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PEDIATRICS Vol. 109 No. 4 April 2002, pp. 585-589

Infant Acceptance of Breast Milk After Maternal Exercise

Kc S. Wright, MS*, Timothy J. Quinn, PhD{ddagger} and Gale B. Carey, PhD*

* Department of Animal and Nutritional Sciences University of New Hampshire, Durham, New Hampshire
{ddagger} Department of Kinesiology, University of New Hampshire, Durham, New Hampshire

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   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Objective. Previous research reported that breast milk lactic acid (LA) levels increase after lactating women complete a bout of exhaustive exercise, resulting in poor infant acceptance of the postexercise breast milk. This highly publicized finding may not apply to more practical, everyday exercise conditions of lactating women. The purpose of the present study was to reexamine the composition and infant acceptance of postexercise breast milk while controlling maternal diet, exercise intensity, and the method, timing, and assessment of infant feeding.

Research Design and Methods. Twenty-four women, 2 to 4 months’ postpartum, completed 3 test sessions: a maximal oxygen uptake test, a 30-minute bout of moderate exercise, and a resting control session. One hour before and 1 hour after each session, participants fully expressed their milk, placed it in a bottle familiar to the infant, fed their infant, and rated their infant’s acceptance of the milk. Each feeding was videotaped and viewed individually by 3 lactation consultants who rated infant acceptance; consultants were blinded to the test sessions. Milk was analyzed for LA and infant milk consumption was measured.

Results. There were no differences in presession versus postsession values for maternal skin temperature, breast milk temperature, and infant milk acceptance as judged by either the mothers or lactation consultants. These results prevailed despite a small but significant increase in breast milk LA premaximal versus postmaximal exercise (0.09 vs 0.21 mM, respectively); there was no difference in milk LA premoderate versus postmoderate exercise, or prerest versus postrest.

Conclusion. These data support the hypothesis that moderate or even high-intensity exercise during lactation does not impede infant acceptance of breast milk consumed 1 hour postexercise.

Key Words: postpartum exercise • lactation • infant feeding • milk acceptance • breastfeeding

Abbreviations: LA, lactic acid • BMI, body mass index • VO2max, maximal oxygen consumption • HR, heart rate • RPE, perceived exertion • LAT, lactic acid threshold


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Physical activity is an important but often underrecognized factor in women’s health. The benefits of physical activity for women have been well-documented and include improved cardiovascular fitness; decreased body fatness; enhanced bone mineralization; decreased risk of colon cancer, hypertension, and diabetes mellitus; and improved mental health.1–3 Recent recommendations from the Centers for Disease Control and Prevention and the American College of Sports Medicine encourage 30 minutes of moderately intense physical activity on most, and preferably all, days of the week.4 Unfortunately, only 22% of adults engage in the recommended level of physical activity,5 with males more likely to participate in vigorous physical activity than females.

The American Academy of Pediatrics recommends that women breastfeed their infants for at least 12 months.6 This recommendation is based on research that provides evidence of the significant health, societal, and environmental benefits of breastfeeding.6–11 There are many barriers, including sociocultural factors, to extended breastfeeding.7 Data that supports the compatibility of exercise and lactation may help to reduce the sociocultural inhibition of extended breastfeeding for some women.

Given the health benefits of both exercise and lactation, recent research has addressed their compatibility. Dewey and colleagues12 demonstrated that exercise improved the cardiovascular fitness of breastfeeding women. Lactating women who exercise retain less weight from pregnancy, have a higher energy expenditure, and have better measures of psychosocial well-being than nonexercising, lactating women.13,14 In addition, exercise during lactation improves the insulin response to a meal and increases high-density lipoprotein concentration.15

Despite these positive findings, one research group reported a negative effect of a single bout of exercise (acute exercise) during lactation. Wallace et al16 demonstrated that lactic acid (LA) concentrations were significantly higher and infant milk acceptance scores significantly lower for postexercise compared with preexercise milk. However, milk LA and infant milk acceptance were measured before and after a maximal treadmill test, only 2 mL of milk were used to assess infant acceptance, maternal diet was not controlled, and infants were fed 30 minutes after exercise. Two subsequent studies from our laboratory confirmed that milk LA increases after maximal exercise, but demonstrated that LA fails to increase after moderate or mild exercise.17,18 Therefore, the aim of the present study was to extend these findings, control for potentially confounding factors, and evaluate how exercise influenced infant acceptance of fully expressed breast milk. We chose to conduct the present study with bottle-feeding rather than breastfeeding to control possible confounding variables, such as milk temperature, and to measure the average LA level from an entire feeding session. The overall goal of our research is to contribute to practical exercise guidelines for lactating women, guidelines that are presently lacking.13,19


