Exercise in general is excellent for health, and muscle-strengthening activities in particular have specific health benefits.
But some of us build muscles much more easily than others. A new study, which now appears in the journal Nature Communications, offers a molecular explanation for why that is.
The new research also explains why some people seem to respond better to endurance training and aerobic exercise rather than muscle training.
Sarah Lessard, Ph.D. — an assistant investigator in the clinical, behavioral and outcomes research section of the Joslin Diabetes Center in Boston, MA — is the first author of the paper.
The molecule in question is a protein called c-Jun N-terminal kinase (JNK). Speaking of it, Lessard says, "It's like a switch [...] If the switch is on, you'll have muscle growth. If it's turned off, you have endurance adaptation in the muscle."
"We've identified an exercise-activated biological pathway that hasn't been studied at all," she adds.
How JNK influences muscle growth
The new study builds on previous work that the same scientists conducted. In their previous study, Lessard and colleagues examined the genetic makeup of rodents they had bred to respond very well or very poorly to endurance exercise.
It was then that the researchers first discovered that the JNK molecular pathway was responsible for how well a mouse would perform on the treadmill.
In the new study, the researchers wanted to know more about the reasons why JNK would have such a vital role.
So, Lessard and team knocked out the JNK gene in a group of mice and compared their behavior with that of normal mice.
The knockout rodents remained healthy and continued to run on the treadmill for long periods of time.
Next, the researchers trained both groups of rodents to run. This was when the scientists remarked that the aerobic capacity of the JNK-free mice had increased drastically, along with their blood vessels and a muscle fiber that builds endurance.
By contrast, in a muscle growth experiment, JNK-free rodents failed to increase their muscle mass, whereas regular mice doubled theirs.
Muscle-building protein linked with diabetes
The new research also included tests in humans. Lessard and colleagues asked healthy volunteers to engage in either weight-lifting exercise or cycling, a type of endurance workout.
The tests found that JNK was highly active in the former kind of exercise but not active in the latter.
Also, the findings may help those who want to build muscle but can't because of various diseases.
"We've begun to figure out how muscle decides whether it will grow or adapt for endurance, which really hasn't been known [...] And we're finding that this process is directly linked to the risk of type 2 diabetes."
Lessard and team have already started to explore this hypothesis by analyzing the JNK pathway in people who have a higher risk of type 2 diabetes.