IRRITABLE BOWEL SYNDROME AND DIET
OBJECTIVE NUTRITION N°68, (MAY 2003)
Dr. Sylvie Tuzet, Hôpital Beaujon-PA-PH, Clichy

Irritable bowel syndrome (IBS) disrupts the lives of several million persons in France.  Can diet play an etiopathogenic role in the development of this disorder?  Several studies have demonstrated that neither fiber supplementation nor food-specific avoidance diets are sufficient to produce a true symptomatic benefit.  Only good overall food hygiene appear to be able to relieve IBS.

The term irritable bowel syndrome (IBS), encompasses all functional manifestations with chronic progression, assumed to be intestinal in nature.  Their perception and daily impact vary highly according to individual subjects.  Even though not life threatening, IBS disrupts the lives of patients: its prevalence is 29% in the populations of industrialized countries, with a marked predominance among females.  In France, 28 million people are estimated to present with IBS, half of which (12.5 million)  consider themselves to be impaired.

EPIDEMIOLOGY OF IRRITABLE BOWEL SYNDROME

In a study conducted on 4,817 subjects representative of the population of France, the most frequent complaints were, in order of importance: exaggerated production of gas (59%), abdominal pain (48%), bloating, (47%), indigestion (40%), constipation (35%),  aerophagia (29%), diarrhea (28%),  bad breath, (22%) and a sensation of incomplete evacuation of stools (19%).  Sixty one percent of the French population presented with functional GI symptoms which resulted in a visit to their doctor in 12.5% of cases and a treatment by prescription or self medication for 27% of the subjects.
In a French study published in 2001, based on self-questionnaires, (1,266 responses) abdominal pains were described in 83% of cases, GI motility disorders involved 77% of patients and were made up of 39% constipation, 18% diarrhea, and 18% alternating diarrhea and constipation; the pain was of more than five years' duration in nearly two thirds of the patients, with the characteristics being the same in both men and women.  Impact on the patient's social life was minor or non-existent in 12% of cases, was considered a social disability in 50% of cases and an occupational disability in 27%, the cause of sick leave in 21% of cases.  Following self-medication for 47% of cases, patients called their general practitioner in 75% of cases. IBS caused 6 to 12 million annual  doctor visits and is the leading cause for visits to gastro-enterologists.  The prevalence of psychiatric disorders in patients with IBS continues to be a subject for debate.  Most often, it involves personality disorders with no major psychiatric disorders.

Since there are no reliable diagnostic signs, (both in terms of imaging and laboratory test parameters),  and to date no specific markers to make a simple positive diagnosis, IBS continues to be a diagnosis of exclusion, in spite of its recognized clinical criteria. (see inset 1)

A study of the relation between IBS and nutrition leads us to ask two questions: can diet play an etio-pathogenic role when low in a nutrient necessary to intestinal physiological balance?  Is there an abnormal digestive sensitivity to a nutrient, with as a corollary, a need to exclude certain foods?

IBS AND DIETARY FIBER

Several epidemiologic studies have suggested that a diet low in fiber may promote the occurrence of colonic motility disorders, thus the occurrence of IBS.  Based on these pathophysiological data, IBS has been treated for years mainly with a fiber-enriched diet.  However, in a case-control study, daily intake of fiber did not differ between a population suffering from IBS and the control population: thus, the pathogenic role of fiber deficiency should be viewed in perspective.
Among ten randomized studies evaluating the efficacy of fiber supplementation, only four have demonstrated a positive effect, with the only symptom clearly improved being constipation.  All of the studies make it possible to conclude that fiber has an overall effect differing little from placebo, whose efficacy is often very high (40-50%).  Recently, a study conducted on 100 patients suffering from IBS subjected to a fiber-enriched diet showed that wheat bran can worsen symptoms.  The efficacy of fiber on IBS symptoms may depend on the nature of the fiber, the harmful effect of wheat bran contrasting with the beneficial effect of mucilage on symptoms.  In the future an improved definition of dietary fiber and the use of modified fiber may improve their tolerability and efficacy.

IBS AND FOOD INTOLERANCES

Some patients describe the regular occurrence of IBS in the immediate or delayed aftermath of ingestion of foods that they are able to identify.  An avoidance diet, then, appears justified.  However, a recent meta-analysis of all studies on food intolerance, concluded that the study methodological quality was still insufficient to draw definitive conclusions on the real nature of food intolerance and the usefulness of avoidance diets.

Among the types of digestive intolerance, which occur in IBS, two deserve specific development: lactose intolerance and fructose/sorbitol intolerance.