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Participants
Twenty-four lactating participants between the ages of 25 to 45 years and 2 to 4 months’ postpartum were recruited to participate in this study. Participants 1) had no history of hypertension, heart disease, or diabetes; 2) had a body mass index (BMI) below Class I obesity19; 3) denied smoking or taking oral contraceptives (with 1 exception, who took a progestin-only birth control); 4) were mild to moderately active during pregnancy and/or lactation; 5) had successful previous experience expressing milk and feeding it to their infant via a bottle; and 6) breastfed their infants every 2 to 3 hours during the day. In addition, breast milk was required to be the source of >=80% of the infant’s nutrition. This research was reviewed and approved by the Institutional Review Board for the Protection of Human Subjects in Research at the University of New Hampshire. In accordance with that approval, written consent, assuring confidentiality, was obtained from all participants.

Participant characteristics are shown in Table 1. Three of the 24 participants delivered their infants by cesarean section; mean infant age for the 14 females and 10 males was 3.0 months. Maternal body composition, BMI, and maximal oxygen consumption (VO2 max) were in the normal ranges for the nonpostpartum female population.20,21


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  		TABLE 1. Participant Characteristics (n = 24)

 
Experimental Design
Participants reported to the exercise physiology laboratory on 4 separate occasions. At the first visit, the participants were oriented to the testing environment, nutrition guidebook, exercise equipment, breast milk expression apparatus, facilities, and research staff. Three test sessions were then scheduled for each participant: a maximal intensity exercise test, a moderate exercise test conducted at an intensity that was relative to each participant’s level of fitness, and a resting control session. Test sessions were separated by a minimum of 3 days.

Exercise Testing and Body Composition
For the first test session, each participant performed a treadmill test to determine VO2 max (Quinton Instruments, Seattle, WA) using a modified Åstrand protocol22 and metabolic cart (SensorMedics, Yorba Linda, CA). Five electrodes were placed and used to monitor heart rate (HR) and rhythm. The participant began the VO2 max test either walking or jogging from a self-selected speed at 0% incline. The incline was increased by 2.5% every 3 minutes. Before exercise and the end of each 3-minute workload, a 25-µL fingerstick blood sample was taken in duplicate and immediately analyzed for LA. Simultaneous to retrieval of blood samples, the participant was queried to rate her perceived exertion (RPE)23 until maximal criteria were met.

LA was determined from the blood samples, and the mean was graphed and subsequently analyzed, in blind fashion, by 3 investigators. The LA threshold (LAT) was determined as the first breakpoint from linearity. Two of the 3 investigators had to agree on the visual breakpoint for acceptance. This method of determining LAT has been previously validated.24,25

For the second and third test sessions, participants performed a 30-minute exercise bout conducted at 20% below the individual LAT and a nonexercise rest session, respectively. These sessions were conducted on separate days and were randomized. In addition, body composition was assessed via skinfold from measurements taken at 5 sites: tricep, subscapula, suprailiac, abdominal, and thigh. Percent body fat was calculated using the equation developed by Jackson et al.26

Nutritional Guidance
Participants were instructed on the use of the Healthy Eating guidebook, which is an effective tool for monitoring and guiding food choices.27 The purpose of using this tool was to eliminate any potentially confounding variables in participants’ diets. The guidebook defines 6 food groups: fat, fruit, high carbohydrate, meat, milk, and vegetable, with serving sizes of foods listed for each group. Daily caloric needs of each participant were calculated by summing resting metabolic needs (from the Harris-Benedict equation28), physical activity expenditure, and 500 kcals to meet the Recommended Dietary Allowance for lactation.29 Each participant was counseled on the number of servings to consume from each food group—this profile insured that 60% of calories were from carbohydrate, 15% from protein, and 25% from fat.

Participants were also asked to refrain from eating cabbage, broccoli, garlic, alcohol, and vanilla during the course of the study as these foods are known to affect the taste of breast milk.30–34 Participants were instructed to practice using the guidebook before the test sessions began and were contacted by phone several days later to enhance their compliance and to answer their questions. They were advised to begin adherence to the guidebook goals >=2 days before the first test session.

