RBP4 and alcoholic fatty liver disease: To test the hypothesis that PNPLA3 protein is involved in retinol metabolism in humans, we compared RBP4 levels between carriers of the 148I allele and 148M homozygotes in a cohort of 146 individuals with biopsy proven non-alcoholic fatty liver disease (see Table 1; Supplementary Material, Table S1 for characteristics).