Identifying elevated child weight from 3 to 24 months: Early transitions into nonparental care and to solid foods

Early entry into nonparental care (NPC) and introduction to solid foods (ITS) have been linked to elevated weight, however, little research exists on the combined influence of these transitions on child weight over time.

the introduction to solid foods (ITS) and this sensitive period leads to the development of eating habits and preferences, 4 and the timing of ITS may be a determinant of elevated weight gain.[7][8] Because infants rely on caregivers for their dietary needs, the contexts in which infants are embedded should be considered when examining infant feeding and weight development.
Infants must quickly learn how to self-regulate their food intake and become responsive to hunger and satiety cues, 9 and these selfregulatory abilities establish a foundation for long-term health.Caregivers are responsible for deciding when to introduce solids, choosing which foods to offer, and understanding and being aware of their child's reaction and cues.1][12] It is, however, important to note that "caregivers" can be parental and nonparental.In the U.S., 42% of children participate in some form of nonparental care (NPC) during the first year of life, with kincare (i.e., with a relative) being the most used form of NPC followed by centre-based and nonrelative care. 13With many children in NPC, nonparental caregivers and settings are valuable targets for promoting appropriate weight development and healthy eating behaviours, [14][15][16][17] especially during sensitive periods such as ITS.
Evidence of the association between NPC usage and child weight is mixed. 186][27][28] Several factors may explain these inconsistencies.First, "early" entry into NPC within the first year of life may pose as a greater risk factor for overweight or obesity, compared to later entry. 29Second, NPC type used may be accompanied with less favourable infant feeding practices (e.g., early breastfeeding cessation, early ITS < 4 months), and consequently, elevated weight gain. 30Kim and Peterson found that the use of relative care was associated with the lowest odds of breastfeeding and use of non-relative care was related to the greatest weight gain.This evidence provides a foundation for approaching the complexity of how early contexts and feeding interactions may influence weight gain.

| Current study
First, we sought to identify groups of children based on early entry into NPC (versus parental care) by 3 months of age and the timing of ITS (before or equal to/after 6 months of age).Our second objective was to examine whether the combined NPC and ITS groups were associated with child weight-for-length/weight-for-height z-scores (WFL/WFHz) from 3 to 24 months of age.We anticipated that, compared to those cared for by parents, children who entered daycare or kincare by 3 months of age would demonstrate higher WFL/WFHz over time.We also anticipated that this association would be most prominent for those who experienced ITS before 6 months.A nuanced approach is needed to determine differences in child weight outcomes based on combinations of care and ITS of which can be used to motivate strategies for coordinating feeding between parental and nonparental caregivers.ing for the study and were informed that they could withdraw at any time.Following consent and enrolment, mothers were contacted via email or phone to schedule home visits and complete surveys.The final sample includes 468 mothers and their infants starting from 1-week postpartum, and we utilized data from home visits and mother-reports at 1 and 6 weeks and 3, 6, 12, 18, and 24 months postpartum.Additional details about the STRONG Kids 2 program can be found elsewhere. 31This research was approved by the Institutional Review Board at the University of Illinois Urbana-Champaign (13448).

| Primary predictors
Three items were used to create the NPC (2 items) and ITS (1 item) groups (NPC-ITS); these three items were selected because they were assessed the closest in time (3 and 6 months).At 3 months, mothers responded to two "childcare" items: whether they had formal NPC arrangements for their child (yes or no [parental care]), and if so, mothers indicated the type of NPC (daycare centre, in-home daycare, nanny, family member, or other).We created two categories of NPC care using the five options provided to mothers, (1) "daycare" included daycare centre and in-home daycare, and (2) "kincare" included a nanny, family member, or friend; mothers identified friends as nonparental caregivers using an open-response item.Beginning around 6 months, mothers indicated their child's age at the onset of ITS.Child age at ITS was used to generate a dichotomous variable: (1) before 6 months, and (2) equal to or after 6 months.The three variables (NPC usage, NPC type, and timing of ITS) were used to generate the combined NPC-ITS groups to reflect six groups: (1) children who were in parental care and experienced ITS before 6 months (Parental care-ITS B6), (2) children who were in parental care and experienced ITS after 6 months (Parental care-ITS A6), (3) children who were in daycare and experienced ITS before 6 months (Daycare-ITS B6), (4) children who were in daycare and experienced ITS after 6 months (Daycare-ITS A6), (5) children who were in kincare and experienced ITS before 6 months (Kincare-ITS B6), and (6) children who were in kincare and experienced ITS after 6 months (Kincare-ITS A6).

