 Recently, plasma trimethylamine N-oxide TMAO level has been proved to be a strong and independent risk factor of cardiovascular disease. TMAO is formed through interaction between gut microflora and the ingested carnitine from food. Studies have shown that plasma TMAO concentration and TMAO productivity of gut microbiota in vegetarians are significantly lower than omnivores. This is probably due to a lack of carnitine in the diet of vegetarians. Therefore, it is worth investigating whether carnitine supplementation could induce TMAO productivity of human gut microbiota as well as to reveal TMAO producers from gut microflora. Therefore, the investigators plan to recruit 20 vegetarian and 10 omnivore volunteers to receive carnitine supplement for weeks, and study the change of plasma TMAO and gut microbiota. The plasma TMAO concentration will be measured by the LC-MS, and the gut microbiota composition will be analyzed by the next generation sequencing. Through bioinformatics analysis, the potential TMAO-producing bacteria might be discovered. The change of plasma TMAO level and gut microbiota composition after carnitine supplementation can provide more understanding on the relationship between carnitine supplement, TMAO and gut microflora. 