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Studies In Human Lactation Milk Volumes In Lactating Women During The Onset Of Lactation And Full Lactation


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Studies In Human Lactation Milk Volumes In Lactating Women During The Onset Of Lactation And Full Lactation

  1. 1. Studies in human lactation: milk volumes in lactating women during the onset of lactation and full lactation3 Margaret C Neville, PhD; Ronald Keller, BA; Joy Seacat, RN, MA; Valerie Lutes; Marianne Neifert, MD; Clare Casey, PhD; Jonathan Allen, PhD; andPhilip Archer, PhD ABSTRACT After validation of test-weighing procedures milk volumes produced by 13 multiparous Caucasian women were followed longitudinally through the first year of lactation. All practiced exclusive breast-feeding for at least 5 mo. Milk transfer to the infant was low on days 1 and 2 and increased rapidly to 498 ± 129 g/d (1± SD) on day 5 and then more slowly to 753 ± 89 g/d during months 3-5. There was a characteristic milk volume for each mother- infant pair that was significantly related neither to milk yield on days 4-6 nor to birth weight. It was, however, strongly related to infant weight at 1 mo, suggesting that infant and/or mater- Downloaded from by on August 22, 2009 nal factors coming into play during the first month of life are strong determinants of subse- quent milk transfer to the infant. Am J C/in Nutr 1988;48: 1375-86. KEY WORDS Lactation, milk intake, breast-feeding, human milk yield, test weighing Introduction Methods Subjects Time is an inescapable variable in all studies of lacta- All subjects in the longitudinal study were multiparous tion. A number ofcross-sectional studies ofmilk output (mean parity 2.9 ± 1 .2, SD), nonsmoking Caucasian women broken down by duration of lactation are available (1, who planned to continue breast-feeding for at least 1 y. None 2). Although these studies provide a series ofviews of the used hormonal contraception during the study (barrier meth- lactation performance ofa sample from a population, in ods, abstinence, or sterilization were used). Mean age at the the absence of longitudinal data on individuals, the cx- birth ofthe study infant was 3 1 .9 ± 4.4 (SD) y (range 25-39 y). tent to which such data predict the temporal course of Median family income was > $35 000/y. All mothers were high lactation in a particular mother-infant pair is not clear. A school graduates and all but two had college degrees. Measure- ment of lactational performance began within 12 h of giving better understanding of the temporal course of lactation birth in all but one subject. The infant ofthis subject, who had may help clarify the factors that govern milk transfer to gestational diabetes, was hospitalized for 2 wk after birth for the infant. Ofparticular interest in this regard is the rela- the respiratory effects of meconium aspiration and evaluation tion between milk transfer during the initiation of lacta- of a possible ventricular septal defect. During this period the tion and later lactational performance. mother pumped her breasts to obtain milk for the infant, who These considerations led us to perform a longitudinal was breast-fed only during maternal visits. Data from this sub- study in 1 3 highly motivated lactating women, focusing ject were analyzed separately and combined with the other data particularly on the first 14 d postpartum. Because the only where stated. A second subject was involved in an auto- amount of data required for each subject in a longitudi- mobile accident 2.5 mo postpartum resulting in hospitalization nal study restricts the subject number, we chose as homo- for 1 wk. She continued to pump her breasts during this time geneous a sample as possible thus reducing the number of maternal variables that might have confounded inter- I From the Departments of Physiology, Pediatrics, and Preventive pretation ofthe results. The study was therefore confined Medicine and Biometrics, University ofColorado School of Medicine, to multiparous, nonsmoking Caucasian women of mid- Denver, CO. 2 Supported by contract 0 1 HD 22801 and grant HD-19547 from dle-to-upper socioeconomic status living in Denver, CO. the National Institutes of Health and grant RR-69 from the General In this paper we report volume data from these subjects. Clinical Research Centers Program of the Division of Research Re- Some data on milk composition and nutritional status sources. ofthe mothers and infants have appeared in brief reports 3 Address reprint requests to M C Neville, Department of Physiol- elsewhere (3, 4). Before reporting the data from these ogy, Box C240, UCHSC, Denver, CO 80262. studies we examine the validity oftest weighing as a mea- Received June 22, 1987. sure ofbreast-milk production. Accepted for publication January 5, 1988. Am J C/in Nutr l988;48: 1375-86. Printed in USA. © 1988 American Society for Clinical Nutrition 1375
