Participants
Forty-six pregnant mothers (39 white, 4 black, 2 Hispanic, 1 Asian) who were in their last trimester of pregnancy and who planned to breastfeed their infants were recruited from advertisements in local newspapers; 65% were multiparous. All but 1 delivered a healthy singleton at term and successfully breastfed their infants during the exposure periods; the remaining mother had fraternal twins. Six additional mothers participated in the study but were disqualified either because they did not breastfeed long enough (
n = 2), the infant refused to eat during both testing days (
n = 3), or the mother did not comply with the experimental procedures (
n = 1). All procedures were approved by the Committee on Studies Involving Human Beings at the University of Pennsylvania.
Exposure Procedures
The pregnant women were randomly assigned to 1 of 3 groups. During the last trimester of pregnancy (33.2 ± 0.3 weeks of gestation), the women were given either bottles of water (Naya, St-André Est, Québec, Canada) or cartons of frozen organic carrot juice (Ferraro, Glendora, CA) and instructions regarding the time of day (10
am to 2
pm) to drink the beverage. Carrot flavor was chosen as the stimulus for several reasons. First, it is a flavor that is transmitted to human milk and is quite distinctive.
20 Second, it was easy to ensure that the nursing mothers’ only source of this flavor was under experimental control.
20 Finally, carrot flavor has been added to the infants’ pap during the time of weaning for centuries.
21The women consumed either 300 mL of carrot juice or water daily for 4 days per week for 3 consecutive weeks during the last trimester of pregnancy and then again during the first 2 months of lactation (1.3 ± 0.1 months). As shown in Fig 1, the mothers in 1 group drank carrot juice during pregnancy and water during lactation (CW; n = 16); mothers in another (WC) group did the opposite (n = 17); whereas those in the control group drank water during both exposure periods (WW; n = 14). There were no significant differences among the groups in the timing of the exposure periods during pregnancy or lactation (F[2,43 df] = 1.42; P = .25). Mothers refrained from eating carrots or drinking carrot juice during and between the 2 exposure periods. To encourage and verify compliance, mothers maintained diet logs of all the foods and beverages that they ate during the exposure period and daily phone or e-mail contact was made with each woman.
| Fig 1.The design of the experimental protocol for the 3 groups of participants. The women consumed either 300 mL of carrot juice or water daily for 4 days per week for 3 consecutive weeks during the last trimester of pregnancy and then again during the first (more ...) |
The characteristics of each group are listed in Table 1. There were no significant differences between the groups in the ages of the mothers (F[2,43 df] = 2.21; P = .12) or infants (F[2,43 df] = 0.54; P = .95), the infants’ body mass index (BMI; F[2,43 df] = 2.31; P = .11), or the sex ratio of the infants (χ2 [2 df] = 1.70; P = .43). There was not a significant interaction between group and sex on the infants’ BMI (F[2,40 df] = 1.66; P = .20). However, there was a significant effect of sex (F[1,40 df] = 4.60; P = .04); posthoc analysis revealed that overall boys tended to have a larger BMI than did girl infants of this age (F[1,44 df] = 3.67; P = .07).
Testing Procedures
A within-participants design that controlled for time of day was implemented. Approximately 4 (± 0.5) weeks after the mothers began complementing their infants’ diet with cereal and before the introduction of carrot foods to their diet, the infants, who were 5.7 ± 0.2 months old, were videotaped at the Monell Center as they fed, in counterbalanced order, cereal prepared with water (1 part cereal, 1 part water) during 1 test day and cereal prepared with carrot juice (1 part cereal, 1 part carrot juice) during another. These test sessions were separated by, on average, 4 (± 0.5) days. All liquids were room temperature before cereal preparation. None of the infants had ever been fed carrot juice or any foods containing carrots. There were no significant differences among the groups in the number of weeks the infants had been fed cereal (
F[2,43
df] = 0.32;
P = .73). The vast majority of the mothers (
n = 39) were still nursing at the time of testing; the remaining 7 women had weaned their infants from the breast, on average, 3.5 ± 0.6 weeks before the first test session.