| Outcomes
Children's weight and length/height were measured by trained research assistants at home visits when they were 3, 12, 18, and 24 months.At 3 and 12 months, child weight (kg) and length (cm) were measured using a digital baby scale (Seca, Model 728); child length was obtained by measuring the distance from head to foot while the child lay flat on a scale.At 18 and 24 months, child weight (kg) and height (cm) were measured using a digital scale (Health-O-Meter, Model 349 KLK) and a calibrated stadiometer (Seca, Model 213), respectively; child height was obtained by measuring the distance from head to foot while the child was standing.The average of two measurements for weight and length/height; however, a third measurement was taken if the difference in height was > 0.50 cm or the difference in weight was > 0.10 kg.Age-and gender-specific WFLz at 3 and 12 months and WFHz at 18 and 24 months were calculated using the 2006 WHO growth charts. 32

| Covariates
At the within-person level, an indicator of time was created based on child age at 3, 12, 18, and 24 months; time was centred at 3 months (0) to control for the association between child WFL/WFHz and time.
At the between-person level, mothers reported, or a trained research assistant measured, child weight (kg) and length (cm) at birth and 6 weeks, respectively; child WFLz at birth and 6 weeks were also calculated using the 2006 World Health Organization (WHO) growth charts. 32Mothers reported their pre-pregnancy weight (kg) and height (cm) at study enrolment; weight (kg) was divided by height squared (m 2 ) to create raw BMI scores and were then compared to the Centres for Disease Control and Prevention BMI ranges to determine weight status (1 = having overweight or obesity [BMI ≤ 30], 0 = not having overweight or obesity [BMI > 30]). 33Mothers reported the primary mode of feeding their child at 3 months (1 = exclusively breastfed/ fed breastmilk, 0 = having some formula), their marital status (1 = married, 0 = not married), and their household income (1 = greater than $5000 per month, 0 = less than $5000 per month).

| Analysis plan
First, descriptive analysis was used to determine the NPC-ITS groups using cross-tabulation of NPC usage and type with the timing of ITS.
Dummy indicators were created for the NPC-ITS group variables and were used in a visual inspection of the data and in the predictive model.Second, multilevel modelling was used to account for the T A B L E 1 Child and maternal characteristics reported at 6 weeks postpartum and child WFL/WFHz scores measured or reported from 1 week to 24 months (n = 468).Data management and descriptive analyses were conducted using Stata 17, 34 and the multilevel regression was conducted using Mplus 8.6. 35Given the longitudinal nature of the data, there is some missing data; missing data in the outcome variable ranged from 2% to 12% over time, with the latter due to attrition at the later timepoints of data collection.However, the full information maximum likelihood (FIML) method in Mplus was used to allow for all cases to be included in the model estimation (n = 468).The maximum likelihood robust estimator was used to obtain robust standard errors.

| Sample characteristics
Descriptive statistics for child and maternal characteristics are provided in Table 1.The majority of mothers identified as white (80%), held a Bachelor's degree or higher (74%), and were married (84%).On average, mothers were 31 years of age at 6 weeks postpartum, and nearly half of the children were male (51%).

| Identifying the nonparental care-introduction to solid foods groups
Table 2 provides descriptive statistics for the NPC usage and type, timing of ITS, and the combined NPC-ITS groups.Of the 468 families, 96% responded to the "childcare" usage item at 3 months and 86% provided their child's age at ITS. Forty-two percent of the sample reported using NPC at 3 months, with the majority using daycare (59%).Just over half of the sample commenced ITS when their child was 6 months or older (53%), and among children who experienced ITS before 6 months, they were, on average, 4.54 months of age.Six groups were identified using NPC usage and type with timing of ITS: 27% were in Parental care-ITS B6M, 31% in Parental care-ITS A6M, 12% were in Daycare-ITS B6M, 14% were in Daycare-ITS A6M, 10% were in Kincare-ITS B6M, and 7% were in Kincare-ITS A6M.Table S1 presents univariate and non-parametric tests examining differences in child and family characteristics by combined NPC-ITS groups;   children who were in parental care by 3 months or were in kincare but introduced to solids after 6 months (Kincare-ITS A6).