  2. 2. 1376 NEVILLE ET AL status were recorded at monthly intervals. All subjects (for hos- pitalized infant, parents) gave informed consent for these pro- cedures, which had been previously approved by the Human Subjects Committee ofthe University ofColorado Health Sci- ences Center. Measurement ofmilk volume production All milk amounts produced during any given measurement period were recorded. Milk received by the infant was mea- Q. sured by test weighing as outlined below. Pumped volumes C., were measured when milk was expressed. Nine subjects were C test weighed at all feeds from birth to 14 d. The other three w were test weighed for at least 9 ofthe first 14 d. All 12 were test weighed weekly from weeks 3 through 8 and then monthly un- 0 til the end ofthe study. One subject undertook a modified, lon- gitudinal protocol, making test weighings on days 6 and 14 and a’ at monthly intervals thereafter. Five-milliliter samples were 4) taken from both breasts at midfeed at regular intervals through- out this study (6); compositional findings will be reported else- U, where. 4) I- Test weighing. Mothers or research nurses were instructed Downloaded from by on August 22, 2009 in the use of the electronic balance for test weighing to obtain a measure of milk transfer to the infant. They estimated any spit-up or spilled milk and those who leaked substantial quanti- ties of milk wore breast pads that were weighed at each feed. Infants were weighed before and after every feed for a given time period, usually 24 h plus two feeds into the next 24-h pe- 20 riod but up to every feed during the first 14 d oflactation. From Bottle Weight Difference per Feed (9) birth to 4 mo, infants were weighed on a Sartorius electronic balance (model 3862MP6, Westbury, NY; accurate to 1 g) set FIG 1. Bottle-fed infants: comparison between milk intake obtained to integrate the values of2O successive weighings. by test weighing of infants with milk intake obtained by weighing the Test weighing ofolder infants. Because movement artifacts bottle before and after the feed. A. Three infants weighing 1.7-3.8 kg were found to be highly significant with older infants, an dcc- had 77 test weighings. B. Fifteen infants were test weighed once each. tronic balance with a swing was devised for test weighing: an The line represents the best-fitting linear regression. For A, the y inter- infant swing was suspended from a Mettler PC24 electronic cept was 1. 1 ± 4.2, the slope, 0.97 ± 0.03, and r, 0.93; for B the y inter- balance (Mettler Instrument Corp, Hightstown, NJ; range 24 cept was -0.01 ± 5.1, the slope, 0.99 ± 0.048, and r, 0.96. kg with a readability of 1 g, integration time 1 s, stability detec- tor update speed of 0.2 s, ME 41648 data output module). Weights and a stability signal were transmitted to a computer and the infant returned to breast-feeding as soon as the mother with printer. Even with this equipment repetitive weights on a returned from the hospital. Three-month data from this moving infant could vary by 300 g. We found it possible to mother were not included in the analysis. Where mastitis was overcome the variation in weights by performing a variation of concurrent with test weighing, as shown by high levels of so- the statistical techniques described by Ansombe (7) for rejec- dium and chloride in the milk (5), the data were excluded from tion of outliers. We called this method iterative trimming (IT, the analysis. see Appendix A). To use IT, 50-100 repetitive weights were Six study infants were male (mean birth weight 3529 ± 337 obtained, the mean and SD ofthe weights were calculated, and [SDI g; mean gestational age 40.0 ± 1 . 1 wk [SD]). Seven study those weights lying > 1 SD from the mean were dropped. The infants were female (mean birth weight 3054 ± 3 17 g [SD]; mean and SD were then recalculated; the procedure was re- mean gestational age, 39. 1 ± 1 .9 wk [SD]). Solids were intro- peated until the SD was < 3 g. duced between 4 and 9 mo (mean 7.0 ± 1.5 mo, SD) with for- Validation oftest weighing. The limitations of test-weighing mula being used occasionally ( 240 mL/wk) after 4 mo in were examined in a number of ways: 1) We evaluated IT for three subjects. Most other infants went directly to cow milk at weighing active infants. 2) We compared milk intake volumes 12 mo. One infant was weaned at 8 mo; all others were in hospitalized, growing, premature infants receiving either weaned at 12 mo. The study continued through weaning bottles or gavage feedings. Research nurses weighed both the for nine subjects; four others discontinued the study before bottle and the infant before and after feeding. 3) We compared weaning. the results of 48-h test weighing with hourly milk production Mothers participating in the validation of the test-weighing values obtained by using a special pumping technique with syn- study had characteristics similar to those in the longitudinal thetic oxytocin. 4) We examined the day-to-day variation in study. Infants used in validation oftest weighing were all hospi- measured volumes. The detailed techniques used for analysis talized in the pediatric intensive care unit of University Hospi- are outlined below. tal, mostly for low birth weight. All were growing well at the Comparison oftest weighing with hourly pumped volumes. time of measurement. Five Caucasian women who had at least some college educa- Data on infant and maternal morbidity; estimated duration tion and who were fully breast-feeding an infant aged 1-6 mo of feeds; intake of formula, water, and beikost; and menstrual test weighed their infants before and after each feed for a 48-h
