As the prevalence of childhood obesity increases, researchers continue to attempt to identify factors contributing to obesity. The purpose of this study was to define the relationship between birth weight, rapid weight gain (RWG), and early childhood obesity in a low-income, inner-city minority population. In this retrospective chart review, researchers documented every medical encounter recorded in the chart from birth to 3 years for 203 3 year old minority children from low-income families living in an urban area. Based on Center for Disease Control and Prevention's growth charts and tables, z-scores at birth, 4 months, and 1 year were calculated and RWG determined. Researchers determined Body Mass Index percentiles at 3 years of age using the last available weight and height between 24 and 38 months of age. Eight percent of children were underweight, 62% were normal weight, 12% were at overweight and 18% were obese. Children who experienced RWG during the first year of life were 9.24 (CI: 3.73–22.91) as likely to become obese as those who did not experience RWG. Neither low birth weight nor being male increased the odds of becoming obese. Low birth weight predicted underweight at 24–38 months. In this high-risk population, children experiencing RWG during the first year of life have a significantly increased risk of being obese during the preschool years. Future research should identify factors leading to RWG, including specific infant feeding practices.

Introduction

In the US, the prevalence of childhood obesity, including obesity among preschool-aged children (2–5 year olds), has steadily increased over the past three decades.[1] Of special interest, preschoolers from minority and low-income families are more likely to be obese than the general population. Among the nation's 2–5 year olds, 13.0% of non-Hispanic Blacks and 19.2% of Mexican Americans are obese compared to only 11.5% of non-Hispanic Whites.[1] Almost 15% of preschool-aged children from low-income families are obese and an additional 16% are overweight.[2] Preschoolers from low-income families living in urban areas, particularly minority children, are at an even greater risk. Of the 3 year olds participating in the Fragile Families and Child Wellbeing Survey from 1999 to 2003 (a survey of low-income families living in urban areas), 18% were obese with an additional 17% overweight.[3] Black and Hispanic children were more likely to be obese (17 and 24% respectively) than their White peers (14%).[3]

While little is known about the adverse health outcomes related to preschool obesity, childhood obesity is associated with numerous physical and mental health issues, including cardiovascular disease,[4] Type II Diabetes Mellitus,[5] orthopedic difficulties,[6] lower self-esteem,[7] and a more negative body image.[8] Obese children are also more likely to become overweight or obese adolescents and adults.[9–11] Preventing early childhood obesity, otherwise known as preschool obesity, could be a critical point in the battle against childhood and adult obesity and weight-related diseases.

In the search for predictors of preschool obesity, some research efforts have focused on the early critical periods for growth and development. These periods include intrauterine growth and early infancy. Traditionally, low birth weight (LBW) is indicative of restricted fetal growth.[12] Many researchers believe this restricted growth programs individuals for future adverse outcomes, including overweight,[13, 14] however, conflicting evidence regarding the relationship between fetal growth, future weight gain and adiposity exists.[15]

Defined as an abnormal acceleration of growth over a specified time period, rapid weight gain (RWG) may be a determinant of future health outcomes, including childhood overweight.[16, 17] Research indicates that RWG, a possible indicator of over-nutrition in early infancy, is a strong predictor for preschool and childhood obesity.[18–20] This study's objective was to define the relationship between low birth weight, rapid weight gain, and early childhood obesity in an urban, low-income, minority population.

Methods

The University of Connecticut's Institutional Review Board approved all protocols for this Health Insurance Portability and Accountability Act-compliant study.

Data Source

The data for this retrospective analysis originated from two randomized chart reviews investigating the prevalence of anemia in minority preschoolers from low-income families living in Hartford, CT during 1999–2002. Chart reviews occurred at pediatric primary care clinics and the major hospitals in Hartford. During the 1999 chart review, medical records of children born between July 1, 1994 and June 30, 1995 were randomly selected for review. Likewise, for the 2002 review, medical records of children born between July 1, 1998 and June 30, 1999 were randomly selected for review. Two hundred, sixty-four charts and 283 charts were reviewed in the 1999 and 2002 studies, respectively. A detailed description of the original chart reviews materials and methodology appears elsewhere.[21]

For this secondary analysis, records must have included sex, birth weight, and height and weight at the same visit at 4 months, 1 year and between 24 and 38 months. The t-tests and Chi-square analyses revealed no significant differences between included and excluded charts. The final complete case sample size for this analysis included 96 medical charts from the 1999 review and 107 charts from 2002. Chi-square analyses and t-tests determined no statistical differences between the two original reviews on all major variables to be used for the secondary analysis. Therefore, the two datasets were combined for a final sample of 203 charts. In 2003, Sanders and Lee[22] reported 13% of kindergarteners in Hartford are overweight. A power analysis using a prevalence of obesity of 13% for the null hypothesis showed that with a sample of 203 a 7% difference in obesity prevalence would be detectable with 80% power at P = 0.05. Thus, the final sample size (n = 203) provided adequate power to detect significance.

