Early Postnatal Calcium and Phosphorus Metabolism in Preterm Infants

Christmann, Viola*; de Grauw, Anne M.; Visser, Reina; Matthijsse, René P.*; van Goudoever, Johannes B.§; van Heijst, Arno F.J.*

Journal of Pediatric Gastroenterology & Nutrition:
doi: 10.1097/MPG.0000000000000251
Original Articles: Hepatology and Nutrition

Objectives: Bone mineralisation in preterm infants is related to the supply of calcium (Ca) and phosphorus (P). We increased the amount of minerals in parenteral nutrition (PN) for preterm infants and evaluated postnatal Ca and P metabolism in relation to mineral and vitamin D (vitD) intake.

Methods: Preterm infants, included on their first day of life, received standard PN, providing a maximum Ca/P intake of 3/1.92 mmol · kg−1 · day−1 on day 3. Ca/P content of formula was 2.5/1.6 mmol/dL, and fortified human milk was 2.4/1.95 mmol/dL. PN supplied 80 IU · kg−1 · day−1 vitD. Formula and fortified human milk contained 200 IU/dL of vitD. During a 5-week period, serum concentrations and urinary excretion of Ca/P were registered and related to the intake of minerals and vitD.

Results: During 12 months, 79 infants (mean gestational age 29.8 ± 2.2 weeks, mean birth weight 1248 ± 371 g) were included. The recommended intake for minerals was achieved by day 5 and for vitD by 4 weeks. Infants developed hypercalcaemia, hypercalciuria, and hypophosphataemia during the first postnatal week, leading to the additional P supplementation in 49 infants. The renal tubular reabsorption of P was >95% until day 9 but decreased <70% after the second week. Alkaline phosphatase was normal at birth, increased to a maximum of 450 IU/L by day 14, and remained above the normal range for the remaining period.

Conclusions: Parenteral intake of P appeared to be too low, leading to mineral imbalances in the early postnatal period, and vitD intake was also below recommendations.

Author Information

*Department of Paediatrics, Subdivision of Neonatology, Radboud University Medical Centre, Nijmegen

Department of Paediatrics, Leiden University Medical Centre, Leiden

Department of Paediatrics, Medisch Spectrum Twente, Enschede

§Department of Paediatrics, VU University Medical Centre, Amsterdam, The Netherlands.

Address correspondence and reprint requests to Viola Christmann, Department of Paediatrics, Subdivision of Neonatology, Radboud University Medical Centre, PO Box 9101, Internal Postal Code 804, 6500 HB Nijmegen, The Netherlands (e-mail: viola.christmann@radboudumc.nl).

Received 12 November, 2013

Accepted 12 November, 2013

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Website (www.jpgn.org).

J.B.G. received honoraria from HiPP GmbH & Co, is receiving grants from Danone and Mead Johnson Nutrition, received payment for lectures, including service on speakers’ bureaus, from Nestle Nutrition Institute, Baxter, Danone and Nutricia Nederland NV, and received royalties from Reed Elsevier. The other authors report no conflicts of interest.

Article Outline

See “Early Mineral Metabolism in Very-Low-Birth-Weight Infants” by Pieltain and Rigo on page 393.

Preterm infants are at risk for the postnatal development of impaired bone mineralisation (1). This is primarily caused by an insufficient supply of calcium (Ca) and phosphorus (P) at the developmental stage, during which these infants exhibit the fastest mineral accretion and thus have the highest requirements during their lifetime (2,3). Therefore, minerals should be supplemented in preterm infants directly after birth, either through parenteral nutrition (PN), formula feeding, or as supplement to human milk (4). In addition to minerals, vitamin D (vitD) needs to be supplemented because human milk contains low amounts of vitD, and infants usually do not obtain enough through contact with sunlight during the first months of life. vitD is essential for the adequate regulation of mineral homeostasis and bone mineralisation (5). During the last decennia, international guidelines presented recommendations of optimal supplementation of Ca and P and vitD (6–12) (Table 1). Daily requirements of minerals have been determined using either the factorial approach or balance studies in stable growing preterm infants (13–15). International recommendations concerning vitD requirements are the subject of debate, but there is consensus that the daily intake should be increased in comparison with former recommendations (11,16). Studies evaluating these recommendations in the early postnatal period of very-low-birth-weight infants have not been performed recently.

