STOMACH STAPLE

Because of Smith’s weight-related illnesses, her doctor recommended that she have bariatric surgery. The doctor explained that bariatric surgery changes the anatomy of the digestive system to limit the amount of food a person can eat and digest before feeling full. Of the different types of bariatric surgery, gastric bypass is the most commonly performed, and also the most drastic: a surgeon uses staples to seal the stomach except for a small pouch at the top. The pouch is about the size of a golf ball. The surgeon then cuts the small intestine and sews part of it directly onto the pouch. Consequently, food is redirected, bypassing most of the stomach and the first section of the small intestine. The rest of digestion continues normally (INFOGRAPHIC 26.3).

Bypassing part of the small intestine contributes to weight loss because the small intestine is the primary organ that extracts nutrients from food. The “small” intestine isn’t actually small. It winds through the lower part of the abdomen for about 20 feet. It’s called “small” because its diameter is smaller than that of the large intestine, which comes later in the digestive tract. The first part of the small intestine (about 10 inches or so) is the duodenum, and this is where chyme from the stomach combines with digestive secretions from several other organs. The pancreas, for example, secretes pancreatic juice, a basic fluid (pH greater than 7), into the small intestine to neutralize the acidic chyme (pH less than 7), which would otherwise damage the small intestine. The pancreas also secretes enzymes that help break organic molecules such as carbohydrates, proteins, and fats into smaller pieces. Enzymes secreted by the small intestine itself further break down macromolecules into building blocks such as amino acids, sugars, fatty acids, and glycerol.

Whereas proteins, carbohydrates, and nucleic acids are easily digested by this powerful mixture of digestive enzymes, fats pose a special challenge. Because they are hydrophobic (see Chapter 2), fats don’t mix well with the watery solutions in the small intestine. This makes it difficult for fat-digesting enzymes to break them down. Helping the process along, the liver secretes bile salts, which are chemically suited to dividing large hydrophobic fat globules into smaller drop-lets—that is, emulsifying them. These bile salts pass from the liver into the gallbladder, which stores them for future use. When we eat a high-fat meal, bile salts pass from the gallbladder into the small intestine, where they help emulsify the fats. Once the fats are emulsified, lipase, a lipid-digesting enzyme secreted by the pancreas, chemically breaks them down to release their constituent fatty acids and glycerol (INFOGRAPHIC 26.4).

The liver and gallbladder are crucial organs of the digestive system. A person with no gallbladder or a dysfunctional one—for example, someone who had gallstones and has had the gallbladder removed—cannot digest fats well. Although the liver may still secrete bile salts, they can’t be stored. If that person eats a high-fat meal, most of the fat taken in will pass through the body undigested and be eliminated in the feces.

Once digested into their smallest subunits, food molecules are taken up by epithelial cells lining the small intestine. This stage of digestion is called absorption. The inner lining of the small intestine is folded into fingerlike projections called villi that are composed of many densely packed epithelial cells. The folds greatly increase the surface area through which the intestine can absorb nutrients. The food molecules then pass into blood vessels of the circulatory system, and the bloodstream transports them throughout the body, where they are used as a source of nutrients to build and maintain cells (INFOGRAPHIC 26.5).

Because most digestion occurs in the small intestine, that structure is a common target for diet pills and other supplements. Alli, for example, which is the first over-the-counter weight-loss pill to be approved by the Food and Drug Administration (FDA), prevents the breakdown of up to 25% of the fat consumed. The drug works by blocking the activity of lipase in the small intestine; consequently, some fat remains undigested and passes out in the feces.

While diet pills can help some people lose weight temporarily, they are largely ineffective in the long term, especially for people who need to lose more than just a few pounds. (They also come with some very unpleasant side effects, in the form of sometimes uncontrollable bowel movements.) Most people who are morbidly obese, Smith included, have already tried various diet pills and lost weight, but not enough to improve their health. Even more troublesome, diet pills aren’t a long-term solution. Over time, most people gain all of the lost weight back, and more, after they stop taking the pills. Smith’s weight fluctuated over the years, and though she weighed 264 pounds when her doctor recommended surgery, this weight was likely not her heaviest, she says.

The other main type of weight-loss surgery, which Smith also considered, does not involve altering the small intestine. Called adjustable gastric banding, the procedure involves partitioning the stomach into two parts by wrapping an inflatable plastic band around the upper part of the stomach to seal it off and create a small pouch. The principle is similar to that of gastric bypass—making the stomach smaller—except that with an adjustable gastric band food still passes through the entire stomach to the intact small intestine. The pouch holds approximately ½ cup of food, whereas the typical stomach holds about 6 cups. When the upper part of the stomach is filled the brain receives a signal that the entire stomach is full. This “misinformation” helps one feel full for longer periods of time, eat smaller portions, and lose weight over time. The band can be adjusted or surgically removed if necessary. Banding is becoming more popular because it’s simpler and has a lower complication rate than gastric bypass surgery (INFOGRAPHIC 26.6). In fact, the FDA recently approved it for more widespread use: now people with a BMI of at least 30 may qualify for the procedure.

Gastric bypass, however, is still the more commonly performed procedure—in part because it’s more effective in reducing weight than adjustable gastric banding. That’s because gastric bypass fundamentally alters the normal path of digestion through the stomach and small intestine; food bypasses not only most of the stomach but also more than half of the small intestine. Consequently, not as many nutrients and associated Calories are absorbed into the bloodstream. Smith chose gastric bypass surgery over adjustable banding because, she says, she “wanted a permanent fix.” She went on: “Bands can always be removed. This was a lifelong decision and commitment for me so the idea of being able to change your mind was not for me.”

Once chyme passes through the small intestine, it moves on to the large intestine. The large intestine functions like a trash compactor—it holds and compresses material that the body can’t use or digest, such as plant fiber. Within the colon, the first section of the large intestine, fiber, small amounts of water, vitamins, and other substances mix with mucus and bacteria that normally live in the large intestine. As this waste travels through the colon, most of the water and some vitamins and minerals are reabsorbed into the body through the colon lining. Bacteria chemically break down some of the fiber to produce nutrients for their own survival and also to nourish cells lining the colon, which is one reason fiber is an important dietary nutrient. As the large intestine expands and contracts, it pushes what ultimately becomes stool into the rectum, from which it is expelled through the anus—a process called elimination (INFOGRAPHIC 26.7).