| Multilevel regression
Table 3 provides the multilevel regression results examining the association between NPC-ITS group membership and child WFL/WFHz

| DISCUSSION
Independently, NPC usage and early ITS have both been identified as potential risk factors for child overweight and obesity. 8,16,30To our knowledge, none have explored combinations of NPC usage and type with the timing of ITS.We sought to further elucidate the relationship between early nonparental use and type with the timing of ITS to determine which combination of factors are the most consequential for child WFL/WFHz across the first 2 years of life.
Consistent with national reports, 13 just under half of the children in the sample were in NPC by 3 months of age.NPC did not appear to be associated with beginning ITS prior to the 6-month recommendation, as proportions appeared equal across groups.This finding contrasts previous research demonstrating that early entry into NPC was linked to early ITS (prior to 4 months). 30Because the recommended timing of ITS recently shifted to around 6 months of age (as opposed to 4-6 months), 12,14 previous research has likely defined "early" ITS as occurring before 4 months of age.We found that children who began ITS before 6 months were between 2 and 5 months of age, and those who began ITS after 6 months were between 6 and 18 months of age.These findings provide insight into utilizing the 6-month recommendation for ITS in relation to entry into NPC.
Overall, child WFL/WFHz increased from 3 to 18 months and then declined at 24 months, exhibiting the adiposity rebound, however, child WFL/WFHz over time differed based on NPC-ITS group membership in the hypothesized direction.Compared to children in parental care, those who were in daycare by 3 months of age, regardless of their ITS timing, exhibited the highest WFL/WFHz over time.
These findings join others who have demonstrated that infants in NPC demonstrate higher weight than those cared for by parents. 30,36r findings also echo a recent systematic review, identifying centrebased care as associated with increased risk of having overweight or obesity 16 ; we found that the timing of ITS did not alter WFL/WFHz outcomes for children in daycare when compared to parental care, and these findings extend the knowledge base of the associations with using centre-based or in-home daycare during early infancy and child weight outcomes, regardless of ITS timing.
Descriptively, child WFL/WFHz was highest for those in daycare, however, children who were in kincare and experienced ITS before 6 months also demonstrated high WFL/WFHz over time, compared to parental care.These findings align with existing research demonstrating risk for elevated weight gain among infants cared for by relatives as opposed to by parents. 16,30,37Benjamin-Neelon and colleagues found that care by grandparents was associated with higher weight at 12 months in a sample of predominantly Black infants. 37Our data suggests that early ITS may place children at a disadvantage in terms of weight outcomes among those in kincare.We observed that the children in kincare who began ITS "on time" exhibited the lowest WFL/WFHz over time compared to all other care-ITS groups.
The timing of ITS appeared to have more influence on those in kincare, which may reflect informal versus formal arrangements.Informal NPC arrangements (e.g., kincare) have been consistently associated with poorer weight outcomes, 16 while formal arrangements (e.g., daycare) often follow guidelines for infant care and feeding to meet state licensing requirements.Because children in daycare demonstrated higher WFL/WFHz regardless of ITS timing, formalized NPC arrangements may eliminate the influence that ITS timing has on child weight.Although formalized care settings may follow guidelines, 38,39 caregivers may be unable to respond appropriately or swiftly to infant hunger and satiety cues due to higher infant-to-caregiver ratios and time constraints on feeding.This scenario may be suggestive of overfeeding or overeating in settings such as daycare where caregivers must simultaneously tend to multiple children.Contrariwise, children in kincare likely received care with a much lower infantto-caregiver ratio.Among those in kincare who experienced ITS after 6 months, the null association may reflect a "goodness of fit" between NPC type and "on time" transition.Our findings both echo existing evidence linking kincare usage to higher child weight and provide insight into under what conditions may improve weight outcomes for children in kincare.
Caregivers play a crucial role in shaping children's lifelong dietary behaviours, 3,[10][11][12] and nonparental caregivers and settings have been identified as key figures in childhood obesity prevention. 10,17,40Early and late ITS, respectively, carry long-term consequences such as risk for obesity and nutritional deficiencies, allergies, and poor eating patterns. 8,41Thus, nonparental caregivers, as well as parents, can support children's weight development and eating patterns by following recommendations for introducing solid foods at developmentally appropriate times.A 2018 study found that only 25% of programs in California reported that infants started solid foods at or around the 6 months. 42Nearly half of programs reported that parent feeding preferences and practices made it hard to introduce solids at the recommended age.Supporting and encouraging both parental and nonparental caregivers, and in particular, kincare providers, to introduce solid foods at or after 6 months of age may be an avenue for promoting child weight development.
For most, NPC is a necessity, not a choice, and as a result, parents and nonparental caregivers share the responsibility of feeding children.It is vital that we identify ways to support parents who rely on NPC and the caregivers who provide care.A recent qualitative study with early childhood professionals revealed that ITS is a common area where parents need more support and reported needing more nutrition education materials for educating and engaging with parents on early feeding practices, including culturally representative information. 43Findings from the current study underscore the need for the development and accessibility of resources for those caring for and feeding infants and young children.
Several covariates emerged as significant predictors of child WFL/WFHz.Time and higher weight at 1 and 6 weeks were associated with increased child weight. 8Maternal marital status was also a significant predictor of child WFL/WFHz; however, this finding may be concomitant with other demographic factors such as household income.Despite evidence from previous research, 2,8,44 we did not find that maternal pre-pregnancy BMI status, household income, nor breastfeeding status were significantly associated with child WFL/WFHz.Some possible reasons for null findings may be due to lack of variability in the covariates: most children in this sample were exclusively breastfed/fed breastmilk through 3 months of age and the household income of the sample was negatively skewed (many families had a monthly income up to or exceeding $5000), both of which may be protective factors against developing overweight or obesity.
Future research with larger samples and greater variability in child and family characteristics would allow for more robust examination of these early life influences.
Our findings complement and extend the previous literature, yet several areas warrant further examination.First, it is unclear whether the type of NPC leads to early ITS.Breastfeeding cessation and early ITS may be concomitant with mothers return to full-time work, and as a result, entry into NPC.However, we do not have this information in the current study sample.Second, the current study does not have access to the infant feeding practices of nonparental caregivers.Less desirable infant feeding practices among nonparental caregivers have been linked to increased child weight, 30 however, providers may lack access to resources or information needed to enact more desirable infant feeding practices. 43Third, NPC has been defined in a myriad of ways.A similar study categorized NPC groups as relative care, informal care, formal care, and combinations of those types of care; infants who were received more than one type of NPC or were in relative care demonstrated higher weight. 37In the current study, NPC groups were categorized using a data-driven approach; various types of NPC were combined to form equitable groups for analysis.Within the defined groups, there may be differences within daycare (centrebased versus in-home care) and kincare settings (relatives, friends, versus nannies), however, we lack the variability or power to detect these differences.

