To understand constipation, it helps to know how the colon (large intestine) works. As food moves through the colon, it absorbs water while forming waste products, or stool. Muscle contractions in the colon push the stool toward the rectum. By the time stool reaches the rectum, it is solid because most of the water has been absorbed.

The hard and dry stools of constipation occur when the colon absorbs too much water or if the colon's muscle contractions are slow or sluggish, causing the stool to move through the colon too slowly.

The most common cause of constipation is a diet low in fiber found in vegetables, fruits, and whole grains and high in fats found in cheese, eggs, and meats. People who eat plenty of high-fiber foods are less likely to become constipated.

Fiber—both soluble and insoluble—is the part of fruits, vegetables, and grains that the body cannot digest. Soluble fiber dissolves easily in water and takes on a soft, gel-like texture in the intestines. Insoluble fiber passes through the intestines almost unchanged. The bulk and soft texture of fiber help prevent hard, dry stools that are difficult to pass.

According to the National Center for Health Statistics, Americans eat an average of 5 to 14 grams of fiber daily,* short of the 20 to 35 grams recommended by the American Dietetic Association. Both children and adults eat too many refined and processed foods from which the natural fiber has been removed.

A low-fiber diet also plays a key role in constipation among older adults, who may lose interest in eating and choose convenience foods low in fiber. In addition, difficulties with chewing or swallowing may force older people to eat soft foods that are processed and low in fiber.

Although treatment depends on the cause, severity, and duration, in most cases dietary and lifestyle changes will help relieve symptoms of constipation and help prevent it.

A diet with enough fiber (20 to 35 grams each day) helps form soft, bulky stool. A doctor or dietitian can help plan an appropriate diet. High-fiber foods include beans, whole grains and bran cereals, fresh fruits, and vegetables such as asparagus, brussels sprouts, cabbage, and carrots. For people prone to constipation, limiting foods that have little or no fiber, such as ice cream, cheese, meat, and processed foods, is also important.