What Is Known
Complementary foods are introduced earlier than recommended in a sizeable number of infants born at term, but little is known about timing or factors associated with the introduction of solid food among infants born preterm.
What Is New
Preterm infants are significantly more likely to be introduced to complementary foods early compared with term infants. The predictors of early solid feeding differ for preterm infants . Given the health implications, specific guidelines for preterm infants should be developed and future research should examine predictors of early introduction in preterm infants .
The introduction of solid foods (also referred to as complementary foods) to complement breast milk, infant formula, or both is recognized as a critical dietary transition for infants that can potentially affect growth and have long-term health consequences. The appropriate time to introduce solid foods is a matter of some debate. The American Academy of Pediatrics (AAP) and the World Health Organization recommend exclusive breast-feeding for 6 months with the introduction of complementary foods and continued breast-feeding thereafter, whereas the European Society for Pediatric Gastroenterology, Hepatology, and Nutrition concludes that introduction should occur no earlier than 4 months and no later than 6 months of age (1–3)
These guidelines do not make specific recommendations for preterm infants (<37 weeks’ gestation) who have increased nutritional requirements and may, depending on gestational age, face delayed gross motor development, feeding problems, gastroesophageal reflux (GER), and respiratory compromise (4–6) preterm infants in the United States come from the American Academy of Family Physicians that recommends introduction of solid foods based on oral–motor readiness, usually achieved at an adjusted age (ie, age of the baby based on his or her due date) of 4 to 6 months (7) preterm infants to solids should be made on an individual basis, considering gestational age, nutritional status and requirements, and gross motor development (5,6,8)
Although the issue of optimal age remains controversial, most researchers agree that there are negative health consequences to the early and late introduction of complementary foods. Early introduction has been linked to rapid weight gain, obesity, type 1 and 2 diabetes mellitus, and allergies and atopic disease, whereas the late introduction of solids may lead to nutritional insufficiency, compromised immunity, and feeding problems (5,6,9–13) (14–19) preterm infants are also introduced to solids early, but little research has identified how early and which infants are at greatest risk (8,19–21) preterm infants in the United States are introduced to solid foods before 4 months of age more often than term infants in a nationally representative sample and to examine whether predictors of early solid food introduction are different for preterm and term infants.
METHODS
Data Source and Sample
We used data from the first round of the Early Childhood Longitudinal Study, Birth Cohort (ECLS-B), a longitudinal study sponsored by the National Center for Education Statistics that followed approximately 14,000 children born in 2001 from birth to first grade. The ECLS-B was designed to provide a comprehensive and reliable data set to better understand early childhood development, health, and education. It collected data on children from diverse backgrounds and oversampled twins, minorities, and other groups to create a nationally representative sample. It excluded children born to mothers younger than 15 years of age and those who died or were adopted before the 9-month assessment (22)
Our study focused on singleton children whose primary caregiver/parent completed the CAPI section that collected retrospective information on newborn (neonatal) and infant nutrition and feeding. The response rate for the parent CAPI was approximately 74% (10,700). Of these, we excluded infants missing data for gestational age or age of solid food introduction (n = 800, 7.5%) and infants introduced to solid food at an adjusted age before 36 weeks because feeding solid food this early in gestation seemed unlikely (n = 50, 0.5%). We also dropped twins and higher-order births (n = 1750, 16.3%), as well as children with congenital anomalies (n = 450, 4.2%), because of potential differences in feeding patterns and practices. It is important to note that given the retrospective nature of this secondary data analysis, we may be able to identify associations between gestational age and age of solid food introduction, but we cannot demonstrate causality.
Variables
Outcome
The primary outcome measure was the age of the infant (in months) when solid foods were first introduced as reported by parents in the CAPI. In the ECLS-B, solid food is defined as cereal and baby food in jars but not finger foods (22) preterm infants . A binary variable, solid food introduction before 4 months or introduction at or after 4 months, was created based on the AAP's guidelines at the time of the first round of the ECLS-B.
Infant Characteristics
Gestational age was computed by the ECLS-B from birth certificate data based on the mother's last normal menstrual period (LMP). If this information was unavailable, the best clinical estimate (CE) was used (22,23) (22,23)
Maternal and Household Characteristics
Maternal characteristics included age, smoking status, and prepregnancy body mass index (BMI); household characteristics included socioeconomic status (SES) and receipt of Special Supplemental Nutrition Program for Women, Infants, and Children benefits. The National Center for Education Statistics created a composite variable for SES using data on household income, education, and occupation. Socioeconomic quintiles were created from these variables (1 = lowest, 5 = highest). These data were collected by the 9-month CAPI and the resident father questionnaires.
