SPP1 and obesity due to melanocortin 4 receptor deficiency: The mean breast milk OPN levels of mothers with a normal BMI, overweight and obesity during lactation were 156.4 ± 46.2 mg/L, 140.8 ± 61.2 mg/L and 78.9 ± 28.8 mg/L (p < 0.001), respectively.