Increased Sedentary Time Predicts Insulin Resistance

Dr. Hendrik J.F. Helmerhorst, from the Medical Research Council Epidemiology Unit, Cambridge, United Kingdom, writes, "Several etiological factors have been identified for impaired insulin sensitivity, including genotype, body composition, inflammation, and lifestyle factors.… Low levels of physical activity and lack of ... MVPA are associated with insulin resistance. Sedentary time has been linked to various cardiometabolic health outcomes, sometimes independent of overall physical activity in cross-sectional analyses."

The Medical Research Council Ely study cohort consisted of 376 middle-aged adults (166 men; 210 women) observed for 5.6 years. At baseline and follow-up, physical activity and sedentary time were objectively measured with individually calibrated, minute-by-minute heart rate monitoring. Sedentary time was calculated from the heart rate observations (in minutes) below an individually predetermined threshold (flex heart rate) and was described as a percentage of total monitored time during waking hours for 4 days. MVPA was defined as the percentage of time spent above 1.75 times resting heart rate. Fasting plasma insulin levels were used as a substitute marker reflecting insulin resistance.

Independent of age, sex, baseline fat mass, fasting insulin, smoking status, and follow-up time, time spent sedentary at baseline was significantly and positively associated with long fasting insulin at follow-up (beta = .003; 95% confidence interval [CI], 0.0006 - 0.006; P = .015). This association was strengthened somewhat after further adjustment for MVPA (beta = 0.004; 95% CI, 0.0009 - 0.006; P = 0.009).

Limitations of this study include observational design, precluding determination of causality; susceptibility of heart rate monitoring to heart rate fluctuations because of external factors; somewhat arbitrary thresholds used for defining sedentary time and MVPA; lack of generalizability to populations differing in age and physical activity; possible undetermined confounders; and possible confounders not accounted for in the analyses. In addition, fasting insulin level is not a direct measure of insulin resistance.

"This is the first study suggesting that increased time spent sedentary is prospectively associated with elevated fasting insulin levels regardless of the amount of time spent in MVPA in healthy middle-aged adults," the study authors conclude. "From a public health perspective, these findings urge the need for recommendations aimed at reducing sedentary time in addition to those aimed at increasing MVPA."