Jornal de Pediatria - Vol. 77, Supl.1 , 2001 S53
0021-7557/01/77-Supl.1/S53
Jornal de Pediatria
Copyright © 2001 by Sociedade Brasileira de Pediatria
REVIEW ARTICLE
Critical analysis of pathophysiological, diagnostic
and therapeutic aspects of metabolic bone disease
in very low birth weight infants
Monique Catache,1 Cléa Rodrigues Leone2
Abstract
Objective: to perform a systematic review on the pathophysiology, diagnosis and approach of
metabolic bone disease in very low birth weight infants.
Sources: literature review of articles published in Medline within the last twenty years.
Summary of the findings: the higher survival of very low birth weight infants was concurrent with the
increased incidence of metabolic bone disease. The process of bone mineral acquisition suffers some
alterations during the neonatal period, including low bone mineral content at birth, insufficient mineral
supply in the neonatal period, and regulatory disorders, which may compromise growth and development
on the long run. The diagnosis is based on the association of risk factors, and biochemical and radiological
alterations. The early intervention in the neonatal period prevents the development of severe metabolic
bone disease, reducing complications during the first year of life.
Conclusions: the early diagnosis of metabolic bone disease allows for early intervention, thus
preventing complications that may originate from the alterations in bone mineral acquisition.
J Pediatr (Rio J) 2001; 77 (Supl.1): S53-S62: very low birth weight infants, metabolic bone disease,
phosphorus deficiency, calcium deficiency.
Introduction
Osteopenia of prematurity in very low birth weight
infants was first reported in the literature in the study of
Ylpo et al.1 Vitamin D deficiency was considered an
important etiological factor, which motivated its use as a
nutritional supplement. As a consequence, a decrease in
vitamin D deficiency was observed in the following decade.
After this period, until 1943, few studies were carried
out on the presence of osteopenia of prematurity. In 1943
Benjamin et al.,2 while carrying out metabolic balance
analysis in preterm newborn infants (PNB), concluded that
maternal milk as the only source of nutrition was insufficient
in terms of minerals, thus favoring alterations in bone
mineralization. In corroboration to these findings, Von
Sidow et al.3 described osteopenia of prematurity in
newborns (NB) on exclusive breastfeeding and without
phosphorus (P) supplementation.
1. Master in Pediatrics, School of Medicine, Universidade de São Paulo.
Assistant Resident.
2. Professor of Pediatrics, School of Medicine, Universidade de São Paulo
(USP). Chief, maternity ward nursery, Hospital de Clínicas, USP.
During the 1960s, studies started to underscore the need
for mineral supplementation, especially of P, in order to
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S54 Jornal de Pediatria - Vol. 77, Supl.1, 2001
ensure appropriate bone mineralization rate (similar to that
in intrauterine life) in PNB and during the first weeks of life.
Shaw et al.4 described a series of minimal nutritional
requirements of very low birth weight NB (VLBW-NB).
These requirements were aimed at allowing for a rate of
growth similar to that of intrauterine life. At the same time,
other authors have underscored the importance of
breastfeeding as a nutritional source for PNB. In 1978,
Forbes et al.5 questioned whether the amounts of calcium
and phosphorus in human milk were sufficient for a bone
mineralization rate similar to that of intrauterine life and for
this group of NB.
Considering that calcium (Ca) and phosphorus (P) are
the most abundant ions in the organism, and are
predominantly distributed in the bone, their homeostasis is
essential for a favorable medium for the mineralization
process. An adequate bone Ca:P ratio favors the formation
of hydroxypatite and, consequently, a more satisfactory
mineralization. Consequently, the amounts of Ca and P
received in both intrauterine life and the neonatal period
exert a significant influence on the homeostasis of this
process. This influence occurs in addition to the effect of
various hormones such as parathyroid hormone (PTH),
1,25-dihydroxyvitamin D [1,25 (OH)2D], calcitonin,
parathyroid hormone-related protein (PTHrP), and other
hormones.
The greater accretion of minerals in intrauterine life
occurs during the third trimester of gestation, and it peaks
at 34 weeks. Consequently, for PNB the amount of Ca and
P received during the neonatal period is fundamental, since
these babies were deprived of the stage of greater accretion
of minerals during gestation.
Initially, the alterations in mineralization observed in
VLBW newborns were called osteopenia of prematurity.
This term was employed in an attempt to characterize a
situation of bone hypomineralization during the evolution
of these NB in the neonatal period. In other studies, this
condition was called rickets of prematurity, due to the X-ray
findings similar to those of vitamin D deficiency. In 1985,
it was determined that metabolic bone disease (MBD) be
the term used. This term comprehends slight bone
hypomineralization and X-ray alterations of rickets with
spontaneous fractures; thus, it includes both previously
described abnormalities.6 Moreover, the term rickets of
prematurity can lead to a misinterpretation in that it deals
with a status of vitamin D deficiency - which is not related
to the physiopathology of MBD.