| Strengths and limitations
The primary strength of the current study is having access to a longitudinal, birth cohort study of children and their families.The study design and sample size allowed for a nuanced examination of NPC, the ITS, and child WFL/WFHz over time.However, several limitations must be acknowledged.First, the demographic characteristics of the birth cohort limit what inferences can be made about children and families who experience greater economic hardships and do not identify as white.Children who are identified as non-white and whose families have less access to resources are more likely to experience overweight and obesity. 2 Additional efforts are needed to improve the demographic diversity of children and families in birth cohort studies to investigate early life factors and sociodemographic characteristics on child growth outcomes.Second, daycare was the dominant form of NPC, resulting in small subsample sizes for the kincare groups. 13Our findings related to kincare should be interpreted with caution and replication with larger samples is needed.Third, children may have entered NPC between 3 and 24 months of age, but limitations of the SK2 data did not allow for the inclusion of a time-varying indicator of NPC usage at 12 and 24 months.

| Conclusion
This study identifies associations between NPC and the timing of ITS with increased child WFL/WFHz across the first 2 years of life.Children who were in daycare exhibited the highest WFL/WFHz over time, regardless of the timing of ITS.This association was more nuanced for children in kincare; participation in kincare and experiencing ITS before 6 months was associated with increased WFL/WFHz over time.For many U.S. families, NPC is essential as most children are being cared for and will continue to be cared for by nonparental adults during infancy and early childhood.There is a need for a partnership approach among parental and nonparental caregivers to support the feeding of infants throughout the transition to solid foods.

