mineral (BMC) data from several different Caucasian populations. In our study, we measured TBW, TBK, and total body BMC in 856 healthy European-American, African-American, and Mexican-American children. When we reconstructed the reference models using our contemporary data, we found that the body's bone, protein, and fat compartments are slightly but significantly different from the earlier models. Our study provides the range of normal body composition of healthy children, aged 5-18 years, and accounts for differences related to gender and ethnicity.

Ethnic differences in androgens, IGF-I and body fat in healthy prepubertal girls

OBJECTIVE

To examine ethnic differences in adrenal androgen production, IGF-I, and IGFBP-1 and -3 in relation to bone age, insulin, and body composition in healthy prepubertal girls.

METHODS

Serum levels of DHEA-S, androstenedione, IGF-I, and IGFBP-1 and -3 were examined in relation to bone age, insulin, and body composition (determined by dual-energy X-ray absorptiometry) in 47 (19 Caucasian, 9 African-American, 19 Mexican-American) healthy prepubertal girls aged 7.5-9.0 years.

RESULTS

Age, weight, height, bone age, androstenedione, insulin, glucose:insulin ratios, and IGFBP-3 levels were not statistically different among groups. Mexican-American girls had higher % body fat than African-Americans or Caucasians (P < 0.001). DHEA-S levels in African-Americans were twofold higher than in Caucasians (P = 0.024), although their % body fat was not significantly different (16.1% and 19.4%, respectively; P = 0.138). DHEA-S levels in Mexican-American girls were intermediate. Bone age and weight were significant covariates for DHEA-S levels. Plasma IGF-I levels were also higher in African-American than in Caucasian or Mexican-American girls (P = 0.009). Covariance analysis showed that IGF-I levels were influenced mainly by ethnicity (P = 0.009) and were independent of bone age. Despite similar insulin levels among groups, IGFBP-1 levels were higher in Caucasians than in Mexican-Americans or African-Americans (P < 0.001).

CONCLUSIONS

In healthy prepubertal girls, DHEA-S concentrations are higher in African-Americans than in Caucasians or Mexican-Americans, even before any clinical evidence of adrenarche. Furthermore, IGF-I concentrations are higher in African-American girls than in Caucasian or Mexican-American girls which may contribute to the higher DHEA-S levels observed. Conversely, higher DHEA-S and IGF-I levels in African-American girls may be indicative of an influence not only of gonadal but also of adrenal androgens on the GH/IGF-I axis.

Effects of rheumatic disease and corticosteroid treatment on calcium metabolism and bone density in children assessed one year after diagnosis, using stable isotopes and dual energy x-ray absorptiometry

OBJECTIVE

To evaluate calcium (Ca) metabolism and bone mineral density (BMD) in children with rheumatic disease (RD) at diagnosis and one year later, and effects of different therapies.

METHODS

We used dual tracer isotope studies and dual energy x-ray absorptiometry (DEXA) to measure Ca metabolism and BMD in 13 children with RD at diagnosis and one year later. Seven subjects were treated with steroids (RD-ST), 6 with antiinflammatory agents (RD-NS), excluding steroids.

RESULTS

Ca balance data for RD subjects were not significantly different from data reported previously for healthy sex and age matched controls. True Ca absorption (Va) was slightly but not significantly greater at study entry in RD-NS subjects (313+/-67 vs 239+/-112 mg/day in RD-ST subjects; p = 0.13). Calculated Ca balance retention (Vbal) was higher at entry in RD-NS (200+/-51 vs RD-ST 60+/-125 mg/day; p = 0.08). One year later, Vbal remained higher in RD-NS (202+/-77 mg/day vs RD-ST 101+/-157 mg/day; p = 0.02). BMD was similar in both groups at entry (RD-NS 0.81+/-0.06 g/cm2 vs RD-ST 0.89+/-0.1 g/cm2; p = 0.07). One year later, BMD was 0.86+/-0.6 g/cm2 in RD-NS versus 0.89+/-0.08 g/cm2 in RD-ST; p = 0.07. Ca kinetic and DEXA studies did not reveal significant alteration of Ca kinetics or significantly lower BMD in steroid treated subjects versus non-steroid treated subjects. However, slightly lower Va and Vbal indicated a possible risk of bone demineralization in steroid treated subjects.

CONCLUSION

Children with RD who are treated with steroids may be at greater risk of bone demineralization at diagnosis that persists through the first year. However, bone loss may take years to manifest when measured by quantitative methods such as DEXA.

