Refraction measurements at 18 to 22 months corrected age were performed in 64 patients (79% of 81 infants surviving to 36 weeks postmenstrual age). The most common reasons for missing the refraction examination were decline by the parents. Spherical and cylindrical refraction errors occurred with similar incidence and magnitude in both study groups (Table 5). Furthermore, there was no association between FOD/BPD and myopia detectable (not shown).
Approximately 88% of urine samples of the control group contained P, whereas Ca was present in 67%. Therefore, most infants in the control group had a sufficient P supply for their actual bone mineralization processes, and Ca was the critical component. Less than 50% of samples contained both Ca and P simultaneously (Fig. 2). In the Ca-supplemented group, >90% of urine samples contained Ca, but <50% of samples contained P and 20% contained both Ca and P, simultaneously. The proportions of urine samples containing Ca, P, or both were significantly different between the study groups (P < 0.02).
Urinary Ca concentrations were higher in supplemented infants compared with controls (Table 6) and weakly correlated to serum Ca concentrations (r = 0.41, P < 0.0001). Urinary P concentrations were significantly lower in supplemented infants compared to controls. Serum Ca concentrations were measured in 201 of 1128 total patient-weeks and were higher in infants randomized to supplementation compared with control infants (Table 6). The highest values were observed during weeks when feeds were tolerated and thus supplements were actually given. Serum P concentrations were measured in 308 of 1128 total patient-weeks and were unrelated to extra enteral Ca supplementation. Weeks of measurements were evenly distributed throughout the hospital stay. Serum Ca concentrations were inversely correlated to serum P concentrations (r = 0.2, P < 0.05). Furthermore, although P supply followed unit standards and feeds were supplemented with P by the manufacturers, serum P concentrations were largely below the threshold for tubular reabsorption and thus were suboptimal (29).
Feeding tolerance was also similar in both groups (Table 7). Feeds were started, 100 mL/kg of feeds were achieved, and parenteral nutrition was discontinued at similar ages in both groups. No difference in the type of feeds or the proportion of breast milk was found between groups. Extra enteral Ca supplementation did not interfere with feeding tolerance because there was a similar cumulative feeding amount in both groups, and no Ca soap bezoars occurred; however, feeding tolerance interfered with extra enteral Ca supplementation because Ca was to be given only when feeding tolerance was good. The infants randomized to extra Ca achieved feeds >100 mL/kg on 2336 of 3719 (63%) patient days, in the control group on 1994 of 3390 (59%) patient days. As a result, infants in the Ca group actually received the Ca supplement on a total of 1924 (52% of total) patient days; therefore, the major reason for not giving Ca was feeds <100 mL kg−1 day−1, in accordance with the protocol.
In this randomized trial, extra enteral Ca supplementation did not increase BMC or affect the head shape. Given these results, the absence of differences in eye refraction errors between study groups is consistent with our hypothesis (17). The urinalyses, however, were distinctly different, highlighting the peculiar balance between Ca and P supply. Control infants excreted Ca in 67% but P in 88% of their urine samples, indicating that Ca deficiency was more abundant than P deficiency and that P supply was sufficient in the vast majority of infants. In the group receiving extra Ca, however, urinary P levels were decreased and many infants ceased to excrete P, indicating that P became the limiting nutrient in the infants supplemented with extra Ca despite a high P content in the feeds.
Because a significant increase in BMC was not detected in Ca-supplemented infants, the apparent P deficiency may not be explained by a higher bone mineral formation in supplemented infants consuming all of the supplied P. Furthermore, weight gain was not affected by the supplement. An alternative sequence may have led to increased fecal losses of P. In this particular study, the P was provided mostly as inorganic phosphate (30), whose bioavailability may be impaired by precipitation as Ca phosphate when Ca salts are added (31). P has a poor bioavailability out of Ca triphosphate (32). We speculate that P absorption may depend on the chemical nature of the actual P supplement and that organically bound P has a higher bioavailability than inorganic P at high Ca supply (8). European formulas usually contain organically bound P (eg, glycerophosphate), which may explain why such a reduced bioavailability of P has not been observed in previous studies of Ca supplementation, which were conducted in Europe (9,10,22). Interestingly, the natural P in breast milk also is organically bound to casein micelles and would thus be optimized for intestinal absorption in the presence of Ca (33). Added inorganic phosphate, however, would not benefit from that mechanism.
Ca absorption of preterm infants is greatly variable and may be <50% (7,19,20). This variability may expose infants to Ca deficiency and thus prevent optimal bone mineralization even when following current recommendations or using current commercial infant formulas. It was expected that absolute Ca absorption would increase because a linear relation between Ca supply and absorption has been demonstrated (34). To meet the Ca demand for intrauterine growth and Ca accretion rates even in patients with low Ca absorption, the Ca dose in the treated group was 2- to 2.5-fold higher than in the most recent European recommendations (35), thus accepting an oversupply in patients with slower growth or better absorption. In the control group, the Ca dose was 1.5- to 1.75-fold higher than in these recommendations (35). Calculations using skeletal growth, necessary P retention, published P bioavailability, and the high P content of commercial formulas and fortifiers indicated that additional P supplementation would not be necessary. The P dose was already 1.3- to 2-fold higher than in the European recommendations (35). This was corroborated by extensive clinical experience from a decade of urinalysis-guided feed supplementation that an extra P supplement is rarely necessary when formula or human milk fortifiers are used (9,10). Therefore, individualized supplement adjustments were not planned to simplify study conduct and blinding. Urinalyses were performed solely to aid in the interpretation of the results. Further studies are necessary to determine whether an urinalysis-guided additional P supplement (organic or inorganic) may improve bone mineral accretion, or if an organically bound P supplement (individualized or flat) is necessary to ensure sufficient bioavailability of P in the presence of generous Ca supplementation (8,32).