  3. 3. HUMAN MILK VOLUME 1377 a) 70- >- 0.-’. 0-C, -C 60- C 0 a) E 50- 30 35 40 a, Test We I g hi Yield E 40- 0 > 30 4, Q. E 20- Downloaded from by on August 22, 2009 I0- I I I C- 0 3 6 9 Time (hours) FIG 2. Hourly volumes ofpumped milk in five lactating women. The points represent the mean across individuals with twice the SEM indicated by the distance between bars. The dotted line represents average milk yield obtained by test weighing and measurement ofall pumped and expressed volumes for 48 h. Inset: Mean and SEM of pumped volumes for hours 2-8 for each woman as a function ofmean hourly milk production obtained by 48-h test weighing. The line represents equality between the two measures. period. They also recorded the volume of all milk pumped feeds for which test weights were collected in the longitudinal from the breasts during this period. Within the succeeding 2 study; this was due to problems with the balance, mother falling wk these women were admitted to the Clinical Research Center asleep before the second weighing, etc. When this occurred, of the University Hospital where their breasts were pumped method 3 was used for the intervals before and after the missed every hour for 8 h according to the following schedule: An dcc- feeds if the total time of test weighing was > 24 h; if the total tric pump (Medela, Inc, Crystal Lake, IL) fitted with dual heads time was < 24 h the data for that day were discarded. Twenty- was used to pump both breasts simultaneously for a minimum five (7%) of373 test-weigh days were discarded for these reasons. of 10 mm or until milk ceased to flow freely. One drop (0.05 Expressed and leaked milk was weighed and added to the total. mL) ofsynthetic oxytocin (Syntocinon#{174}, Sandoz Pharmaceuti- Corrections to test-weighing data. Two types of correction cal, East Hanover, NJ) was administered intranasally and the were applied to data obtained by test weighing, a correction pumping was continued for another 5 mm. Milk volumes were for the insensible weight loss by the infant during the feeds, obtained by weighing the collection vials before and after amounting to 2 g. kg h’ (9, 10), and a correction . for specific pumping using a Sartorius electronic balance. gravity (1.032) when the data were to be expressed in volume rather than weight units. Because these two corrections by and Data analysis large offset each other, infant data are not corrected and are Determination ofdaily milk intake. To overcome the problem stated in grams/day. This facilitates comparison with data from of deciding what feeds to include in a day (8), we analyzed the other studies (8). data three ways: we calculated 1) the total milk produced during Standard statistical methods from the program ABSTAT the first 24 h of the total interval, 2) the milk produced during (Anderson-Bell, Canon City, CO) were used to obtain means the last 24 h of the total interval, and 3) a value derived for an and variances across subjects. Multiple-linear-regression tech- interval from the beginning ofthe first feed to the beginning of niques and Student’s I test were used in the analysis of individ- ual temporal trends and other relations among variables. the last feed. For this derived value the summed weight ofall but the first feed was divided by the number of hours in the total Results interval and multiplied by 24 to obtain the daily yield. The daily yields obtained by the three methods were compared and the Validation oftest weighing median value, usually the value obtained by method 3, was re- Test weighing ofbottle-fed infants. Test weights were ported. Unusable data were obtained for 54 of the total 3484 compared with the weight of milk taken from the bottle
  4. 4. 1378 NEVILLE ET AL a 0 E C 0 C., .a 0 1, E 0 > Downloaded from by on August 22, 2009 Time (Days Post Partum) FIG 3. Rates ofmilk production in exclusively breast-feeding women. A. The corrected daily milk yield is plotted for days 1 through 6; thereafter lumped values derived from regression analysis as described in the text are plotted for several segments of the postpartum interval. B. Means (small circles), SD, and range of values from A. Large circles represent a subject with gestational diabetes. as shown in Figure 1 . For the three infants measured at the mean hourly pumped milk volumes obtained from every feed for a period of 3-5 d each (A), test weighing five women over an 8-h period in a hospital. For compar- gave significantly different values (1 ± SEM, 50.4 ± 1.9 ison, the horizontal dashed line gives the mean 48-h test g/feed) from bottle weighing (52.9 ± 2 g/feed; one-tailed weighing volume obtained at home expressed as an paired t test, p < 0.00 1). When the data were corrected hourly rate of milk production. Volumes from the first for insensible weight loss using an average feed length of two pumpings were higher than the mean volumes ob- 30 mm, the mean value from test weighing was 52.7 tamed by test weighing because of the presence of resid- ± 1 .9 g/feed, not significantly different from the value ual milk; thereafter, the hourly pumped volumes did not obtained from weighing the bottle (one-tailed paired t differ significantly from