Measurement of true absorption, endogenous fecal excretion, urinary excretion, and retention of calcium in term infants by using a dual-tracer, stable-isotope method

Intestinal Ca2+ absorption revisited: A molecular and clinical approach

Ca2+ has an important role in the maintenance of the skeleton and is involved in the main physiological processes. Its homeostasis is controlled by the intestine, kidney, bone and parathyroid glands. The intestinal Ca2+ absorption occurs mainly via the paracellular and the transcellular pathways. The proteins involved in both ways are regulated by calcitriol and other hormones as well as dietary factors. Fibroblast growth factor 23 (FGF-23) is a strong antagonist of vitamin D action. Part of the intestinal Ca2+ movement seems to be vitamin D independent. Intestinal Ca2+ absorption changes according to different physiological conditions. It is promoted under high Ca2+ demands such as growth, pregnancy, lactation, dietary Ca2+ deficiency and high physical activity. In contrast, the intestinal Ca2+ transport decreases with aging. Oxidative stress inhibits the intestinal Ca2+ absorption whereas the antioxidants counteract the effects of prooxidants leading to the normalization of this physiological process. Several pathologies such as celiac disease, inflammatory bowel diseases, Turner syndrome and others occur with inhibition of intestinal Ca2+ absorption, some hypercalciurias show Ca2+ hyperabsorption, most of these alterations are related to the vitamin D endocrine system. Further research work should be accomplished in order not only to know more molecular details but also to detect possible therapeutic targets to ameliorate or avoid the consequences of altered intestinal Ca2+ absorption.

Intestinal absorption and renal reabsorption of calcium throughout postnatal development

Calcium is vital for many physiological functions including bone mineralization. Postnatal deposition of calcium into bone is greatest in infancy and continues through childhood and adolescence until peek mineral density is reached in early adulthood. Thereafter, bone mineral density remains static until it eventually declines in later life. A positive calcium balance, i.e. more calcium absorbed than excreted, is crucial to bone deposition during growth and thus to peek bone mineral density. Dietary calcium is absorbed from the intestine into the blood. It is then filtered by the renal glomerulus and either reabsorbed by the tubule or excreted in the urine. Calcium can be (re)absorbed across intestinal and renal epithelia via both transcellular and paracellular pathways. Current evidence suggests that significant intestinal and renal calcium transport changes occur throughout development. However, the molecular details of these alterations are incompletely delineated. Here we first briefly review the current model of calcium transport in the intestine and renal tubule in the adult. Then, we describe what is known with regard to calcium handling through postnatal development, and how alterations may aid in mediating a positive calcium balance. The role of transcellular and paracellular calcium transport pathways and the contribution of specific intestinal and tubular segments vary with age. However, the current literature highlights knowledge gaps in how specifically intestinal and renal calcium (re)absorption occurs early in postnatal development. Future research should clarify the specific changes in calcium transport throughout early postnatal development including mediators of these alterations enabling appropriate bone mineralization. Impact statement This mini review outlines the current state of knowledge pertaining to the molecules and mechanisms maintaining a positive calcium balance throughout postnatal development. This process is essential to achieving optimal bone mineral density in early adulthood, thereby lowering the lifetime risk of osteoporosis.

Effect of added calcium, phosphorus, and infant formula on calcium and phosphorus dialyzability in preterm donor human milk

OBJECTIVES

We studied the effect of preparing donor human milk (DHM) with commonly used nutritional additives on the dialyzability of calcium and phosphate. We hypothesized that the additives to DHM would decrease the dialyzability of calcium and phosphate when prepared according to hospital protocols.

METHODS

An in vitro system simulating premature infant digestion was developed to measure dialyzability of calcium and phosphate in DHM. Dialyzable calcium and phosphate were measured after in vitro digestion in DHM before and after preparation of DHM with the following additives according to hospital protocols: calcium glubionate, sodium potassium phosphate, calcium glubionate and sodium potassium phosphate added together, Similac human milk fortifier, Similac NeoSure, or Enfamil Enfacare.

