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In preterm infants, longitudinal growth patterns have stronger association with clinical outcomes than cross-sectional metrics. For qualitative growth, a one-time body composition measurement at near term is common, and here we explore the potential use of a novel estimated longitudinal body composition metric (adjusted fat-free mass deficit) using birth anthropometrics

Background: Body composition assessment using noninvasive air displacement plethysmography (ADP) can help determine the quality of postnatal growth in infants. The accretion rates of fat mass (FM) and fat-free mass (FFM), both are known to change in various clinicopathological situations in a discordant fashion, can also help evaluate the short-term impacts of nutritional interventions on body composition. Objectives: To determine the FM and FFM accretion rates from 30 to 45 weeks' postmenstrual age (PMA) and whether these rates are different between male and female infants. Methods: We used previously published normative data with median FM and FFM values for infants at 30-45 weeks' PMA (Norris et al., 2019). Weekly gains in FM and FFM in g/week and g/kg/week were calculated using early one-point and average two-point methods. Results: FM and FFM accretion rates calculated by the early one-point method were higher than the average two-point method. Male and female infants had similar FM and FFM accretion rates. Constant accretion rates of FM and FFM were not aligned with individual weekly accretion rates, which showed a twofold-fourfold change. A composite index (FFM/FM accretion rate ratio), which we named the "body composition accretion ratio" (BCAR), was more sensitive than the individual weekly accretion rates and showed a ninefold change during the study period. Conclusions: Weekly FM and FFM accretion rates can help assess quality of postnatal growth in young infants, but BCAR can be a more useful, sensitive index for early identification of body composition changes and may possibly guide nutritional interventions.