In 2005, we implemented a parenteral nutrition (PN) solution that contained more Ca and P to improve the mineral supply for preterm infants. At that time, the Dutch recommendations for vitD supplementation were 80 IU · kg−1 · day−1 with PN and 400 IU/day with enteral feeding (17).

As a quality control in relation to the implementation of the revised PN solution, we performed an observational study evaluating mineral homeostasis during the first 5 postnatal weeks. In this study, we evaluated mineral homeostasis in relation to postnatal mineral and vitD intake. We hypothesised that the use of this PN solution would not lead to disturbances in Ca and P metabolism.

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Study Population

This prospective cohort study was conducted throughout 2005. Preterm infants born at <34 weeks of gestational age (GA) who were admitted to our level III neonatal intensive care unit (Radboud University Nijmegen Medical Centre, Nijmegen, the Netherlands) were recruited on the first day of life. Infants with major congenital malformations or asphyxia were excluded from evaluation.

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Nutritional Protocol

Our PN solution consisted of standard prepared components. The PN solution contained: 2.5 mmol/dL of calcium gluconate (calcium gluconate 10%; B. Braun, Melsungen, Germany) and 1.6 mmol/dL of sodium-glycerophosphate (Glycophos; Fresenius Kabi BV, Zeist, the Netherlands). All of the infants received nutrition according to the standard institutional nutritional protocol. PN was started directly after birth. Mineral intake was increased daily by incremental increases in the amount of PN (Table 2). The maximum parenteral intake according to the European Society of Pediatric Gastroenterology, Hepatology, and Nutrition (ESPGHAN) recommendations in 2005 was achieved at day 3 with 3 mmol · kg−1 · day−1 of Ca and 1.92 mmol · kg−1 · day−1 of P (10). The recommendations provided by Tsang et al (9) were used for enteral intake. vitD was supplemented with PN using Vitintra infant (40 IE/mL of vitD, Fresenius Kabi, Hertogenbosch, the Netherlands). The infants received the generally advised doses of 2 mL · kg−1 · day−1, resulting in a parenteral intake of 80 IE · kg−1 · day−1 by day 4 (10).

Enteral feeding was started on the first day of life with daily increments, whereas PN was gradually reduced to maintain a daily fluid intake within the protocol range. The maximum enteral fluid intake for enteral nutrition and PN was the same (Table 2). The stepwise increase in the enteral volume was left to the discretion of the attending neonatologist. Human milk was enriched with a commercially available fortifier (human milk fortifier) (Nutrilon Neonatal BMF, Nutricia, Zoetermeer, the Netherlands) from an intake of 50 mL/day onwards. HMF added 1.6 mmol/dL of Ca, 1.5 mmol/dL of P, and 200 IE/dL of vitD; therefore, fortified human milk was assumed to contain 2.4 mmol/dL of Ca and 1.95 mmol/dL of P. The human milk fortifier provided minerals such as calcium glycerophosphate and calcium lactate. If human milk was not available, the infants received a preterm formula (Nutrilon Neonatal Start, Nutricia). The formula contained 2.5 mmol/dL of Ca, 1.6 mmol/dL of P, and 200 IE/dL of vitD. Because fortified human milk and preterm formula were both supplemented with vitD, the infants received no further supplements.

Additional Ca and P were administered using 10% calcium gluconate or sodium glycerophosphate for PN or a potassium phosphate (KPO4) and calcium chloride (CaCl2) suspension for enteral supplementation. The decision to start additional supplementation of minerals was left to the discretion of the attending neonatologist and was based on serum mineral concentrations, urinary excretion of Ca and P in spot urine samples, and serum alkaline phosphatase (18). We aimed for serum P (sP) ≥2.0 mmol/L, serum Ca (sCa) ≥2.0 mmol/L, serum alkaline phosphatase (sAF) <300 IU/L, and urinary excretion of P (uP) >0.4 mmol/L and uCa >1.2 mol/L and a tubular reabsorption of P (trP) >80%. If one of the parameters was below or beyond the target, then this could lead to changes in additional supplementation of Ca and/or P; usually, 0.5 to 1.0 mmol · kg−1 · day−1 were added.

