Fast Twitch Fibers Reduce Fat PDF Print E-mail
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Written by Karel Nunnink   

Fast Fibers Reduce Fat and Speed Metabolism

In a widely reported Boston University Medical School study, published in the February 6, 2008, issue of Cell Metabolism, researchers found that replacing slow and enduring (type 1) muscle fibers with stronger and faster (but less enduring) type 2 fibers produced a reduction of fat mass and insulin resistance. (Endurance training normally develops type 1 fibers, and strength training builds type 2.)

This was accomplished using a genetically engineered mouse (MyoMouse), with a muscle-growth regulating gene called Akt1. The special gene could be turned on and off at will by the researchers. Activating Akt1 caused the mice to grow type 2 fibers, without exercise. (Needless to say, mice don’t pump iron on cue.) When the gene was de-activated, the mice again develop a predominance of type 1 fibers.

That’s a pretty neat trick and probably not feasible in humans, but the implications are huge. With the gene on, the mice take on the characteristics of a lean and powerful sprinter or a weight lifter. Turned off, the mice revert to their original state.

The MyoMouse is obese and insulin resistant when the gene is deactivated. It is fed a high-calorie diet with lots of fat and sugar, like you might get at a fast food restaurant. The kicker is that the transformation took place with no change in diet. The mice with the activated gene continued to eat the same fattening diet.

“Remarkably, type 2 muscle growth was associated with an overall reduction in body mass, due to a large decrease in fat mass. In addition, blood tests showed that these mice became metabolically normal [with no insulin resistance],” said senior author Kenneth Walsh, PhD, a professor of Medicine at Boston University. “This work shows that type 2 muscle just doesn’t allow you to pick up heavy objects, it is also important in controlling whole body metabolism,” he said.

“It appears that the increase in type 2 muscle fiber orchestrates changes in the body through its ability to communicate with other tissues,” Walsh continued.

Importantly, this is a previously unappreciated benefit of powerful, type 2 muscle fibers. It suggests that strength training may be especially beneficial to overweight and insulin resistant individuals.

This finding also helps to explain why most people get fatter as they age.

Fat gain has come to be thought of as a normal part of aging.

“Beyond the age of 30, humans lose approximately 6 pounds of muscle mass per decade. Thus a 50-year-old may be relatively good at playing tennis or jogging because type 1 muscle is preserved, but a measurement of grip strength or core body strength could show appreciable declines,” Dr. Walsh explained. That, of course, would indicate a loss of type 2 muscle, and probably fat gain as well.

The new study suggests what can be done about it.

In a commentary published in Cell Metabolism along with the study, Brooke and Leslie Leinwand of the University of Colorado at Boulder wrote: “These findings indicate that interventions designed to increase skeletal muscle mass…may prove to be critical weapons in the fight against obesity and obesity-related [ailments], including diabetes, heart disease, stroke, hypertension, and cancer.”

The key point here is that weight training appears to be as effective as aerobics for fat loss and weight control—and perhaps more effective. The rap against barbell training has always been that it doesn’t burn many calories. The MyoMouse study, where fat mass was lost all over the body without an increase in physical activity, suggests that calorie-burning exercise isn’t necessary—it helps, of course, but is not essential. Endurance training burns lots of calories, but strength training appears to do something even more important: It turns the metabolism into a fat-burning machine.

Strength training advocates have been saying this for a long time—and this study provides strong evidence that it’s true. Weight trained muscle keeps the metabolic furnace burning bright.

Here’s the hopeful message: Lift weights throughout life, and you will not only maintain muscle mass, you will gain less fat and be healthier as you age. Combine strength training with aerobic exercise and you almost cannot miss. (Hmmm, seems that I’ve heard that before.)

If you are not a super mouse, it would also be a good idea to go easy on concentrated-calorie foods and refined carbohydrates.


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