Figure 1. Spline regression of serum concentration of p,p′-DDT on serum 25-hydroxyvitamin D (25(OH)D) levels (ng/ml).

The curve was adjusted for gender, age, race, BMI, smoking status, outdoor activity, and vitamin D supplement. The distribution of p,p′-DDT among study subjects was added below the X-axis. The increasing trend of vitamin D was mainly driven by only a very small number of subjects with very high p,p′-DDT.