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Purpose: Restriction of animal foods and choline may affect plasma trimethylamine- N-oxide (TMAO). In vegetarians, we investigated the association between TMAO concentrations and the strictness of the diet or sex. We also studied the biological variations of TMAO in vegans. Methods: Concentrations of plasma TMAO and choline metabolites were measured in 38 vegans and 67 lacto-ovo-vegetarians (group 1: mean age ± SD = 50 ± 15 years). Group 2 consisted of 66 vegans (29.2 ± 7.3 years) that was tested twice within 3 months of intervention with vitamin B12 or a placebo. Results: In group 1, plasma TMAO did not differ according to the strictness of the diet (both means 3.7 µmol/L). In lacto-ovo-vegetarians, men had higher TMAO and betaine, but lower trimethylamine than women. In group 2, the intervention with vitamin B12 had no effect on plasma TMAO or choline metabolites. The mean within-subject change of TMAO within 3 months was −0.3 (95 % confidence intervals = −0.7-0.1 µmol/L). TMAO increased after 3 months (mean 1.7 to 2.8 µmol/L) in vegans with a lower baseline dimethylglycine (2.2 µmol/L), while it declined (from 2.7 to 1.9 µmol/L) in vegans with a higher dimethylglycine (3.1 µmol/L). The intra-class correlation coefficients were 0.819 for TMAO, 0.885 for betaine and 0.860 for dimethylglycine. Conclusions: Plasma TMAO was not related to the strictness of the vegetarian diet. Metabolisms of TMAO and dimethylglycine are interrelated. Intra-individual variations of TMAO are low in vegans. Changes of fasting plasma TMAO >80 % upon retesting are likely to exceed the biological variations. [ABSTRACT FROM AUTHOR]