APOA1 and vitamin D deficiency: Krul-Poel et al. (2018) found that low serum 25-hydroxyvitamin D [25(OH)D] levels are significantly linked to higher insulin resistance in women with PCOS—independent of BMI, season, and ethnicity—and also that severe vitamin D deficiency is associated with the lowest levels of HDL cholesterol and apolipoprotein A1 [31].