Losing "weight" on a raw foods diet is a definite probability. But the weight might be excess fat and water retention due to detoxing. By weight I am assuming you are talking about muscle. It is not that complicated to keep muscle while on a raw food diet. The Vegan Effect diet program I created is 60-90 percent raw. Raw before dinner if that helps. I allow myself one cooked meal a day. For the most part the diet is a Low fat/high carb/high fruit/high alkaline diet.  The most important thing to remember is getting enough calories. My athletes (including raw body builders) typically eat 1000 calories of fruit for breakfast in the form of a green smoothie. Usually post workout. You need to eat roughly 3000 calories to maintain or grow muscle mass. Some of my mass building athletes eat as much as 5000 calories. Albeit at a higher fat ratio than our standard 70/10/20 carb/fat/protein ratios.