Calories. The amount of weight you can lose by using fruit smoothies as meal replacements depends upon the number of calories you conserve. Losing 1 pound requires burning or saving about 3,500 calories. If you normally eat about 700 calories at a meal and replace one meal per day with a 300-calorie smoothie, you could lose about 3/4 pound per week. The key to losing weight healthfully and keeping it off is following a balanced diet.