Patterns of early life body mass index and childhood overweight and obesity status at eight years of age

Childhood obesity is a major global public health threat. Globally, 5.6% of girls and 7.8% of boys were obese in 2016 [1]. In the United States (US), 17% of children were obese and another 15% were overweight in 2010 [2]. Childhood obesity increases the risk of type 2 diabetes, cardiovascular disease, metabolic syndrome, and later life obesity, and has adverse effects on pulmonary, musculoskeletal, and psychosocial functioning [3‐5]. There is a compelling need to prevent childhood obesity, since there are few effective non-pharmacological or non-surgical interventions to reduce excess adiposity [6].

Understanding the natural course of adiposity during infancy and childhood may help identify obesity risk factors or periods of heightened susceptibility. Previous studies have reported that earlier adiposity rebound, crossing major weight centiles during the first 2 years of life, and trajectories of rapid weight gain during infancy and childhood BMI are associated with increased childhood and adulthood adiposity and risk of obesity [5, 7‐17]. However, few studies have examined these patterns among US children using measures of body composition, like bioelectric impedance, to define later childhood overweight or obesity status [13].

To improve our understanding of the patterns of adiposity associated with risk of childhood obesity and overweight among US children, we used data from 215 children with 1070 serial anthropometry measures from ages 4 weeks to 5 years to determine if overweight or obese status at age 8 years was associated with BMI or changes in BMI in the first 5 years of life. We hypothesized that children who were overweight or obese at age 8 years would have more rapid increases in BMI in the first 5 years of life compared to children who were lean.

At age 8 years, 51 (24%) children were overweight, but not obese, and 22 (10%) children were obese, according to WHO definitions. Compared to children who were normal weight at age 8 years, children who were overweight or obese were more likely to have been breastfed for 0 to < 6 months and have mothers who were younger at delivery, non-Hispanic Black, less educated, poorer, and unmarried (Table 1). From ages 4 weeks to 5 years, 8.2–23.2% of children had a BMI z-score > 1 and 1.4–9.4% of children had a BMI z-score > 2 according to WHO definitions (Additional file 1: Supplemental Table 2). A total of 37 (17%) children were overweight and 29 (13%) were obese at age 8 years according to BIA-derived body fat percent.

The pattern of BMI z-scores among children who became obese at age 8 years was significantly different compared to children who were normal weight or overweight at age 8 years (Wald χ² p-values< 0.001). Compared to children who were normal weight at age 8 years, children who were obese had higher BMI z-scores at all ages, including age 4-weeks, and the magnitude of these differences increased as children aged (Tables 2 and 3, Figs. 1 and 2, and Additional file 1: Supplemental Tables 3 and 4). Children who became obese at age 8 years had positive rates of BMI z-score change at all ages, but the annual rates of change were greater between 4 weeks and 2 years than between 2 and 5 years.

Children who were overweight at age 8 years had statistically significantly higher BMI z-scores than normal weight children at all ages, but their pattern of change in BMI z-scores was not significantly different than normal weight children (Wald χ² p-value = 0.84). Children who became overweight at age 8 years had similar BMI z-scores as obese children at ages 4 weeks and 1 and 2 years, but their mean BMI z-score plateaued after age 2 years and was statistically significantly lower at ages 3, 4, and 5 years compared to children who became obese (Tables 2 and 3). The annual rates of change in BMI z-scores among children who became overweight at age 8 years were positive before age 2 years and then approximately 0 after age 2 years (Tables 2 and 3).

When we examined BMI z-score trajectories according to normal weight, overweight, and obesity status at age 8 years assessed with BIA, our results were similar to those when we used BMI z-score to define adiposity at age 8 years (Fig. 2, Table 3, Additional file 1: Supplemental Table 4). There were no notable differences in the patterns of child BMI z-score by overweight/obesity status when we stratified by child sex (results not shown). Our results were not appreciably different when we excluded children who were small for gestational age or preterm (results not shown).

In this cohort, we observed that children who were overweight or obese at age 8 years had higher BMI z-scores between 4 weeks and 5 years of age than their normal weight peers. Compared to children who were normal weight or overweight at age 8 years, children who were obese had greater gains in BMI during the first 5 years of life with the greatest gains in BMI during the first 2 years of life. Children who were overweight had similar rates of BMI gains as children who were normal weight, but they had higher BMI than normal weight children at all ages, including at age 4 weeks.

