Boosting insulin levels can restore muscle-building process in elderly

WASHINGTON - While most people have always seen insulin as a hormone that helps the body process sugar, scientists have now found that increasing its levels above the normal range can restore the impaired muscle-building process in the elderly.

This finding springs from a study led by University of Texas Medical Branch at Galveston researchers.

The study follows a recent finding that loss of responsiveness to insulin plays a major role in the loss of physical strength, which occurs as people grow older.

“Insulin is normally secreted during food intake,” said Dr. Elena Volpi, senior author of a paper on the study published in the September issue of Diabetologia.

“When you give insulin intravenously and increase the blood insulin levels to the same amount produced after a meal, you see that in young people it stimulates protein synthesis and muscle growth, while in older people it really doesn’t. But when we gave seniors double the insulin they would normally produce after eating, their muscles were stimulated like those of young people,” she added.

During the study, Volpi’s team worked with 14 elderly volunteers to examine the response of thigh muscle to the two different blood insulin levels, established by infusion into the thigh’s main artery.

The researchers took blood samples from catheters inserted in the femoral artery and vein of each subject enabled the researchers to calculate blood flow and muscle protein synthesis, and muscle biopsies allowed them to measure levels of signaling molecules involved in muscle protein growth.

They say that the data showed that a blood insulin level double that produced by a typical meal seems to turn back the clock on elderly thigh muscle.

“While we had called this ‘insulin resistance’ in the past, we didn’t really have evidence that you can get an elderly person’s muscle to grow if you give it a lot more insulin, which is what we needed to truly say this is insulin resistance,” Volpi said.

According to her, the phenomenon is also quite different from the insulin resistance seen in diabetes.

“These were older subjects with perfect glucose tolerance. So what we have identified is a novel kind of insulin resistance that’s not related to sugar control,” she said.

The researchers attribute this new kind of insulin resistance to age-related changes in the vascular system - in particular, changes in the endothelium, the single-cell-thick layer that lines blood vessels.

The endothelium controls blood flow by increasing or decreasing the diameter of capillaries (the smallest blood vessels), and regulates the release of oxygen, nutrients, water and other blood-borne cargo through the capillary walls and into muscles and other body tissues.

“Having a capillary dilation induced by insulin is important, because it exposes more muscle to the nutrients and hormones and everything flows better and gets stored away better. But in even healthy older people, this dilation response doesn’t work, because they have this endothelial dysfunction,” Volpi said.

The researchers are presently studying whether using drugs to dilate muscle blood vessels during insulin exposure can improve muscle growth in older people.

“Preliminary data suggest that this treatment may be effective, but these data are not yet published. On the other hand, in a paper we published two years ago in Diabetes, we showed that a single bout of aerobic exercise - a staple of diabetes treatment - may also improve muscle growth in response to insulin in older nondiabetic people,” Volpi said. (ANI)