Meal-replacement shakes and diet soda are drinks that may promote weight loss. Experts do not recommend diet soda for long-term dieting, but it may help to reduce overall calorie intake for regular soda drinkers.
In order to increase satiety, you should make meal replacement shakes with protein powder, fiber and a small amount of healthy fat. For weight loss, drink a shake rather than eating solid food two times per day. In general, the shakes have a lower calorie content than a cooked meal.
You can use diet soda as a low-calorie substitute for sweets, but it does not have any inherent weight-loss benefits.