In this case-control study, preterm infants were compared with term infants in the cardiometabolic outcomes at school age, and important factors associated with outcomes such as post-discharge growth, weight, lean body mass and fat mass were considered and adjusted. Early school-aged children who were born preterm had higher BP, more insulin resistance, and higher fasting glucose than those who were term infants, while weight and fat mass were lower in the preterm group. Insulin resistance and BPs were associated with preterm birth, independent of lean mass index and fat mass index. The weight z-score change from the neonatal period to school age was not associated with BPs but had a correlation with insulin resistance in the multivariate analysis.
Patterns of growth, as well as alterations in adipose tissue, were different between SGA infants and preterm infants. Previous SGA studies reported that they had less adipose tissue at birth, becoming similar after the neonatal period [17], and a higher fat distribution was observed in childhood and adolescence [18, 19]. However, for the preterm infants, lower BMI and fat mass were frequently seen compared with term infants until infancy and school age [20, 21], even though they had higher BMIs and fat mass with adverse cardiometabolic outcomes in adulthood [22–25].
Although the interaction between obesity and insulin resistance is a key pathogenesis factor in the development of metabolic syndrome [26], cardiometabolic problems in preterm infants at school age have been reported, despite a lower BMI during this period [10, 27]. In the present study, to elucidate the effect of body composition more clearly, fat mass and lean body mass were measured and adjusted, and the results showed that preterm birth was independently associated with increased BP and HOMA-IR.
Moreover, as catch-up growth is an important factor in the development of insulin resistance [28], weight z-score change until school age was considered and adjusted, and it was found to be associated with insulin resistance. The weight z-score was much lower in preterm infants at discharge compared to term infants, with a subsequent increase by early school age, resulting in a higher weight gain velocity in preterm infants in this study, even though the weight z-score was still lower than that of term infants. As with SGA infants [29, 30], preterm infants experienced the development of insulin resistance during rapid weight gain [31].
On the other hand, while BPs were associated with preterm, weight z-score change was not associated with BPs at school age. Although the mechanisms of elevated BP in children born prematurely are not fully understood, impaired development of the glomeruli with decreased nephrons, microvascular growth arrest, and sympathoadrenal overactivity might be contributing factors [9, 32, 33]. These conditions were associated with preterm birth and related morbidities during a neonatal intensive care unit stay, rather than the pattern of growth beyond the neonatal period. However, the influence of weight gain velocity on increased BP should not be ignored because a longitudinal cohort study from the UK showed that growth gain velocity from 1 year of age until adolescence was correlated with systolic and diastolic BP [34].
When preterm infants were further categorized according to their growth velocity, cardiometabolic factors were consistently higher in the preterm infants with improved growth group, compared to term infants. However, there were no differences in the HOMA-IR and fasting glucose between term infants and preterm infants with negative growth, and HOMA-IR was even lower in the preterm infants with negative growth group than in the group with positive growth. Interestingly, systolic BP was higher in both preterm infant groups, regardless of growth velocity after discharge, and there were no differences in the systolic and diastolic BPs between the two preterm groups. Notwithstanding the small sample sizes used for these subgroup analyses, they did demonstrate an impact of the growth pattern in preterm infants on adverse cardiometabolic findings, as shown in the multivariate analysis.
There are several limitations to our study. A relatively small patient population was analyzed, and the growth velocity of the early post-discharge period, such as the time between discharge and one year of age, was not compared. Also, the fat mass was not measured by dual energy X-ray absorptiometry (DEXA). However, bioelectrical impedance analysis is a useful method for estimating body composition and has been used in both clinical and research fields in the pediatric population [35, 36]. The aforementioned New Zealand Very Low Birth Weight Study also used bioelectrical impedance as a method to measure fat mass [5].