ELSEVIER Early Human Development 38 (1994) 131-138 Sodium in hand and pump expressed human breast milk Sandra LangaTb, Clive J. Lawrence*a, Richard L’E. Ormeb aDepartment of Mathematical Statistics and Operational Research, Lover Building. North Park Road, University of Exeter, Exeter, EX4 4QE, UK bDepartment of Child Health, Royal Devon and Exeter Hospital, Exeter, UK Received 7 February 1994; revision received 20 May 1994; accepted 24 May 1994 Abstract Sodium content was analysed in the expressed breast milk from 30 mothers. Measurements were taken up to 24 days postpartum, during which time the mothers expressed manually or by means of a pump. Statistical analysis has confirmed not only the known decline in sodium levels postpartum but has also shown a significantly higher sodium content in manually expressed milk compared to that from pump expressed milk. This finding suggests that the method of expression may alter the sodium concentration in human breast milk, thus poten- tially reducing the amount of sodium supplementation required by some preterm infants. Keywords: Sodium; Hand and pump expression of breast milk; Cross-over design 1. Introduction Sodium content in human breast milk is known to vary. It is relatively high in the first few days of lactation, 50-85 mmol/l, falling markedly to approximately 7-14 mmol/l by 7-14 days [ll]. Further rises have been observed to occur both at the cessation of lactation and when ovulation returns [9]. Several studies have also shown sodium concentration to be consistently higher in breast milk produced by mothers of preterm infants compared to mothers of term infants [3,7-9,11,16]. For * Corresponding author. 0378-3782/94/$07.00 0 1994 Elsevier Science Ireland Ltd. All rights reserved SSDI 0378-3782(94)01543-X
Transcript
ELSEVIER Early Human Development 38 (1994) 131-138
Sodium in hand and pump expressed human breast milk
Sandra LangaTb, Clive J. Lawrence*a, Richard L’E. Ormeb aDepartment of Mathematical Statistics and Operational Research, Lover Building. North Park Road,
University of Exeter, Exeter, EX4 4QE, UK bDepartment of Child Health, Royal Devon and Exeter Hospital, Exeter, UK
Received 7 February 1994; revision received 20 May 1994; accepted 24 May 1994
Abstract
Sodium content was analysed in the expressed breast milk from 30 mothers. Measurements were taken up to 24 days postpartum, during which time the mothers expressed manually or by means of a pump. Statistical analysis has confirmed not only the known decline in sodium levels postpartum but has also shown a significantly higher sodium content in manually expressed milk compared to that from pump expressed milk. This finding suggests that the method of expression may alter the sodium concentration in human breast milk, thus poten- tially reducing the amount of sodium supplementation required by some preterm infants.
Keywords: Sodium; Hand and pump expression of breast milk; Cross-over design
1. Introduction
Sodium content in human breast milk is known to vary. It is relatively high in the first few days of lactation, 50-85 mmol/l, falling markedly to approximately 7-14 mmol/l by 7-14 days [ll]. Further rises have been observed to occur both at the cessation of lactation and when ovulation returns [9]. Several studies have also shown sodium concentration to be consistently higher in breast milk produced by mothers of preterm infants compared to mothers of term infants [3,7-9,11,16]. For
* Corresponding author.
0378-3782/94/$07.00 0 1994 Elsevier Science Ireland Ltd. All rights reserved SSDI 0378-3782(94)01543-X
132 S. Lung et al. /Early Hum. Dev. 38 (1994) 131-138
this reason it has been suggested that such milk is better adapted to meet the increas- ed requirements of preterm infants [7,8], whilst obviating the need for supplemen- tation.
This study was undertaken to determine whether manual (hand) or mechanical (pump) expression in any way affects the composition of breast milk. In preliminary work, it had been observed that the sodium content appeared to vary according to the method of expression, with greater concentrations of sodium occurring in manually expressed milk.
