MRI assessment on the impact of a low FODMAP diet on the gastrointestinal tract

Abstract

Introduction: Poorly absorbed fermentable carbohydrates (FODMAPs) escape digestion in the small bowel, and enter the colon to be fermented by gut bacteria thus increasing luminal volume. However, in susceptible individuals e.g. Irritable Bowel Syndrome (IBS) sufferers, this may result in gastrointestinal symptoms, due to the rapid emptying of the small bowel producing increased luminal water and distension in the colon from gas producing bacteria. Previous studies have demonstrated the effect of fermentation on FODMAPs by assessing microbial concentration and breath hydrogen. However, the physiological changes in the lumen, particularly in the colon, remain unclear. Aims: The main purpose of this thesis is to assess the effects of FODMAPs on luminal volume, using magnetic resonance imaging, and how they impact appetite, gastrointestinal symptoms and function. The hypotheses generated and tested were that: i) FODMAPs increase luminal volume, causing symptoms in IBS patients ii) Restriction of FODMAPs reduces luminal volume in patients and healthy volunteers Methods: Four studies were undertaken, with the first being an observational study and the remainder being RCTs. The first study was a pilot study to compare a validated dietary assessment method with a reference method to quantify FODMAP intake in healthy volunteers (n=37). The second study assessed the effect of a low FODMAP diet versus a high FODMAP diet on gastrointestinal physiology and function in healthy volunteers (n=37; FOG). The third study measured the effect of different physical forms of FODMAPs on gastrointestinal physiology in healthy volunteers (n=18; APPLE). The final study compared the effects of a low FODMAP diet versus habitual diet in patients with IBS (n=27; CRAIGS). Results: Findings from the first study suggested that the 7-day food diary is more reliable in assessing energy, nutrient and FODMAP intake within the UK population. In the FOG study, colonic volume increased in both groups and fasting breath hydrogen levels declined on the low FODMAP diet. However, fasting breath hydrogen levels increased in those on a high FODMAP diet when supplemented with oligofructose. In the third study, altering the physical form of a meal had an effect on small bowel water content only during the late phase, whereas gastric emptying in healthy volunteers was fastest with apple juice, followed by apple puree and whole apples respectively. In the CRAIGS study, the low FODMAP diet lowered the increase in colonic volume when compared with the habitual diet. Conclusions: A high FODMAP diet, whether from dietary intake or supplementation, can lead to an increase in colonic volume. In IBS patients this impacts on their overall global symptoms. The unexpected increase with a low FODMAP diet, and the inconclusive effect of fermentation as a result of low breath hydrogen levels, suggests that the role of foods substituted as part of the low FODMAP diet needs further investigation. Meanwhile, altering physical properties of FODMAPs has an impact on luminal volume, but this also needs further investigation. A detailed dietary intake using a 7 day food diary during pre and post dietary intervention is recommended as a measure of compliance