Newborn infants have similar FOD/BPD across all gestational ages (16,36), but increasing values, indicating dolichocephalic deformation, were described for older preterm infants (16,36,37). Two factors have been considered to affect head shape: bone mineral deficiency (16) and positioning (38). Apparently, extremely mineral-deficient bone is too soft to counter the forces of gravity working on the conventionally sideways-positioned heads. From preliminary data, some authors have suggested a protection against myopia of prematurity when bone mineral deficiency and dolichocephalic deformation can be avoided by supplementing with Ca and P (17,18). In this study, infants were slightly dolichocephalic at study entry in both groups, as the FOD/BPD values were higher (1.44 ± 0.11 and 1.43 ± 0.09, Table 2) than the previously reported 1.27 ± 0.073 SD (16). The FOD/BPD ratio did not change during the study in either group, which may be interpreted as sufficient bone mineralization to prevent further dolichocephalic deformation in most infants. Alternatively, staff in the neonatal unit may have used alternative head positioning because this was not controlled.
We anticipated measuring the refractions of 68 infants, but only 64 were actually measured because of several parents declining the refraction examination during their follow-up visit. The data show no trend toward a refraction difference between the study groups, which makes it unlikely that a significant difference may have been detected with 4 more measurements.
The incidence of myopia in preterm born infants varies (39–45). In this study, the incidence of myopia was rather low in comparison to 30 to 50 years ago, when many very-low-birth-weight infants developed myopia and dolichocephalic modification of their head shape (1–6). Most infants in the present study were normopic or slightly hyperopic at follow-up, which is normal for this age group. Astigmatism was predominantly with the rule, which is also physiologic but different from a preterm population published previously (40). Furthermore, the study groups did not differ in their refraction results. According to our hypothesis, refraction errors increased in the presence of head deformities. Because Ca and P supply appeared to be sufficient in either group to prevent bone demineralization and skull deformities, there were no skull deformities, which could cause refraction errors. The FOD/BPD values of the 4 myopic infants at follow-up were between 1.25 and 1.38, showing an even less dolichocephalic head shape than the mean of the study group; therefore, the myopia in these infants cannot be attributed to the head shape and, because it was so rare, not even to prematurity.
In general, BMC values increased with body weight and therefore with postnatal age (24,46–52), but varied with the demographics of the studied infants. A DXA setup similar to ours has been successfully used in Gambian newborns (51), resulting in the expected BMC values for the Gambian population. In our study, total body BMC results were slightly higher than in a recent report on term infants measured with a different equipment, whereas femur BMC values were lower (50), but all were within the published range. Furthermore, our values were greatly correlated to body weight, as expected. Differences in instrument calibration and image processing may play a part, but they do not invalidate our conclusion that there was no difference in BMC between our study groups. Overall, the constant head shapes and the absence of any fractures indicate that bone mineralization was at least sufficient to prevent these adverse effects, although this finding must be interpreted with caution because fractures were not systematically sought.
Infants in both treated and control groups received at least 1.5-fold more Ca and P than recommended by the European Society for Paediatric Gastroenterology, Hepatology, and Nutrition (ESPGHAN) Committee on Nutrition (3–3.5 mmol Ca and 1.9–2.9 mmol P/kg/day) (35). The compositions of modern standard preterm infant formulas and human milk fortifiers appear to exceed these recommendations. Whether the lower amounts recommended by the ESPGHAN committee also support bone mineralization sufficiently to prevent dolichocephalic head deformation cannot be determined from our data.
No difference in the incidence of complications between the study groups, most notably renal and gastrointestinal complications, were seen in the present study and in previous studies of Ca and P supplementation (9,53). Intestinal obstruction, Ca soap bezoars, the milk curd syndrome (54,55), or renal function impairment were not observed. Thus, Ca supplementation appeared to be safe. Furthermore, feeding tolerance was similar in both study groups and thus not influenced by the supplement. Feeding intolerance was frequent in our study group, which is common in the enrolled patients (9,10) and cannot be excluded experimentally in a clinical trial. It is common to withdraw feeds quickly for any abdominal symptoms to prevent NEC. Feeding intolerance affected patients equally, independent of the randomization.
In this randomized trial of extra enteral Ca supplementation in ELBW infants, weight gain was within the reported range for preterm infants (56,57). Furthermore, sufficient bone mineralization was achieved with both standard feeds and Ca-supplemented feeds to prevent severe dolichocephalic deformation. Therefore, our hypothesis that dolichocephalic deformation is associated with myopia and against the rule astigmatism cannot be proven or excluded. Furthermore, the incidences of myopia and against the rule astigmatisms were reassuringly low and not different from the normal population (35). In the infants receiving extra Ca supplements, urinary excretion of P was reduced, indicating that bioavailability of P had deteriorated and P had become the limiting mineral of bone mineral formation. We suggest that future recommendations regarding the composition of formulas for preterm infants may account for the chemical nature of P compounds and their bioavailability in the presence of Ca. If a Ca phosphate precipitation was indeed the reason for the decreased P supply in Ca supplemented infants, then formulas containing inorganic P as used in the present study cannot be further improved by adding extra Ca.
The authors thank the nurses of the regional newborn intensive care units at the University of Alabama at Birmingham Hospital and the Children's Hospital of Alabama for excellent cooperation in performing the present study, and the Department of Clinical Chemistry at the University Hospitals of Ulm, Germany, for performing the urine Ca and P analyses. Furthermore, the authors thank R. Tsang for reviewing the manuscript.
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bioavailability; bone mineral content; bone mineral density; phosphorus; preterm infant
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