HCAR2 and type 1 diabetes mellitus: Under normal physiologic conditions, the GPR109A may have little relevance because of low levels of β-HB or nicotinate in circulation; however, in conditions such as uncontrolled type 1 diabetes, when circulating levels of β-HB are elevated to levels that could have a maximal impact on receptor activity, the expression of the GPR109A in the basolateral membrane of RPE assumes great significance [18].