Micronutrient concentration in preterm breast milk on postpartum days 7 (n = 36), 14 (n = 35), 21 (n = 34), and 28 (n = 28). (A) Sodium by gestational age. (B) Sodium by race and volume. Among blacks at a volume of 65 mL a significantly greater mean sodium was seen at day 7 than at day 14 (P = 0.0218) but was lower than at day 28 (P = 0.0004), day 14 was lower than day 28 (P < 0.0001), and day 21 was lower than day 28 (P = 0.0003). Among blacks at a volume of 150 mL a greater mean sodium was seen at day 7 than at day 14 (P = 0.0253) and lower than at day 28 (P = 0.0009). (C) Potassium by race. Among blacks, day 7 was greater than day 28 (P = 0.0099). Among whites, day 7 was greater than day 21 (P = 0.0001) and day 28 (P = 0.0001). (D) Calcium by race. Calcium was lower for blacks than for whites at day 7 (P = 0.0284) and day 14 (P = 0.0295). Among whites, day 7 calcium was greater than day 21 (P = 0.0102). (E) Chloride by gestational age and volume. Among those delivering at ≥28 weeks of gestation at a volume of 65 mL, day 7 was lower than day 21 (P = 0.0369) and day 28 (P < 0.0001), day 14 was lower than day 28 (P < 0.0001), and day 21 was lower than day 28 (P < 0.0001). Among those delivering at ≥28 weeks of gestation at a volume of 150 mL, day 7 was lower than day 21 (P = 0.0301) and day 28 (P = 0.0001), day 14 was lower than day 21 (P = 0.0083) and day 28 (P < 0.0001), and day 21 was lower than day 28 (P = 0.0002). Among those delivering at ≥28 weeks of gestation at a volume of 300 mL, day 14 was lower than day 21 (P = 0.0003) and day 28 (P = 0.0019). At a volume of 65 mL, those delivering at <28 weeks of gestation had lower mean chloride than those delivering at ≥28 weeks of gestation at day 7 (P = 0.0014), day 14 (P < 0.0001), day 21 (P < 0.0001), and day 28 (P < 0.0001). At a volume of 150 mL, those delivering at <28 weeks of gestation had lower mean chloride than those delivering at ≥28 weeks of gestation at day 7 (P = 0.0014), day 14 (P < 0.0001), day 21 (P < 0.0001), and day 28 (P < 0.0001). At a volume of 300 mL, those delivering at <28 weeks of gestation had lower mean chloride than those delivering at ≥28 weeks of gestation at day 7 (P = −0.0324), day 21 (P = 0.0070), and day 28 (P = 0.0057). (F) Phosphorus by race. (G) Magnesium. Day 7 was greater than day 21 (P = 0.0148). (H) Vitamin D by gestational age and volume. Among those delivering at <28 weeks of gestation at a volume of 300 mL, day 7 was greater than day 14 (P = 0.0005), day 21 (P = 0.0048), and day 28 (P = 0.0016). At a volume of 65 mL and day 28, those delivering at <28 weeks of gestation had lower mean vitamin D than those delivering at ≥28 weeks of gestation (P < 0.0001). At a volume of 150 mL and day 28, those delivering at <28 weeks of gestation had lower mean vitamin D than those delivering at ≥28 weeks of gestation (P = 0.0002). (I) Zinc by race. Among whites, day 7 was greater than day 21 (P = 0.0011) and day 28 (P = 0.0045), and day 14 was greater than day 21 (P = 0.0257).