the test-weighing mean. The vol- test, p > 0.2). Figure lB shows milk intake per feed plot- umes obtained from hours 3 to 8 were averaged for each ted as in Figure lA for 15 different infants measured at a subject and compared to the mean volume obtained by single feed each. For these data the test weight was less 48-h test weighing by that subject in her home, again cx- than the bottle weight by a mean of 2.5 ± 1 .3 g (SEM). pressed as grams/hour(Inset, Fig 2). The mean difference This value is not significantly different from the pre- between the two techniques across individuals was 0.7 g/ dicted insensible weight loss of 2.8 g/feed. The random h (- 3%), suggesting that test weighing in the home by error ofthe daily difference between test weighing of the motivated, educated subjects carries an acceptably low infant and weighing of the bottle before and after feeds error and does not interfere with lactation performance. amounted to ±3% of the weight of formula taken for both groups, suggesting that the process oftest weighing Milk yield and transftr in thefirst week of lactation itselfis associated with an acceptably small error. The upperportion (A) of Figure 3 presents individual Comparison oftest weighing with hourlypumped milk patterns of milk yield during the period of exclusive volumes. The question of whether test weighing in the breast-feeding in our 12 normal subjects. After the first 6 home can produce representative data was addressed by d postpartum the data are grouped to reduce the variance comparing results from 48-h test weighing with milk ofthe individual yields (see below). The lower portion of yields obtained by hourly pumping with synthetic oxyto- the figure (B) shows the mean, SD, and range of values cm used to obtain a complete letdown. Figure 2 shows for the same periods. These average data indicate a grad-
  5. 5. HUMAN MILK VOLUME 1379 1400 1200 U, E a 4, E 0 > 400 a Downloaded from by on August 22, 2009 200 0 I 2 3 4 5 6 7 8 9 10 4 Days Postpartum FIG 4. Milk production during the onset of lactation. Milk volume production from the present study (open circles) is compared with infant intakes from the present study (closed circles) and from references 1 1 (open squares, n = 9) and 12 (open triangles, n = 9). Data from references 13 and 14, obtained by manual expression ofall milk for the first 10 d oflactation, is depicted by closed triangles (n = 7). Distance between bars represents 2 SEM. ual increase in milk yield for the first 36 h followed by an postpartum. Data in two of these studies (1 1 , 1 2) repre- abrupt increase during hours 49-96 (mean slope 200 sent infant intakes obtained by test weighing the infant mL/d) after which yield levels off. as in the present study. Data from the third study (13, During the first 6 d only four subjects actually followed 14) were obtained by breast pump or hand expression. the mean pattern shown in Figure 3B. Three subjects showed a more gradual increase over the first 1 3-30 d Milk yield and transfer duringfulllactation and four produced large volumes of milk on day 4 fol- The mean daily intakes are tabulated along with the lowed by a decreased yield as lactation progressed. The yields in Table 1 Milk . yield increased slowly from a difference between total milk yield and milk transferred mean of ‘.-550 mL/d on day 5 to “.-750 mL/d at 1 mo to to the infant during the onset of lactation is shown in “.-850 mL/d at 5 mo. The differences between the mean Figure 4. The difference is greatest on day 4 140 g), (fl’.- values at 2 mo (yield, 744 ± 3 1 mL/d [SE]; transfer 694 primarily reflecting the last group ofmothers who tended ± 27 g/d [SE]) and 5 mo (yield, 849 ± 33 mL/d [SE]; to produce more milk than their infants took in and used transfer 838 ± 37 g/d [SE]) were significant (p < 0.02) a breast pump to reduce engorgement. By 10 d the yield indicating that both variables continue to increase in all but one of these subjects had leveled off to within through the period ofexclusive breast-feeding. The range 30 mL of infant intake. One subject (shown by the + in of individual daily yields was large, 800 mL/d during Fig 3A) continued to overproduce, leaking considerable week 2 decreasing to 400 mL/d by 2 mo. quantities of milk into breast cups, until weaning was Figure 5 shows a comparison between our data and begun after 5 mo. infant intake volumes from literature studies that fit the The data from one subject, the hospitalization of following criteria: data were obtained by test weighing whose infant forced her to remove most milk by breast the infant, exclusive breast-feeding was validated, three pump during the first 14 d postpartum, are shown as or more subjects were studied, and milk transfer was bro- large circles in Figure 3B. Despite use ofthe breast pump, ken down by monthly interval. The mean values from her volumes fell within the range of the breast-feeding the 1 5 qualifying studies are similar to those observed in subjects. our study, suggesting that milk transfer to the infant in Figure 4 also shows the data from the three other de- these Denver mother-infant pairs is representative of tailed studies (1 1-14) of milk volume in the first 2 wk milk transfer in most populations throughout the world.