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Data Registration

Nutritional intake was registered on a daily basis during the first 2 weeks and weekly until week 5. Serum values of Ca (sCa, mmol/L), P (sP, mmol/L), alkaline phosphatase (AF, IU/L), and creatinine (sCreat, μmol/L) were measured according to a standard protocol. Spot urine samples were used to evaluate Ca (uCa, mmol/L), P (uP, mmol/L), and Creat (uCreat, mmol/L) excretion.

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Data Analysis

Nutritional intake was calculated according to the notations in the patient charts, including parenteral or enteral intake and additional supplementation of Ca, P, and vitD. Ca and P metabolism was evaluated by comparing the concentrations in blood (sCa and sP) and urinary excretions (uCa and uP). Our laboratory reference served as the normal range for sCa (2.2–2.6 mmol/L), sP (2.03–2.9 mmol/L), and AF (80–280 IU/L). The trP was used as an indicator of P availability (normal range 85%–95%). The ratios of Ca and P to creatinine (uCa:Creat and uP:Creat) were calculated retrospectively and compared with the percentiles based on the data presented by Aladangady et al (19). AF was used as a marker for changes in bone metabolism. All of the data were categorised into the subgroups of infants according to GA.

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Statistical Analysis

The data were collected within a cohort study evaluating weight gain in relation to different nutritional intakes. For the primary objective of weight gain, we determined that a study group size of 66 patients was required to identify a significant difference with an α = 0.05 and a power of 0.80. The data are presented as the mean ± standard deviation (SD), unless otherwise indicated.

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Ethical Considerations

The study was approved by the local ethics committee. The informed consent of parents was not necessary according to our institutional review board because the PN solution was a standard medical prescription that was changed to improve quality. The nutritional protocol, record keeping, and blood and urine sampling were part of the standard care used in our department.

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Patient recruitment and cohort inclusion are presented in Figure 1. A total of 79 infants were evaluated. The mean GA was 29.8 ± 2.2 weeks, and the mean birth weight was 1248 ± 371 g. The nutritional characteristics are presented in Table 2. Only the total group data are presented.

The nutritional intake of Ca, P, vitD, and protein is shown in Figure 2. The daily intake increased with time according to the nutritional protocol. The mean duration of PN was 10.6 ± 3.6 (range 6–25) days (Table 3). The highest mean daily intake of Ca was 3.3 ± 1.5 mmol/kg, which was achieved at week 3. The mean total Ca intake was within the recommended range (9,10). Only 7 infants received additional Ca supplementation during short periods to treat hypocalcaemia (Fig. 2A). The mean total P intake was within the lower range of the ESPGHAN PN recommendations at day 2 but increased to just above the maximum intake for PN (2.3 mmol · kg−1 · day−1) after day 6, when enteral nutrition and PN provided a similar amount of intake. The maximum daily intake including parenteral, enteral, and the additional supplement (3.3 ± 1.2 mmol/kg) occurred in week 3 (Fig. 2B). The total Ca and P intake was slightly above the more recent ESPGHAN recommendations for enteral nutrition (11). Infants received a total protein intake of mean (SD) 2.6 ± 0.6 and 3.2 ± 0.7 g · kg−1 · day−1 during the first and second weeks (Fig. 2C).

vitD intake was calculated according to the recorded intake. Human milk was provided to 91% of our infants and reached a mean enteral intake of >50 mL/day at day 4. Subsequently, human milk was fortified and enriched with vitD. After day 5, the daily vitD intake increased >160 IU. The recommended daily intake of 400 IU was only reached 4 weeks after birth (Fig. 2D).

