Increased Insulin Resistance Linked to Lack of Exercise

Several studies have shown that lack of physical exercise leads to obesity and insulin resistance, which is responsible for type 2 diabetes. The exact mechanism that links physical inactivity with obesity and insulin resistance is unknown. This review analyzed the results of previous studies that were done on healthy and physically active volunteers who turned toward a more inactive lifestyle for two weeks. The relationship between physical inactivity and obesity, and insulin resistance was studied. It was seen that reduction in physical activity increased insulin resistance and induced fat deposition around the abdomen.

Being overweight and obese are direct risk factors in the incidence of numerous other ailments. The rates of obesity are rising alarmingly worldwide. According to the World Health Organization, nearly 400 million people worldwide were obese in 2006, and it is estimated that this figure would rise to 700 million by 2015. Obesity is linked to type 2 diabetes, and with a rise in the incidence of obesity, the number of patients with type 2 diabetes is also increasing. Insulin resistance develops when the insulin becomes ineffective at lowering the levels of sugars in blood. This is a common cause of type 2 diabetes. Both insulin resistance and obesity are thought to be caused by a multitude of factors in a person’s lifestyle. Most notable among these factors is lack of physical exercise. While several studies have investigated the association between physical inactivity and obesity in individuals who were already obese, few studies have actually imposed inactivity on the study participants to analyze its effects. This review attempted to evaluate the outcomes of such studies.

This review evaluated previous studies that had assessed the effects of lower daily physical activity on insulin resistance and obesity in humans. In the studies, obesity was measured by examining the central adiposity, i.e., deposition of fat around the abdomen. Since insulin resistance cannot be mimicked in animals such as laboratory rats, this parameter was evaluated only in humans.

Key findings
* One of the reviewed studies involved healthy young men who were accustomed to at least 10,000 steps (a measure of daily ambulatory activities) per day. During the experiment, they were asked to maintain a normal diet but reduce the exercise levels to approximately 1,500 steps a day for 14 days. The reduction in activity was by 85 percent.
* At the end of the study, it was seen that central adiposity rose by 7 percent in the participants. On the other hand, there was a weight loss of 1.2 kg on an average. This meant that while lowering activity does not significantly increase weight, it tends to raise central adiposity.
* The study also showed that the capacity of ventilation by the lungs reduced by 7 percent and lean body weight in the legs reduced by 2.8 percent after the period of inactivity. Reduction of these two factors significantly raises the risk of early death.
* The authors also deduced that with lack of exercise, there is excess of unburned energy that increases insulin resistance and the risk of increase in inflammation and oxidative stress that are associated with type 2 diabetes.

Next steps/Shortcomings
The authors admit that the reviewed studies were conducted for a short period; short-term changes in insulin resistance and central obesity may not predict long-term pathological changes that may lead to diabetes. While human studies that aim at imposing a long-term reduction in physical activities on participants are difficult to conduct, animal studies are a possibility.

This review showed that reduction of physical activity for just 14 days could cause significant changes in insulin sensitivity and increase central adiposity, and thus, raise the risk of type 2 diabetes in apparently healthy young human volunteers. The authors agree that the study reviewed here was a short term one and the changes in the insulin sensitivity pattern and central adiposity are acute and may not translate to more chronic and permanent ones. However, they conclude that “chronic inactivity provides a permissive environment for the development of pathological processes.” They suggest future studies on humans and animals to evaluate the link between inactivity, obesity and type 2 diabetes.

For More Information:
Metabolic Disruptions Induced by Reduced Ambulatory Activity in Free Living Humans
Publication Journal: Journal of Applied Physiology, 2011
By John P. Thyfault; Rikke Krogh-Madsen; University of Missouri, Columbia and University of Copenhagen, Denmark

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