Strength training, in addition to the endurance training, may benefit overweight individuals, suggests a new study conducted on mice.
In the mice-based study, the researchers from Boston University School of Medicine (BUSM) found that the use of barbells might be as important to losing weight and improving health as the use of running shoes.
The discovery builds upon the fact that skeletal muscle consists of two types of fibres. Endurance training such as running increases the amount of type I muscle fibres, while resistance training such as weightlifting increases type II muscle fibres.
Using a mouse genetic model, the researchers demonstrated that an increase in type II muscle mass could reduce body fat which in turn reduces overall body mass and improves metabolic parameters such as insulin resistance.
The research team genetically engineered a mouse, called the MyoMouse, to grow type II fibres by activating a muscle growth-regulating gene. The gene, called Akt1, was engineered in such a way that it could be turned on and off at will by researchers. Even without exercise, activating the gene made the MyoMouse physically stronger.
When the gene was de-activated, the mouse returned to its original strength. While stronger and faster than a regular mouse, the MyoMouse did not run with as much endurance on a treadmill, a finding that is consistent with the growth of type II rather than type I muscle.
These findings demonstrate that the mouse was genetically programmed to have the characteristics of a lean and powerful sprinter rather than those of a gaunt marathon runner.
In the study, the Akt1 gene was turned off and the MyoMice were fed a high fat/high sugar diet with a similar caloric composition as a meal from a fast food restaurant. Over an eight-week period, the mice became obese and insulin resistant and developed fatty acid deposits in their liver, a condition referred to as hepatic steatosis or fatty liver disease.
The researchers then activated the Akt1 gene in the animals, which led to the growth of type II muscle fibres.
"Remarkably, type II 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 and their fatty liver disease rapidly resolved," said senior author Kenneth Walsh, PhD, a professor of medicine and head of Molecular Cardiology at the Whitaker Cardiovascular Institute at BUSM.
The beneficial changes occurred despite the fact that the mice continued to eat the same high-calorie diet and did not display any increase in physical activity.
"This work shows that type II muscle just doesn't allow you to pick up heavy objects, it is also important in controlling whole body metabolism," added Walsh.
Further analysis found that the mice burned fat because of changes in the physiology and gene expression of their fat and liver cells.
"Thus, it appears that the increase in type II muscle fiber orchestrates changes in the body through its ability to communicate with these other tissues," he said.
These findings indicate that type II muscle has a previously unappreciated role in regulating whole body metabolism through its ability to alter the metabolic properties of remote tissues.
The data also suggests that strength training, in addition to the widely prescribed therapy of endurance training, may be of particular benefit to overweight individuals
The study is published in Cell Metabolism.