The Ca, P and sAF concentrations in blood and urine are presented in Figure 3. Directly after birth, the mean sCa was 2.2 ± 0.24 mol/L, which continuously increased to a maximum of 2.7 ± 0.24 mmol/L on day 5. Between days 4 and 8, the mean sCa remained above the upper reference limit of 2.6 mmol/L. Forty-five percent of all infants had an sCa >2.6 mmol/L on day 5. At the same time, 34% infants had an sP level <1.8 mmol/L. For the remaining observational period, the mean sCa remained below the upper normal range. The mean sP concentration was below the lower reference range of 2 mmol/L from birth until day 9, and then increased within the normal range for the remaining observational period. At birth, the mean sAF was within the normal range (mean 194 ± 62 IU/L), but thereafter, it steadily increased to a maximum of 480 IU/L by day 7 and remained >300 IU/L for the remaining observation period, thus exceeding the upper reference limit.

The mean uP, evaluated with spot urine samples, was low until day 9, but generally above the recommended surplus of 0.4 mmol/L. The urinary excretion of P increased steadily until week 5. The mean urinary excretion of Ca was below the recommended surplus of 1.2 mmol/L until day 2. The mean uCa remained above the surplus for the rest of the observational period and tended to increase toward the end of the observational period. The highest uCa excretion occurred between days 3 and 8 (see Fig. S1, http://links.lww.com/MPG/A290 [Urinary excretion of Ca and P determined from spot urine samples. Dotted line: surplus; P: 0.4 mmol/L; Ca: 1.2 mmol/L (18); data presented as mean ± standard deviation]).

The mean uCa:Creat ratio increased steadily after day 3, remaining >3.8 mmol/mmol (the 95th percentile) between days 4 and 8 and >2 mmol/mmol (the 75th percentile) for the rest of the follow-up (Fig. 3) (18). The mean uP:Creat ratio remained <0.5 mmol/mmol (the 10th percentile) until day 9 and increased >16 mmol/mmol (75th percentile) after the second week (Fig. 3) (19). In accordance with the low mean uP:Creat ratio, the trP remained >95% during the first week. After the second week, the mean trP decreased to <70% (range 58%–68%).

During the second week, 49 of 79 infants received additional supplementation of P. This was continued in 25 infants until week 5. Infants who received additional P until week 5 had a significantly lower GA and lower birth weight (P < 0.001) and received enteral nutrition earlier. They had lower sP concentrations and lower uP during the first week, and higher sAF concentrations compared with infants without additional P. The trP was low in all infants during the last 3 weeks independent of any P supplementation (see the subanalysis of additional P supplementation, http://links.lww.com/MPG/A291).

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Our results demonstrate that the mineral intake of our infants met the recent recommendations for parenteral and enteral nutrition for preterm infants (9–11) (Table 1). Nevertheless, we observed hypercalcaemia, hypophosphataemia, hypercalciuria, and high tubular phosphate reabsorption during the first week and hyperphosphaturia with a low trP during the second half of the study period. Furthermore, the sAF increased above the normal range. The recommended vitD intake was only achieved by week 5. These data indicate a great need for P supplementation in the early postnatal period and a mineral imbalance in the second half of the observation period, probably related to low vitD intake.

Increasing Ca and P intake in PN has been shown to improve bone mineral content in preterm infants (20). The use of sodium-glycerophosphate in PN offers the opportunity to increase the mineral contents of standard PN solutions (10,21).

