How Insulin Sensitivity and Fat Oxidation Relate to Early, Late Sleep Chronotype

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"Collectively, this work highlights and supports chronotype as a potential risk factor related to type 2 diabetes and cardiovascular disease risk," investigators wrote.

Sleep chronotypes are known to indicate various circadian-mediated metabolic and horomonal profiles, but a team of investigators aimed to further understand how the different chronotypes are affected by resting and exercise fuel use as it relates to insulin sensitivity.

Those with early chronotypes have a tendency for insulin sensitivity due in part to their physical activity. However, the interaction between the chronotype variations in resting and/or exercise fuel oxidation and insulin action was not previously understood, according to investigators.

"Collectively, this work highlights and supports chronotype as a potential risk factor related to type 2 diabetes and cardiovascular disease risk," the team wrote.

The Study

The investigation was led by Steven K. Malin, PhD, Department of Kinesiology and Health, Rutgers Uniersity, and the investigators used the Morningness-Eveningness Questionnaire (MEQ) and included invidividuals with metabolic syndrome according to the adults with metabolic syndrome (ATP III) criteria to classify early chronotypes and late chronotypes.

The cross-sectional study included a total of 51 adults who were separated into a chronotype category and then assessed to determine if there were impacts related to each chronotype. Participating individuals had a sedentary lifestyle with less than 60 minutes each week of structured exercise, did not smoke, have cardiovascular disease, respiratory complications, known metabolic disease (such as type II diabetes, non-alcoholic fatty liver disease, etc.), or cancer (within the last 5 years).

This study was part of a larger clinical trial in which all individuals had metabolic syndrome classified by ATP III guideline.

Anyone using medication that affected substrate metabolism, blood flow or insulin sensitivity were excluded. Those who met the criteria underwent an electrocardiogram (resting/exercise), clinical biochemistries, as well as a medical examination.

The Results

The early chronotype group consisted of 24 adults, 19 were women, with the average age of 54.2 years, and 27 adults, with 23 women, were included in the late chronotype group with the average age of 55.3 years. While the demographics of the groups were similar, investigators found that those in the early chronotype had less sedentary behavior and more physical activity during the morning and midday than those who were categorized as late chronotypes, which correlated with some related measures.

When compared with the late chronotype, a higher measure of maximum oxygen intake during exercise (VO2 max), non-oxidative glucose disposal (NOGD), and resting fat oxidation (FOX) was observed in the early chronotype. During exercise, both chronotypes showed an increase in carbohydrate (CHO) reliance, though the early chronotype used more fat.

The key findings of the investigation indicated that adults with metabolic syndrome in the early chronotype utilize more fat as energy when engaing in fasted and moderate-high exercise compared with adults with a late chronotype. Overall, during high intensity exercise investigators observed elevated exercise fat oxidation was associated with non-oxidative glucose metabolism.

"These findings occur independent of workload, suggesting that chronotype may be characterized by unique alterations in metabolism. Indeed, late chronotype participants were more sedentary and had lower aerobic fitness than early chronotypes. Interestingly, more light physical activity was positively related to metabolic insulin sensitivity, and higher fitness was related to increased exercise fat oxidation," investigators concluded.

The study "Early chronotype with metabolic syndrome favours resting and exercise fat oxidation in relation to insulin-stimulated non-oxidative glucose disposal" was published in Experimental Physiology.