Researchers from the Dana-Farber Cancer Institute, Massachusetts, Harvard Medical School, University of California and Denmark’s Odense University Hospital, among others, have found that some of the most widely recognised effects of exercise on muscle are mediated by irisin, a hormone secreted by muscle cells. The study has been published in the latest issue of scientific journal Nature.
“Irisin is induced with exercise in mice and humans, and mildly increased irisin levels in the blood cause an increase in energy expenditure in mice with no changes in movement or food intake. This results in improvements in obesity and glucose homeostasis.
The hormone has been found to act in two ways. It promotes the conversion of inert (white) yellow fat to the metabolically more active brown fat, and facilitates insulin action, thus decreasing blood glucose levels. While white fat is used to store energy (calories),brown fat seems to be more involved in burning it. The researchers found irisin to be present in the blood of human volunteers who had undergone 10 weeks of exercise.
When irisin was introduced in sedentary mice, the physiological effects of exercise like higher metabolism, stabilisation of blood glucose levels through better uptake of glucose from blood — one of the mechanisms by which insulin works — were mimicked in the laboratory.
In the last few years, brown fat has begun to increasingly intrigue scientists. In many ways, it physically resembles muscle more than it does typical fat. While little is known about it, it seems to play a major role in maintaining normal weight. One study found that the more brown fat a person has, the lower their BMI will be. Another study found that mice with a genetic abnormality that eliminated their brown fat became obese even without overeating.
Brown fat is common in infants but was once thought to disappear in adults. Recent studies have shown that it merely decreases. It’s found mainly in the neck and upper chest of adults. In infants, brown fat is an important regulator of body temperature, helping to keep the infant warm. Unlike white fat, when brown fat is metabolized (burned), most of its energy content is converted into heat. Adults don’t seem to need this system of keeping warm. It’s not yet known precisely what function brown fat serves in adults. So far, most studies of brown fat have been done in mice, because adult mice have more brown fat than adults humans do.
In the current study, the Dana-Farber researchers injected small amounts of irisin into the muscles of sedentary adult mice that were both obese and pre-diabetic. Within 10 days of treatment, the mice showed better control of blood sugar and insulin levels — possibly preventing the onset of full-blown diabetes — and also lost a small amount of weight. The researchers suspect that longer therapy would have led to greater weight loss.
Irisin was named after a Greek messenger Goddess (Iris) because it’s a chemical messenger. It’s normally present as part of a larger protein in a muscle cell’s outer membrane, where it lies dormant and inactive. Exercise (and, most likely, other unknown factors) cause this protein to be split, releasing irisin, which exits the muscle cell and carries its message to other cells of the body. Ultimately, some white fat cells are told to convert to brown fat cells and islet cells of the pancreas are told to produce more insulin.
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