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.