Aim: Leptin, a hormone that regulates food intake and energy metabolism, is present in breast milk and thus may be involved in body composition differences between breastfed and formula‐fed infants. The aim of this study was to evaluate whether diet and gender affect plasma leptin concentration in breastfed and formula‐fed infants during the first months of life. Methods: Anthropometric and bioelectrical impedance measurements [total body water (TBW) calculated with the Fjeld equation] were made and venous blood plasma samples were analysed for leptin concentration in healthy, exclusively breastfed or formula‐fed Italian infants in the first year of life. Infants were subdivided in two ways: three groups (periods) in relation to age, and five groups in relation to weight. Results: The average serum concentration of leptin was 7.35 ng ml‐1. Serum leptin values were higher in breastfed than in formula‐fed infants. Breastfed infants in group 1 had a statistically higher serum leptin concentration (2500–3749 g). There were no significant differences in anthropometric measurements, body mass index or skinfold thickness between breastfed and formula‐fed infants. In the periods I and II, breastfed infants had a significantly higher TBW than formula‐fed infants. Males had a significantly higher TBW than females in periods I and II. Breastfed infants in group 2 (3750–4999 g) had a significantly higher TBW than formula‐fed infants. Conclusion: The data on TBW, weight and skinfold thickness suggest that the higher leptin concentration observed in breastfed infants in the first months of life may be due not only to adipose tissue production but also to human milk.

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A preliminary observation is reported concerning total and high-density lipoprotein blood cholesterol levels in 36 infants at 3 months of age fed with a hypoantigenic milk formula. The values are compared with those of 66 breastfed and 76 conventional formula-fed infants. Total and high-density lipoprotein-cholesterol levels in hypoantigenic formula-fed infants (152 and 65.7 mg dl(-1), respectively) were comparable to those of breastfed infants (148 and 61.8 mg dl(-1)) and significantly (p < 0.05) higher than those of conventional formula-fed infants (130 and 42.5 mg dl(-1)).