  6. 6. 1380 NEVILLE ET AL TABLE 1 Milk yields and intakes Days Numberof Number of postpartum Corrected milk yield subjects Infant milk intake subjects n (range) mL/d g/d 1 56± 65(-ll-155)t 7 44± 7l(-31-149)t 6 2 185± 103(12-379) 10 182± 86(44-355) 9 3 393±158(226-745) 11 371±153(209-688) 10 4 580±250(306-1010) 11 451 ± 176(164-694) 10 5 563± 145(354-929) 12 498± 129(323-736) 11 6 558±156(360-888) 10 508±167(315-861) 9 7 610± 187(421-1008) 8 573± 167(406-842) 7 8 657 ± 236 (442-1223) 9 581 ± 159 (410-923) 8 9 606 ± 105 (485-872) 10 580 ± 76 (470-720) 9 10 682±254(395-1283) 10 589± 132(366-866) 9 11 654±168(410-1001) 8 615±168(398-934) 8 14 668 ± 163 (423-1024) 10 653 ± 154(416-922) 9 21 (19-23) 703 ± 108 (601-935) 10 65 1 ± 84 (554-786) 10 28(24-32) 794± 178(550-1134) 13 770± 179(495-1144) 13 35(33-39) 726± 135(506-1054) 12 668± 117(465-930) 12 Downloaded from by on August 22, 2009 42(40-46) 743 ± 113(601-926) 12 71 1 ± 1 1 1 (554-896) 12 49(47-53) 749± 124(586-986) 10 709± 115(559-922) 10 56(54-63) 744±111(560-914) 13 694± 98(556-859) 12 90(84-98) 772± 127(616-943) 12 734± 114(613-942) 10 120(117-126) 754± 103(603-889) 13 711 ± 100(570-847) 12 150(145-162) 849± 119(736-1170) 13 838± 134(688-1173) 12 180(172-185) 790±134(523-1027) 13 766±121(508-936) 13 l801 848± 99(719-1027) 9 820± 79(710-936) 9 210 (208-221) 738 ± 161 (486-963) 12 721 ± 154 (486-963) 12 2lOf 872± 67(796-945) 6 848± 63(796-945) 6 240(230-256) 645± 195(288-1006) 10 622±210(288-1002) 9 240t 804± 135(720-1002) 4 818± 158(720-1002) 3 270(275-287) 639 ± 221 (242-889) 12 618 ± 220(223-871) 12 270j 853± 46(787-889) 5 817± 91(681-871) 4 300(295-316) 578 ± 248 (143-896) 11 551 ± 234(129-894) 11 300: 838 - - 1 720 - - 1 330(317-344) 578±233(132-861) 9 554±240(120-860) 8 360(346-375) 427±250(73-772) 9 403±250(65-770) 8 a 1± SD. Range given in parentheses. t Negative values are due to insensible weight loss. t Includes only subjects giving < 100 kcal/d beikost. Longitudinal correlations in milk output yield, intake, slope, and day-to-day variation (Table 2). Data for months 6-9 are not included in Table 2 because There is no simple mathematical relation to describe the longitudinal course of milk-volume production. We only five subjects (four shown in Fig 3A and one in Fig 3B) continued exclusive breast-feeding during this inter- divided the postpartum interval into six segments corn- prising days 2-6, days 7-14, days 15-28, days 29-56, val (Table 1). Because the slopes and variations were sim- days 60- 150, and days 154-240. Days 2-6 represent the ilar for yields and intakes, only the data for intake are onset of lactation. We found the mean volume for each shown. It is the mean yields from this analysis that are individual from days 4 through 6 to be highly correlated plotted for each individual in Figure 3B. (r > 0.9) with daily volumes from days 2 to 6. This mean Milk volume intake increased rapidly from days 7 to volume was used as representative ofthe pattern of lacto- 14 (slope 12 mL/d) and more slowly from days 14 to 28 genesis in each individual. The rate of change of yield (mean slope 7 mL/d). The mean rate ofincrease thereaf- and transfer over the remaining time segments as well as ter was 1 mL/d from the individual slopes corresponding the mean milk yields and transfers were determined by to the increase in mean milk intake described above. In linear regression for each individual. The SD about the the five subjects continuing full breast-feeding from regression line (S.) was used to assess the day-to-day months 5 to 8, there was a slight, statistically insignificant variability within subjects. These quantities were then decrease in milk output over this final interval. averaged across subjects to give the values for the mean The mean day-to-day variability within subjects was
  7. 7. HUMAN MILK VOLUME 1381 ll00 1000 0 .C 900 0 2) S 0. 800 C, E 700 a) C 600 C H 500 Downloaded from by on August 22, 2009 400 I -I- 1 2 3 4 5 6 7 8 Months Postpartum FIG 5. Milk intakes during established lactation. The lines show the smoothed mean from this study and ±1 SD. Points are data from the literature obtained by test-weighing offully breast-fed infants (see Appendix B). C, Pao et al (15); N, Hofvander et al (16); 0, Butte and Calloway (17); #{149}, Butte et ai (18); Steenberger , et al (19); A, Dewey et al (20); i, Dewey et al (21); D, Salmenpera et al (22); 9, Walgren (23); *, Janus et al (24, 25); U, Whitehead and Paul (26); Q, Stuffet al (27); 0, Chandra (28); V, Carvalho (29); Prentice , et al (2); and 5, present study. fairly constant throughout lactation (SD 60-80 mL/d, intake on days 4-6 and milk intake through day 28 (Fig giving a maximum upper 95% confidence limit of 120 6A). Thereafter, there was no significant relation. A sim- g/d; Table 2). The interindividual variation in intake de- ilar pattern of correlation between milk yield days 4-6 creased from a high of 1 50 mL/d (CV ‘-.-22%) at 1 mo to and milk intake was observed (Fig 6A). When milk yield 80 mL/d (CV 10%) after 6 mo. Two types of evidence and intake on days 4-6 were correlated with milk yield suggest that there is a characteristic rate of milk transfer through lactation, a similar pattern emerged (data not for each mother-infant pair after the first month of lacta- shown). Results obtained using the Spearman rank cor- tion: 1) During months 2-5, when all subjects were fully relation were similar. The conclusion is that milk trans- breast-feeding, there was a highly significant difference fer in full lactation cannot be predicted from milk yield (p < 0.00 1) between the highest (878 ± 26 mL/d, SEM) or intake during the first week postpartum. and the lowest (59 1 ± 33 mL/d, SEM) daily milk yields. The relation between milk intake and the weight of the 2) The correlation coefficient for milk transfer in an mdi- infant at the beginning and end of each time interval is vidual mother-infant pair during month 2 compared shown in Figure 6B. The correlation coefficient between with months 3-5 was 0.63 (p < 0.02, n 13). = mean milk intake and infant weight at the beginning of the segment was “.0.4 (p > 0.1, n 12) for the first = Sources ofvariability in milk transfer to the breast -ftd month increasing to -‘.--0.6 (p < 0.05, n = 13) after 1 mo infant (Fig 6B). The correlation coefficient between milk intake To determine whether the yield at the onset of lacta- and infant weight at the end of the interval was signifi- tion, presumably controlled by such maternal character- cantly higher. By 150 d there was a very strong relation istics as breast development and hormonal milieu (30), between infant weight and milk intake at that time (p was related to milk production in full lactation, we exam- < 0.0 1) as has been found by others (2, 3 1). This increase med the correlation of mean milk yield and intake for in correlation with time implies, but certainly does not days 4-6 with milk intake in full lactation (Fig 6). There prove, that milk intake is the independent variable in the was a highly significant relation (p < 0.01) between milk relationship between infant weight and milk intake.
