Non-alcoholic fatty liver disease (NAFLD) is a multifactorial condition often observed in obese patients. Here, the inventors showed that plasma levels of phenylacetic acid (PAA, a metabolite resulting from microbial proteolysis associated with steatosis) correlates with steatosis and induces lipid accumulation and BCAA utilization in primary human hepatocytes and to some extent in mice. Accordingly, this metabolite could be used as a biomarker of NAFLD for stratifying the patients according to their risk.