Using stable isotopes to assess mineral absorption and utilization by children

Adequate mineral intake is a crucial part of a healthy diet for children-it supports appropriate growth and development and provides protection against childhood conditions like anemia and helps to prevent future adult diseases such as osteoporosis. Challenges in performing and interpreting studies in infants and children have hampered the accurate assessment of their mineral utilization. Many of the most powerful techniques used in adults, such as radioisotope testing, are not appropriate for use in children. In recent years, advanced mineral stable-isotope techniques have been developed to fill this gap. Pediatric applications include studies of calcium absorption and kinetics during puberty and evaluation of the calcium-iron interaction in infants and toddlers. The effects of genetics in determining calcium absorption and bone turnover may become an important research area. The goals and methods of ongoing mineral stable-isotope research in infants and children are examined in this report. In the past, the cost and difficulties in obtaining isotopes have limited such research. This situation has improved considerably, although relatively few nutrition research laboratories are prepared to perform sample analyses.

Monitoring childhood obesity: assessment of the weight/height index

The body mass index (BMI), defined as weight/height, is often used to monitor childhood obesity. BMI values for 979 children (438 White, 283 Black, and 258 Hispanic) aged 3-18 years living in the Houston, Texas, metropolitan area from 1994 to 1998 were compared with percentage of fat (%Fat) measurements obtained by using dual-energy x-ray absorptiometry. The associations between %Fat and BMI were statistically significant (r2 = 0.34-0.70, p < 0.0005) and were gender and ethnic dependent (p < 0.0005), indicating that BMI can provide a general description of the adiposity characteristics of a healthy pediatric population. However, BMI was a poor predictor for the individual child, with a standard error for %Fat of 4.7-7.3% of body weight. It is advantageous to identify accurately, as early as possible, those children who truly have excess adiposity, but this assessment should not be done at the risk of falsely mislabeling a significant number of healthy children as overweight or obese.

Effect of growth hormone treatment on calcium kinetics in patients with osteogenesis imperfecta type III and IV

Using a dual stable isotope technique, the effect of growth hormone (GH) on whole body calcium (Ca) metabolism was studied in children (ages 5-14 years) with type III (n = 9) and IV (n = 8) osteogenesis imperfecta. Each subject was studied twice: at baseline and following a GH (0.1-0.2 U/kg per day) treatment period of 1-1.5 years. Subjects were given 42Ca intravenously and 44Ca orally. The sera and urine 42Ca and 44Ca isotopic enrichments were followed over 7 days using thermal ionization mass spectrometry. The SAAM program was used to fit a three-compartment model to the tracer data. No significant differences were observed between: (1) children with type III and IV disease; or (2) baseline studies of boys and girls within each disease type. However, GH treatment significantly increased: (1) the exchangeable calcium pool (EP) in type III patients (2086 vs. 4422 mg/day, p = 0.02); and (2) the parameter associated with bone calcium accretion in type IV patients (Vo+: 973 vs. 1560 mg/day,p = 0.03) with boys responding with a significantly greater increase than girls (p = 0.008). Although not statistically significant, a trend toward an increase in Vo+ in type III patients and in EP in type IV was observed following treatment. Our observations imply that more Ca was available for bone mineralization following GH treatment in these subjects.

Effects of high compared with low calcium intake on calcium absorption and incorporation of iron by red blood cells in small children

BACKGROUND

The potential benefits of increasing calcium intake in small children must be balanced with the potential risk to iron utilization from high calcium intakes.

OBJECTIVE

This study was designed to evaluate the relation between calcium intake and calcium absorption and iron incorporation into red blood cells.

DESIGN

We performed a multitracer, crossover study of the absorption of calcium and red blood cell incorporation of iron in 11 preschool children aged 3-5 y who had been adapted for 5 wk to low- (502 +/- 99 mg) and high- (1180 +/- 117 mg) calcium diets. Stable-isotope studies were performed by using 44Ca and 58Fe given orally with meals and 46Ca given intravenously.

RESULTS

Iron incorporation into red blood cells 14 d postdosing was similar (6.9 +/- 4.2% compared with 7.9 +/- 5.5%; NS) with the low- and high-calcium diets, respectively. Total calcium absorption (181 +/- 50 compared with 277 +/- 91 mg/d; P = 0.002) was greater in children with the higher calcium intake.

CONCLUSIONS

Our findings indicate that small children may benefit from calcium intakes similar to those recommended for older children without adverse effects on dietary iron utilization.