RESULTS

The percentage of dialyzable calcium in DHM with added calcium and calcium and phosphate together was greater than the percentage of dialyzable calcium in DHM with added powdered infant formulas (P<0.0001). Dialyzable calcium was greater in DHM with added calcium and with added calcium and phosphate than in all other treatment groups (P<0.0001). Dialyzable calcium in DHM without additives was not different from dialyzable calcium in DHM with added phosphate or with added powdered infant formulas. Dialyzable phosphate did not differ between the treatment groups.

CONCLUSIONS

The addition of calcium alone or calcium and phosphate together increased calcium dialyzability in DHM significantly, whereas the addition of powdered human milk fortifier or formulas did not. The addition of calcium or calcium with phosphate together to DHM may provide the most dialyzable calcium.

Addition of Calcium and Phosphorus to Preterm Donor Human Milk and the Impact on Protein, Fat, and Calcium Digestibility In Vitro

Infants born prematurely are at risk for metabolic bone disease and may need increased minerals for normal bone mineralization. In these situations, supplementation of human milk with calcium and phosphorus is common in the United States. The interaction of these nutrients and their carriers with other nutrition components of human milk has not been systematically investigated. The primary goal was to study the effect of calcium and phosphorus supplementation on the digestibility of protein, free fatty acids, and calcium in preterm donor human milk (DHM). An in vitro model of the premature infant’s gastrointestinal tract was used to simulate digestion. Protein, free fatty acids, and ionized calcium were measured before and after in vitro digestion in milk with and without added calcium glubionate and sodium potassium phosphate. Calcium and phosphorus supplementation did not negatively affect total protein, protein breakdown, protein digestibility, or fat breakdown when compared with unfortified DHM. Supplemental calcium increased ionized calcium, which may replace ionized calcium lost during milk expression, storage, and processing. Supplemental calcium glubionate and sodium potassium phosphate do not affect protein or fat breakdown in vitro. DHM contains less ionized calcium than fresh human milk because it loses CO2 during expression and processing. Adding supplemental calcium increases ionized calcium in DHM.

Percent true calcium absorption, mineral metabolism, and bone mineralization in children with cystic fibrosis: effect of supplementation with vitamin D and calcium

OBJECTIVE

To assess whether percent true calcium absorption (alpha) is normal in children with cystic fibrosis (CF) and to assess whether supplementation with 2,000 IU vitamin D(3), 1 g calcium, or both will alter alpha, mineral metabolism, and/or bone mass in children with CF.

STUDY DESIGN

Fifteen children ages 7-13 were randomly assigned to one of four different orders to receive all four 6-month treatments including placebos. Change in 25-hydroxyvitamin D (25-OHD), 1,25-dihydroxyvitamin D (1,25(OH)(2)D), PTH, bone turnover markers, and minerals after 6 months, and bone mineral content (Hologic 1000W) after 9 months was measured. alpha was measured by a dual stable isotope method using (48)Ca intravenously and (46)Ca orally and measuring (48)Ca, (46)Ca, and (42)Ca in a 24-hr urine using High Resolution Inductively Coupled Mass Spectroscopy (HR-ICP-MS). Analysis used Wilcoxon Sign Ranks.

RESULTS

alpha was in the normal range and did not differ by treatment (P 35 +/- 10%, Ca 38 +/- 23%, D 36 +/- 11%, D + Ca 46 +/- 21%). One gram calcium did not increase serum or urine calcium. Two thousand IU D(3) did not increase 25-OHD or change 1,25(OH)(2)D. Serum and urine minerals, markers of bone turnover and bone mineral gains did not differ by treatment.

CONCLUSIONS

alpha is normal in children with CF. One gram calcium and/or 2,000 IU D(3) does not change alpha or increase 25-OHD, serum calcium, or mineralization. Longer trials of a significantly higher dose of vitamin D(3) shown to increase serum 25-OHD are needed to assess effects on mineral metabolism and bone mass accrual. However, study of therapeutic options other than calcium and vitamin D should be encouraged.

