Within- & Between- Subject Variation in Dietary Intake of FODMAPs among patients with IBS

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Dietary assessment methods available have been recognised as having both random and systematic errors that can affect the interpretation of nutritional studies. However, if these errors are minimised the remaining measured variability is representative of the true variability in nutrient intake. As well as between-subject variation, within-subject variation also exists and is a measure of true day-to-day variation in dietary intake of an individual. In recent years, a diet low in fermentable oligo-, di-, monosaccharides and polyols (FODMAPs) has gained increasing interest as a means of managing symptoms of IBS patients. However, the intake patterns of FODMAPs in this patient group remain unclear, for example, types, amounts and variability in FODMAPs consumed.

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The aim of this study was to characterise the within- and between-subject variation in FODMAP intake in patients with IBS. The number of days required per subject to estimate an individual’s nutrient intake and the number of days required to correctly rank individuals into quartiles of FODMAP consumption was also calculated.

The baseline dietary data collected for this study came from two clinical studies undertaken at a specialised centre for functional GI disorders in Sweden. The first study aimed to characterise pathophysiologic traits of IBS and the second study was a randomised controlled trial comparing two dietary regimes used in the management of IBS symptoms. The same inclusion criteria were used in both studies: >18 years of age diagnosed with IBS according to ROME III criteria. Patients were excluded if they had other GI conditions which explained their symptoms. In the dietary intervention study, only subjects with moderate to severe symptoms (IBS Severity Scoring System > 175) were eligible. All subjects completed a paper-based dietary record in which all foods and drinks consumed during four consecutive days (Wednesday- Saturday) was recorded. Time of consumption and type of food consumed were noted along with quantities in grams or by household measures e.g. slices. Cooking method and foods labels were noted where applicable. All diet records were entered into specialist dietary software designed to calculate the energy and nutrient composition of foods. This was then linked to a food composition table and to a Swedish database with FODMAP content developed in house by one of the authors.

A total of 151 women and 46 men were included in the analyses. Group mean BMI was normal for both sexes and subgroups of IBS were evenly distributed for both as well. However, most women reported having severe IBS symptoms (57%), whereas half of men reported moderate symptoms. The reported median total FODMAP intake was 18.7g (range 3.7-73.4) for women and 22.8g (range 3.6-63.2) for men and was similar across the various days of the week. Lactose was found to contribute the most to total FODMAP intake in this Swedish population and for each group of FODMAPs, there was more variation between subjects than within subjects.

The coefficient of variation in FODMAP intake within subjects was relatively high compared with that for macronutrients and was more similar to that of dietary fibre and sugars, which was expected. The fact that the between-subject variation was larger than within-subject variation explains why ranking of individuals into quartiles of FODMAP intake can be achieved with a good degree of precision using a few days of observations. This is because it is easy to distinguish between low and high consumers. However, if the objective is to capture intake at the individual level, the large within-subject variation in FODMAP intake causes difficulty in assessing habitual FODMAP intake. To obtain a good level of precision would not be achievable using a reasonable number of days of food records. It is recognised that quality of dietary data declines in relation to the number of days recorded.

This study provides a basis for planning studies aiming to assess intake of FODMAPs among individuals with IBS. If the study objective can be met by ranking individuals into quartiles of FODMAP intake, then dietary records or repeated 24 hour recalls can provide accurate data using a 4 day food record. However, to capture the intake of less frequently consumed foods high in FODMAPs, these are not suitable. Instead, the authors suggest the use of a semi-quantitative food-frequency questionnaires specifically aimed at capturing intakes of FODMAP-rich foods consumed both regularly and less frequently. The challenge of assessing absolute FODMAP intake remains unsolved.