2. Materials and methods
The subjects of this study were 30 mothers of infants requiring admission to the Exeter Neonatal Unit. The mothers were asked to express their milk for up to 24 days postpartum, using either manual or mechanical expression for 6 days before changing, where appropriate, to the other method. The intention of the study was to try to randomly allocate as many mothers as possible to commence with one of the two methods of expression. In six of the 30 cases, this was not possible because they were commenced on mechanical expression before eligibility for the study had been assessed. It was felt that these mothers, who were willing to change their meth- od of expression at 6 days, should be included in the analysis. Milk samples were obtained on days 6, 12, 18 and 24. Samples consisted of 2-ml aliquots of fore, mid and hind milk obtained by expressing for 4 min into one container, then 4 min into a second container and then expressing the remaining milk into a third container. The samples were taken from each container. Samples were obtained at each expres- sion over a 24-h period at given times, thus up to six sets of samples were obtained from the individual mothers each sample day. All samples were frozen at - 15°C until analysis of the material was possible. The dry chemistry slide method, carried out on a Kodak Ektachem 700XR, was used to determine measurements of sodium con- tent. Analysis was conducted on whole milk with no interference with the sodium concentration by turbid or lipaemic samples.
As far as the first two periods of 6 days were concerned, there were two cases of hand (in days l-6) followed by hand (in days 7-12), seven of pump-to-pump, 14 cases of pump-to-hand, four cases of hand-to-pump, and three cases with insufficient information from the two 6-day periods to be included in the analysis.
With one exception, the mothers all delivered preterm infants (Table 1). The mean (and standard deviation) gestational ages in the hand-to-hand, pump-to-pump, pump-to-hand, and hand-to-pump groups were 32.00 (2.0), 29.86 (l.O), 31.21 (1.1) and 32.5 (0.9), respectively, with no statistically significant differences indicated.
The data analysed in this paper are presented in Table 1, which characterises the layout of the study as a 2 x 2 cross-over design [lo]. Such data can be analysed using a general linear model to answer questions such as: Are there any cross-over effects from the first to the second period of 6 days? Do the mean sodium levels significantly differ for the two periods? And more importantly, is there a significant difference in sodium concentration between hand and pump expression?
Outline details of the model and statistical analysis used are given in the Appendix.
S. Lung et al. /Early Hum. Dev. 38 (1994) 131-138 133
Table I Mean sodium concentrations (mmol/l) for each subject, in each group, at 6 and 12 days post-partum (with gestational age given in parentheses)
Group n Subject no. Mean sodium concentrations (mmol/l) (gestational age)
at 6 days at 12 days
Hand >> hand 2 1 (34) 30.83 16.00 2 (30) 13.00 8.67
134 S. Lang et al. /Early Hum. Dev. 38 (1994) 131-138
Ethical approval for this study was given by the Exeter Health Authority and informed consent obtained from each mother prior to enrolment into the study.
3. Results
The sample means and standard deviations of sodium concentrations for the four groups at 6 and 12 days postpartum are given in Table 2, and subject profiles for each group are shown in Fig. 1. Despite the relatively small sample numbers in the groups, there is some visual evidence that hand-expression produces slightly higher sodium concentrations. Any such effect, if it is present at all, will to some extent be masked by the known decline in sodium concentration that has been shown to take place over the period 0Y16 days postpartum [l 11. Also given in Table 2 are indica- tions of the mean milk volumes; in some instances these were not known accurately, and were very variable. Despite these reservations about the milk volume data, the majority of mothers produced less milk with manual expression than with mechani- cal expression. A small number produced similar quantities, but more importantly all the mothers who manually expressed were able to produce sufficient quantities for their infants’ requirements.
In order to determine whether or not there are any differences in sodium concen- trations between hand and pump expression, a statistical analysis of the data from
Group 1: Hand n Hand.
_. 0 5 10 15
Days
Gn~up 3: Pump >> Hand
Group2Pump~Pump
0 5 10 15
Days
Group 4: Hand )P Pump
-. 0 5 10 15
Days
0 5 10 15
Days
Fig. 1. Subject profiles (day 6 >> day 12) for each of the groups.