General background on the hypothalamic-pituitary-thyroid (HPT) axis

This article reviews the thyroid system, mainly from a mammalian standpoint. However, the thyroid system is highly conserved among vertebrate species, so the general information on thyroid hormone production and feedback through the hypothalamic-pituitary-thyroid (HPT) axis should be considered for all vertebrates, while species-specific differences are highlighted in the individual articles. This background article begins by outlining the HPT axis with its components and functions. For example, it describes the thyroid gland, its structure and development, how thyroid hormones are synthesized and regulated, the role of iodine in thyroid hormone synthesis, and finally how the thyroid hormones are released from the thyroid gland. It then progresses to detail areas within the thyroid system where disruption could occur or is already known to occur. It describes how thyroid hormone is transported in the serum and into the tissues on a cellular level, and how thyroid hormone is metabolized. There is an in-depth description of the alpha and beta thyroid hormone receptors and their functions, including how they are regulated, and what has been learned from the receptor knockout mouse models. The nongenomic actions of thyroid hormone are also described, such as in glucose uptake, mitochondrial effects, and its role in actin polymerization and vesicular recycling. The article discusses the concept of compensation within the HPT axis and how this fits into the paradigms that exist in thyroid toxicology/endocrinology. There is a section on thyroid hormone and its role in mammalian development: specifically, how it affects brain development when there is disruption to the maternal, the fetal, the newborn (congenital), or the infant thyroid system. Thyroid function during pregnancy is critical to normal development of the fetus, and several spontaneous mutant mouse lines are described that provide research tools to understand the mechanisms of thyroid hormone during mammalian brain development. Overall this article provides a basic understanding of the thyroid system and its components. The complexity of the thyroid system is clearly demonstrated, as are new areas of research on thyroid hormone physiology and thyroid hormone action developing within the field of thyroid endocrinology. This review provides the background necessary to review the current assays and endpoints described in the following articles for rodents, fishes, amphibians, and birds.

Urinary mineral excretion in preterm neonates during the first month of life

BACKGROUND

Estimation of urinary parameters in preterm infants is a useful method for identifying metabolic derangements.

OBJECTIVES

A prospective, longitudinal, hospital-based study was designed to examine the variability and the associations in renal excretion of calcium (Ca), magnesium (Mg), phosphate (P) and sodium (Na) in formula-fed preterm infants during the first month of life.

PATIENTS

Thirty-four infants <32 weeks gestational age, clinically stable, not receiving nephrotoxic drugs.

METHODS

Measurements of serum and 8-hour urinary mineral and creatinine (Cr) concentrations were made in all infants during three periods (at 7-10, 14-17 and 21-26 days postnatally). The urinary parameters, FENa, FEP, UCa/UCr, UMg/UCr were calculated. 24- hour urinary excretion was estimated by extrapolation of the 8-hour values.

RESULTS

The 24-hour excretion values (median and range) (mmol/kg) during the three study periods were, respectively, for Ca: 0.027 (0.015-0.15), 0.030 (0.007-0.12), 0.031 (0.008-0.12), for P: 0.26 (0.07-0.83), 0.29 (0.06-0.67), 0.41 (0.22-0.70), for Mg: 0.025 (0.007-0.14), 0.025 (0.008-0.16), 0.027 (0.010-0.10) and for Na: 2.85 (0.8-15), 1.45 (0.3-17), 1.56 (0.6-4.3). Na excretion declined while the excretion of the other minerals remained stable. A positive correlation was observed between excretion of Ca, Mg and Na (Ca vs. Na r = 0.63, p < 0.0001; Ca vs. Mg r = 0.65, p < 0.0001; Na vs. Mg r = 0.38, p = 0.012) as well as between the renal parameters FENa, FEP, UCa/UCr, UMg/UCr and the respective 24-hour excretion values (r = 0.80, 0.86, 0.84, 0.81) CONCLUSIONS: Urinary excretion of Ca, P, and Mg in preterm formula-fed infants is stable during the first month of life while urinary Ca, Na and Mg are closely correlated during the same period.

