HDL and Reverse Cholesterol Transport Biomarkers.

High-density lipoprotein (HDL) is a protein-lipid nanoparticle that has predominately been characterized by its cholesterol concentration (HDL-C). Recent studies have challenged the presumed inverse association between HDL-C and cardiovascular events, suggesting a more U-shaped association. This has opened new opportunities to evaluate more novel measures of HDL metabolism, such as HDL particle number (HDL-P) and one of HDL's key functions, cholesterol efflux. Both HDL-P and cholesterol efflux are inversely associated with incident cardiovascular events and may perhaps be better targets for intervention. This review includes recent research on the emerging U-shaped association between HDL-C and cardiovascular events, recent observational studies related to HDL-P, and the effects of established and novel interventions on cholesterol efflux.