  8. 8. 1382 NEVILLE ET AL TABLE 2 Characteristics ofmilk intake in exclusively breast-feeding mother-infant pairs Time interval (days postpartum) 7-14 15-28 30-59 60-150 Subjects(n) 12 12 13 13 Meandailyproduction(mL/d±SD) 661 ± 175 739 ± 164 751 ± 107 787 ±88 Mean daily intake (g/d ± SD) 6 15 ± I 30 689 ± 148 707 ± 104 753 ± 89 Meanslopeforintake(g/d±SEM) 12.4± 3.4 7.0± 2.3 0.8± 1.1 1.3± 0.6 Mean day-to-day variation in intake (g/d ± SEM) 56.2 ± 12 55 ± 6 63 ± 12 79 ± 18 Meanfeedfrequency(feeds/d±SEM) 7.5± 1.1 8.2± 0.8 8.1± 0.6 7.3± 0.5 Mean feed duration (mm ± SEM) 17 ± 2 18 ± 2 17 ± 2 16 ± 2 Days0-28 Days3O-59 Days6O-l50 r p r p r 0 Correlation between mean daily intake and Infant birth weight 0.30 NS 0.06 NS 0.44 >0.10 Infant weight at 1 mo 0.69 <0.02 0.57 <0.05 0.66 <0.02 Infant weight gain 1 mo postpartum 0.79 <0.01 0.84 <0.01 0.6 1 <0.05 Downloaded from by on August 22, 2009 Total milk intake 1 mo postpartum I .0 - 0.64 <0.05 0.49 <0.10 a Subject 3 excluded from analysis up to day 30 because ofgestational diabetes. There were at least three times per subject within each interval. r is correlation coefficient, p is probability that there is no correlation using a two-tailed test. A one-tailed test would give higher levels of significance; statistical measures obtained from linear regression analysis with n = 12. Although birth weight is not a strong predictor of milk vander et al (I 6) found no significant differences between intake throughout lactation, infant weight at 1 mo is (Ta- 24-h test weighings performed in the home and those ble 2). Weight gain during month 1 is, in turn, strongly performed by the same mothers in a hospital. Butte et al correlated with milk intake over this interval (p < 0.01). found (33) that deuterium oxide dilution and test weigh- Thus, lactation performance during weeks 2-4 postpar- ing in the home give remarkably similar values for milk tum is a strong predictor ofmilk output during the subse- volume production. Our comparison ofmilk production quent period of full lactation as shown by the significant values obtained by test weighing with those obtained by relation between total milk intake during month 1 post- consecutive pumping suggests that intelligent, well-moti- partum and subsequent milk intake (Table 2). vated women are able to obtain accurate data in the field. The mean feed frequency and duration (Table 2) var- These studies, as well as the broad agreement between ied little during lactation. There was no significant rela- milk volume intakes recently measured throughout the tion between milk intake and feed frequency, feed dura- world (Fig 5), indicate that when carried out with appro- tion, the product of feed frequency and duration, or in- priate equipment by reliable mothers or research work- fant sex (p > 0.1) at any time between days 5 and 150. ers, test weighing is a consistent measure of milk in- Because ofthe limited subject number and the homoge- take (9). neous sample, we did not examine the effects of maternal A serious problem in determining mean milk transfer variables such as age, parity, or weight gain during preg- for an individual mother-infant pair is the day-to-day nancy. variability in volumes obtained by test weighing, a prob- 1cm that has been extensively discussed, particularly by Butte and Garza ( 1 8) and by Woolridge et al (37). In the Discussion present study, regression analysis oftest weights obtained Test weighing as a measure ofmilk volume during several 24-h periods showed that mean intraindi- vidual variation averaged 60-80 g/d throughout lacta- Test weighing was evaluated in a large number of labo- tion. Thus, an individual 24-h test weigh result can differ ratones with a wide variety of methodologies (8, 16, 18, from the true mean milk transfer by 1 5% (95% confi- 27, 32-38). In assessing the available comparisons be- dence interval). The precision of the mean estimate can tween formula weight and test weighing, it becomes clear be increased ifseveral days oftest weighing are averaged that good results were achieved where an integrating (8, 27, 38). Three to 4 d oftest weighing will usually re- electronic balance was used (1 8, 37) and errors in estima- duce the coefficient ofvariation (CV) to 5%. tion of intake from the bottle were avoided by careful weighing before and after the feed. Recording errors were Onset of lactation reduced ifthe balance had a print-out. With some excep- tions (32, 36), good results were also achieved in the field Including this study four investigations of milk vol- with mothers carrying out their own test weighing. Hof- ume production during the onset of lactation are avail-