To minimize possible effects attributable to different levels of satiation and familiarization, each test session began at approximately the same time of day that the infants were usually fed cereal and the mothers drank the juice or water during the exposure periods (10 am to 2 pm). The infants were last fed ~2.5 ± 0.2 hours before testing and there was no significant difference in this duration of time between the 2 test sessions (paired t [45 df] = 0.35; P = .73). In addition, the type of cereal offered was the brand that the infant was currently being fed and, thus, was familiar.
The infants sat in a chair that had a tray attached to its arms and their mothers fed them with utensils and the infants wore bibs that were identical to those used at home.22 The mothers refrained from talking or making faces during the feeding sessions to eliminate any potential influence of the mother’s verbal or facial responses on her infant’s behaviors; replays of the videotapes verified that this was indeed the case. Mothers were instructed to feed the infants at their customary pace until they refused the cereal 3 consecutive times, using the criterion that the infant exhibited such behaviors as turning his or her head away, pushing the spoon away, crying, or becoming playful. The experimenter, who was unaware of the experimental conditions of the participants, then signaled to the mother to stop feeding. The experimenter sat behind the video camera, which was placed at the far corner of the testing room ~10 to 12 feet from the mother–infant dyad and was out of view of the mother and her infant.
Immediately after each feeding session, the mothers, who were unaware of the hypothesis of the study, then rated, how much that they thought their infant liked the cereal on a 9-point scale. Intermediate ratings were to be marked at the appropriate locations between the extremes such that ratings could range from 1 (did not like at all) to 9 (liked very much). The infants’ behaviors were monitored by videotape and the amount of cereal consumed by the infant was assessed by weighing the bowl immediately before and after each feed on a Mettler pm 15 top-loading balance (Greifensee, Switzerland). All food that spilled onto the tray or bib was placed in the bowl before weighing. Mothers were not told how much their infants had ingested.
Trained raters, who were unaware of the experimental conditions, scored the videotaped records of the first 2 minutes of each feed in real time.22 Because previous research15,23,24 has shown that negative facial configurations are more discriminating than positive configurations in gauging the infants’ hedonic responsiveness, the videotape analyses focused on frequency of negative facial responses (eg, nose wrinkling, brow lowering, upper lip raising, gaping, head turning) made in response to each spoonful of cereal proffered. During scoring, the sound was turned off so that the raters would not be influenced by the infants’ vocalizations. Reliabilities for each measure were determined by correlating the scoring of at least 2 raters. The mean Pearson product–moment coefficients for the scoring of all behaviors were above 0.80.
Questionnaires on Mother’s Eating Habits
All but 1 mother completed an 8-item scale that measured their variety-seeking tendency with respect to foods,
25 during both pregnancy and lactation. Each mother was also queried about the frequency with which they consumed carrots and other foods and rated how much they liked eating carrots.
Statistical Analyses
For each infant, we determined the frequency of negative facial expressions, the mothers’ ratings of their infants’ enjoyment of the food, total cereal intake, and the length of feedings on each of the 2 test sessions. These data were then analyzed in separate 2 × 2 analyses of variance with flavor (plain, carrot) as the within-participant factors and treatment group (groups CW, WC, and WW) as the grouping factor. Significant effects in the analysis of variance were probed by posthoc tests. All summary statistics are expressed as mean ± standard error of the mean (SEM).
Because this is a within-participants design study, proportional difference scores are more informative than are mean group data. Therefore, for each infant, we calculated proportional responses by dividing each infant’s response to the carrot-flavored cereal by his or her response to the carrot cereal plus plain cereal (carrot/[carrot + plain]; Table 2 and Fig 2). Scores above 0.50 indicate increased display of negative facial expressions, maternal ratings of infants’ enjoyment, or increased intake when feeding the carrot-flavored cereal relative to the plain cereal.
| TABLE 2. Effect of Exposure on Infants′ Acceptance and Enjoyment of Plain and Carrot-Flavored Cereal |
| Fig 2.The infants’ relative acceptance of the carrot-flavored cereal as indicated by display of negative facial expressions (left panel), mothers’ ratings of their infants’ enjoyment of the cereals (middle panel), and intake (right panel). (more ...) |