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Past studies have looked at the relationship between the body's metabolism and the operation of the body's biological clock. This study is the first to definitively prove that the body's circadian biological clock controls insulin activity.
A hormone produced in the body's pancreas, insulin helps in controlling the body's fat and carbohydrate metabolism. Carbohydrates are broken down during the digestive process into simple sugars called glucose; the glucose is then absorbed into the bloodstream. Insulin also works on transferring glucose into the body's cells, allowing excess glucose to be removed from the blood. When insulin's ability in removing glucose from the blood is diminished, it is known as insulin resistance. Researchers found that, during inactive phases, the body is more sensitive to insulin than high activity periods. As such, glucose is changed into fat during inactive phases and engaged in tissue building or other forms of energy during active phases.
"That is why it is good to fast every day...not eat anything between dinner and breakfast," commented Carl Johnson, a professor of biological sciences at Vanderbilt University, in a statement.
According to the Cleveland Clinic, the circadian rhythm, otherwise known as the "internal body clock," manages the body's 24-hour cycle of biological processes and is found in both plants and animals.
Science Daily writes that circadian rhythms are endogenously generated, but can also be modulated by external factors such as sunlight and temperature.
The researchers believe this study shows that in addition to the certain eating habits which lead to obesity and diabetes, explanations are also found for such conditions having a positive correlation to disrupted sleep schedules and late work shifts.
The results of the study were recently published in the journal Current Biology.
"Our study confirms that it is not only what you eat and how much you eat that is important for a healthy lifestyle, but when you eat is also very important," explained Shu-qun Shi, a postdoctoral fellow who conducted the study with research assistant Tasneem Ansari in the Vanderbilt University Medical Center's Mouse Metabolic Phenotyping Center, in a prepared statement.
The team of investigators utilized mice, as their circadian rhythm mirrors that of humans. While humans work and play during the day, mice sleep during the day and are active at night. Apart from the different wake times, the scientists discovered that the internal timekeeping system of humans and mice operate almost identically at the molecular level. Found in the brain's hypothalamus, the suprachiasmatic nucleus (SCN) in the brain has a master circadian clock that manages cells, many of which have their own molecular clocks.
"People have suspected that our cells' response to insulin had a circadian cycle, but we are the first to have actually measured it," noted Owen McGuinness, a professor of molecular physiology and biophysics at Vanderbilt, in the statement. "The master clock in the central nervous system drives the cycle and insulin response follows."
The researchers used different approaches to disrupt the circadian clocks of individual mice. Measurements were taken at different hours to better understand whether there was a pattern. In one experiment, mice were placed in an environment that was constantly lit. The light disrupted their circadian cycles and the mice were stuck in an inactive/fasting phase. They ended up developing a higher portion of body fat and gaining more weight, even though they ate less food than their counterparts. Other issues were related to weight gain, such as the increased risk of developing diabetes and cardiovascular disease as a result of obesity.
In another experiment, they used genetically modified mice that lacked the gene needed for proper biological clock function. These mice were stuck in an insulin-resistant mode that was similar to being in an inactive/fasting phase. When they consumed a high-fat diet, they usually gained more weight. When given the protein that normally comes from the missing gene, the circadian rhythm was re-established thus lowering insulin resistance.
Based on the findings, the investigators believe that insulin action and blood sugar metabolism are linked to internal mechanisms that keep track of time and the different periods of the day. They also propose that it is probably better to eat a light supper and no snack before dinner.
"Mediterranean diets in which the main meal is eaten in the middle of the day are probably healthier," concluded Johnson in the statement.
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