Milk Expression and Analyses
Participants were asked to nurse their infants approximately 2 to 3 hours before reporting to the laboratory. All milk expression was performed in a private milk expression room; participants were given a clean towel and alcohol pad to wipe the breast area free from any skin lotion or perspiration residue before each expression session. Skin probes were placed over the participant’s breasts to monitor skin temperature. Milk was collected by complete expression of both breasts using a dual-chamber electric breast pump (Medela, Inc, McHenry, IL). The collecting bottles were insulated with aluminum foil-covered bubble wrap to maintain the temperature of the expressed milk. Milk temperature and volume were measured immediately after expression (Checktemp, Hanna Instruments, Woonsocket, RI). Milk was mixed thoroughly and an aliquot of the milk was assayed, immediately and in duplicate, for LA using a portable LA analyzer (Yellow Springs Inc, Yellow Springs, OH). Values obtained via the portable analyzer are identical to those obtained using an independent enzyme-coupled spectrophotometric assay.17

Infant Feeding and Assessment
Within 1 minute after milk temperature and volume were recorded, expressed milk was transferred into a feeding bottle familiar to the infant. The mother fed her infant the expressed milk while a video camera recorded the infant feeding. The camera was positioned so that the infant’s face, jaw, and contact with the nipple of the bottle were in full view. Each feeding session was recorded for 10 minutes. At the conclusion of the feeding, the mother rated her infant’s acceptance of the milk in that specific session compared with a typical nursing period. An ordinal response scale from 1 to 10 was used with verbal anchors of poor (= 1), fair (= 4), good (= 7), and excellent (= 10). The volume of any remaining, unconsumed milk was measured and recorded.

After completion of the 3 test sessions by each participant, the 6 vignettes of infant feeding were randomly scrambled and transferred to videotape with 1 vignette duplicated for a reliability check. Each videotape was reviewed by 3 independent lactation consultants who were certified by the International Board of Lactation Consultant Examiners. Consultants rated infant acceptance using the same ordinal response scale as the mothers and recorded their scores. On receiving the scores, 1 of the investigators confirmed that the duplicate vignette scores on a videotape were identical or 1 point apart; if they were 2 or more points apart, scores from that lactation consultant were excluded from analysis.

Statistical Analyses
Data were analyzed using a repeated measures analysis of variance design followed by Bonferroni multiple comparisons test; (InStat Graphpad Software, San Diego, CA). Dependent variables included milk LA, milk temperature, skin temperature, infant milk acceptance, RPE, and volume of milk consumed. The independent variable was the test session. Spearman correlation was determined between the mother’s infant acceptance score and the mean lactation consultant’s score for 138 feeding sessions. Statistical significance was set at P < .05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Breast milk LA was significantly increased (P < .05) 1 hour after the maximal exercise test compared with the preexercise value (Fig 1). However, there was no significant difference in breast milk LA concentration before or after moderate exercise, which was conducted at 59% of VO2 max, 72% of maximal heart rate and a perceived exertion of 13 or "somewhat hard." There was no difference in breast milk LA before or after the rest session.



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  		Fig 1. Breast milk LA before and after exercise or rest. Participants expressed breast milk 1 hour before and 1 hour after completion of exercise or rest; milk was analyzed immediately for LA (N = 24).

 
Neither maternal milk nor skin temperatures were significantly different between any test period (Table 2). Volume of milk consumed by the infants (mL/kg) after maximal exercise or rest did not differ from the volume consumed before maximal exercise or rest. Volume consumed after the moderate exercise was statistically less than before moderate exercise, but was not different from the volume consumed after maximal exercise or rest. Milk consumption at each feeding averaged 15.3 mL/kg, approximately 104 mL.


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  		TABLE 2. Measures of Infant Acceptance, Milk Temperature, and Skin Temperature*

 
Of the 72 videotape evaluations by lactation consultants, 6 evaluations failed to meet the duplicate vignette reliability check and were excluded from calculation of the infant acceptance evaluation score. Mean infant acceptance of milk as evaluated by the consultants for all feedings ranged from good to excellent (Table 3; range: 7.23–8.47). There was no significant difference in the consultant’s evaluation of infant acceptance before or after maximal exercise, moderate exercise, or rest. Of note was the high infant acceptance of postmaximal exercise milk despite a small but significant increase in LA in this milk compared with preexercise milk (Fig 1).


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  		TABLE 3. Infant Acceptance of Breast Milk: Maternal Versus Consultant Score*

 
Infant acceptance before and after each test session as evaluated by mothers was in agreement with the evaluations of the consultants with 1 exception: mothers rated the postmaximal exercise milk acceptance significantly higher than consultants (Table 3). There was a positive correlation between the maternal and consultant evaluations of infant acceptance (r = 0.31; P < .05).

Recommended caloric intake for the participants was significantly higher than actual caloric intake (Fig 2A; P < .05). However, the participants did achieve the macronutrient goal throughout the study period of 60% carbohydrate, 15% protein, and 25% fat (Fig 2B).



