Data plot and RCS regression for men aged 50 to 90 years. The plots show SF concentrations with median Hb (A; red line) and median eZnPP (B; red line), as well as RCS regressions with 5 knots for Hb (C; the vertical dashed line indicates the plateau point) and eZnPP (D; the vertical dashed line indicates the minimum point), in a healthy sample of US men aged 50 to 90 years (n = 1884) participating in NHANES III, 1988 to 1994. Shaded areas between the dashed lines represent 95% CIs.