Abstract: The present invention provides a method for measuring the trimethylamine N-oxide (TMAO) production capacity in a subject, which comprises the following steps: (a) making the subject intake a specific dosage of carnitine; and (b) obtaining a body fluid sample of the subject at a specific time point after the subject ingests the carnitine and detecting the TMAO content in the body fluid sample. Different from the general technical detection on the market that only detects the gut microbiome composition, the invention can directly detect the ability of gut microbiota to produce TMAO in the human body. In addition, compared with directly detecting the concentration of TMAO in the blood, the invention gives a better predictive effect of gut microbiota functional phenotypes.