Statistical Analysis
Data for term and preterm infants were first analyzed together to identify whether preterm infants were fed earlier or later than term infants and then separately to identify the differences in the factors associated with early feeding. The descriptive characteristics were computed using cross-tabulations and significance determined using simple logistic regression. Multiple logistic regression analysis was then used to determine the adjusted odds of being fed solid foods before the adjusted age 4 months. Multivariable models were chosen based on bivariate analyses and factors identified by the previous research (8,16,18,20,21,24,25) P < 0.20) included living with a partner, number of siblings, number of days hospitalized after birth, and amount of maternity leave.
All of the analyses used STATA 12.0 (StataCorp, College Station, TX). All of the unweighted sample sizes were rounded to the nearest 50 per the regulations of the restricted data license agreement for ECLS-B case-level data. Taylor linearization for standard error estimation was used to account for multistage sampling and unequally weighted design. Adjustments for the complex survey design were made using the sample weight and primary sampling units provided by the ECLS-B (22)
Human Patients
The University of Maryland Baltimore's institutional review board approved the present study. The Institute of Education Statistics Data Security Office at the Department of Education also reviewed the present article and approved its publication.
RESULTS
All of the descriptive characteristics are shown in Table 1 . The sample included 7650 children, 1850 preterm infants and 5800 term infants. Nearly half of the full sample was not breast-fed for any length of time, and more than half were enrolled in some form of child care. The majority of mothers were nonsmokers with a normal BMI.
TABLE 1: Sociodemographic characteristics of sample at 9 mo (n = 7650), unweighted number and weighted percentages
Differences between the term and preterm populations became apparent once the sample was stratified. The infants from the preterm sample were more likely to be non-Hispanic black and to live in households in the lowest 2 socioeconomic quintiles. Preterm infants were also more likely to have never been breast-fed and to live in households that received Special Supplemental Nutrition Program for Women, Infants, and Children benefits.
More preterm infants were introduced to solid foods early (before 4 months of adjusted age) than term infants (64.5% vs 23.8%, P < 0.001). The mean age of the introduction of solid foods in the term and preterm population was 17 and 13 weeks’ adjusted age, respectively. Table 2 shows the weighted unadjusted odds ratios for the sample characteristics and age of introduction. Term and preterm infants with significantly increased odds of early solid food introduction included those with mothers younger than 30 years of age or current smokers, whereas infants with significantly decreased odds included those of Asian race/ethnicity. No other factors were associated with timing in the same way for both populations.
TABLE 2: Weighted unadjusted odds ratios for timing of introduction of solid foods
Table 3 provides the findings from weighted adjusted logistic regression models for significant predictors of early solid food introduction. In the full sample, being born preterm had an independent association with early introduction. The infants born between 22 and 32 weeks’ completed gestation had a 9.90 (95% confidence interval [CI] 5.45–18.00, P < 0.001) odds of early introduction to solid foods compared with term infants, whereas infants born between 33 and 36 weeks’ completed gestation had a 6.19 (95% CI 4.58–8.36, P < 0.001) odds. Infants with significantly increased odds of early solid food introduction included those with a mother who was a current smoker (1.77 [95% CI 1.41–2.21]), as well as those who came from a household below the highest SES quintile, were never breast-fed (1.61 [95% CI 1.30–2.00]), were breast-fed <1 month (1.97 [95% CI 1.53–2.54]), or entered day care earlier (1.66 [95% CI 1.39–1.97]), whereas being of Asian race/ethnicity reduced the odds (0.43 [95% CI 0.31–0.60]).
TABLE 3: Weighted adjusted odds ratios of predictors of early introduction of solid foods
The only similar predictors, once the term and preterm populations were analyzed separately, were race/ethnicity and maternal smoking. In both populations, Asian infants had a lower odds of being introduced to solid food early (0.41 [95% CI 0.27–0.61] and 0.52 [95% CI 0.31–0.88], respectively). Infants whose mothers were current smokers had a greater odds of early introduction (1.80 [95% CI 1.42–2.29] and 1.50 [95% CI 1.04–2.17]). Maternal BMI was also a significant predictor in both populations but in different directions; in term infants, maternal obesity was associated with early introduction to solid food (1.38 [95% CI 1.07–1.77]), whereas preterm infants born to underweight mothers had a lower odds (0.47 [95% CI 0.26–0.88]).
The remaining predictors differed in the term and preterm populations. The characteristics significant only in the term population included infant sex, household SES, duration of breast-feeding, and age of child care initiation. Maternal age <35 years (except mothers ages 20–24 years) was significant for early introduction of solid food only among preterm infants .
