I'd like to see the diets in the study that are specified as the "calorie-reduced diets". (Can't seem to find the paper). If it's the same as the Standard American Diet, this muscle loss is quite explainable. I think the mitigation is relatively easy though, if you want to shift the p-ratio, recommending a daily high protein shake would do a lot to stave off muscle loss (and even more if resistance training is applied of course). The exercise addition is probably the hardest to adhere to.

I'd be surprised if either mice or human cells eat "the Standard American Diet"