Influence of Chronotype on Cardiovascular Risk

Early chronotype with metabolic syndrome favors fat oxidation at rest and exercise relative to insulin-stimulated non-oxidative glucose disposal.

Chronotype is a circadian classification that identifies an individual’s preference for performing an activity or recognizing alertness during different periods of the day. Early chronotypes ( EC), i.e., preference for waking up early and/or engaging in activities earlier in the day, tend to have a reduced prevalence of cardiovascular disease (CVD) risk, including low triglyceride levels and C-reactive protein, with high-density lipoprotein (HDL). Furthermore, ECs tend to engage in more general physical activity (PA) than their late chronotype (LC) counterparts.

The exact biological mechanisms by which chronotype confers increased disease risk are unknown, but insulin sensitivity is an important etiologic factor in the progression to type 2 diabetes and CVD. Additionally, the inability to switch between lipid and carbohydrate fuel sources from the fasted state to the fed state, known as metabolic inflexibility , may precede insulin resistance.

Carbohydrates are considered the main source of energy during moderate to high intensity exercise in healthy controls, although training is known to reduce glycogen/ glucose flux dependence and increase fat oxidation. Somewhat discordant with observations of fasting fat oxidation is that people with insulin resistance, prediabetes, and/or type 2 diabetes have lower muscle glycogen utilization and higher fat oxidation during exercise than their healthy counterparts.

Several factors have been purported to explain this apparent paradox (e.g., excess lipids and/or impairment of oxidative capacity), but no study to date has examined the impact of chronotype on exercise fuel selection. This is biologically relevant since the circadian rhythm is a fundamental characteristic in the risk of chronic diseases, in part, through alterations in energy metabolism. Indeed, we recently reported that individuals classified as CD are more insulin sensitive in relation to insulin-stimulated carbohydrate utilization, a marker of metabolic flexibility .

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Our activity patterns and sleep/wake cycles may influence our risk for diseases such as type 2 diabetes and heart disease. New research published in Experimental Physiology found that sleep/wake cycles cause metabolic differences and alter our body’s preference for energy sources. Researchers found that those who stay up later have a reduced ability to use fat for energy, meaning fat can build up in the body and increase the risk of type 2 diabetes and cardiovascular disease.

Metabolic differences relate to how well each group can use insulin to promote glucose uptake by cells for energy storage and use. People who are "early birds" (individuals who prefer to be active in the morning) rely more on fat as a source of energy and are more active during the day with higher levels of aerobic fitness than "night owls." On the other hand, ’night owls’ (people who prefer to be active later in the day and at night) use less fat for energy at rest and during exercise.

Researchers at Rutgers University, New Jersey, USA, classified participants (n = 51) into two groups (early and late) based on their "chronotype" , our natural propensity to seek activity and sleep in different moments. They used advanced imaging to assess body mass and body composition, as well as insulin sensitivity, and breath samples to measure fat and carbohydrate metabolism.

The participants were monitored for a week to assess their activity patterns throughout the day. They ate a calorie- and nutrition-controlled diet and had to fast overnight to minimize the impact of the diet on the results. To study fuel preference, they were tested at rest before completing two 15-minute bouts of exercise: a moderate-intensity session and a high-intensity session on a treadmill. Aerobic fitness levels were assessed through an incline challenge in which the incline was raised 2.5% every two minutes until the participant reached the point of exhaustion.

The researchers found that early risers use more fat for energy both at rest and during exercise than night owls. Early risers were also more sensitive to insulin.

Night owls , on the other hand, are insulin resistant , meaning their bodies require more insulin to lower blood glucose levels, and their bodies prefer carbohydrates as an energy source over fat. The poor ability of this group to respond to insulin to promote fuel use may be detrimental, indicating an increased risk of type 2 diabetes and/or heart disease. The cause of this shift in metabolic preference between early birds and night owls is still unknown and needs further investigation.

Lead author Professor Steven Malin, from Rutgers University, New Jersey, USA, said: "Differences in fat metabolism between ’early birds’ and ’ night owls’ show that the circadian rhythm of our body (sleep/wake cycle) could affect how our bodies use insulin. A sensitive or impaired ability to respond to the hormone insulin has important implications for our health. This observation advances our understanding of how our body’s circadian rhythms affect our health. "Because chronotype appears to affect our metabolism and the action of hormones, we suggest that chronotype could be used as a factor to predict an individual’s risk of disease."

“We also found that early birds are more physically active and have higher fitness levels than night owls , who are more sedentary during the day. "More research is needed to examine the link between chronotype, exercise and metabolic adaptation to identify whether exercising earlier in the day has greater health benefits."