Improved survival of PNB lead to an increase in incidence
of MBD. In this sense, there has been an increased interest
in elucidating the physiopathology of MBD and in defining
a scheme for adequate prevention and treatment of this
metabolic abnormality. In addition, the factors that govern
bone mineral acquisition have been given more attention in
view of normal growth, especially in the sense of increasing
peak bone mass as a strategy for the reduction of fractures
in adult life.7
Critical analysis of pathophysiological, diagnostic... - Catache M et alii
The greatest challenge in the handling of MBD is the
absence of well-defined biochemical and/or radiological
abnormalities that appear in early stages as indicators of
mineral deficiency. Numerous studies have tried to define
laboratory data as indicators of Ca and P deficiency, but
none of these attempts were successful. Thus, the definition
of parameters for early diagnosis of MBD is still to be done
in order to allow for early detection and intervention.
Moreover, these parameters would also allow for prevention
of a severe clinical status that may compromise long-term
growth of PNBs.
Intrauterine metabolism of minerals
Calcium is the most abundant cation in the organism.
Ninety-eight percent of bodily Ca is found in the bones, thus
constituting one of its main inorganic compounds.8-11
Phosphorus is the second most abundant ion in the
organism. It is mainly distributed in the bone, in which 80%
is found in the skeleton and 9% in skeletal muscle. The
remaining P is distributed in lipids of the cellular membrane,
which are composed of high energy (adenosine triphosphate
- ATP), intracellular proteins for signal translation,
ribonucleic acid (RNA), and deoxyribonucleic acid
(DNA).8-11
During all stages of growth, bone mineral acquisition is
dependent on delivery of adequate mineral amounts of
vitamin D. Moreover, it is also dependent on perfect hormone
control that favors mineralization and limits bone
mobilization, thus promoting an increase in bone mineral
content.7,12
During intrauterine life, Ca is actively transported against
a concentration gradient in the direction mother-to-fetus at
the placenta. The umbilical cord levels are more elevated
than those of the mother.7
Serum P levels of the mother tend to lower during
gestation as a result of the increased requirements of the
fetus.13 Concentration of P in the fetus during the third
trimester of gestation is greater than that of the mother. The
placental transfer of P is an active process against a
concentration gradient; it uses the sodium (Na) gradient as
a source of energy with coupled transport of P.
Intrauterine accretion of minerals occurs after the 24th
week of gestation until the end of the term. There is a
constant ratio of Ca:P, which is ideal at 2:1 at bone level and
at 1.7:1 in the body. Maximum peak accretion of minerals
was reported between 34 and 36 weeks of gestation; during
this stage, different authors have reported an accretion of Ca
of approximately 100-120 mg/kg/day, and of P of 60-75
mg/kg/day.9,10,14,15
During gestation, there is an adaptation in the metabolism
of the mother, which favors an increase in intestine absorption
of Ca and in bone reabsorption in order to fulfill fetal
requirements.7
Critical analysis of pathophysiological, diagnostic... - Catache M et alii
These activities are a result of an increase in circulating
levels of 1.25 (OH)2D, which favors bone absorption and
mobilization. The described alterations do not depend on
maternal reserves nor on vitamin D supplementation.
Various hormone factors favor the process of intrauterine
mineralization: the parathyroid hormone-related protein,
which plays an important role in maintaining the
transplacental gradient of Ca; the production of 1.25 (OH)2D
in the placenta, which can be involved in regulating the
production of Ca carrier proteins; the low PTH
concentrations, which limit the mobilization of bone
minerals; the presence of elevated concentrations of
calcitonin in the fetal period, during which it plays its most
important physiological role favoring mineral deposition;
the release of insulin-like growth factors (IGF-I), which
stimulate bone growth and increase mineralization; the high
levels circulating estrogen in maternal blood, which favor
mineralization.7 The result of the association of these
factors is an increase in bone formation and decrease in
reabsorption, which favors a rapid gain of minerals during
the third trimester of gestation (during which 80% of bone
mineralization takes place).7,14-18
Regulation of metabolism of minerals
Many hormones are involved in the regulation of the
metabolism of minerals.
The vitamin D, through the formation of active
compounds, acts on maintaining serum Ca levels, thus
stimulating intestinal absorption of the vitamin through
activation of local enzymes (Ca/Mg ATPase and Ca/Na
ATPase); on promoting bone mobilization, on increasing
Ca renal reabsorption acting together with PTH. The active
compound of vitamin D (1.25 (OH)2D) leads to an increase
in intestinal absorption of P through an active process and
through favoring its uptake process. In addition, at the bone
level, the active compound stimulates the formation of
matrix proteins, thus favoring the process of
mineralization.8-10,12
The secretion of the parathyroid hormone, a hormone
produced in the parathyroid gland, is inversely related to the
serum concentrations of Ca and Mg.9 The PTH increases
activity and number of osteoclasts through the release of
osteoblast factors; enhances tubular reabsorption of Ca and
reduces reabsorption of P; elevates the activity of 25 (OH)
D-1 alpha-hydroxilase leading to the formation of 1,25
(OH)2 D; all of these actions are aimed at restoring the
values of calcemia.7,9,12
During fetal life, the synthesis and secretion of PTH are
probably suppressed by the high serum concentrations of
Ca of this period.7,19
The parathyroid hormone-related protein (PTHrP ) is
involved in the metabolism of minerals and its specific
functions in mineral homeostasis are still not wellunderstood. The PTHrP is found in the placenta and in fetal
Jornal de Pediatria - Vol. 77, Supl.1 , 2001 S55
tissue around 8 to 12 weeks of gestation; this suggests an
important role in the process of bone mineralization.8,9
PTHrP has functions similar to those of PTH in both bone
absorption and mineralization, renal P excretion, Ca
reabsorption, and production of calcitriol.
