I've got a great client -- she works hard, moves really well, and asks very good questions.
When she started with me, it was her first time lifting and she made progress. But recently wasn't as consistent because of a running related injury -- she was training for (and completed!) a half-marathon!
We were back in the lab getting some training in and she asked if I thought she had lost some definition. I said yes her arms and back didn't look as toned because she hadn't been as consistent and had taken some time off.
(Some of you might be thinking, jeez Sam that's a little ruff bro don't be mean. And to that I say: shut your mouth; you're not being helpful; it's not mean, it's the truth. What's mean is lying and giving her a false sense of reality)
So then she started peppering me with questions about how muscle is lost, if it will come back, and what about her marathon training helping maintain muscle.
They were excellent because it made me think critically and forced me to give solid answers other than just a banal bro sciencey type answer -- "oh yeah marathons make ya flabby, but there's muscle memory so yeah just chill."
I told her I'd get back with some research on the topic. Here is what we learned because of her questions.
First a quick lesson on how you gain muscle.
When you lift, you break muscle. The muscle fibers come back thicker, creating new muscle fibers w/ nuclei and everything. eat, lift, sleep, repeat and you end up putting on some size.
This new muscle looks great but also requires more energy for the body to maintain (hence people w/ muscle have a higher metabolism).
What happens when you take time off from lifting?
Research shows that for about 2 weeks not much size or strength is lost, but after that there is a drop off, particularly in size.
There was a study of muscle mass and strength loss of bed ridden individuals. After 2 weeks, their lean mass decreased by .5% per day and their strength decreased by 1.5% per day, but a loss none the less (link).
There was also an interesting study that showed if you maintained your protein intake, but lowered your other caloric intake to equalize your caloric output/intake, your muscles maintained size for longer.
And, NO, Muscle doesn't turn to fat.
Why do you lose muscle size?
First, let me say that "size" doesn't necessarily mean being ripped. When people want to "tone their arms" they don't actually lose fat in that particular area, they just increase muscle size pushing against the fat in that particular area giving them a more "toned" look.
Next, when you stop working out -- either long-term because of new life circumstances or your goals shift from lifting -- your body isn't using those muscles as much.
First, not working out minimizes blood flow and cellular water levels of the muscles, so the initial loss of size are those things (there are probably some fancy science words for it, but Hey, I'm not rocket surgeon so you're out of luck.) (link)
Next, the body recognizes that maintaining those unused muscles is USING more energy than those muscles are contributing to body's goals, so eventually the body starts to metabolize the tissue (link).
The good news on that front is, that process TAKES A LONG TIME for muscle to waste away. True muscle wasting occurs from malnutrition, severe disease like cancer, or significant injury like spinal cord injury.
IN ADDITION, the control center of that muscle cell -- THE NUCLEI -- never goes away. So when your goals shift back to lifting or you decide to stop being a lazy pud, your muscle fibers will grow around that nuclei again. Hence the idea of "muscle memory".
did my marathon training help with toning?
It might have helped with toning of the legs, but likely didn't contribute to toning of the arms as the body recognized those muscles weren't being utilized and needed to decrease energy expenditure to those areas.
Also, as stated above, a lot depends on your diet when you go away from lifting. Maintaining a high level of protein intake (assuming you had a high level when you were training), will help prevent muscle loss.
Flip side of the coin -- what if I switch from distance running to weight lifting
First of all, congrats on picking up lifting. It's really good for you. You'll feel better and look better nekked.
Next, research shows cardiovascular capabilities are lost faster and to a greater extent than your muscle size and strength.
So if you do switch to lifting and want to maintain cardio capabilities, I'd say add in one day of a medium-paced distance run to help keep your cardio up.
Stay consistent with your lifting. If you have to stop or your goals change, keep your protein and supplement intake high as if you were lifting. Your gains may decrease, but as long as you get back to lifting, they won't disappear forever.