AUTHOR CONTRIBUTIONS
JB refined the concept for this manuscript, conducted data cleaning, analysis, and interpretation, and wrote and edited the manuscript.AL provided feedback on the concept for this manuscript and wrote and edited the manuscript.MF contributed to the concept for this manuscript and wrote and edited the manuscript.BF received funding for the research.BM developed the initial concept for this manuscript.BF and BM supervised the research and edited the manuscript.All authors reviewed and approved the manuscript for publication.

2 | METHOD 2 . 1 |
Sample and procedureData were drawn from the STRONG Kids 2 longitudinal, birth cohort study on early childhood health and development.Women were recruited from healthcare facilities and birthing classes during their third trimester from 2013 to 2017 in east-central Illinois.Exclusion criteria included premature birth (<37 weeks), birth conditions precluding normal feeding (e.g., cleft palate), and low birth weight (<2.50 kg).Mothers provided written informed consent when register- repeated measures nested within individuals over time(3, 12, 18, and 24 months of age) and to examine whether the NPC-ITS groups were associated with child WFL/WFHz over time.Children in any NPC (daycare or kincare) by 3 months were the primary focus; children who were in parental care by 3 months (regardless of ITS) were included as the reference group for comparison, and the regression coefficients can be interpreted as compared to children who were in parental care.Child and maternal characteristics included as covariates in the predictive model were child WFLz at birth and 6 weeks, mode of feeding (1 = exclusively breastfed/fed breastmilk through 3 months), maternal pre-pregnancy BMI status (1 = having overweight or obesity), maternal marital status (1 = married), and household income (1 = greater than $5000 per month).

F I G U R E 1
Average weight-for-length/weight-for-height z-scores over time by known groups of early into nonparental care (compared to parental care) and timing of introduction to solid foods (ITS) (black lines = parental care; orange area = daycare, green area = kincare).ITS B6 = introduction to solid foods before 6 months; ITS A6 = introduction to solid foods after 6 months.T A B L E 3 Multilevel regression predicting child WFL/WFHz from 3 to 24 months (n = 468).
over time.Children who were in daycare by 3 months and experienced ITS before or after 6 months (Daycare-ITS B6 and A6) demonstrated greater WFL/WFHz from 3 to 24 months (β's = 0.15 and 0.18, p < 0.01), compared to their parental care counterparts.Similarly, children who entered kincare by 3 months and experienced ITS before 6 months (Kincare-ITS B6) also demonstrated greater WFL/WFHz over time (β = 0.16, p < 0.01).The association between kincare use and ITS after 6 months (Kincare-ITS A6) with child WFL/WFHz was not significant, however, the association was trending in the negative direction (β = À0.05,p = 0.450).There were several significant associations with time and child and maternal characteristics.At the within-person level, time was a significant predictor of child WFL/WFHz over time (β = 0.30, p < 0.001), suggesting that child WFL/WFHz significantly increases from 3 to 24 months.At the between-person level, child WFLz at birth and 6 weeks were both positively associated with WFL/WFHz over time (β's = 0.19 and 0.43, p < 0.01), and children whose mothers were married demonstrated lower WFL/WFHz from 3 to 24 months (β = À0.15,p < 0.01).No significant associations emerged for exclusive breastfeeding/fed breastmilk, maternal pre-pregnancy BMI status, or household income.

Table S2
Descriptive statistics of nonparental care usage and type and introduction to solid foods.
6 months (Kincare-ITS A6) appeared to have the lowest WFL/WFHz across all NPC-ITS groups.For all children, WFL/WFHz appears to increase across the first 18 months of life and then declines starting at 24 months.In general, it appears that early entry into daycare (regardless of ITS timing) and kincare with ITS before 6 months (Kincare-ITS B6) may be related to elevated weight, compared toT A B L E 2