Calcitonin is a peptide produced in the parafollicular
cells (C cells) of the thyroid. It acts on a receptor connected
to the G protein inhibiting the absorption of osteoclasts
(which is induced by PTH) and the maturing of precursor
cells in mature osteoclasts, thus reducing the mobilization
of bone Ca and P.7
The action of calcitonin leads to a decrease in tubular
reabsorption of Ca, Mg, P, and Na increasing free water
clearance. It is possible that the bioactivity of calcitonin
modulates the action of PTH in target organs.9
During fetal life, it is possible to observe an increase in
calcitonin levels, which block the effects of PTH and
PTHrP in osteoclasts. These levels are higher than those of
adult life. This suggests that the most important physiological
role of calcitonin is in mineralization during intrauterine
life.7,12
Metabolism of minerals during the neonatal period
Reported serum Ca concentrations during the neonatal
period are of 3 mmol/l in the umbilical cord. These levels
are higher than those reported for the mother. However,
after birth, the serum Ca concentrations decrease and reach
the nadir during the first 48 hours of life. After this period,
they increase to levels higher than those of adult life, which
last throughout childhood in order to promote a positive
balance of Ca and with the objective of attaining normal
skeleton development and growth.20
Intestinal absorption is the determining factor of the
supplying of minerals. Ca is absorbed in the duodenum by
two different processes: an active process with saturable
absorption and dependent on adequate levels of 1,25
(OH) 2 D; and a passive process dependent on the
concentration gradient between the intestinal lumen and
serum concentration.21,22
The absorption varies from 30 to 65%; it nears 65%
when the baby is being breastfed and remains around 40%
during administration of special formulas for preterm
newborns.10,17 Absorption is influenced by protein-mineral
interaction, bioavailability of salts used in the diet, absolute
amount of Ca in diet, Ca:P ratio in the milk used, and
presence of carbohydrates - which favor absorption.
Moreover, absorption is markedly influenced by the action
of 1,25 (OH)2 D, which increases absorption from 35 to
68%.10
Rowe et al. 23 showed a Ca absorption rate of
approximately 74% and retention of approximately 70%.
Thus, it is important to evaluate the salts used in diet once
their insolubility leads to lower retention and compromises
the process of mineralization.7,10,17
S56 Jornal de Pediatria - Vol. 77, Supl.1, 2001
At the renal level, Ca in glomerular filtrate corresponds
to 60% of the plasmatic concentration. Reabsorption occurs
mainly at the proximal tubule (50%); renal excretion
corresponds to only 1 to 2 % of the filtered Ca.10,24
The serum concentration of phosphorus in the umbilical
cord is of 1.8 to 2.3 mmol/l. This concentration increases
during the first 48 hours independently of intestinal
absorption. This increase is possibly influenced by low
renal excretion of this ion during the neonatal period. Serum
levels remain elevated throughout childhood, which suggests
a correlation with skeletal growth rate. Normal values are
2.3 + 0.2 mmol/l.9
The absorption of P occurs mainly in the duodenum and
in the jejunum. It occurs by simple, facilitated diffusion (8095%) and it is dependent on both intestinal concentration,
according to P delivered in the diet and in active sodium
transport, and on the action of 1,25 vitamin D. The earlier
is the most important determinant mechanism of the
absorption of P.10,25
Absorption of P can be as high as 90%, as is the case of
breastfeeding. Absorption of P is considered optimal except
with soy milk, in which case the presence of phytates
compromises absorption because it forms complexes with
P.8,17,21,22 The absorption and retention rates for P observed
in the study by Rowe et al.23 were, respectively, of 76 and
74%.
Regulating of absorption of P is carried out basically by
the kidney, in which case there can be an important tubular
reabsorption with urinary P levels that are practically
undetectable.7,24,25 At the kidney level, 80% of filtered P is
reabsorbed at the proximal tubule, 10% in the distal tubule,
and 10% is excreted in urine.
Due to the importance of the diet as the source of Ca and
P in the neonatal period, strategies for delivery of P to
newborns at risk for MBD should consider the amounts of
P in different types of milk. Moreover, these strategies
should consider the probable absorption rates of Ca and P.
