The three most important elements after RNY 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.
Many experience problems drinking protein shakes. I hated them. But in the beginning I drank them, three 16-ounce protein shakes a day, in order to meet my protein requirements. You do not have to like protein shakes only tolerate them. There are many varieties of protein shakes available today. Experiment until you can find one you can tolerate. I used Muscle Milk Light (Vanilla Creme) powder blended with water and a half a banana. That worked for me. But on the opposite end of the spectrum, you might try premixed Isopure.
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.
The fluid requirement is met by a combination. It is not only the water that you drink each day. But also the water you drink when you take your vitamins and medicine. It is the milk you drink. It is the fluids content of the protein shakes. It is the water component of the soups you take. It is met by flavored water such as Crystal Light. It is met by sugar free popsicles. It is met by tea and decaf coffee. It is a combination of all the fluids that you consume during the day.