Field Notes

Sitting Time Associated With Increased Risk of Chronic Diseases

The more you sit, the higher your risk of chronic diseases.

Richard Rosenkranz, PhD, an assistant professor of human nutrition at Kansas State University, examined the associations of sitting time and chronic diseases in middle-aged Australian men in a study published in the International Journal of Behavioral Nutrition and Physical Activity. The research is part of the 45 and Up Study, the largest long-term study of aging in Australia, involving more than 267,000 people.

The study's sample included 63,048 men aged 45 to 65 from the Australian state of New South Wales. Study participants reported the presence or absence of various chronic diseases along with their daily sitting time, categorized as fewer than four hours, four to six hours, six to eight hours, or more than eight hours.

Compared with those who reported sitting four hours or fewer per day, those who sat for more than four hours per day were significantly more likely to report having a chronic disease such as cancer, diabetes, heart disease, or high blood pressure. The reporting of chronic diseases rose as participants indicated they sat more. Those sitting for at least six hours were significantly more likely to report having diabetes.

"We saw a steady stair-step increase in risk of chronic diseases the more participants sat," Rosenkranz explains. "The group sitting more than eight hours clearly had the highest risk."

The study is relevant to office workers sitting at desks and those sitting for long periods of time, such as truck drivers, he says.

"We know with very high confidence that more physically active people do better with regard to chronic disease compared with less physically active people, but we should also be looking at reducing sitting," Rosenkranz says. "A lot of office jobs that require long periods of sitting may be hazardous to your health because of inactivity and the low levels of energy expenditure."

Researchers discovered consistent findings in those who had a similar physical activity level, age, income, education, weight, and height. Participants who sat more reported more chronic diseases, even if they had a similar BMI compared with those who sat less.

In general, people should get more physical activity and sit less, Rosenkranz says. "It's not just that people aren't getting enough physical activity, but it's that they're also sitting too much," he explains. "And on top of that, the more you sit, the less time you have for physical activity."

The study focused on men because they have higher rates of diabetes and heart disease, but it’s probably applicable to adults across gender, race, and ethnicity, according to Rosenkranz. However, little is known about children and sitting with regard to chronic disease.

Researchers said that although most of the current evidence suggests a causal connection, they can’t be certain in this study whether volumes of sitting time led to the development of chronic diseases or whether the chronic diseases influenced sitting time.

— Source: Kansas State University


Circadian Clock Linked to Obesity, Diabetes, and Heart Attack

Disruption in the body’s circadian rhythm can lead not only to obesity, but it also can increase the risk of diabetes and heart disease.

That is the conclusion of the study published in Current Biology, the first to show definitively that insulin activity is controlled by the body’s circadian biological clock.

“Our study confirms that it’s not only what you eat and how much you eat that’s important for a healthful lifestyle but when you eat is also very important,” says postdoctoral fellow Shuqun Shi, PhD, who performed the experiment with research assistant Tasneem Ansari in Vanderbilt University Medical Center’s Mouse Metabolic Phenotyping Center.

In recent years, several studies in both mice and humans have found a variety of links between the operation of the body’s biological clock and various aspects of its metabolism, the physical and chemical processes that provide energy and produce, maintain, and destroy tissue. It generally was assumed that these variations were caused in response to insulin, which is one of the most potent metabolic hormones. However, no one actually had determined that insulin action follows a 24-hour cycle or what happens when the body’s circadian clock is disrupted.

Because they’re nocturnal, mice have a circadian rhythm that’s the mirror image of that of humans: They’re active during the night and sleep during the day. Otherwise, scientists have found that the internal timekeeping system of the two species operates in nearly the same way at the molecular level. Most types of cells contain their own molecular clocks, all of which are controlled by a master circadian clock in the suprachiasmatic nucleus in the brain.

Insulin action (the hormone’s ability to remove glucose from the blood) can be reduced by several factors and is called insulin resistance. The study found that normal "wild-type" mouse tissues are relatively resistant to insulin during the inactive/fasting phase, whereas they become more sensitive to insulin (therefore better able to transfer glucose out of the blood) during the high-activity/feeding phase of their 24-hour cycle. As a result, glucose is converted primarily into fat during the inactive phase, and used for energy and other tissue building during the high-activity phase.

The researchers also examined what happened to insulin action when the circadian clocks of individual mice were disrupted. One approach they used was to study special “knock-out” mice that had one of the genes necessary for proper biological clock function removed. They found these mice appeared to be locked in an insulin-resistant mode around the clock comparable with the inactive/fasting phase. After feeding on a high-fat diet, they tended to gain more weight and carry more fat than wild-type mice. However, supplying them with the protein produced by the missing gene reestablished their circadian rhythm, reduced their insulin resistance, and prevented them from gaining excess fat.

Another approach was to place normal wild-type mice in a constantly lit environment that disrupted their circadian cycle. In this case, they found the mice were locked in the inactive/fasting phase, developed a higher proportion of body fat, and gained more weight on a high-fat diet than wild-type mice despite actually eating less food. Obesity and the insulin resistance that accompanies it increase the risk of diabetes and cardiovascular disease.

According to the researchers, this helps explain the increased frequency of obesity and diabetes among night-shift workers and people suffering from disruption of their circadian clocks and normal sleep patterns.

The researchers also found that high-fat diets disrupted the circadian clock of wild-type mice living in a normal day/night cycle. As a result, their insulin cycle defaulted to the inactive/fasting phase, which helps explain why high-fat diets lead to weight gain.

— Source: Vanderbilt University