  9. 9. HUMAN MILK VOLUME 1383 A B 1.0 - C a) U 0.8- a) P’(o.oI 0 U ::-_ C 0 0.6 :?P ‘ C a) 0 U a) 0 C H Downloaded from by on August 22, 2009 I I I I I I 4-6 7-14 5-2830-60 61-150 4-6 7-14 15-2830-60 61-150 Days Post Partum FIG 6. Relation of milk intake at various times postpartum with milk yield and intake days 4-6 postpartum (A) and infant weight (B). Correlation coefficients were determined by regression analysis on 12 subjects who practiced exclusive breast-feeding from the onset oflactation. The dashed horizontal lines give the critical values for two-sided levels ofsignificance for sample correlation coefficients under the null hypothesis ofno correlation. A. Closed triangles depict correlation coefficients between mean milk yields days 4-6 postpartum and milk transferred to the infant during the stated interval. Open triangles represent the correlation between milk transfer days 4-6 and subsequent milk transfer. B. Closed circles represent the correlation between average milk intakes for that interval and infant weights at the beginning ofeach ofthe intervals. Open circles represent the correlation between average milk intakes for that interval and infant weights at the end ofeach ofthe intervals shown on the abscissa. able (Fig 4) ( 1 1 , 1 2, 14). In all studies volumes were low ous milk secretion after day 2 resulting from falling ma- for the first 2 d, increased markedly on days 3 and 4, and ternal progesterone and possibly estrogen levels (30). leveled offafter day 5. The actual volumes obtained were Healthy full-term infants obviously possess nutrient similar in Denver, Perth (1 1), and Edinburgh (12) where stores that allow them to cope with low milk supplies data were obtained by test weighing the infant. In the during this period; however, milk output from the breast study by Macy et al (13, 14), all milk was obtained by may not be adequate for premature or other infants with manual expression and milk volume production rose an atypical requirement for fluids or glucose during the more sharply starting on day 3 and reached values nearly first 2 d after birth (39). double those of the present study by day 5. Although Macy used wet nurses in earlier studies, these data were Milk output duringfull lactation obtained from Detroit multiparas ostensibly selected by We found that milk transfer increased rapidly to ‘‘.-700 much the same criteria as the subjects in our study. The g/d during the first month of full breast-feeding, leveling volume differences suggest that the potential for milk off to a steady increase of < 1 g/d thereafter to give a production in early lactation is larger than infant need mean intake of - 800 mL/d at 6 mo. Volumes in this and can be fully realized by complete emptying of the study were similar to those obtained elsewhere by test- breasts on a consistent basis (3, 3 1). Several subjects in weighing the infant (Fig 5). (A set of early values from this study did produce more milk than the infant con- Picciano et al [40] are systematically lower than the more sumed in early lactation (Fig 4), providing additional cvi- recent values from this same group shown in Figure 5. dence that infant demand rather than maternal capacity The discrepancy may be because test weighing was per- limits milk intake after day 3, at least in some subjects. formed in the early group of infants 3 d after routine Milk volumes obtained on days 1 and 2 were similar DPT vaccination [MF Picciano, personal communica- in all four studies and were low, the mean for day 2 being lion].) Isotope dilution gives similar values. Coward et 175 mL/d. At this stage maternal steroid hormone 1ev- al (41) reported data obtained by this technique in The els probably inhibit milk production, the onset of copi- Gambia and in Papua New Guinea; these fall neatly
  10. 10. 1384 NEVILLE ET AL within the ranges shown in Figure 3B, ie, 795 mL/d dur- ables influencing milk production. It is gratifying, there- ing months 1 to 4 in The Gambia and 670 mL/d in fore, that our data differ in no significant respect from Papua New Guinea during the same postpartum inter- milk-volume data obtained in larger, cross-sectional val. Mean values currently in the literature obtained by studies from a variety of populations throughout the test weighing the mother were not corrected for insensi- world. Mean milk transfer is low during the first 2 d post- ble weight loss and were elevated (42): although the tech- partum, rises rapidly on days 3 and 4, and then increases nique can be used with accuracy (43), correction for in- more slowly to reach maximum levels of ‘-800 mL/d at sensible weight loss is essential (9). 6 mo postpartum in exclusively breast-feeding women. The mean intraindividual CV in all the studies shown The pattern of milk transfer for a given mother-infant in Figure 5, weighted for subject number, was 1 7% com- pair appears to be largely set by the end of the first pared with a value of 14% in our study. The consistency month. The interindividual CV is 16% in most studies; ‘ among the milk-volume data derived from 16 studies this variation is related most closely to infant size and from nine countries suggests that there is now sufficient may be more a function ofinfant demand than any ma- data in the literature on exclusively breast-feeding ternal variable (2, 3, 32). Future studies focusing on in- women during the first 8 mo postpartum so that refer- fant factors that influence the demand for breast milk ence data can be established for the temporal depen- in the first month postpartum may be of considerable dence ofhuman-milk-volume transfer. Further, analysis importance in understanding the growth of the breast- ofour longitudinal data suggests that the increase in milk fedinfant. 