Irritable bowel syndrome is a functional disorder of the gastrointestinal tract that impacts a person's quality of life by causing abdominal pain and altered bowel habits, but does not cause any damage to the gastrointestinal tract (see reference 1 pg s2 column 1 under The utility of diagnostic criteria in irritable bowel syndrome). Treatment options vary depending on severity, and may include behavioral changes or medication. While many people with IBS report certain trigger foods, such as lettuce, exacerbate their symptoms, there's currently no evidence to support these claims (see reference 1 pg s 4 under Diet and Irritable Bowel Syndrome).
Food and IBS
Sixty percent to 70 percent of those with IBS believe that a food sensitivity or allergy is the cause of their illness, according to a January 2009 article in a supplemental issue of "The American Journal of Gastroenterology." (see reference 1 pg s17 under 2.6). While this theory has not been fully investigated, the data so far, which includes 2 randomized controlled studies, shows no link between a food sensitivity or allergy and exacerbation of IBS symptoms (see reference 1 pg s17 under 2,6). However, the authors of the article report that more research is necessary.
Reaction to Lettuce
Although very rare, some people may experience an allergic reaction when eating lettuce, which might include irritation to the mouth or skin, or, in more serious cases, anaphylaxis (see reference 2 under Discussion para 1, reference 3 pg 155 under Methods para 1). Proper allergy testing, which might include skin prick tests or blood tests, may be necessary to rule out a true allergy (see reference 1 pg s17 under 2.6 para 2). Seek immediate medical attention if after eating lettuce you experience difficulty breathing, severe hives or swelling, tightness in your throat, abdominal pain or fainting.