A study published in the journal Experimental Physiology highlights that people with early chronotype use more fat during rest and exercise and exhibit more sensitivity to insulin. They remain more physically active throughout the day and have a lower risk of type 2 diabetes.
Background
Chronotype refers to the physiological preference of an individual to be active and alert during different periods of the day. It influences a person’s sleep-awake cycle, physical activity, alertness, appetite, and core body temperature.
Early chronotypes (early risers) prefer to wake up and start daily activities early in the morning. They tend to have a lower risk of cardiovascular and metabolic diseases. In contrast, late chronotypes (late risers) stay up late and feel more active and alert in the evening. They tend to have disturbances in energy metabolism and a higher risk of insulin resistance.
In the current study, scientists have evaluated the dynamics of energy metabolism among early and late chronotypes during rest and exercise.
Study design
The study population included 51 adults with metabolic syndrome. They were categorized as early chronotypes (n=24) or late chronotypes (n=27) based on their response to a Morningness–Eveningness questionnaire.
The Energy metabolism preferences of the participants were assessed by allowing them to perform moderate-to-high intensity exercise on a treadmill.
Both resting and exercise carbohydrate and fat oxidation were determined to measure energy fuel preference. In addition, heart rate and rating of perceived exertion were assessed. Physical activity patterns, body composition, and insulin sensitivity for non-oxidative glucose disposal were also determined.
Important observations
The analysis of metabolic parameters revealed that early chronotypes have higher VO2max (maximum oxygen utilization during exercise) and non-oxidative glucose disposal than late chronotypes.
The level of physical activity was higher among early chronotypes. They were more active in the morning and midday compared to late chronotypes.
At resting conditions, early chronotypes showed higher fat oxidation than late chronotypes. During moderate and high-intensity exercises, both groups exhibited increased carbohydrate oxidation. However, early chronotypes maintained a higher level of fat oxidation during all exercise conditions.
During moderate exercise, the maximum oxygen utilization correlated significantly with fat oxidation and metabolic flexibility (carbohydrate or fat preference). A significant correlation was also observed between body mass index (BMI) and afternoon sedentary behavior.
Both body weight and insulin sensitivity significantly correlated with light physical activity. Notably, a significant correlation was observed between fat oxidation and non-oxidative glucose disposal during high-intensity exercise.
Study significance
The study reveals that early chronotypes with metabolic syndrome utilize more fat during rest and exercise than their late chronotype counterparts. This metabolic activity in early chronotypes does not depend on the level of physical fitness and light physical activity per day.
Early chronotypes also have higher insulin sensitivity than late chronotypes, reducing their susceptibility to type 2 diabetes. They remain less sedentary throughout the day and perform more physical activity in the morning and midday, which further help improve metabolic insulin sensitivity.
Although both early and late chronotypes can shift fuel preference toward carbohydrate oxidation during exercise, late chronotypes prefer carbohydrates over fat as an energy source.
As mentioned by the scientists, a variation in the patterns of the sleep-wake cycle and circadian rhythm (internal body clock) could be responsible for metabolic differences between early and late chronotypes.
Professor Steven Malin from Rutgers University, New Jersey, a senior author on the study, said,
Night owls are reported to have a higher risk of obesity, type 2 diabetes and cardiovascular disease when compared with early birds. A potential explanation is they become misaligned with their circadian rhythm for various reasons, but most notably among adults would be work.”
If late chronotypes are forced to wake up early because of workplace demands, they may feel more tired and sleepy throughout the day because of the misalignment with their circadian pattern. This particular physiological pattern of people should be considered while discussing the health risks of night-shift work.
In this context, Prof Malin said,
If we promote a timing pattern that is out of sync with nature, it could exacerbate health risks. Whether dietary patterns or activity can help attenuate these is an area we hope becomes clear in time.”
