Figure 1.

Proteins involved, or hypothesized to be involved, in vitamin A (VA) metabolism within the lumen of the upper gastrointestinal tract. βC: β-carotene and all other provitamin A carotenoids; PLB1: phospholipase B; PNLIP: pancreatic lipase; PNLIPRP2: pancreatic lipase-related protein 2; RET: retinol; RP: retinyl palmitate and all other retinyl esters. Proteins followed by a question mark have been hypothesized to be involved because RET and βC are not soluble in water, and thus, non-micellarized VA is assumed to be associated with proteins. A dotted arrow means the pathway is suspected to exist, but there is no evidence thereof yet.