Regulating mineral homeostasis
At birth, there is an increase in release of PTH during the
first 48 hours of life, probably as a response to the fall in
serum levels of Ca. As the calcemia starts to normalize,
there is a reduction in concentrations of PTH, which near
the adult values around the first week of life.7,15
In the NB, the levels of 1,25 (OH)2 D increase during the
first 24 hours of life. Next, these levels vary according to the
delivery of Ca and P in the diet, probably as a result of the
need for greater intestinal absorption of these elements in
order to maintain mineral homeostasis.7,9,17
Special attention should be given to the delivery of
vitamin D and sun exposure during this period, considering
that these are the two main sources of the metabolite. It is
Critical analysis of pathophysiological, diagnostic... - Catache M et alii
also important to underscore that maternal milk delivers
amounts of vitamin D that are not sufficient to maintain
adequate serum levels.10
Serum concentrations of calcitonin are greater in
umbilical cord blood than in maternal blood. There is a
marked increase in this concentration during the first days
of life for values five- to tenfold higher than those of adults
independently of the serum concentrations of Ca. The role
of calcitonin during the neonatal period is still not wellunderstood.9 Apparently, the increase in concentration is
stimulated by the action of the catecholamines and glucagon,
which are released during birth.7,15
Providing Ca and P in the neonatal period
After birth, the main nutritional source is maternal milk.
The milk is responsible for satisfying the needs of the
newborn and for promoting better adaptation to the neonatal
period.
Nutritional handling of VLBW newborns is still
controversial since the basic requirements of these babies
are not well-established. The delivery of nutrients is aimed
at achieving the same rate of growth of intrauterine life
according to the postconcepcional gestational age. This rate
is based on increase in weight and height and on the values
of retention of nutrients and minerals.26
The milk of mothers of term newborns has an average
Ca content of 340 mg/l and P content of 140 mg/l. These
values present slight variations during the different stages
of lactation.27 The ration of CA:P in maternal milk is of
1.8:1 to 2.2:1.
In case cow milk is used in replacement of maternal
milk, it is important to consider that despite the greater
mineral content of the earlier, most of it is excreted as
calcium palmitate resulting, thus, in lower absorption.28
The use of cow milk as a nutritional source for VLBW
newborns requires special attention as to what concerns its
mineral content. The mineral concentrations found in the
maternal milk vary from 20 to 25 mg/dl for Ca and from 10
to 15 mg/dl for P. The ratio of Ca:P is of 1.8 to 2.2:1.7,10
Consequently, for an enteral delivery of 200 ml/kg/day and
considering an absorption rate of 60% for Ca and of 90% for
P, the maternal milk provides approximately 60 mg/kg/day
of Ca and 30 mg/kg/day of P.14,17,29 These rates are much
lower than those predicted during the third trimester of
gestation, which is the period of greater mineral acquisition
for the baby.
In order to improve the delivery of minerals to exclusive
breastfeeding, VLBW newborns supplementation of human
milk in the form of powder or liquid with added minerals,
calories, and proteins. The fortified human milk allows for
an accretion more similar to that of intrauterine life. 25,30,31
As a result, it is possible to deliver approximately 60 mg/dl
of Ca and 33 mg/dl of P.29
Critical analysis of pathophysiological, diagnostic... - Catache M et alii
The Ca and P solutions, used as a nutritional supplement,
are an alternative for a more adequate delivery of minerals.
This allows for the correction of abnormalities in
mineralization in the absence of the option of fortified
human milk.32
Another alternative for providing the nutritional
requirements of preterm newborns is through the use of
specially designed formulas. These formulas provide
approximately 70 to 140 mg/dl of Ca and 40 to 70 mg/dl of
P. These values near those of intrauterine life. 10
Consequently, it is possible to observe a variation in delivery
of Ca of 110 to 216 mg/kg and of P of 76 to 108 mg/kg for
an enteral feeding of 200 ml/kg/day.14
The activity of vitamin D in human milk is insufficient
for maintaining adequate serum levels of 25 (OH)D; thus it
is recommended that 400 IU/day of vitamin D be used as a
nutritional supplement.19
Metabolic bone disease
The term metabolic bone disease (MBD) refers to
abnormalities in bone mineralization of VLBW newborns
in comparison to normal fetal skeleton and as a result of
insufficient delivery of minerals during the neonatal
period.15,17
The MBD does not have a characteristic clinical
presentation. It can develop hindering longitudinal growth;
maintaining head circumference; and presenting severe
rickets with craniotabes, rachitic rosary, and epiphysial
dysplasia in the long bones (especially the wrists) in some
cases developing with spontaneous fractures.16,33 These
newborns may present clinical status of delayed-onset
respiratory distress due to the lack of thoracic cavity support
and with the development of atelectasis and the deterioration
of bronchopulmonary dysplasia.15 Clinical manifestations
of the disease appear between the 6th and the 12th weeks of
life.14
Currently, it is understood that MBD affects all riskgroup newborns in its lighter form, which is called
osteopenia.17,34 This develops to more severe forms of the
disease in approximately 30% of these newborns.16 Others
have reported an approximate incidence of 55% for newborns
with less than 1,000 grams and of 23% for those with less
than 1,500 grams; fractures were reported in 24% of
cases.14,17 The incidence of MBD according to the delivery
of nutrients is of 40% in breastfed, preterm newborns; 20%
in partially breastfed preterm newborns; and 16% in special
formula-fed preterm newborns.35
It is important that this disease that affects VLBW
newborns be understood due to the alterations in growth
with the development of the disease. Many studies have
shown a reduction in bone mineral content in exclusively
breastfed, VLBW preterm infants in comparison to preterm
infants fed with a nutrient enriched formula during the first
year of life.36,37 Schanler et al.,38 carried out a 2-year
Jornal de Pediatria - Vol. 77, Supl.1 , 2001 S57
follow-up of preterm newborns after hospital discharge.