Calcium nutrition and metabolism during infancy

Calcium is a vital mineral for the developing newborn infant. This review discusses perinatal and neonatal calcium metabolism, with an emphasis on enteral calcium absorption and the nutritional factors affecting calcium bioavailability including the three major endocrine hormones involved in calcium metabolism: parathyroid hormone, vitamin D, and calcitonin. The placenta transports calcium to the fetus throughout pregnancy, with the largest amount of fetal calcium accumulation occurring in the third trimester. At birth, the newborn transitions to intestinal absorption to meet the body's calcium needs. Most calcium is absorbed by paracellular passive diffusion in the small intestine. Calcium intestinal absorption is affected by the type and amount of calcium ingested. It is also affected by the amount of intestinal calcium that is bound to dietary fats and proteins. One major consequence of decreased calcium absorption is metabolic bone disease in which there is a failure of complete mineralization of the bone osteoid.

Urinary excretion of calcium and phosphate in preterm infants

The aims of this study were to determine reference ranges for the urinary calcium (UCa/Cr) and phosphate (UPO(4)/Cr) creatinine ratios and to study factors influencing these ratios in a representative population of preterm infants managed according to current nutritional guidelines. Spot urine samples were obtained from 186 preterm infants (gestation 24-34 weeks) for measurement of UCa/Cr and UPO(4)/Cr ratios as part of a routine metabolic bone screening program, once every 2-4 weeks from the 3rd to the 18th week of life. Data were also collected on gender, appropriate or small for gestational age (SGA), nutrition [total parenteral nutrition (TPN), preterm or term formula, and breast milk], plasma Ca, P0(4), urea, and electrolytes and on the use of drugs (frusemide, dexamethasone, and theophylline). Data from infants treated with any of these three drugs were analyzed separately and not included in establishing the reference ranges for UCa/Cr and UPO(4)/Cr. The mean gestational age of the study population was 28 weeks (range 24-34 weeks). The 95th percentile for UCa/Cr at 3 weeks of age was 3.8 mmol/mmol and decreased significantly with increasing postnatal age (P<0.001). The 95th per-centile for UPO(4)/Cr was 26.69 mmol/mmol at 3 weeks of age, but this did not change significantly with increasing postnatal age (P=0.296). On univariate analysis there was no significant association of UCa/Cr and UPO(4)/Cr with gender and type of enteral nutrition. The UCa/Cr was lower in infants who were SGA (P=0.013) and with low plasma Ca (P=0.008). Infants on TPN had significantly higher UCa/Cr (P =0.019) and lower UPO(4)/Cr ratios(P<0.001). Multivariate analysis confirmed the decrease in UCa/Cr ratio with increasing postnatal age, but the SGA effect was eliminated. The use of furosemide(P<0.001) and theophylline (P=0.003) was associated with a significant increase in the UCa/Cr ratio. The use of dexamethasone was also associated with an increase in UCa/Cr ratio, but this did not achieve statistical significance (P=0.339). The use of furosemide, theophylline,and dexamethasone had no effect on UPO(4)/Cr. We report a reference range for UCa/Cr and UPO(4)/Cr ratios and factors influencing these ratios in a representative population of preterm infants between 24 and 34 weeks gestation, managed according to current nutritional guide-lines.

Nutrient requirements for preterm infant formulas

Achieving appropriate growth and nutrient accretion of preterm and low birth weight (LBW) infants is often difficult during hospitalization because of metabolic and gastrointestinal immaturity and other complicating medical conditions. Advances in the care of preterm-LBW infants, including improved nutrition, have reduced mortality rates for these infants from 9.6 to 6.2% from 1983 to 1997. The Food and Drug Administration (FDA) has responsibility for ensuring the safety and nutritional quality of infant formulas based on current scientific knowledge. Consequently, under FDA contract, an ad hoc Expert Panel was convened by the Life Sciences Research Office of the American Society for Nutritional Sciences to make recommendations for the nutrient content of formulas for preterm-LBW infants based on current scientific knowledge and expert opinion. Recommendations were developed from different criteria than that used for recommendations for term infant formula. To ensure nutrient adequacy, the Panel considered intrauterine accretion rate, organ development, factorial estimates of requirements, nutrient interactions and supplemental feeding studies. Consideration was also given to long-term developmental outcome. Some recommendations were based on current use in domestic preterm formula. Included were recommendations for nutrients not required in formula for term infants such as lactose and arginine. Recommendations, examples, and sample calculations were based on a 1000 g preterm infant consuming 120 kcal/kg and 150 mL/d of an 810 kcal/L formula. A summary of recommendations for energy and 45 nutrient components of enteral formulas for preterm-LBW infants are presented. Recommendations for five nutrient:nutrient ratios are also presented. In addition, critical areas for future research on the nutritional requirements specific for preterm-LBW infants are identified.

