High and Low Fiber Diets

By Nancy Ling, Registered Dietitian

People often are confused as to when they should be following a high-fiber diet, and when they should be following a low-fiber diet. Foods such as whole wheat bread, fruits, vegetables, and bran cereals are often seen as healthy by many individuals, but are they always good for you? Let's see what they foods contain and when they can be advantageous, and when they might cause problems.

What is Dietary Fiber?

Whole grain products, fruits and vegetables all contain an undigestible plant matter called dietary fiber. Dietary fiber is only found in plant products, and never in animal products. Though dietary fiber cannot be digested or absorbed, it plays a role in nutrition because our gastrointestinal tracts require it for bulking. Dietary fiber absorbs water, thereby bulking the stool and making the stool softer and bowel movements more regular. In other words, dietary fiber is desirable in the diet to promote bowel regularity and a healthy colon.

The average American diet of meat, dairy products, refined breads and cereals, processed food, and very little fruits and vegetables contains too little dietary fiber for health. This is why people are often encouraged to eat more fruits and vegetables and to choose whole grain cereals and breads more often.

Who Should Follow a High-Fiber Diet?

Ideally, all healthy individuals without any disease should try to follow a diet high in dietary fiber. This amounts to approximately 20 to 25 grams of fiber per day, the amount that can be obtained by following a diet with 5 or more servings of fruits and vegetables, along with 4 to 6 servings of whole grain breads or cereals per day. A serving of fruit or vegetable is a medium-sized fruit or vegetable or 1/2 cup of raw fruit or vegetable. A serving of whole grain bread or cereal is a slice of bread or 1/2 cup of cereal. A diet high in dietary fiber is not only good for the colon, but also is heart-healthy and may help control body weight. At least 8 cups of water should be consumed per day in conjunction with a high-fiber diet.

People who have a family history of colon cancers, or who have irritable bowel syndrome or chronic constipation are advised to adhere to a high-fiber diet with plenty of fluids. They are encouraged to follow a plant-centered diet, where at least 2/3 of their plate should be filled with a variety of raw and cooked vegetables and whole grains. Fruit should be a primary snack, and whole grain bread products should be selected. Minimizing restaurant meals and processed food is also helpful in increasing the fiber content of meals, since most restaurant and processed foods are low in fiber.

Who Should Follow a Low-Fiber Diet?

So when it is useful to have a low-fiber diet? It is helpful to keep fiber intake below 10 grams a day when the bowel is having some acute problems, such as abdominal pain, delayed gastric (stomach) emptying, diarrhea, strictures, obstructions, diverticulitis or flare. A stricture is a narrowing of the intestiine that makes it harder for material to pass through, and an obstruction is a blockage in the intestine. When such situations occur, dietary fiber can irritate the bowel because it is bulky and undigestible material, thereby causing pain. Dietary fiber also slows stomach emptying, which may worsen delayed gastric emptying. Individuals with this diagnosis, or those who feel full easily, can benefit from following a low-fiber diet.

In the case of diarrhea, fiber in the diet can make bowel movements more frequent, or it can improve diarrhea by absorbing some of the fluid that is in the intestine -- the effect is not always predictable. But often times, when someone has diarrhea, the best thing might be to avoid too many raw fruits and vegetables, and stick to white breads and avoid whole grains. Then small amounts of fiber, such as that found in oatmeal, can be added back into the diet slowly.

When the bowel has strictures, obstructions, or diverticuli, a low-fiber diet may also be helpful to avoid exacerbating the problem. A high-fiber diet can cause a narrowing in the bowel to become blocked, or can cause food particles to be stuck in the diverticuli, causing more inflammation. Hence, a low-fiber diet avoiding seed and nut products can help in these situations.

When one has a chronic inflammatory bowel disease such as Crohn's disease or ulcerative colitis, a regular diet is generally used, but a low-fiber diet may be useful during a flare-up. A low-fiber diet during a flare can help reduce the symptoms of abdominal pain and improve dietary tolerance. Avoidance of raw fruits and vegetables and whole grains is generally recommended during this time.

The thing to remember with a low-fiber diet is that it should be used short term to help treat an abdominal condition that is acute and generally painful or uncomfortable. Once the diarrhea, abdominal pain, inflammation or obstruction has resolved, fiber should be added back to the diet gradually so the intestine can adapt to the dietary fiber and resume regular bowel movements.