Variables

For the primary study, data collected from each record included all medical encounters, including but was not limited to birth records, sick and well child doctor visits, emergency room visits, and in-hospital stays. When available, researchers recorded weights and heights for each visit, as well as demographic variables, including ethnicity, sex, type of insurance, and age of mother at birth. For this secondary analysis, additional variables were generated through manipulation of already established variables.

BMI Percentile and Weight Class

After 2-years-of-age, height and weight from the same visit were converted into body mass index (BMI). BMI was calculated using weight in kilograms divided by height in meters squared. Then, using Center for Disease Control and Prevention's SAS program (CDC 2005), growth chart percentiles were calculated for BMI-for-age and sex. The dependent categorical variable Weight Class was determined using BMI-for-age and sex percentile between 24 and 38 months of age. Underweight was defined as < 5th percentile for BMI-for-age; normal weight was ≥5th percentile but < 85th percentile; overweight was ≥85th percentile but < 95th percentile; and obesity was ≥95th percentile. A special category of obesity, extreme obesity, was defined as ≥97th percentile.

Weight Change and Rapid Weight Gain

Researchers calculated z-scores for weight at birth, 4 months, and 1 year relative to the CDC's reference population. Weight change during the first 4 months and first year old life was defined as changes in weight-for-age z-scores between birth and 4 months and birth and 1 year, respectively. Rapid weight gain (RWG) was defined as an increase in weight-for-age z-scores ≥0.67 standard deviations (SD) between birth and 4 months. A second category of RWG included children whose weight-for-age z-scores increased ≥0.67 SD between birth and 1 year of age. While both 0.67 and 1 SD increases in weight-for-age z-scores are accepted in the literature as cut-offs for rapid weight gain (4,5), 0.67 SD was chosen as it is a broader inclusion criteria that presumably would make finding significant differences between groups more difficult.

Statistical Methods

The final data set included complete cases only (n = 203). Chi-square and t-tests were used to describe associations between weight class and RWG at 4 months, RWG at 1 year, birth weight, and sex. Multiple regression was used to estimate the strength of the association between BMI percentile and RWG and birth weight. Sex was included in the model to control for the potentially confounding effects of this variable on observed associations.

Statistical significance was set at P ≤ 0.05. To confirm findings from the complete case-only analysis, researchers conducted a second analysis of the full 547 chart data using multiple imputation (MI) to manage missing data. All analyses were performed using SAS version 9.1 (SAS Institute, Cary, NC).

Results

Subject characteristics (n = 203) are described in Table 1. Almost 18% of children were obese between 24 and 38 months, with an additional 12% overweight. Male children comprised 61% of the sample. Indicating a low-income and minority sample population, Medicaid insured 85% of subjects, 54% of subjects were Hispanic, and 32% were African-American. Nine percent of children were born LBW (< 2,500 g) and 19% were born preterm (< end of 37 weeks gestation). Twenty-four percent of children included in this study were born to mothers younger than 19 years of age. None of the children were exclusively breastfed for more than 2 weeks. At 4 months of age, 54% of children had experienced RWG since birth, while 40% of subjects experienced RWG from birth to 1 year.

Unadjusted Analyses

Unadjusted odds ratios for birth weight, sex, RWG at 4 months and RWG at 1 year according to weight class at 24–38 months are presented in Table 2. Children falling into the weight class "normal weight" were used as the reference sample. Children who experienced RWG between birth and 1 year of age were 9.2 times (95% CI: 3.7–22.9) more likely to be obese at 24–38 months and 31.2 times (95% CI: 4.0–245.9) more likely to be extremely obese at 24–38 months than those who did not experience RWG. Children born with a LBW were 4.2 times (95% CI: 1.1–15.5) more likely to be underweight at 24–38 months than those born normal weight. Researchers found no significant associations between sex or RWG at 4 months and BMI category at 24–38 months (P > 0.05).

Adjusted Analysis

To account for variables potentially confounding the association between RWG and preschool obesity, investigators used multivariate logistic regression model. LBW, RWG, and sex were regressed against obesity at 24–38 months. A chi-square test of independence revealed the variables RWG at 4 months and RWG at 1 year were not independent of each other; therefore, RWG at 1 year but not RWG at 4 months was included in the model. Results indicated an independent association between RWG at 1 year and obesity at 24–38 months (OR = 11.7; 95% CI: 4.5–30.0). Neither LBW nor sex were significant predictors for obesity (P > 0.05) in the multivariate model.

Logistic regression analyses were repeated using MI on the full dataset (n = 547). Results were similar to those analyses using only complete cases (n = 203) (results not shown).

Discussion

In this sample population of low-income, minority children living in an urban area, 18% of the 24–38 month old children were overweight with 7% of those being extremely obese. Compared to their normal birth weight peers, children born with a low birth weight were over four times more likely to be underweight between 24 and 38 months. Additionally, children were over nine times more likely to be obese and 31 times more likely to be extremely obese during early childhood if they experienced RWG between birth and 1 year of age.

