The interplay between diet, gastrointestinal complaints and gut microbiota: more than a gut feeling?

Even though functional bowel complaints are not life-threatening disorders, they can have an immense impact on quality of life (QoL), and can co-exist with anxiety and depression. Fully understanding these complaints and exploring the potential of the diet to reduce symptoms is therefore pivotal for a large group of people, as functional bowel disorders, of which Irritable Bowel Syndrome (IBS) is the most well-known, can occur in up to one fifth of the global population. The majority of IBS patients report that symptoms can be induced by specific foods, so-called dietary triggers, but these differ largely between patients, and it is unclear what is causing these differences. It has also been suggested that the gut microbiota in IBS patients is altered, however a specific microbial signature remains elusive.In Chapter 2, a large online cross-sectional survey was performed, which included 1601 IBS patients. In this survey, associations between self-reported dietary triggers and IBS symptoms were investigated. No differences were found between IBS patients of different subgroups (e.g. different IBS subtypes or severity groups). In general, greasy foods, onions, cabbage, spicy and fried foods were most often mentioned as triggers. IBS severity was positively associated with the severity of a response and number of dietary triggers a patient responded to, while QoL was negatively correlated with the response to dietary triggers. No differences were seen in self-reported response to dietary triggers between patients with and without anxious and depressive symptoms. Only 30% of the IBS patients that made alterations in their diet was guided by a dietician, showing a clear need for dietetic supervision in this population.Next, a longitudinal study including two timepoints with 4 weeks in between was performed, which included 91 IBS patients and 30 matched controls (Chapter 3). This study investigated the dynamics of gut microbiota and SCFA levels in different IBS severity groups, and the association between stool pattern, diet, depression, anxiety and QoL over time. We observed large time-dynamics, as already 36% of the IBS patients changed severity group and 53% had a different stool pattern within 4 weeks of time. No consistent differences between IBS patients and controls or IBS severity groups were observed in microbial alpha diversity or composition, and SCFA levels. The relative abundance of Bifidobacterium was consistently lower in IBS compared to controls while that of Terrisporobacter and Turicibacter were higher. These genera could be potential targets for future microbiota-mediated treatment strategies in IBS. Importantly, Chapter 3 shows that longitudinal studies are crucial to discriminate consistent associations between different datasets from coincidental observations. Due to the large within-person variation over time, biological interpretation from cross-sectional studies is thus very limited and may partly explain the inconsistency between studies.Constipation is another frequently occurring bowel disorder, which is characterized by hard stools and infrequent bowel movements, and can reduce QoL. Constipation can be the result of an unhealthy lifestyle characterized by a low fiber intake. A high fiber intake is considered positive for health, regardless of having constipation or not, as this is associated with a reduction of risk for several diseases. A fiber intake of 30 g/day for females and 40 g/day for males is recommended in the Netherlands, but Dutch median intakes are currently 18 and 23 g/day, respectively. Sustainably increasing dietary fiber intake remains a challenge. Personalized dietary advice (PDA) has been suggested as a promising method to sustainably improve dietary intake. Moreover, a successful screening is essential for such high-fiber interventions, but this has been shown difficult as current dietary assessment methods are more elaborate than needed for recruitment. This places an unnecessary burden on both participants and researchers.The development and validation of a short fiber screening questionnaire was described in Chapter 4. The 18-item FiberScreen assessed dietary intake of fruits, vegetables, whole grains, pasta/rice/potatoes, legumes and nuts/seeds over the past 2 weeks, and was compared with a food frequency questionnaire (FFQ) in 87 adults without gastrointestinal complaints and 29 adults with constipation complaints. The 18-item FiberScreen had a good correlation with the FFQ, and differences between the estimates were relatively small. Average completion time was only ~4 minutes, compared to 45-60 minutes of the FFQ. The 18-item FiberScreen was therefore shown to be a suitable questionnaire to screen for a low fiber intake in Dutch adults.In Chapter 5 and 6, the development and validation of a digital high-fiber PDA in 81 adults without gastrointestinal complaints and 25 adults with constipation was described. The PDA was based on a participants’ habitual diet and gender, and participants could substitute habitually low-fiber products for high-fiber alternatives and add high-fiber foods to their PDA to meet the recommendations. In Chapter 5, it was shown that a digital high-fiber PDA was effective in increasing dietary fiber intake, even up to 3 months after the intervention when still given access to the PDA. In Chapter 6, the further optimized high-fiber PDA increased fiber intake, reduced symptoms and improved stool consistency and QoL in adults with constipation. No effects of the high-fiber PDA on stool frequency, gut microbiota or SCFA levels were observed. In both chapters, the high-fiber PDA was well-accepted and improved self-reported knowledge of fibers.In conclusion, my thesis adds important new knowledge regarding associations between diet, gastrointestinal symptoms and the gut microbiota in IBS patients and developed practical tools for the screening and stimulation of dietary fiber intake. The results from this thesis show dietary triggers for IBS complaints are highly personal. Furthermore, bowel complaints and the gut microbiota are highly fluctuating over time and do not seem associated with each other or with a change in fiber intake. PDA based on habitual diet and gender is effective in increasing dietary fiber intake in the general population, and can subsequently reduce complaints in adults with constipation. The interplay between bowel complaints, the gut microbiota and the diet is complicated, indicating that it is too soon to develop a PDA for reducing complaints in IBS patients.&nbsp