Background & aims: Breastfeeding is the gold standard infant feeding. Data on macronutrients in relation to longitudinal body composition and appetite are very scarce. The aim of this study was to investigate longitudinal human milk macronutrients at 1 and 3 months in association with body composition and appetite during early life in healthy, term-born infants. We hypothesized that infants receiving higher caloric human milk would have more body fat mass and satiate earlier. Methods: In 133 exclusively breastfed infants (Sophia Pluto Cohort), human milk samples at 1 and 3 months were analyzed for macronutrients (fat, protein, carbohydrate) by MIRIS Human Milk Analyzer, with appetite assessment by Baby Eating Behavior Questionnaires. Fat mass (FM) and fat-free mass (FFM) were measured by PEA POD and DXA, and abdominal FM by ultrasound. Results: Milk samples showed large differences in macronutrients, particularly in fat content. Protein and energy content decreased significantly from 1 to 3 months. Fat and carbohydrate content tended to decrease (p = 0.066 and 0.081). Fat (g/100 ml) and energy (kcal/100 ml) content at 3 months were associated with FM% at 6 months (β 0.387 and 0.040, resp.) and gain in FM% from 1 to 6 months (β 0.088 and 0.009, resp.), but not with FM% at 2 years. Carbohydrate content at 3 months tended to associate with visceral FM at 2 years (β 0.290, p = 0.06). Infants receiving higher caloric milk were earlier satiated and finished feeding faster. Conclusions: Our longitudinal data show decreasing milk protein and energy content from age 1 to 3 months, while fat and carbohydrate tended to decrease. Macronutrient composition, particularly fat content, differed considerably between mothers. Milk fat and energy content at 3 months associated with gain in FM% from age 1 to 6 months, indicating that higher fat and energy content associate with higher gain in FM% during the critical window for adiposity programming. As infants receiving higher caloric breastfeeding were earlier satiated, this self-regulatory mechanism might prevent intake of excessive macronutrients. Online trial registry: NTR, NL7833.