Background: Numerous studies have documented short- and long-term differences in growth and body composition based on whether an infant is breastfed or formula-fed. However, little is known about whether these differences result from the substance consumed (breastmilk vs. formula) or from the delivery method (breast vs. bottle). This prospective pilot study compared the growth and body composition patterns of 19 predominantly breastfed/nursing infants (NG) and 18 infants fed significant quantities of breastmilk by bottle (BG) during the first 6 months of life.

Methods: Infants were measured in a laboratory setting each month by trained staff. Growth measures (length, weight, and head circumference) were compared to World Health Organization growth standards. Body composition, including relative fat mass (%FM), was measured using an air-displacement plethysmography system (Pea Pod(®), Life Measurement, Inc., Concord, CA).

Results: With the exception of small differences in length and weight scaled for length (body mass index and weight-for-length z-score) present at birth and continuing for the first 1-2 months of life, NG and BG infants were similar in weight, weight-for-age z-scores, head circumference, fat mass, and %FM. However, BG infants were three times more likely to exceed the 85(th) percentile for weight velocity during the 4-6-month age interval than NG infants (33% vs. 10%, respectively), but this did not reach statistical significance in this sample size (p = 0.12).

Conclusions: This pilot study suggests the delivery method (breast vs. bottle) for breastmilk may not dramatically affect growth for the first 4 months of life; however, future research with larger samples will be needed to carefully evaluate longer-term growth patterns in infants fed breastmilk by bottle.