The authors observed that the differences observed in the
first year of life were not observable at the end of the second
year. The bone mineral content (BMC) caught up to that of
term newborns with the same postconcepcional age.
However, James et al.39 carried out a comparative study
with full term and preterm infants at a postconcepcional age
of 40 weeks. Results indicated that the latter presented a
significantly lower height, weight, and BMC even with the
administration of an adequate diet for preterm newborns.
Lucas et al.40 observed that preterm newborns who were
given nutrient-enriched milk formula presented significant
increases in linear growth and weight gain than those who
were given standard term formula. This is probably a result
of a more adequate nutritional and mineral content in the
enriched milk.
These differences become less significant as other types
of foods become the main nutritional source of infants. This
occurs also as their growth rate decreases and, consequently,
so do their requirements of Ca and P.34.,41
Associated risk factors
The following conditions during the neonatal period
favor the development of MBD12,14,16,17,25:
– All newborns with birth weight less than 1,500 g,
especially those smaller than 1,000 g are susceptible to the
development of MBD. This predisposition is more important
in the case of newborns with lower birth weights, who have
higher growth and mineralization rate requirements to be
met in order to mirror intrauterine life. In this sense, these
infants are more at risk for being affected by postnatal
mineral and nutritional deficiency;
– Newborns with gestational age less than 32 weeks,
especially because they present lower mineral reserves at
birth. This is a result of these infants being deprived of the
stage of greater accretion of minerals, which occurs during
the third trimester of gestation;
– Prolonged use of parenteral nutrition, which delivers
less Ca and P in comparison to intrauterine life. Moreover,
there is the possibility of aluminum contamination and its
toxic effects that inhibit bone mineralization. Another
limiting factor of the delivery of minerals in prolonged use
of parenteral nutrition is the volume administered to each
newborn; small volumes limit the amount of minerals due to
the risk of precipitation in the solution. The solubility of the
solution depends on the salts used, duration of storage,
room temperature, pH, amino acid concentration, and
dextrose;
– Treatment with diuretics, which leads to renal loss of
Ca due to inhibition of the absorption of electrolytes in the
ascending limb of the loop of Henle. Calciuria will be
proportional to renal loss of Na;
– Situations that lead to a delay in the start of enteral
nutrition, either due to obstacles to its introduction, to
S58 Jornal de Pediatria - Vol. 77, Supl.1, 2001
limitation of the volume used (as in cases of respiratory
failure, bronchopulmonary dysplasia, patent ductus
arteriosus, necrotizing enterocolitis, and so on;
– Low delivery of minerals in the diet, either due to
insufficient content in milk or insufficient volume of milk;
– Bronchopulmonary dysplasia, which presents various
associated risk factors for the development of MBD,
including deficient enteral delivery of minerals (due to the
limitation in volume administered); use of diuretics (leading
to renal loss of Ca); use of corticoids (with the reduction in
bone mineral content due to reduction in absorption or renal
loss); sedation and immobilization (can lead to loss of bone
mass);
– The use of corticosteroids, which is associated with a
reduction of intestinal absorption of Ca, renal loss of Ca,
and reduction in BMC.
Physiopathology
One of the factors that certainly contribute to the
development of MBD is the fact that preterm newborns
present low mineral reserves at birth (80% of bone mineral
accretion occurs during the third trimester of gestation). To
attain the same rate of intrauterine mineral accretion in
these newborns is an important objective for the prevention
of MBD, as long as the rate is attained without producing
undesirable metabolic effects.17,25
Preterm newborns with exclusive breastfeeding develop
a phosphorus deficiency syndrome. The reduction in the
delivery of this mineral stimulates an increase in the
production of 1,25 (OH)2D with a resulting increase in
intestinal absorption of Ca and P.25,38,39,42 Moreover, the
release of PTH is inhibited, which leads to a reduction in
renal loss of P and Ca and, consequently, to hypercalciuria.