Despite the increased need for minerals, the supplementation of high amounts of minerals is often regarded as a potential risk factor for the development of hypercalcaemia, hypercalciuria, and, thereafter, nephrocalcinosis (22). We observed hypercalcaemia, a high urinary excretion of Ca between days 4 and 8, and a uCa:Creat ratio >95th percentile in the same period. Hypercalcaemia can be caused by either high Ca intake or hypophosphataemia. High nutritional Ca intake will increase urinary Ca excretion. Furthermore, several medications, metabolic acidosis, renal disease, and vitD deficiency are known to have a calciuretic effect (23–25). Aladangady et al demonstrated a significant relation between a high Ca:Creat ratio and the use of xanthine derivatives, such as caffeine (19,23). All of the infants received caffeine directly after birth or shortly before extubation from ventilation. Additionally, hypophosphataemia will lead to increased urinary Ca excretion (25). Our cohort had low sP concentrations and high renal P retention during the first week. Balance studies have demonstrated improved Ca retention after increasing phosphate intake (26,27). Aladangady et al (19) demonstrated that a low uP:Creat ratio is related to P insufficiency and is independent of medication. The decrease in the uCa:Creat ratio and the increase in the uP:Creat ratio after day 8 in our cohort may be the result of P supplementation. Therefore, our results reflect an increased need for P during the early postnatal period. Several studies recently suggested that this increased need may be related to a higher amount of amino acids administered with PN, compared with older studies used for present guidelines (28–30).

During the second half of the observation period, we recognised a different pattern with still elevated uCa excretion, an increasing phosphaturia, low trP, and elevated AF, whereas the mineral intake mainly remained the same. Because many infants received additional P at that time, this may be interpreted as a result of an unnecessary supplementation. Furthermore, we used potassium phosphate as an oral supplement, whereas organic salts may have had an improved bioavailability (11). Nevertheless, infants who did not receive additional P during the last week demonstrated the same increase in urinary excretion of P and Ca (see the subanalysis of additional P supplementation, http://links.lww.com/MPG/A291). At that time, the mean vitD intake was below the recommended daily intake of 400 IU of vitD. Insufficient vitD may explain the high Ca excretion, whereas indirectly, this may have inhibited the renal tubular reabsorption of P (5). Our data are in accordance with previously performed balance studies in preterm infants that demonstrated that vitD is absorbed well and positively influences mineral homeostasis (31). In these studies, infants received 1200 IU/day of vitD.

Present recommendations concerning nutritional intake and mineral homeostasis are developed for either PN or enteral nutritional intake and the recommended Ca:P ratio encloses a wide range. There is a discrepancy between the guidelines presented by Tsang et al and the ESPGHAN and ESPEN recommendations (9,10). According to Tsang et al, the lower limit of parenteral P intake is higher and the range of Ca:P ratio lower than recommended by ESPGHAN (Table 1). Looking at our data, the American recommendations seem to be more appropriate.

Most very-low-birth-weight infants receive a combination of PN and enteral nutrition with an increased amount of protein and energy directly after birth. Present recommendations are based on studies performed in stable growing infants of several weeks, whereas recent studies indicate that nutritional deficits developed during the first postnatal weeks may have a long-lasting effect on future development (32,33).

Our study has several limitations. We evaluated mineral homeostasis but did not evaluate bone mineral content using dual-energy x-ray absorptiometry. Nevertheless, mineral imbalance, especially hypercalciuria, has been related to impaired bone mineralisation (34,35). We observed a high urinary excretion of minerals in our patients. Therefore, we believe that despite the high mineral intake, our patients were at risk for developing suboptimal bone mineralisation, with the long-term consequence of reduced length (36). In addition, we did not measure vitD concentrations, although the intake could be calculated from charts and a positive relation between enteral vitD intake and the vitD concentration in blood has been demonstrated in preterm infants (31). The latest ESPGHAN guidelines recommend a daily intake of 800 to 1000 IU/day of vitD in enterally fed preterm infants (11). This recommendation does not state at what time enteral vitD supplementation should be initiated. Furthermore, the recommendation for parenteral supplementation has not been reevaluated with regard to the revised target value of 75 mmol/L (30 ng/mL) of 25-hydroxy vitD (37).

Early postnatal mineral homeostasis with high mineral intake and high doses of vitD in preterm infants has not yet been investigated. Our data suggest that the optimal dosing of minerals and vitD should be reevaluated in very-low-birth-weight infants in the early postnatal period in combination with more aggressive PN and enteral nutritional intake.

In conclusion, our data demonstrate that the PN intake of P appeared to be too low, leading to mineral imbalances in the early postnatal period, and vitD intake was also below recommendations.

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bone metabolism; calcium; phosphorus; preterm infant; vitamin D

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