S. Lang et al. /Early Hum. Dev. 38 (1994) 131-138 135
Table 3 Analysis of variance
Source of variability Degrees of freedom
Sum of squares
Mean sum of squares
F-value
Between subjects Within subjects:
Periods (Days l-6,7-12) Treatments (hand, pump) Carry-over Treatments x carry-over Residual Total
the four groups was carried out. The model used to associate the sodium measure- ment with the various factors is given in the Appendix; the experiment constitutes a special case of a class of cross-over designs first introduced by Balaam [2]. The model includes components to represent possible effects of the different time periods (days l-6 and days 7-12); different treatments (hand and pump expression); cross- over effects; and any interaction effects of these main factors. Using the basic data (Table l), which comprises sample mean values based on up to six measurements, and the model, the analysis of variance (Table 3) was calculated; each sum of squares in the Table is the measure of variability associated with the corresponding source factor adjusted for the effects of all the preceding factors in the Table. The results of this analysis indicate that the hypothesis of no cross-over effects should be accepted (F-value = 2.14, P = 0.16), and no significant difference in the effects of the two periods was found (F-value = 2.05, P = 0.17). The important finding was that significantly higher sodium concentrations were found in the hand-expressed sam- ples than in those that were pump-expressed (F-value = 16.76, P < 0.001). Despite the small numbers in two of the groups, the statistical analysis is able to properly assess the significance of the various factors.
4. Discussion
In many neonatal units, sick and preterm infants receive their mothers’ expressed breast milk. This has clear immunological, developmental and nutritional benefits, particularly for preterm infants who require the higher levels of protein and certain other micro-nutrients that preterm breast milk is known to contain [1,4,13].
It is evident from this study that the method of expression used by a mother may subtly affect breast milk composition. The differences in sodium content observed in this study between milk expressed manually and mechanically do not appear to have been previously reported. There are clearly fundamental differences between the two principal methods of extracting human milk. The main feature of mechanical expression is intermittent suction applied directly to the breast via a glass or plastic
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funnel. Manual expression, a more tactile method, employs rhythmic compression of the lactiferous sinuses to obtain the milk. In this country, mechanical expression is the most widely used method. It is considered to be both quicker and easier to use, more efficient in emptying the breasts, and in extracting larger volumes of milk than manual expression [5,6,17].
The inverse relationship between volume and sodium levels which has been observed in this study, is in agreement with the findings of other studies, in particular that the greater the volume of milk produced, the lower the sodium concentrations when measured in mmol/l. Koo and Gupta [ 1 l] showed a gradual reduction in sodi- um concentration ‘as the supply of mature breast milk became established’. Increas- ed sodium levels associated with reduced milk supply have been reported to occur at the time of ovulation [9]; with the cessation of lactation, when involution occurs; and to a somewhat lesser extent if milk secretion is inhibited by irregular expression of the breasts [9]. In a further study 1151 examining the case of a lCday-old infant suffering hypematraemia assumed to have been caused by inadequate lactation, it was noted that the mother’s very low milk supply on expression contained a very high concentration of sodium (74 mmol/l). This fell markedly as her supply in- creased.
Whilst hypematraemia is not a condition normally associated with infants fed suf- ficient quantities of expressed breast milk, hyponatraemia is a more common condi- tion in preterm infants of less than 33 weeks, due to renal immaturity and increased urinary sodium losses [14,16]. In one study [8], hyponatraemia occurred in 15% of babies receiving preterm breast milk, compared to 20% and 50% of babies receiving a preterm formula and mature breast milk, respectively. To reduce the risk of hyponatraemia in preterm infants both the ESPAGN guidelines and the World Health Organisation [3,16] recommend the use of the mother’s own milk because of its higher sodium content. The regular monitoring of plasma sodium levels of these infants ensures that supplements, usually in the form of sodium chloride, are given when necessary.
