Optimizing Muscle Protein Synthesis: the Crucial Impact of Protein Quality and Quantity, and the Key Role of Resistance Training | Luc van Loon, Ph.D.

Now, the problem is, when you become sick or you get surgery, you have two issues: you exercise less or you become less physically active, but you also eat less.

So, it’s a double whammy downwards. And that’s what makes us so vulnerable to a short period of inactivity or sickness.

You take a group of group of young, healthy people, who presumably have lots of anabolic capacity. You put them in a cast for 1 week, and you immediately demonstrate, upon removing the cast, that the leg that has been immobile for a week is 35% less responsive to protein assimilation than the other leg. That’s an anabolic resistance factor of 35%, that is far greater than what you see in an aged individual.

So, with 1 week of inactivity, you (can) make a young leg or young muscle respond completely like a senescent muscle.

And now the fun thing: you take an aged individual, who might have 20% or 30 % less anabolic response to protein than a young person. If you exercise them, you can bring them up to the same level as a young person.

These two factors suggest that activity might be the main determinant of anabolic resistance, and an aging individual or age within an individual is simply a proxy for activity.

So, muscle itself doesn’t seem to get that old. It actually still responds completely normal and we can normalize for age by physical activity.