BACKGROUND: With the increasing focus on obesity, growth patterns in infancy and early childhood have gained much attention. Although the adiposity rebound has been in focus because of a shown association with adult obesity, not much has been published about the infant peak in body mass index (BMI).

OBJECTIVE: This study links age and BMI at infant peak to duration of breastfeeding and body composition at 3 y of age.

DESIGN: Frequent weight and height measurements for 311 Danish children in the SKOT (Complementary and Young Child Feeding - Impact on Short and Long Term Development and Health; in Danish) cohort were used to estimate BMI growth curves for the age span from 14 d to 19 mo by using a nonlinear mixed-effects model. BMI growth velocity before peak and age and BMI at peak were derived from the subject-specific models. Information about pregnancy and breastfeeding was assessed from background questionnaires. Assessment of body composition at age 3 y was made based on bioelectrical impedance, weight, and height.

RESULTS: A longer duration of exclusive breastfeeding was associated with an earlier peak in infant BMI (P = 0.0003) and a lower prepeak velocity (P < 0.0001). BMI level at peak and prepeak velocity was positively associated with fat and fat-free mass at age 3 y (all P < 0.0001), whereas a later age at peak was associated with a lower fat mass, fat mass index, and fat-free mass index at age 3 y (all P < 0.001).

CONCLUSIONS: BMI peak characteristics are strongly associated with both duration of exclusive breastfeeding and body composition at 3 y of age. Thus, a better knowledge of characteristics and determinants of the early BMI peak is likely to improve our understanding of early development of obesity.