13 transfer from months 2 to 5 (- 1 g/d) reflects a general We acknowledge the helpful discussions of Mary Frances Picciano pattern observed in most individuals. During the first Downloaded from by on August 22, 2009 and an unknown referee in the interpretation of these studies. Most month of lactation, the individual temporal patterns important, however, was the unstinting enthusiasm and cooperation of tend to deviate rather widely from the population pattern the mothers who served as subjects through a tedious year oftest weigh- and must be studied on an individual basis. ing, milk samples, and diet records. Without the dedication of these subjects to furthering our knowledge ofthe physiology and nutritional Sources ofvariability among individuals significance oflactation, these studies could not have been done. We conclude from our longitudinal data that there is a characteristic milk-volume transfer for each mother- References infant pair after the first month oflactation. The volume 1. Morrison SD. Human Milk. Yield, proximate principles and inor- transferred was not related to yields or intakes at the on- ganic constituents. Farnham Royal, UK: Commonwealth Agricul- set oflactation, suggesting that maternal variables associ- turalBureaux, 1952. ated with lactogenesis do not regulate milk production 2. Prentice AM, Paul A, Prentice A, Black A, Cole T, Whitehead RG. during full lactation. This conclusion is strengthened by Cross-cultural differences in lactational performance. In: Hamosh the observation that mean yield in mothers who overpro- M, Goldman A eds. Human lactation 2: maternal and environ- duced during early lactation (eg, those who removed sig- mental factors. New York: Plenum Press, 1986:13-44. nificant volumes by breast pump or manual expression) 3. Neville MC, Oliva-Rasbach J. Is maternal milk production limit- was not different from mean yield in nonoverproducers ing for infant growth during the first year of life in breast-fed in- fants? In: Goldman A, Atkinson 5, eds. Human lactation 3: the at any time during lactation (data not shown). Volume effect of human milk on the recipient infant. New York: Plenum transfer after the first month was significantly related to Press, 1987:123-33. infant weight (p < 0.02). Because the correlation be- 4. Neville MC, Seacat J, Lutes V, Keller RP, Allen J, Casey C. Lacto- tween milk transfer and infant weight was stronger at the genesis in women: milk volumes for the first two months postpar- end ofeach time interval examined that at the beginning tum. Fed Proc l985;43:668(abstr). (Fig 6B), we suggest that some factor or factors other than 5. Neville MC, Allen JC, Watters C. The mechanisms of milk secre- infant weight per se are the primaiy determinants of tion. In: Neville MC, Neifert MR, eds. Lactation: physiology, flu- transfer from mother to infAnt. Future studies might well trition and breast-feeding. New York: Plenum Press, 1983:49-104. focus on the relation between milk output in early lacta- 6. Neville MC, Keller RP, Seacat J, Casey CE, Allen JC, Archer P. tion and such factors as milk composition, the infant’s Studies on human lactation. I Within-feed and between-breast variation in selected components of human milk. Am J Clin Nutr growth potential as expressed in parental and grandpa- l984;40:635-46. rental size, the strength of the infant’s suck, and infant 7. Anscombe FJ. Rejection of outliers. Technometrics 1960;2:123- basal metabolic rate. 147. 8. Woolridge MW, Butte N, Dewey KG, Ferris AM, Garza C, Keller Conclusion RP. Methods for the measurement of milk volume intake of the breast-fed infant. In: Jensen RG, Neville MC, eds. Human lacta- The major objective ofthis study was to provide longi- tion: milk components and methodologies. New York: Plenum tudinal data on milk-volume production and infant in- Press, 1985:5-20. take in a small, relatively homogeneous group of exclu- 9. Neville MC. The measurement of milk transfer from mother to sively breast-feeding women. Such data are necessary to breast-feeding infant. J Pediatr Gastroenterol Nutr l987;6:659- provide a temporal basis for the design of cross-sectional 662 (editorial). studies with a large enough number of subjects to allow 10. Hendrikson EC, Seacat JM, Neville MC. Insensible weight loss in reliable statistical analysis of maternal and infant vari- children under one year ofage. Acta Paediatr 1985;74:678-80.