Abstract: Objective To einvestigate the association between elevated relative fat mass （RFM） and metabolic diseases in individuals with normal weight, and to characterize the distribution of this phenotype within the South China population. Methods The analysis utilized baseline data from the South China Cohort （SCC）, a prospective study on chronic diseases in a natural population, collected between 2018 and 2020. Participants aged 25-89 years were included. This study examined the distribution of the normal-weight high RFM phenotype across various subgroups and its association with the prevalence of diabetes, hypertension, and dyslipidemia. Results Among normal-weight individuals, the proportion of high RFM was significantly greater in females （25.34%） than in males （18.03%）, a disparity that widened with increasing age. A higher prevalence of elevated RFM was observed in subgroups characterized by low income （23.86%）, lower educational attainment （32.69%）, ethnic minority status （28.15%）, alcohol consumption （22.99%）, and non-smoking status （23.68%）. Within the normal-weight cohort, individuals with high RFM exhibited an increased risk for diabetes （OR=1.58）, hypertension （OR=1.67）, and dyslipidemia （OR=1.68） compared to their low RFM counterparts. Conclusion RFM effectively identifies metabolically at-risk individuals with normal body weight, thereby facilitating early intervention strategies to promote health outcomes.

Key words: Relative fat mass, Metabolic diseases, Normal weight, Diabetes mellitus, Hypertension, Dyslipidemia