Our finding of more rapid gains in adiposity in the first 2 years of life among children who became obese is consistent with prior studies observing increased risk of later life obesity among infants who grow rapidly in the first 2 years of life [8‐11, 24]. Additional studies have tried to identify patterns and trajectories of early childhood adiposity associated with risk of obesity in child- or adulthood with relatively consistent results [12‐15, 25]. Our results are generally consistent with the results of most prior studies [12, 13, 15, 25]. Glavin et al. found an increased risk of obesity and overweight at age 8 years among Norwegian children with more rapid gains in adiposity during infancy and more adiposity in later childhood, findings consistent with our own [12]. In a study of US children, greater gains in adiposity in the first 3 years of life were associated with measures of body composition at age 6–10 years; these associations were strongest for adiposity gains between age 2–3 years [13]. A study of French adults reported that BMI trajectories between birth and age 10 years characterized by above average adiposity were positively associated with both BMI and waist circumference in adulthood [15]. In a longitudinal study of Chilean children, prevalent, but not incident obesity and overweight status in the first 4 years of life was associated with increased risk of being overweight or obese at age 7 years [25]. Finally, and in contrast to the current and prior work, BMI in adults born and living in India was more strongly correlated with BMI and increases in BMI in later childhood [14]. However, a large proportion of this cohort was underweight during childhood, which may explain these results.

Children who became overweight in this cohort did not experience more rapid gains in BMI in the first 5 years of life compared to lean children; however, they did have higher BMI at all ages, with these elevations becoming relatively stable at ages 3, 4, and 5 years. A prior study reported that children who were overweight at age 5 years had 4-times the risk of being obese by age 14 years compared to children who were lean [5]. Thus, children who are overweight in early childhood, but not obese, represent a susceptible sub-population who are at higher risk of developing obesity. Additional research is necessary to understand factors that increase the risk of this transition [26].

The causal link between rapid gains in adiposity during infancy and risk of obesity is not clear. Cole has argued that this is a statistical phenomenon and not a physiological one, as individuals who are obese later in life are more likely to have a higher weight earlier in life and more likely to be crossing major weight centiles earlier in life [27]. Even if rapid gains in adiposity during infancy are not causally related to obesity risk, they may be useful clinical markers for predicting subsequent risk of obesity [10]; however, additional work is needed to confirm this as simple measures of childhood adiposity may not have strong predictive value for adult adiposity or disease risk [28].

Our study has several strengths and limitations. First, we used prospectively collected and longitudinal research-quality anthropometry measurements to characterize children’s BMI trajectories. In addition, we used BMI and BIA to define overweight and obesity status at age 8 years. However, our sample size was modest when examining the number of children who were obese or overweight at age 8 years. Despite this, we observed large and statistically significant, albeit imprecise, differences in mean BMI z-scores or rates of BMI z-score change when comparing children who were obese, overweight, or normal weightat age 8 years. Second, we used linear fixed effect models to characterize average BMI and rates of BMI change over the first 5 years of life according to age 8-year adiposity status. While prior studies have classified longitudinal measures of adiposity and related these to subsequent adiposity status, it was unlikely that our sample size was large enough to identify unique classes with sufficient sample size [29]. Indeed, prior studies using these methods with larger sample sizes (n > 1000) still observed relatively small cell sizes for some trajectories of adiposity (e.g., n < 20) [15]. Third, while we adjusted for several determinants of childhood obesity, we could not adjust for child diet or physical activity. In addition, our modest sample size precluded us from examining differences in subgroups of children; however, we did observe similar patterns of adiposity gain among children without in utero growth restriction or who were born at term. Fourth, we examined a group of healthy and typically developing US children. Still, our findings may not be generalizable to other populations if there are environmental or genetic factors that modify the relations between growth and risk of childhood obesity. Finally, we examined several features of early childhood growth and some of our results may be chance findings. However, these features were consistent when we used either BMI or BIA to define overweight/obesity status at age 8 years, thus reducing the possibility of spurious results.

Our results, from this modestly sized cohort, contribute to the body of literature suggesting that the risk of childhood obesity has origins in early in life. Efforts to identify interventions to prevent childhood obesity should focus on the periods of gestation and infancy as obesity risk may be malleable during these periods.