Utilization of supplemental iron by premature infants fed fortified human milk

We measured red blood cell iron incorporation (RBC-inc) in 13 human milk-fed premature infants (birthweight 1037 +/- 289 g, gestational age 27 +/- 2 wk, weight at start of study 1571 +/- 426 g) who were receiving full tube-feedings of human milk fortified with a commercial human milk fortifier (FortHM). The relative RBC-inc of supplemental iron (2 mg/kg/d of ferrous sulfate) was assessed using 57Fe sulfate mixed directly into a 24-h volume of FortHM, and 54Fe sulfate given as a bolus between two FortHM feedings the next day. RBC-inc was similar between the two methods of supplemental iron administration (4.7 +/- 2.5% vs 4.6 +/- 1.5%, respectively). Although these values are lower than RBC-inc expected from iron native to human milk, the relatively large amount of iron in the supplements contributed most of the iron incorporated into RBC by the infants. There was a significant positive correlation between the reticulocyte count and RBC-inc. As the high nutrient (especially calcium) content of the FortHM did not interfere with iron utilization, adding iron directly to FortHM, or incorporating it into commercial fortifiers, may be a practical method to provide iron to premature infants.

Calcium metabolism before, during, and after a 3-mo spaceflight: kinetic and biochemical changes

The loss of bone during spaceflight is considered a physiological obstacle for the exploration of other planets. This report of calcium metabolism before, during, and after long-duration spaceflight extends results from Skylab missions in the 1970s. Biochemical and endocrine indexes of calcium and bone metabolism were measured together with calcium absorption, excretion, and bone turnover using stable isotopes. Studies were conducted before, during, and after flight in three male subjects. Subjects varied in physical activity, yet all lost weight during flight. During flight, calcium intake and absorption decreased up to 50%, urinary calcium excretion increased up to 50%, and bone resorption (determined by kinetics or bone markers) increased by over 50%. Osteocalcin and bone-specific alkaline phosphatase, markers of bone formation, increased after flight. Subjects lost approximately 250 mg bone calcium per day during flight and regained bone calcium at a slower rate of approximately 100 mg/day for up to 3 mo after landing. Further studies are required to determine the time course of changes in calcium homeostasis during flight to develop and assess countermeasures against flight-induced bone loss.

Combination growth hormone and estrogen increase bone mineralization in girls with Turner syndrome

Turner syndrome is characterized by osteopenia and impaired skeletal growth. Neither feature is normalized by current modes of hormone therapy. The purpose of this study was to determine whether GH would increase protein anabolism and provide additional benefit to a regimen of estrogen replacement on calcium metabolism in girls and women with Turner syndrome. Using stable isotopes of calcium and leucine, we determined calcium absorption, urinary calcium loss, calcium retention, deposition into bone, leucine rate of appearance from protein, leucine incorporation into protein, and leucine oxidation in seven girls (10-17 y of age) and four adult females (16-34 y of age) with Turner syndrome, before and after 3 mo of GH treatment. All adults were treated with estrogen (ethinyl estradiol, 50 micrograms/d) and progesterone before and throughout the study. Three girls received no estrogen, and four girls were treated with low-dose estrogen (ethinyl estradiol, 5 micrograms/d) in combination with GH. The addition of estrogen to GH treatment resulted in a significant increase in calcium absorption and deposition in girls. GH did not affect calcium kinetics in adults already receiving estrogen/progesterone replacement therapy, nor did GH alone affect calcium kinetics in girls, and neither GH nor estrogen affected protein metabolism. These data suggest that the addition of low-dose estrogen to a regimen of GH improves bone deposition and calcium metabolism in girls with Turner syndrome and that estrogen is facultative for GH effects on bone.

Vitamin D receptor gene Fok1 polymorphism predicts calcium absorption and bone mineral density in children

The vitamin D receptor (VDR) gene has been implicated as one of the major genetic components of osteoporosis. We evaluated the relationship between markers of mineral status and restriction fragment length polymorphisms of the VDR gene in 72 healthy children age 7-12 years. Using stable isotope techniques and dual-energy X-ray absorptiometry, we measured dietary calcium absorption, bone calcium deposition rates, and total body bone mineral density (BMD). The Fok1 polymorphism at the VDR translation initiation site was significantly associated with BMD (p = 0.02) and calcium absorption (p = 0.04). Children who were FF homozygotes had a mean calcium absorption that was 41.5% greater than those who were ff homozygotes and 17% greater absorption than Ff heterozygotes. BMD was 8.2% greater in the FF genotype than the ff genotype and 4.8% higher than the Ff genotype. These results suggest a substantial relationship between the VDR gene and bone metabolism at one or more levels, including dietary absorption of calcium and BMD in growing children.

