FNDC5 and metabolic syndrome: Likewise, contradictory evidence was found for the association of irisin plasma levels with cardiovascular and metabolic parameters such as glucose, insulin, HOMA-IR, HDL-C, LDL-C, and TG levels in healthy individuals, as well as in those with obesity and the metabolic syndrome (4-6,9,10,26,27); however, it remains unclear whether these associations reflect a true cause-effect relationship.