© The Author(s) 2016. High infant weight gain is associated with subsequent overweight and obesity and so may contribute to related comorbidities such as metabolic syndrome and cardiovascular disease. The growth acceleration hypothesis proposes that early and rapid growth in infancy can shape the metabolic profile to increase susceptibility to obesity. Although breastfeeding reduces the infant's risk of subsequent overweight and obesity, high infant weight gains are observed in this population. We report the case of an infant with excessive weight gains that persisted throughout 6 months of exclusive breastfeeding. The mother chose to continue breastfeeding despite medical advice to wean, and high weight gains continued for a further 8 months of breastfeeding and complementary foods. This is the first reported case of an exclusively breastfed infant with excessive weight gains (> 97th percentile) with contemporaneous measures of 24-hour breast milk intake and doses and concentrations of protein, fat, and energy. We found a high breast milk protein dose, which is associated with increased weight gain and lean body mass but not necessarily adiposity. It is likely that other influences also contributed to the high infant weight gain. High infant weight gain is multifactorial, with evolving evidence for the role of adipokines and genetic markers. Advice to replace breast milk with formula affects the dose-dependent protection from noncommunicable diseases afforded by breast milk and may not be an effective strategy for reducing adiposity.