This inhibition of the release of PTH can provide a protective
effect as to what concerns bone mobilization. The effect of
1,25 (OH)2 D, however, will remain, stimulating the action
of osteoblasts and leading to mobilization of Ca and P
through activation of osteoclasts. The continuation of
deficient delivery of minerals will result in an increasing
mobilization, with a consequent deterioration of bone
loss.14,15,43 In this situation, despite a concomitant
deficiency of Ca due to low nutritional content of the
mineral, it is possible to observe a significant renal loss of
the ion due to the absence of bone deposition, which is the
result of an inadequate Ca:P ratio.44
The studies developed by Koo et al.34,45 have shown
that serum levels of 25 (OH)D were normal with the
supplementation of 400 IU/day of vitamin D. This
corroborates the hypothesis that the deficiency of vitamin D
is not a determinant factor for MBD. In turn, the finding of
elevated levels of 1,25 (OH)2D is suggestive of mineral
deficiency. The increase in production of this metabolite is
an attempt to intensify intestinal absorption of Ca and P with
the objective of restoring normal bodily content.
Critical analysis of pathophysiological, diagnostic... - Catache M et alii
Other studies have suggested that MBD is a state of
increase bone turnover. The presence of elevated serum
alkaline phosphatase (indicating osteoblastic activation)
and high urinary hydroxyproline (a marker of bone
mineralization) indicated increased bone turnover.30,33
Seemingly, local alterations related to the release of
prostaglandins and PTH-related peptide can be involved in
this process of bone reabsorption.33
Studies with radioisotopes showed that urinary Ca in
preterm newborns have their origin in both the diet and bone
tissue. Moreover, the hypercalciuria observed in preterms,
even with adequate mineral supplementation, with good
retention rates, suggests the presence of high bone turnover
in these newborns.46
Other authors have described the occurrence of MBD in
newborns who received “preterm” human fortified milk.
Apparently, the use of special formulas improves growth
and BMC with the increase in delivery of Ca, P, and
proteins; this is achieved, however, without eliminating
relative deficiency of P, indicated by a BMC lower than
expected considering a similar period of intrauterine life.47
Studies evaluating bone mineral accretion of preterm
infants indicated rapid accretion of minerals between 40
and 60 weeks of postconceptional age, which at times was
fivefold higher than in term infants, comparatively.48
Considering these findings, studies have raised the
hypothesis that there is a phase of rapid mineral accretion
starting at 40 weeks’ postconception in preterm infants that
substantially reduces the perinatal mineralization deficit.49
Diagnosis
There is no defined diagnostic method for MBD.
Compromise of the clinical status appears late and, in
certain situations, the diagnosis is not obtained.
It is still necessary to develop precise diagnostic methods
for this disease in order to reduce the number of complications
affecting these newborns, who are already affected by a
series of complications during the neonatal period due to
their prematurity.
Biochemical alterations
Biochemical dosing has been carried out in preterm
newborns for early detection of mineral deficiency. These
alterations, which precede the appearance of variations in
bone densitometry and radiological findings, are observed
during the third week of life.
The presence of serum levels of P lower than 3.6 mg/dl
in newborns on exclusive maternal breastfeeding is
suggestive of the deficiency of this mineral; this indicates
the need for monitoring these infants for the development of
MBD.14,17,50
Studies have shown that newborns being fed
exclusively their mother’s milk presented lower levels of
P than those receiving special formulas or mineral
Critical analysis of pathophysiological, diagnostic... - Catache M et alii
supplementation.23,51,52 Newborns fed with preterm
formulas can present associated MBD when their dosage of
Ca is normal or low and of P is lower than 5.7 mg/dl.50 Some
authors have already described these alterations with weeks
of life.12
Supplementation of vitamin D at 400 to 800 IU/day
allows for normal serum levels of 25 (OH)D. This would be
enough to maintain a normal status of vitamin D in the
organism. The dosages of 1,25 (OH)2D, in turn, were
elevated.34,45
The activity of alkaline phosphatase involves
predominantly the bone isoenzyme, and only 10% involves
the intestinal isoenzyme. It is a phosphotranspherase located
on the bleb of the matrix of osteoblasts, and it is responsible
for the transfer of phosphate residues into the blebs, where
the process of crystallization on the connection with Ca is
started. With the progress of crystallization, there is a
rupture of the blebs with an outflow of alkaline phosphatase
to the circulation. In the presence of mineral deficiency,
there is an increase in synthesis of alkaline phosphatase
without the mineralization being processed. This leads to an
increase in serum levels of alkaline phosphatase.
The study carried out by Lucas et al. 40 showed that
breast-fed newborns with lower concentrations of Ca and P
presented higher levels of alkaline phosphatase. The increase
in alkaline phosphatase occurred in the first three weeks of
life, with peak values between the fifth and sixth week.
These values remained higher for a longer period of time in
newborns with MBD alterations.12,15
Despite the fact that these studies hint at the importance
of the measurement of alkaline phosphatase for the diagnosis
of MBD (alkaline phosphatase values sixfold higher than
the upper limit indicate the need for investigation of MBD),14
this measurement does not, however, give a final diagnosis
of the disease; other laboratory findings are required for the
final diagnosis. Alkaline phosphatase values do not allow
for final diagnosis of MBD because they can change,
especially in preterm newborns due to the high bone turnover
that is characteristics to these patients and that can be
related to the higher growth rates.