DISCUSSION
Timing of Solid Food Introduction
The present study's main finding is that almost two-thirds of preterm infants (adjusted age) and one-quarter of term infants were introduced to solid food before 4 months of age. When compared with term infants, preterm infants were significantly more likely to be introduced to solids early. Furthermore, the more premature infants born between 22 and 32 weeks’ completed gestation were at a greater risk of early introduction to solids compared with both later-born preterm infants (33–36 weeks) and term infants. These results should be interpreted with caution and in the context of other previous research. We are limited in our ability to clarify whether the observed epidemiologic association is truly a causal effect.
That said, our study findings are consistent with the previous research showing that a significant proportion of all of the infants are introduced to solid foods early (14–19,26,27) preterm infants are significantly more likely to be introduced to solids foods early compared with term infants is in accordance with a small number of European studies that specifically looked at early introduction and gestational age and found that a large proportion of preterm infants (61%–95%) were introduced to solid foods before 4 months (8,20,21) (8,20)
One potential explanation is that parents of preterm infants may consider chronological age as opposed to corrected age, which in turn influences what age they introduce solids. The AAP recommends that providers use correction for degree of prematurity up to at least 24 months adjusted age when considering a child's growth and development (28) (28)
Variations in the introduction of solid foods by gestational age may also be because of physiological and developmental factors. In general, preterm infants experience more GER than term infants and when it is severe, symptoms of irritability and unexplained crying may result (4,7) preterm infants , have found that caregivers misunderstand infant cues and fussiness and feed to soothe infants who may not be hungry (29–31) (7,32) preterm infants are responding to perceived symptoms of GER by feeding infants thickened milk or formula and other solid foods.
Another consideration is parent/caregiver attitudes and cultural beliefs around introducing solid foods (33) (19,34,35) preterm infants may have similar beliefs and/or other attitudes that arise from feeding problems and stress about infant growth that are not as common among parents of term infants; this may differentially affect their decision to introduce solids earlier (36)
Predictors of Early Introduction
In the 3 European studies specifically concerning early introduction of solid foods in preterm infants , the authors identify a number of predictors including degree of prematurity, male sex, formula feeding, younger maternal age, lower level of maternal education, and maternal smoking (8,20,21) preterm infants , Asian race/ethnicity and lower maternal prepregnancy BMI, which are not mentioned in the previous research.
The present study also suggests that the predictors of early solid food introduction are different for term and preterm infants . Among term infants, breast-feeding was associated with decreased odds of early feeding, whereas high maternal prepregnancy BMI, maternal smoking, low SES, and earlier day care initiation were all independent predictors. Studies examining all infants have supported these predictors, so it is unclear why these results are seen exclusively in the term sample (16,19,24,37,38) preterm infants was relatively small and a high percentage was fed solids before 4 months, making it difficult to understand the relation with covariates without a larger sample size. Qualitatively, the relations seen in the term sample were also seen in the preterm sample but lacked precision surrounding the estimates.
There are a few limitations to the present study. First, in the ECLS-B, the timing of solid food introduction was based on maternal report when study children were approximately 9 months of age. This means that age of introduction could not be validated, and recall bias is possible. Second, there were no data collected concerning why parents introduced solid foods before the recommended age. Given the focus on establishing whether preterm infants are introduced to solids earlier than term infants and identifying predictors in the 2 populations, we have presented hypotheses for future study. Third, the ECLS-B calculated gestational age using LMP, a traditional but unreliable measure of the duration of pregnancy. Despite its limitations, the more random errors associated with LMP, such as menstrual cycle irregularity, make it more appropriate for population-based data analyses. Where LMP was unavailable, other CEs such as gestational age assessment of the infant were used. CEs are known to have wide variations in accuracy, and concordance with LMP estimates can be poor. Therefore, they have been regarded for the most part as an adjunct to dating based on LMP (39,40)
The study also has a number of strengths. It is the first to use a nationally representative sample of US infants to examine the timing of solid food introduction by gestational age. Other studies have examined this topic in US infants, but none to our knowledge have specifically compared preterm and term infants. Finally, the ECLS-B data collection instruments cover a vast array of child and family characteristics, allowing the examination of multiple factors that could affect the timing of solid food introduction.
CONCLUSIONS
Using a nationally representative sample, our study demonstrates that almost two-thirds of preterm infants and one-quarter of term infants were introduced to solid food before 4 months of age, and infants born more prematurely had the greatest odds of early introduction. Although preterm infants are a heterogeneous population and some of these infants may have been ready to eat solid foods, it is likely that some were not. Further research to understand why parents of preterm infants are introducing solids so early is needed to help clinicians advise and educate parents who may not fully understand the consequences of early solid food introduction.
That said, clinicians face a lack of evidence and guidelines regarding how to advise parents. For now, clinicians must provide individual advice based on the infant's gestational age at birth, early nutritional intake, current nutritional status and requirements, and developmental progress and readiness (6) preterm infants .
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