Influence of prenatal iron and zinc supplements on supplemental iron absorption, red blood cell iron incorporation, and iron status in pregnant Peruvian women

BACKGROUND

It is estimated that 60% of pregnant women worldwide are anemic.

OBJECTIVE

We aimed to examine the influence of iron status on iron absorption during pregnancy by measuring supplemental iron absorption, red blood cell iron incorporation, and iron status in pregnant women.

DESIGN

Subjects were 45 pregnant Peruvian women (33+/-1 wk gestation), of whom 28 received daily prenatal supplements containing 60 mg Fe and 250 microg folate without (Fe group, n = 14) or with (Fe+Zn group, n = 14) 15 mg Zn, which were were consumed from week 10 to 24 of gestation until delivery. The remaining 17 women (control) received no prenatal supplementation. Iron status indicators and isotopes were measured in maternal blood collected 2 wk postdosing with oral (57Fe) and intravenous (58Fe) stable iron isotopes.

RESULTS

Maternal serum ferritin and folate concentrations were significantly influenced by supplementation (P < 0.05). Serum iron was also significantly higher in the Fe than in the Fe+Zn (P < 0.03) or control (P < 0.001) groups. However, the supplemented groups had significantly lower serum zinc concentrations than the control group (8.4+/-2.3 and 10.9+/-1.8 micromol/L, respectively, P < 0.01). Although percentage iron absorption was inversely related to maternal serum ferritin concentrations (P = 0.036), this effect was limited and percentage iron absorption did not differ significantly between groups.

CONCLUSIONS

Because absorption of nonheme iron was not substantially greater in pregnant women with depleted iron reserves, prenatal iron supplementation is important for meeting iron requirements during pregnancy.

Calcium absorption and kinetics are similar in 7- and 8-year-old Mexican-American and Caucasian girls despite hormonal differences

To assess the possibility of ethnic differences in mineral metabolism in prepubertal children, we compared measures of calcium metabolism in 7- and 8-y-old Mexican-American (MA) and non-Hispanic Caucasian (CAU) girls (n = 38) living in southeastern Texas. We found similar fractional calcium absorption, urinary calcium excretion, calcium kinetic values and total-body bone mineral content in the MA and CAU girls. In contrast, parathyroid hormone (PTH) concentrations were greater in MA girls (4.01 +/- 0.47 vs. 1. 96 +/- 0.50 pmol/L, P = 0.005) than in CAU girls. Serum 25-hydroxyvitamin D concentrations were lower in MA girls (68.9 +/- 7.7 vs. 109.4 +/- 8.4 nmol/L, P = 0.001) than in CAU girls, but 1, 25-dihydroxyvitamin D concentrations did not differ between groups. Seasonal variability was seen for 25-hydroxyvitamin D concentrations in girls of both ethnic groups, but values in all of the girls were >30 nmol/L (12 ng/mL). We conclude the following: 1) greater PTH levels in MA girls than CAU girls are present without evidence of vitamin D deficiency; and 2) differences in 25-hydroxyvitamin D and PTH concentrations between MA and CAU girls do not have a large effect on calcium absorption, excretion or bone calcium kinetics. These data do not provide evidence for adjusting dietary recommendations for mineral or vitamin D intake by MA girls.

Methodologies for using stable isotopes to assess magnesium absorption and secretion in children

OBJECTIVE

To provide methodological information regarding the absorption and excretion of dietary magnesium by children and adolescents.

METHODS

Recently, stable isotope techniques for assessing magnesium absorption and excretion have been developed which allow for these studies to be safely performed in subjects of all ages. In the report, we describe the dosing and sample requirements for such studies.

RESULTS

Our data demonstrate that, after oral and intravenous dosing of isotopes, a complete 72-hour urine collection will allow for determination of fractional magnesium absorption. In our study, urinary, but not endogenous fecal magnesium excretion, was closely correlated with magnesium intake (r = 0.47, p = 0.02 vs r = 0.08, p = 0.69). As endogenous fecal magnesium excretion is small relative to urinary magnesium excretion, measurement of endogenous fecal magnesium excretion is not needed to make a reasonable estimate of net magnesium retention for most studies. Using high-precision analytical techniques, an intravenous dose of 25Mg of approximately 0.2 to 0.3 mg/kg would be adequate for absorption measurements.

