"Gastric surgery is not magic," says Brad Watkins, MD, FACS, co-founder of the Northwest Weightloss Center in Kirkland, Washington. "It's not a panacea, but a very powerful tool to assist you with your weight loss efforts."

There are three primary types of obesity surgery: laparoscopic gastric banding, known as LAP band surgery, gastric bypass and biliopancreatic diversion (BPD). The goal of these procedures is to reduce the amount of food your body can absorb, either by making you feel fuller faster or by removing your body's capacity to absorb calories.

LAP band is the least invasive of these surgeries; a band is placed around the opening of your stomach, restricting the amount of food that can be eaten at a given time. Both gastric bypass and BPD restructure your digestive system, with gastric bypass minimizing the size of your stomach and BPD shrinking the stomach and bypassing most of the small intestine. Both of these surgeries are called "malabsorptive"; by detouring parts of the gastrointestinal system, fewer calories—and nutrients—can be absorbed. These patients will have to take numerous multivitamins and nutrient supplements for the rest of their lives.

The rules that will govern your dietary habits after surgery will differ a lot based on which procedure you undergo. This should be one of the many major considerations you make when choosing a bariatric surgery; if you are unable to stick to the eating plan post-operation, major problems will arise and you may even put on more weight.

Introducing Your New Stomach
Expect your doctor to have a post-surgical dietary plan that will ease you into eating solid foods. Post-surgical diets vary from doctor to doctor, but are generally divided into phases, progressing slowly from liquids to mushy foods to more solid foods. Before you eat anything, however, your doctor will examine you after the surgery to confirm that either the LAP band is correctly in place or that the sutures from the gastric bypass or BPD are secure.

At first, you'll only be allowed to consume clear liquids including water, sugar-free juice, clear broth, sugar-free gelatin and flat soda in small amounts at a time. No other food can be consumed for one to two weeks after surgery. This will give your body time to heal. When your body adjusts to these liquids, your doctor will gradually allow a full-liquid diet, with the major component being a nutrient-rich protein shake. This liquid diet can last anywhere from two to six weeks after surgery.

Eventually, you'll begin to eat soft and puréed foods. Baby foods can be eaten, but be sure that, no matter what, everything you eat is completely smooth in texture, so you don't rip out a suture. Eggs and low-fat cheeses are a good option, as are puréed tuna or chicken. Eventually, you will be allowed to begin to eat soft, solid foods. Mashed potatoes, oatmeal and cooked vegetables are good options to help you transition into tougher foods.

Sabotaging Your Own Success
So now that you've made it through the hardest steps, it's time to transition into a routine eating pattern. This does not mean that you can go back to your old eating habits, however.

For all patients, every meal needs to be nutrient dense. Some find that eating five or six small meals throughout the day works better than eating three larger meals. And since your stomach can only hold about 1 ounce of food at a time, you'll need to chew your food thoroughly into a mush before swallowing to aid absorption, and remember to stop eating when you feel full.

Since a LAP band procedure leaves the stomach completely intact, you are still able to digest food normally and absorb nutrients—and calories—as you did prior to surgery. As a result, LAP band patients only need to take a chewable multivitamin for the few weeks following surgery. However, many LAP band patients gain weight after gastric surgery due to "grazing," or constantly eating throughout the day. If a LAP Band patient nibbles on food all day, he or she can continue eating a high-calorie, high-fat diet without ever feeling full and never lose weight.

For gastric bypass and BPD patients, failure to take all vitamin and mineral supplements continuously after surgery is by far the biggest problem. As they begin to heal from surgery and move into a more routine life, it becomes easy for you to skimp on the expensive pills that you are supposed to take because, for the most part, you still feel completely healthy.

"It's not the kind of thing you're going to feel tomorrow or next week," says Kevin Montgomery, MD, FACS, co-founder of the Northwest Weightloss Center. "But down the road it'll really effect you."

Common illnesses after these surgeries include osteoporosis from calcium deficiency, anemia from iron deficiency and permanent nerve damage from thiamin deficiency.

Many people find BPD appealing because it eliminates your stomach's role in the digestive process; food just passes right through, never making you feel full, but it leaves very few nutrients absorbed in the process. Some patients think they can just eat and eat and let the food end up in the toilet. Additionally, the vitamins BPD patients are given often go undigested because the high amount of food consumption pushes them through too quickly, without anytime to dissolve and be absorbed, leading to, as Dr. Watkins put it, "very expensive diarrhea."

Protein consumption is especially emphasized in malabsorptive bariatric surgery patients, as protein is necessary to help you heal after surgery. But it is also important later on. Proper protein consumption ensures that the weight you lose is fat, not muscle. LAP band patients can continue to digest protein normally, so this is less of a concern. Part of the reason gastric bypass and, especially, BPD offers the fastest weight loss, is only because these patients do not get enough protein; their body begins to decompose their muscles, called "muscle wasting", to make up for the missing protein, leading to very rapid weight loss, but also weakness and possibly loss of mobility from the muscle loss.

Upsetting Your Stomach
If you do not follow the diet plan specified by your doctor, you probably will experience some discomfort. However, be advised that not all people tolerate the same foods equally well after surgery. LAP band patients often have problems digesting rice or white bread; gastric bypass and BPD patients have problems with sugary foods. As you experience discomfort, keep notes of the food you ate. Identifying these trigger foods can help you stay as comfortable as possible.

Dumping syndrome is probably the most common problem after gastric bypass or BPD. Because sugar is no longer well digested in the stomach, it passes straight into the intestine. The sugar then absorbs water, causing diarrhea that leads to nausea, weakness, dizziness and a rapid pulse. To prevent this discomfort, sugars and high-fat foods should always be avoided. If it's too late, and you begin to experience symptoms of dumping syndrome, lay down to slow the movement of food to the intestine.

Mental Weight-Loss
In many cases, the hardest lifestyle factor to change is one's psychological need for food. In many cases, morbidly obese people use food as a source of comfort, rather than a source of nutrition, leading to their extreme weight.

"[Some patients] have trouble changing their relationship with food," Dr. Watkins says. "They ate when they were bored; they ate when they were depressed, and they just can't do that anymore; they need to find something else."

Having a multidisciplinary team set up before you have the obesity surgery will help you transition both physically and mentally into your new body. Many women—85 percent of the people who receive weight—loss surgery-find themselves confused when treated differently by men and coworkers. A dietician, psychologist and a support group can help you cope with these changes you may not have expected and give you new ways to handle stress and find comfort.