Neonatal period: body composition changes in breast-fed full-term newborns.

Background: Weight loss during initial days of life in healthy infants is known to consist of loss of both body solids and total body water. However, the nature of these body composition changes needs further investigation.

Objective: To investigate the nature of changes in body composition that accompany weight loss during the first days of life in healthy term newborns.

Methods: Body composition of healthy full-term newborns was assessed using air-displacement plethysmography. Cross-sectional (n = 262) and longitudinal samples (n = 28) were assessed during the first 4 and 5 days after delivery, respectively.

Results: In the cross-sectional sample, mean body weight decreased significantly through day 4 (p < 0.001), mean fat mass (FM) and %FM decreased significantly (p = 0.005 and p = 0.031, respectively) by day 3. There was a significant decrease in mean fat-free mass on days 3 and 4 (p = 0.01 and p < 0.001, respectively). In the longitudinal sample, there was a significant decrease in mean body weight, FM and %FM (p < 0.001, p = 0.001, p = 0.013, respectively) by day 4. On day 5 there was a significant increase in mean body weight, FM and %FM (p < 0.001, p = 0.024, p = 0.036, respectively) when compared to day 4. There was no significant difference in mean FM and %FM values between day 1 and day 5.

Conclusions: These results indicate that body composition changes are constituted by a reduction in both fat and fat free mass. However, a greater %FM is lost and consequently regained by day 5 when compared to fat-free mass.