The Fetal Programming Hypothesis asserts that restricted fetal growth, indicated by LBW, leads to future excessive weight gain and obesity.[13] Ravelli et al.[23] found that boys whose mothers were pregnant during the Dutch famine of World War II were at an increased risk of becoming severely obese at 19 years of age. Loos et al.[24] reported that LBW leads to increased fat and decreased muscle mass in adulthood. However, evidence exists to contradict this hypothesis. Some researchers have found birth weight is positively associated with future weight.[3, 9, 25] Kimbro et al.[3] reported that in a sample population similar to the current study, birth weight was a significant positive predictor for weight at 3 years of age, concluding that LBW is a protective factor against obesity in early childhood. Still others report no association between birth weight and future weight outcomes.[10] Adding to the enigma surrounding birth weight and future weight, the present study showed that LBW was indicative of underweight at 24–38 months. The difference in outcomes may be attributable to the sample population differences, as income level, ethnicity, and geographic living conditions varied for each study. Furthermore, the full impact of fetal programming on weight may not emerge until later in life.

In the past, researchers have reported children experiencing RWG during critical periods of growth are at an increased risk for childhood[18–20, 26, 27] and adult obesity.[28, 29] While Dennison et al.[20] studied the effects of RWG on early childhood weight in a low-income, multi-ethnic cohort, the present study is the first in the US to examine the relationship between RWG and preschool obesity in a low-income, minority, urban population. Confirming the findings from Dennison et al. and other studies, this study's results indicate RWG during the first year of life strongly predicts early childhood obesity.

Two factors may explain the relationship between early predictors of weight and preschool weight class: genetics and environment.[30] One hypothesis is that the link between birth weight, RWG and preschool weight class is genetic. This hypothesis purports that children who experience RWG during infancy are more likely to become obese during preschool, because these children are genetically predisposed to being larger. Beginning at birth, these children's predestined growth plots begin to express themselves, leading to excessive weight gain during infancy and eventually obesity during the preschool years. Likewise, children born LBW could be genetically bound to being smaller. In this case, beginning in utero, these children's genetic makeup inhibits normal growth, causing the children to be small at birth. This pattern continues into the preschool years, resulting in underweight at 2–3 years of age.

An alternative explanation connects preschool weight class and early infancy predictors using an environmental approach. If a child is placed in a setting that leads to RWG during infancy, this same setting could also lead to continued excessive weight gain and obesity during the preschool years. Similarly, if a child is deprived of adequate nourishment in utero, leading to LBW, the same poor circumstances might exist outside the womb, increasing the chances that a child would continue to be underweight into early childhood. While these two hypotheses can rationalize the association between birth weight, RWG, and preschool weight class, a combination of the two stands as the likely origin of abnormal weight gain in young children.

If RWG is an indicator of a poor environment and over-nutrition in early infancy, these outcomes indicate early infant feeding practices likely contribute to excessive weight gain by age three. Infant feeding practices include many different feeding behaviors, such as parenting style, timing of introduction to solids, inclusion of cereal in the bottle, breast feeding versus formula feeding, and variety and portion size of foods and beverages fed to a child. Children served larger portion size increase food consumption and caloric intake at that meal,[31–33] however, little is known about the impact of larger portion sizes on caloric intake in infants. Additionally, parent feeding style may play an important role in RWG during the first year of life. An authoritative parent feeding style, characterized by encouragement while allowing choices and options, is associated with increases in milk and vegetable consumption in preschool-aged children; and an authoritarian parent feeding style, characterized by feeding actions that are restrictive, forceful and controlling, is linked to decreases in vegetable consumption.[34] However, researchers have yet to investigate how parent feeding style impacts total caloric consumption in infants. Future research should explore which feeding practices specifically contribute to RWG during infancy. Once these sources are determined, healthcare providers can develop intervention strategies that focus on appropriate feeding behaviors for infants and young children.

Potential limitations of this study should be taken into account. RWG significantly predicted preschool obesity. However, potential links between RWG, preschool obesity, and infant feeding practices could not be investigated in this study due to inconsistent documentation of infant feeding practices in the medical charts. In general, data collected were incomplete as a result of omission of information by healthcare providers, missed well child check-ups, and the transient nature of Hartford residents. Yet, researchers attempted to increase completeness of data by recording every medical encounter available. When children transferred use of services to a different healthcare provider within the city, researchers tracked the changes and continued the chart review at the new location. In spite of these efforts, missing data still occurred. Therefore, the decision to use only complete cases for this secondary analysis resulted in a decreased sample size from the original dataset. Nevertheless, applying MI to the original dataset confirmed outcomes from the complete case analyses (results not shown).

Conclusions

A large proportion of low-income, minority preschool-aged children living in urban areas are obese or overweight. RWG at 1 year of life significantly predicts obesity at 2–3 years of age in this population, while LBW does not. Factors leading to RWG have not yet been elucidated. Future research endeavors should focus on possible causes of RWG during the first year of life, including infant feeding practices.