CONCLUSIONS

The cost and availability of isotopes and their analysis are such that it should be feasible for increasing numbers of investigators to make use of these techniques.

Multicompartmental analysis of magnesium and calcium kinetics during growth: relationships with body composition

Stable isotope techniques permit a unique approach to the assessment of magnesium metabolism, including absorption, excretion, pool sizes, and turnover. Findings from our recent studies on magnesium and calcium kinetics indicate close relationships between the mass of the magnesium exchangeable pool, efflux from this pool, and body weight. In this report, we further evaluate these data by relating the kinetic values with other measures of body composition. We found that fat-free mass represents the single body composition parameter that most closely correlates with magnesium kinetic data. Body composition has a stronger relationship with magnesium kinetics than with calcium kinetics. These relationships provide justification for basing dietary magnesium requirements in children on body composition measures such as body weight or, where available, fat-free mass. Further studies to evaluate these relationships are indicated in situations where either magnesium status or body composition is abnormal.

Insights into bone metabolism from calcium kinetic studies in children

Changes in the mineralization rate of the skeleton during childhood are related to normal growth and pubertal development. These may be affected by genetic factors, including race and gender, and by the presence of abnormalities of growth or hormonal abnormalities such as occur in children with chronic illnesses. We have used multicompartmental studies to examine calcium kinetics in healthy children ranging in age from premature infants of 1-2 kg body weight through adolescence. These studies are performed using orally and intravenously administered stable isotopes of calcium. Sample collection requires multiple blood samples to be obtained during the initial time period after isotope dosing, which is feasible in older children using an indwelling catheter. We have found that the peak rate for both bone calcium deposition and removal occurs in girls during the year before menarche. Peak kinetic rates decrease in an exponential fashion post-menarche. On a body-weight basis, the greatest rates of bone calcium deposition and removal are in infants, especially premature infants.

Iron absorption and red blood cell incorporation in premature infants fed an iron-fortified infant formula

This study was designed to identify differences in red blood cell (RBC) incorporation and iron absorption in premature infants between iron provided in a premature infant formula compared with iron provided as a supplement between feedings. We used a triple stable isotope technique in which 13 infants received 57Fe mixed with Enfamil Premature Formula on d 1 of the study, and 54Fe with a multivitamin supplement between meals on d 2. Two weeks later, blood was drawn for isotope analysis and 58Fe was given i.v. The percentage RBC incorporation of the 54Fe and 57Fe was calculated, and the percent absorption of these tracers was estimated by dividing by the percentage of 58Fe identified in RBCs 14 d after its infusion. We found a small, but significantly greater, percentage of RBC incorporation of the 54Fe given as a supplement compared with the 57Fe given in the formula (9.7 +/- 3.8% versus 7.8 +/- 3.1%, p = 0.02). The RBC 57Fe incorporation was closely correlated with the reticulocyte count (r = 0.80, p = 0.001), but not the serum ferritin or the Hb concentration. Approximately 68% of an i.v. dose of 58Fe was incorporated into RBCs. These findings indicate 1) iron is incorporated well into RBCs from preterm infant formula, with only a small increase in incorporation when given as a supplement, and 2) the reticulocyte count, but not the Hb concentration, is a good measure of RBC iron-incorporating capacity.

Development and regulation of calcium metabolism in healthy girls

The major components of calcium metabolism, as evaluated by a dual-tracer stable isotope method, were determined in 100 studies of 68 healthy girls, aged 5-18 y and analyzed from a developmental and regulatory viewpoint. Bone calcium deposition and removal rates were closely correlated with the size of the exchangeable bone calcium compartment. All three quantities, as well as intestinal calcium absorption, peaked at or near menarche. Both bone calcium deposition and removal rates were positively and linearly correlated with calcium absorption. However, in this correlation, because bone calcium deposition increased 70% faster than calcium absorption, most of the increase in the bone calcium compartment and its turnover must have occurred in response to something other than intestinal calcium input; presumably this occurred in response to developmental signals. Nevertheless, the constancy of the serum calcium in the face of a large intestinal calcium input and the modest way in which excretion overcame the calcium load in this population point to the importance of the exchangeable bone calcium compartment, in dynamic equilibrium with the bone mineral, as the site at which most of the load is taken up. In this population of girls, as in older women, this increase in the skeletal calcium balance resulted from a decrease in the bone calcium removal rate that was greater than the corresponding increase in the bone calcium deposition rate.