Association of long-term change in waist circumference with insulin resistance

Recent studies have shown that fat accumulation is associated with insulin resistance; however, the risks associated with long-term changes and fluctuations in central fatness are less clear. This study examined the longitudinal relationship between waist circumference (WC) and insulin resistance using three dimensions of WC: baseline WC, slope of linear changes in WC, and fluctuation of WC around the slope during 20 years of follow-up. Anthropometry, insulin resistance (homeostasis model assessment (HOMA _IR)), and lifestyle factors were obtained in a population-based, prospective observational study (Coronary Artery Risk Development in Young Adults (CARDIA)) during 1985-2006, excluding participants who had been diagnosed with diabetes at any examination. After adjusting for socio-demographic and lifestyle factors, the evolution of HOMA _IR from CARDIA year 15 to 20 was 6.9% higher per standard deviation of year 0WC (P trend < 0.0001) and 6.3% higher per standard deviation increase in the change in WC over the long term (P trend < 0.0001). However, WC fluctuations around the linear change were not associated with insulin resistance or its evolution. The level of HOMA _IR increased substantially with steeper linear WC slope among initially thinner participants at baseline, whereas this association tended to be weaker in those with higher initial WC (P interaction < 0.0001). We conclude that year 0WC and long-term increment in WC are associated with worsening insulin resistance. However, the association of HOMA _IR with slope of WC change may vary across the range of initial WC.