In newborns with MBD, it is possible to observe
important hypophosphaturia for almost 100% of tubular
reabsorption of P, whereas the expected excretion of P
should be of approximately 2.0 to 2.3 mmol/l for the
preterm infant.12,53
The measurement of fractional excretion of P was (FeP)
was reported with a reduction of 20 to 3% during the first
week of life. FeP values reached minimum values after three
months of age.53 Moreover, others have observed an
increased fractional excretion of Ca in cases of P deficiency.
The values observed were higher than 4 mg/kg/day with
previous reports of values as high as 35 mg/kg/day.17,25
The literature indicates that beginning of mineral
supplementation lead to phosphaturia, which was previously
absent, and to reduction of calciuria to values of
approximately 6 mg/kg/day.46
Jornal de Pediatria - Vol. 77, Supl.1 , 2001 S59
Measurements of the Ca/Creatinine relation showed
values of 1 to 1.7 mmol/l in newborns being fed their
mother’s milk; moreover, there were results greater than 0.6
in the presence of MBD due to the increase in Ca excretion.24
Some authors24,44 have advocated the monitoring of
urinary Ca and P as a means to evaluate the presence of bone
mineralization. During an early stage, in which the delivery
of minerals is limited, the authors observed elevated calciuria
with practically no excretion of phosphorus (less than 1 mg/
kg/day). As the delivery of minerals improved, it was
possible to observe urinary excretion of phosphorus and
reduction in excretion of calcium, which indicated an increase
in bone mineral accretion. In this sense, urinary excretion of
Ca and P could be used as a parameter for assessment of
delivery of minerals and catch up mineralization in the
monitoring of MBD.44 Mancini et al.54 carried out a study
with the objective of identifying early markers of mineral
deficiency. The authors showed that values of calciuria at
timed 6-hour collections are higher in newborns who
developed MBD; these values were always together with
cases of hypophosphaturia. The authors collected samples
between the third and fifth weeks of life and underscored the
importance of monitoring calciuria for the diagnosis of
mineral deficiency.
Based on these studies, at the Maternity of the Pediatrics
Department at the Hospital de Clínicas of the FMUSP, the
recommendations are to monitor calciuria and phosphaturia
at timed 6-hour collections and between the third and fourth
weeks of life. These procedures are used for early detection
of phosphorus deficiency. The objective of these sample
collections are to adjust the delivery of minerals in cases of
elevated urinary excretion of Ca (greater than 4 mg/kg/day)
associated with reduced excretion of P (less than 1 mg/kg/
day) (Figure 1). It is understood that increased Ca excretion
together with hypophosphaturia is suggestive of inadequate
delivery of minerals to the patient. In this case, it is necessary
to reevaluate enteral feeding not only of Ca and P, but also
of energy and proteins.
Radiological alterations
Abnormalities in radiological examinations are a result
of an alteration in the process of bone remodeling. These
alterations are a consequence of inadequate intake of
minerals, which leads to a deficiency in mineralization and
occur in the presence of an increase in the formation of the
bone matrix.12
Radiological alterations have been reported in 64% of
newborns with birth weight less than 1,000 grams. These
alterations occur only in cases of important bone
mobilization, with a 30 to 40% reduction in bone mineral
content.14,17,55 In general, the modifications are detected
after 6 weeks of life and indicate an abnormal process of
remodeling with an increase of the bone matrix. This occurs
without concomitant mineral accretion and are usually
detected later in the infant’s life. 12
S60 Jornal de Pediatria - Vol. 77, Supl.1, 2001
Figure 1 - Algorithm used for the detection of mineral deficiency
Koo’s score is used for the description of these
radiological alterations55:
– Grade 1: presence of bone rarefaction;
– Grade 2: presence of bone rarefaction associated with
metaphysial alterations, shadows, and subperiosteal bone
formations;
– Grade 3: associated with the presence of spontaneous
fractures.
Bone densitometry alterations
The measurement of BMC using densitometry is being
developed in an attempt to monitor the speed of mineral
accretion in newborns. This measurement represents the
amount of hydroxypatite per centimeter of bone.51
The measurement of BMC using single photon
absorption (g/cm) was initially described as an efficient and
rapid method for verifying sequential changes in BMC on
the same location. Normally, in this technique, radium is
used for the measurement. However, the dosage of one
single location compromises precision and reproducibility
of the method; in which case whole body methods are
preferable.14 Studies have shown that single photon
absorption does not affect bone metabolism as a whole
since it uses one single location. Moreover, the measurement
is carried out on the cortical bone, thus not including the
metaphysial regions with elevated cellular activity.56
Bone width (BW) can also measured in order to establish
its relation with BMC. Studies have shown a reduction of
Critical analysis of pathophysiological, diagnostic... - Catache M et alii
BMC in preterm infants when compared to values of
intrauterine life. The ratio BMC:BW decreased due to the
increase in bone matrix formation during a stage of rapid
linear growth associated with mineralization limited by low
mineral content in diet.30,57
The use of two-photon bone densitometry (g/cm2 of the
scanned area) presents better precision and uses wholebody measurements. It is not affected by adjacent tissues
since it uses the spinal column and hips. This technique
differentiates bones from soft tissue and air, thus reducing
their interference in the measurements. Two-photon
densitometry requires an extended period of time to be
carried out. In this sense, the newborn has to be sedated.