Postnatal attainment of intrauterine macromineral accretion rates in low birth weight infants fed fortified human milk

HYPOTHESIS

Provision of more bioavailable mineral sources as human milk supplements enables very low birth weight (VLBW) infants to meet the intrauterine accretion rate for calcium and phosphorus.

DESIGN

Comparison of currently formulated human milk fortifier with previous formulation.

SETTING

Neonatal level II and III nurseries.

PATIENTS

Twenty-six healthy, VLBW infants, whose mothers chose to breast-feed.

INTERVENTIONS

We tested the effects of two formulations designed for VLBW infants as human milk supplements and differing primarily in their quantity and source of Ca, P, and magnesium. The study interval began with a milk intake of 100 ml.kg-1.day-1 and ended when a body weight reached 2.0 kg.

MAIN OUTCOME MEASURES

Net absorption and retention of Ca, P, and Mg during a nutritional balance study conducted once during the study interval, growth during the entire study interval, and bone mineral content of the radius were measured at the beginning and end of the study interval.

RESULTS

The newer Ca gluconate-glycerophosphate preparation (given to group CaGP) resulted in greater net absorption and retention of Ca and P (p < 0.01) than in infants given Ca phosphate (group CaTB). Mg retention was greater than (in group CaGP) or equivalent to (in group CaTB) the intrauterine accretion rate. Radius bone mineral content was significantly greater in group CaGP than in group CaTB (p < 0.001). Volumes of the fortified human milk preparation needed to meet the needs for gain in body weight were higher in group CaGP than in group CaTB (p < 0.001).

CONCLUSIONS

Intrauterine accretion rates for Ca and P can be achieved when VLBW infants are fed human milk supplemented with Ca gluconate-glycerophosphate. Supplementation of human milk with Mg may not be indicated. In this study, greater intakes of Ca and P, and not improvements in bioavailability, result in improved net retention and bone mineral content of VLBW infants.

Calciuria and aminoaciduria in very low birth weight infants fed a high-mineral premature formula with varying levels of protein

To assess the influence of protein intake on renal excretion of calcium and amino acids and on bone mineralization in preterm infants, we randomly selected within weight group strata 27 infants who weighed < 1500 gm at birth (nine per group) to be fed a high-mineral (calcium, 940 mg/L; phosphorus, 470 mg/L) premature formula with one of the following protein contents: formula A, 3.0 gm/100 kcal; formula B, 2.7 gm/100 kcal; and formula C, 2.2 gm/100 kcal. Mean (+/- SD) daily weight gain was greater in infants receiving the higher protein intakes for the first 30 days (formula A, 24.8 +/- 5.1 gm; formula B, 20.5 +/- 3.8 gm; formula C, 16.2 +/- 5.9 gm (analysis of variance: p < 0.01; C < A, p < 0.05)). Bone mineral content did not differ at any time point, and all groups had a high prevalence of generalized aminoaciduria (4 weeks: formula A, 56%; formula B, 71%; formula C, 75%). Urinary calcium corrected for creatinine (in milligrams per milligram) increased as protein content decreased (2 weeks: formula A, 0.16 +/- 0.10; formula B, 0.20 +/- 013; formula C, 0.44 +/- 0.33 (C > A, C > B, p < 0.05); 4 weeks: formula A, 0.23 +/- 0.15; formula B,0.34 +/- 0.47; formula C, 0.49 +/- 0.22 (C > A, p < 0.01). We conclude that the high mineral content and other components of premature formulas result in a higher growth rate and may increase protein requirements. Failure to meet protein requirements may result in underutilization of absorbed calcium and increased renal excretion of calcium. In preterm infants, higher protein intake probably supports rather than jeopardizes bone mineral accretion, and reduces rather then increases calciuria.