The three most important elements after gastric bypass surgery are to meet your daily protein, fluid and vitamin requirements. Food is secondary because your body is converting your stored fat into the energy that drives your body. Thus you lose weight.
Weight loss is achieved after surgery through meal volume control. You begin at 2 ounces (1/4 cup) per meal and gradually over the next year and a half increase the volume to 1 cup per meal. With this minuscule amount of food, it is next to impossible to meet your protein daily requirements by food alone, so therefore you need to rely on supplements such as protein shakes.
Without sufficient protein, your body will scavenge protein from other areas of your body, such as your muscles. Protein is extremely important after surgery as its job in the body is to build and maintain tissues including your body's major organs and skeletal muscles. Protein deficiency, when continued over a long period of time can cause a disease known as protein caloric malnutrition. Common symptoms are poor healing, fatigue, hair loss and muscle wasting. Immediate post op, protein promotes healing of the staple line and incisions.
Your daily protein requirement is met by a combination of the amount of protein you obtain from food combined with the amount of protein from protein supplements (protein shakes, protein bars)