This aspect of the technique is found to limit its use.56
Quantitative CT scans allow for the evaluation of mineral
density of the lumbar spine. These measurements are
tridimensional. CT scans are, however, affected by the
amounts of fat in which case the trabecular bone density is
underestimated. The greatest disadvantage of this method is
the high dosage of radiation.
The latest technology used is that dual energy X-ray
absorptiometry (DEXA) measurements of bone density,
which is based on densitometry by two-photon absorption
associated with X-rays. This allows for better precision and
reduced duration of exposure. This method still need to be
approved for pediatric patients. 56,58
The study carried out by Minton et al.59 showed that
preterm newborns require a period of approximately 12
weeks to achieve mineral accretion similar to that of 5
weeks of intrauterine life; the authors also observed that the
periods with greater mineral accretion correspond to those
of improvement in enteral feeding of minerals.
Other studies have shown that newborn infants who
were fed fortified or preterm formulas presented greater
BMC than those who were fed exclusively their mother’s
milk. The reported BMC of these infants is still lower than
that of intrauterine life.47,51
Recommendations
All preterm newborn infants with risk for developing
MBD should be investigated. Early detection of mineral
deficiency would allow for prevention of the more severe
forms of the disease.
At the Maternity of the HC-FMUSP, it is recommended
tat monitoring begin between the third and fourth week of
life with timed 6-hour urine sample collections. These
samples should be used to detect mineral excretion suggestive
of mineral deficiency, in other words, to detect significant
hypophosphaturia (less than 1 mg/kg/day) and increases in
urinary excretion of Ca (greater than 4 mg/kg/day).
In the presence of hypophosphaturia and hypercalciuria,
administration of Ca and P supplementation is recommended.
Critical analysis of pathophysiological, diagnostic... - Catache M et alii
Despite the controversy in recommendations for amounts
of Ca and P (with reported values ranging from 77 to 231
mg/kg/day of Ca and 55 to 148 mg/kg/day of P), the values
of 200 and 100 mg/kg/day for Ca and P, respectively, have
been sufficient for maintaining a mineralization curve similar
to that of intrauterine life.26 Some studies have shown that
150 and 77 mg/kg/day of Ca and P, respectively, for a 2:1
ratio are sufficient; these values allow for avoiding sideeffects of high dosages of these minerals, such as
nephrocalcinosis, extraskeletal calcifications, and excessive
accumulation of Ca and P in bone.14,23
In face of the requirements of minerals for the prevention
of MBD, it is recommended that newborn infants on
exclusive breastfeeding be given mineral supplementation
through fortified human milk or formulas of Ca and P. After
the beginning of Ca and P supplementation, a reduction on
urinary excretion of Ca should be expected. This reduction
should occur together with normalization of urinary levels
of P (greater than 1 mg/kg/day).50
The duration of mineral supplementation is still a
controversial matter. It is known that newborns who are fed
preterm formulas presented bone mineralization at six
months of age comparable to that of term newborn infants.
In turn, preterm newborn infants who are fed their mother’s
milk do not attain comparable bone mineralization until
they are 2 years old.38 Studies have also reported important
differences in BMC at 3 and 9 months of age in newborn
infants who were fed mineral supplementation.14
At our services, thus, we recommend mineral
supplementation until 40 weeks of corrected age, either
using fortified human milk or preterm formulas in case it is
not possible to feed maternal milk.32
The study carried out by Cooke et al.60 shows that there
are differences in rate of growth after hospital discharge
between infants who were fed conventional formulas and
preterm formulas. This suggests the importance of nutrition
after discharge for faster catch-up. Schaenler et al.37 observed
that BMC values for preterm newborn infants at one year of
age caught up to those of term infants when commercial
formulas were used; in turn, infants who were breastfed and
only fed supplementation during hospital stay caught up
only at two years of age.
Some authors recommend the use of mineral
supplementation up to 3 months of life, in an attempt to
include the stage of greater risk for fractures in these
newborn infants.14,44
In addition to mineral supplementation, it is also
recommended that vitamin D be administered at 400 IU/day
during the first year of life.
The increase in survival of very low birth weight
newborns has stimulated the understanding of MBD that
affects these newborns. Studies carried out on MBD indicate
that the compromising of bone mineralization can affect
older ages. This underscores the importance of the diagnosis
Jornal de Pediatria - Vol. 77, Supl.1 , 2001 S61
and intervention as early as possible, with the objective of
preventing long-term repercussions of MBD.
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Correspondence:
Dra. Monique Catache
Rua Gabriel dos Santos, 503 - apto. 31 - Santa Cecília
CEP 01231-011 - São Paulo, SP, Brazil