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  		Fig 2. Recommended versus actual macronutrient and energy intake. At the outset of the study, each participant was advised on dietary intake using the Healthy Eating guidebook22 and each recorded her intake while participating in the study (N = 24).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The present study is the first to demonstrate that infant acceptance of expressed breast milk is the same 1 hour before and 1 hour after maternal exercise at both moderate and maximal intensities. This evaluation was conducted by objective lactation consultants who were blinded to the treatments. Mothers, who were potentially subjective in their evaluations, also failed to score a difference in infant acceptance of milk before versus after exercise. The average acceptance values from lactation consultants and mothers ranged from good to excellent. These results persisted despite a small but significant increase in breast milk LA 1 hour after maximal exercise compared with 1 hour before exercise (0.21 vs 0.09 mM).

Our findings conflict with previous research that reports infant acceptance of breast milk declines after maximal exercise.16 However, this discrepancy may be attributed to several methodological differences. The first is the manner in which infant milk acceptance was measured: we assessed overall feeding by soliciting evaluations from the objective lactation consultants. The second is that lactating women were tested at both maximal intensity and at 20% below their LAT; the latter is a moderate level of exercise intensity. Measurements of ventilation, O2, HR, and RPE are representative of moderate exercise and the values are consistent with those reported in previous research using moderate exercise.12,14,17,18 The third is that complete breast expression from both breasts was performed to obtain a sample for analysis, and the milk was fed via bottle rather than via medicine dropper.16

Milk consumption at all sessions was normal for 3-month-old infants.33 Preexercise or rest milk consumption was often greater than that postexercise or rest, although only after 1 session—the moderate exercise session—was this difference statistically significant. This may reflect the lack of control over the time interval between the last home feeding and the first laboratory feeding, whereas the laboratory presession to postsessions feedings were always 2.5 hours.

Of note is that all posttest session values have somewhat higher infant acceptance scores, compared with the pretest session values as evaluated by both the mother and the consultants (Table 3). This is especially evident in the maternal evaluations for the premaximal versus postmaximal exercise test. Because the maximal test was the first test session for every participant, the lower pretest value may represent insufficient acclimation to the testing environment, for both the mother and the infant. On the morning of testing, several participants anecdotally reported busy schedules in getting both themselves and their infants prepared for participation. Furthermore, several participants reported that although their infant had readily accepted feeding previously from a bottle, the feeding was often given by a spouse or care provider. Thus, several infants may have experienced some nipple confusion in accepting the bottle in a new environment with the mother from whom they usually nurse.

Other research demonstrates that lactation and exercise are compatible.12,14,15 However, 1 case of postexercise nursing difficulty has been reported.35 A lactating woman who resumed her distance running, reported that her infant nursed very well 1 to 2 hours postexercise. Within an hour after the postexercise feeding session, the infant regularly displayed relentless crying and apparent discomfort. The infant’s fussiness could not be attributed to LA because samples of her breast milk before and after exercise were analyzed for LA with no discernible differences. The possibility remains that other chemical components in the postexercise breast milk were altered by exercise, although our previous work demonstrates no change in milk pH, lipid, ammonium, or urea as a result of moderate or even intense exercise.17,18

Breastfed infants experience a variety of flavors attributable to the composition of the maternal diet.31 Attempts were made in the present study to control other variables that might influence an infant’s taste for breast milk, including alcohol, garlic, vanilla, and cruciferous vegetables. Because maternal diet also influences breast milk composition, participants in the present study were instructed to consume calories of similar macronutrient composition to meet their energy needs. Although actual total caloric intake (2063 kcals) was significantly lower than recommended (2511 kcals), total calories consumed were comparable with that found in similar research with lactating women.14,36,37

The notion that a significant number of lactating women experience postexercise nursing difficulties has been suggested16 but not substantiated. Although the results of the current study are compelling, they do not mimic the au naturale condition of feeding the infant directly from the breast. There may be other unidentified factors associated with exercise, such as body sweat or pheromone secretion, that could influence infant acceptance of milk from the breast postexercise. We are currently conducting clinical research to assess this possibility.

This study and others12,14,15,17,18 support the notion that moderate exercise and lactation are compatible. Health providers should encourage lactating women to exercise comfortably when they feel ready, as long as there are no contraindications. Women can and should enjoy the benefits of exercise while reaping the benefits of lactation for themselves and for their infants.


   ACKNOWLEDGMENTS
 
This study was supported by the Women’s Sports Foundation through the 1999 Quaker Rice Cakes Nutrition and Exercise Research Grant.

We thank Miranda Gaudette, Tracy Postek, and Jenny Klooster for technical assistance.


   FOOTNOTES
 
Received for publication Jul 5, 2001; Accepted Oct 10, 2001.

Reprint requests to (G.B.C.) Department of Animal and Nutritional Sciences, University of New Hampshire, Durham, NH 03824. E-mail: gale.carey{at}unh.edu


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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PEDIATRICS (ISSN 1098-4275). ©2002 by the American Academy of Pediatrics



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