The answer,
Cut off protein intake A BIT (you need a minimum, so don't go bellow a certain value, you can find these values for you body weight on the internet). The body when protein levels are to low, goes into survival mode and keeps repairing/building the most used body systems including muscles and chucks the rest. If you cut all proteins, you start to "eat up" almost essential things first and then go for the essential like the heart; that's why many anorexic have big long term damages.
Cut carbs, carbs are "protein sparing" since the muscles can use them for food right away, but if you don't have a carb store and the body needs energy very quickly (say a lion is charging you :-), the body will have to go for muscles (next readily accessible) and then fats (fats take longer to come on line, so the can't respond to quick energy demands, and there is a maximum rate of conversion which makes them not suited for very big energy expenditure in short periods of times (that's what carbs are good for)). When someone HITS the WALLS in a marathon is when they're carb stores are depleted, past that point, if this happens too early in the marathon, you'll end up finishing much slower on your fat reserves, if its at the end, you can access muscles for a few miles and finish at a good pace.
You'll notice that marathoners are lanky and very lean, its because of the mechanism I talked here. The body has decreased their muscle mass enough so the carbs they take in pre-race is enough to carry them through the race without touching the muscles. Before, maybe they had more muscle mass and they hit the wall before the end; the body has adjusted their metabolism to function.
The muscle used by the body for fuel are the least used one. The body is trying to reduce its metabolism so it does not run out of carbs the next time, by decreasing its muscle mass. So, if you bicycle with a decent resistance and your body runs out of carbs, it would have a tendency to eat up the upper blody mass.
Going back to the marathonners, they always have a leaner (in muscle mass) upper body proportionallly than their lower body and core.
Increase polysaturated and monosatured fats intake, these are harder for the body to use as fuel and thus there's a bigger chance that it will have to tap into the muscle reserves for short term relief when it needs an energy burst.
Slightly diminish caloric input in general (through the rebalancing of food intake talked earlier) to slightly less than needed to sustain your current body mass (this can be calculated precisely if you know your lean mass and fat %).
I'd say mixed training, with high intensity, high reps with low weights used for the upper body and more weights/resistance but again high intensity, lots of reps for the lower body (bicycling at a good spped, against the wind, or on ondulating small hills is great for the lower body) is good. In between the high intensity stuff, do lower intensity cardio which will empty the carbs store even more so when you increase the intensity they will be quasi depleted and thus the body has to go for something else. This only works if you've got a caloric deficit in your diet, if you eat more of carbs and proteins and do exactly the same thing, you'd gain muscle mass instead of losing it.
Do weight training, with a decent amount of weight, to increase caloric outake even more, on body areas where you want to maintain muscle mass throughout your diet, or even gain some. Like the buttocks and the core.
IF you do high intensity exercise in the morning before eating, again the carb stores are empty and the body will use muscles for fuel.
Well, my answer is a bit jumped :-). but essentially corect .