- Lactose intolerance

As a result of a relative (physiological or pathological) deficiency in lactase in some patients, lactose, a disaccharide can be poorly absorbed with two consequences: an osmotic effect and fermentation by colonic flora with highly variable intolerance symptoms (i.e. bloating, abdominal pain, even osmotic diarrhea.)
In France, lactase deficiency involves 20-40% of the population, only half of whom present with clinical signs of lactose intolerance.  No correlation exists between intestinal activity of lactase and GI symptoms: some lactase-deficient subjects can be asymptomatic even with normal intake of lactose.  The prevalence of lactase deficiency does not seem to be higher in patients presenting with IBS than in a control population.  Several studies using the "breath test method" demonstrated that many subjects with IBS are erroneously labeled as lactose-intolerant and that there is a clear placebo effect linked to the elimination of lactose.  Pragmatically, in subjects who complain of IBS and ingest large amounts of milk, it is logical to try reducing lactose intake and in particular the drinking of milk in the morning on an empty stomach.  In patients having a genuine intolerance, fermented dairy products, in particular yogurt are better tolerated, since bacterial lactase present in yogurt compensates for the intestinal lactase deficiency.

- Fructose and/or Sorbitol intolerance

Recently emphasis has been placed on intolerance to sugars such as fructose and sorbitol.  Diarrhea and/or chronic abdominal discomfort are signs of fructose and/or sorbitol malabsorption.  This type of malabsorption is the subject of renewed interest in IBS study.  Indeed, the western diet is changing, with a decrease in the fraction supplied by starch in favor of other sugars in the carbohydrate caloric ration.  In Europe and the US, over half of simple dietary sugars come from additive sweeteners incorporated in industrial baked goods, prepared dishes, or fruit juices.  These can come in the form of either natural sweeteners such as fructose, or as sugar substitutes-the polyols, mainly in the form of sorbitol (which exists naturally in some fruits: apple, cherry, pear) arousing major interest due to their non-cavity causing characteristics and are therefore found in chewing gum, candy,  and also so-called "sugar-free" dietary products.

The absorption mechanism of fructose has not yet been fully elucidated.  The analysis of fructose absorption demonstrated that, on the one hand major inter-individual differences exist and, on the other hand, the frequency of malabsorption has been underestimated. Similar observations have been made for sorbitol with malabsorption possible even with small doses and possibly enhanced by concomitant absorption of fructose.  However, the studies are not all in agreement, for some of them no relationship has been demonstrated between the malabsorption, its extent, and the occurrence of symptoms.  The symptomatic effects of a diet low in fructose and sorbitol have not been studied in depth and never according to a controlled design to date.

Lactose intolerance and malabsorption of fructose and/or sorbitol may not be independent syndromes; an American team has recently demonstrated that malabsorption of several sugars may be observed when a lactose intolerance exists.

IBS AND FOOD ALLERGIES

This is one of the most controversial topics of gastrointestinal disorders.  Compared to simple GI intolerance, food allergy differs by the existence an atopic context, with non digestive and oro-pharyngeal manifestations associated upon the ingestion of foods (I.E. urticaria, systemic pruritus, rhinitis, asthmatiform respiratory impairment, headache sometimes with migraine characteristics, overall malaise and arthralgia).  The GI symptoms are also varying: pain and/or bloating, nausea, vomiting, diarrhea with expulsion of mucous.  They generally occur within an hour of ingestion of the food, but may be delayed by up to six to eight hours after the meal.

IBS AND AVOIDANCE DIETS

When food intolerance reactions are suspected, avoidance diets have not demonstrated evidence of systematic benefit.  To temporarily exclude a food which is manifestly poorly tolerated to evaluate the symptomatic benefit appears to be a pragmatic and logical approach.  However, it is always necessary to weigh the potential symptomatic benefit of these avoidance diets against the risk of inducing nutritional disorders in patients who are likely to be obsessive and who totally  exclude several types of foods for prolonged periods.

CONCLUSION

Proper dietary and lifestyle measures are the mainstay of IBS management.
Such measures consist in advising the patient to eat meals in a quiet environment (over at least twenty minutes, not while working), with adequate chewing and sufficient fluid intake of between 1.5 and 2 liters per day.  After enthusiastic interest in fiber supplementation for treatment of IBS, this measure can no longer be considered as remarkably effective.  Generally, except for the avoidance of foods which promote fermentation (i.e. pulses, white string beans, cabbage...and very sweet foods) and diets too rich in fat for patients with dyspepsia, no diet should be recommended to IBS patients.

Dr Sylvie TUZET
Hôpital Beaujon -AP-HP, Clichy

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