The effect of maternal energy balance in conjunction with body energy reserves on lactation performance has not been fully elucidated in humans. Energy balance was computed from changes in weight and body composition over 6 mo postpartum in 21 Otomi Indian women with relatively low postpartum body mass indexes (BMIs; in kg/m2, 21.4 +/- 0.9) and 19 with relatively high BMIs (25.7 +/- 1.9). Body fat was determined by deuterium dilution. Milk production was estimated by 24-h test weighing. Macronutrient content of human milk was measured by standard techniques. In both groups weight declined over the 6 mo postpartum (P = 0.04). In the lower BMI group only, body fat (kg, % of wt) decreased significantly (P = 0.04). Milk fat concentration was positively correlated with body fat at 3 and 6 mo (r = 0.32 and 0.40; P = 0.04 and 0.01, respectively). Energy balance was not associated with milk production or composition, nutrient secretion into milk, or infant growth velocity, except at 6 mo, when energy balance was positively correlated with milk energy and fat concentration in the lower BMI group (r = 0.55; P = 0.01). Fat concentration was lower in the lower BMI group (P = 0.04). Because of the inverse correlation between milk production and milk fat at 3 and 6 mo (r = -0.47 and -0.43, respectively; P = 0.01), fat secretion into milk did not differ between groups. Infant growth velocities did not differ significantly between groups. In conclusion, negative energy balance in conjunction with lower energy reserves did not adversely affect lactation performance or infant growth in this population of Otomi Indians.