Ethnicity has been reported as a prominent factor affecting NMR; however, all the subjects in the current study were Caucasian (Finns). Age, sex, and BMI accounted for 8.9% and 6.1% of variance in NMR in the FinnTwin and YFS samples, respectively. This is in line with a previous study showing that various non-genetic factors account for altogether 8% of inter-individual variance in NMR [17]. However, in the FINRISK2007 sample age, sex, and BMI only accounted for 0.5% of variance in NMR, which may be due to older age and longer overall duration of smoking with more cessation than among the young adult FinnTwin and YFS samples.
