The cause of irritable bowel syndrome (IBS) is unknown. There is no apparent disease of the bowel involved. Rather, IBS is related to highly stimulated or reactive nerves and muscles in the large intestine.

Stress, anxiety, food intolerances and allergies may all play a role in triggering the symptoms of IBS. In some cases, severe gastrointestinal infections may be associated with the development of IBS even after the infection resolved.

The following are associated with triggering symptoms in patients with IBS:

• Large meals. Digesting large amounts of food at one time can be difficult for patients with IBS. Too much food passing through the digestive tract at once can lead to nutrient and fluid absorption problems, triggering IBS symptoms.
  
  
• Gas. When the intestines are distended due to trapped gas, this can cause abdominal pain and cramping in patients with IBS. Activities such as chewing gum or eating meals quickly can lead to the inadvertent swallowing of air and cause gas.
  
  
• Certain beverages. Foods high in fat (e.g., French fries, potato chips), foods containing artificial sweeteners (e.g., sorbitol), dairy products and chocolate may cause diarrhea, aggravating symptoms in patients with IBS.
  
  
• Specific drinks. Alcohol and drinks that contain caffeine (e.g., coffee, tea, soda) can stimulate digestion, causing or aggravating diarrhea in patients with IBS. Carbonated beverages can cause gas, further worsening IBS symptoms.
  
  
• Emotional stress. Many nerves inside the intestines connect them to the brain. These nerves control functioning and can react adversely to stress. Intestinal muscles may contract too much or too little, leading to chronic diarrhea or constipation. In patients with IBS, the large intestine appears to be extra-sensitive to emotional stress – even slight conflict can cause an adverse response in the intestines, triggering IBS symptoms.
  
  
• Medications. Some medications can negatively affect the digestive system (e.g., cause diarrhea or constipation) and may aggravate symptoms in patients with IBS.
  
  
• Menstruation. Women with IBS may experience more severe symptoms during their menstrual periods.
  
  
• Nicotine. Smoking may aggravate symptoms in patients with IBS.

Factors that may increase a person’s risk of developing IBS include:

• Gender. Although it is not clear why, many more women are diagnosed with IBS than men in Western nations. In the United States, 80 percent of patients with IBS are women, according to the American College of Gastroenterology (ACG). This gender difference may be a cultural phenomenon, however, because in other areas of the world (e.g., India) it is just the opposite – more men than women are diagnosed with IBS.
  
  
• Age. IBS can occur at any age. However, young people are at greater risk of developing IBS. It typically begins in the late teens or early 20s. IBS rarely begins in late adult life.
  
  
• Traumatic life events. Some studies have shown that IBS is more common in people with a history of physical, verbal or sexual abuse.