Why should the sodium concentration of hand-expressed milk differ at all from pump-expressed milk ? Possible physical and physiological explanations of the dif- ferent levels might follow intercellular leakage and/or cellular damage caused by over-vigorous manual expression - although this did not appear to be so in the cases of this study. Mastitis or acute inflammation of the breast tissue is reported to cause high sodium concentrations in the breast milk. This is thought to result from damage to the mammary epithelium causing increased permeability of the gland [9]. Potential differences in composition may also arise from hitherto un- known variations in hormonal activity causing different permeability states in the mammary epithelium. In dairy animals, for example, it has been demonstrated [12] that injections of oxytocin cause changes in milk composition, with decreased lactose and potassium and increased sodium and chloride levels. Prolactin administered in late lactation has been observed to reduce both sodium and potassium concentra- tions to mid-lactation levels. Animal experiments also indicate that milk composi- tion can be altered by the administration of insulin, thyroxin, pilocarpine and adrenaline. With the exception of pilocarpine, the other substances are all present
S. Lang et al./Early Hum. Dev. 38 (1994) 131-138 137
in both the human body and human breast milk [ 161. If it is possible to increase the sodium levels in some mothers by using manual expression, what other constituents may be affected? Is it possible, by changing the nature of the stimulation we apply to the breast, to affect the levels of other electrolytes, nutrients and hormones? Is expression with our present mechanical pumps producing milk which is of the opti- mum quality for the nourishment and growth of infants who receive expressed breast milk?
The bioavailability of micro-nutrients in breast milk is generally considered to be higher [16] than in formula milk. For the preterm and sick infant who might other- wise require supplementation, this feature of breast milk is important. Therefore, if a method of expression can itself influence sodium concentration then training of the mother to use that method could reduce even further the need for, and extent of, supplementation. Whilst it is desirable to have a reserve of the mother’s milk in the refrigerator, it is far from certain that quantity equates with quality. It may be beneficial for a mother to express her milk in a way that produces a quantity which is closer to the requirements of the infant, and which may result in a more energy- rich milk of higher quality.
Acknowledgements
We wish to thank Mr S. Bottomley, senior Biochemist at Wonford Biochemistry Laboratory, Wonford Hospital, Exeter for undertaking the analysis required in this study. Also the staff and parents of the Exeter Neonatal unit for their assistance and co-operation. The mechanical pumps used in this study were loaned by Egnell- Ahmeda. This study was supported by a grant from the Northcott Devon Medical Foundation.
Appendix
Details of the analysis of cross-over designs, in which repeated measurements are made on the same experimental unit (in this case, subjects) can be found in several textbooks (see, for example, Jones and Kenward [lo]).
The following gives an outline of the model and analysis used in this paper. Assuming there are I groups of subjects each receiving the t treatments in a dif-
ferent order (specified in our case by the group definitions given in Table 1). The sodium concentrations Y, the observed response, are assumed to be related to the possible sources of influence in the following linear model structure:
ygik = m + Pj + &,I + ck,jI] + sgk + egik
where
m = general mean; pi = effect of thejth period,j = l,...J; tk,,l = effect of the treat- ment administered in period j of group g, g = l,...,G; CC~_~] = effect of the carry- over of the treatment administered in period j-l of group g; sgk = random effect of
138 S. Lang et al. /Early Hum. Dev. 38 (1994) 131-138
kth subject in group i, k = l,..n,; etik = random error effect for kth subject in period j in group g.
Using appropriate statistics, it is possible to test the following hypotheses:
Hi: that the carry-over effects c~$~] are equal, and, assuming this hypothesis is accepted as reasonable, it is then possible to examine:
Hz: that the period effects pi are equal (i.e. no differences, in this case, between 6 days and 12 days).
H,: that the treatment effects are equal (i.e. no differences between hand and pump expression of breastmilk).
In the analysis presented in this paper, G = 4 (Hand X= Hand, Pump >> Pump, Pump >> Hand, and Hand >> Pump)‘with nl = 2, n2 = 7, n3 = 14 and n4 = 4, and J = 2 (l-6 days, 7-12 days).
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