This present study cross-sectionally investigates the relationship between dietary fibre intake and bowel function in adults. Findings of this present study suggest the presence of a strong relationship between fibre status and stool frequency, and an association between fibre intake (g/day), and improved stool type and bowel function habits.

Assessment of fibre status revealed suboptimal intakes in this sample population. Recommendations from EFSA suggest an intake of above 25 g a day is sufficient to provide health benefits relating to optimal gastrointestinal functions [27]. Results of the present study suggest only 32.7% of the cohort met these recommendations. Authors of the National Adult Nutrition Survey [29] found 80% of participants did not meet EFSA recommendations. This suggests the 67.2% of those recorded to have suboptimal fibre intakes in the present study, may be an underestimation, possibly due to respondent bias and over-reporting of intakes of high-fibre groups. Alternatively, the reported intakes may be accurate and reflect an over-representation of health-conscious individuals given the large response rate from the faculty of health sciences. In terms of contribution from food groups, breads & cereals contributed over a third of daily fibre intake, which is aligned with the general population [29]. Gender differences were observed in terms of the amount and types of high fibre foods consumed, with females more likely to have a high fibre intake and significantly higher intakes of fibre from fruits, vegetables, and breads and cereals than males. This pattern was also observed in the National Adult Nutrition Survey [29]. It must be noted however in this cohort that intakes in both genders displayed wide standard deviations across food groups, thus the results should be interpreted with caution.

The prevalence of hard stools, indicative of constipation, was 9.1% in this cohort, which is comparable to reported community levels [12, 13]. It was observed that 72% (18/25) of those reporting a hard stool type, and 68.7% (46/67) of those defecating every 2 to 3 days or less, had a low-fibre status, thus confirming the protective effects of dietary fibre against constipation symptoms. This association endorses similar findings from an RCT conducted by Woo et al., as described earlier [21] and by other comparable studies [22]. There are many possible mechanisms which may underpin this association [3]. Insoluble fibre, found in wheat bran, vegetables, and whole grains, is beneficial for constipation as it dilutes the colon of harmful substances, absorbs undesirable colonic contents, and stimulates peristalsis, facilitating digestion [30, 31]. Gut motility is also stimulated by the short-chained fatty acids produced from fibre fermentation. SCFA are known to do this by enhancing mineral absorption and increasing the bacterial load in the colon, consequently assisting faecal transit [32]. In analyses of fibre intake by sources, significant associations were found between stool frequency and all high-fibre sources except breads and cereals, suggesting fruits, vegetables, seeds, nuts, and legumes have a greater potential to affect faecal transit. Previous studies have found fruit and vegetable fibre is more readily fermented than cereal fibre which may explain its stronger association [33, 34]. However, it must be acknowledged that the association between fibre sources and bowel frequency may be confounded by total fibre intake.

In the current study, an inverse association was present between fibre intake, and bowel dysfunction symptoms, suggesting fibre has protective effects against disordered bowel habits. These symptoms are like that of IBS, and include abdominal cramping, bloating, flatulence, diarrhoea, and constipation. Oka et al. [15] estimated the global prevalence of IBS at around 9.2% (using Rome III criteria) which is lower than the prevalence of those with reported moderate to severe symptoms of bowel dysfunction in this present study (13.1%), suggesting possible over-reporting of IBS symptoms. Oka et al. [15] also reported that woman have a higher prevalence of IBS (OR: 1.46, p < 0.001) compared to men, which corresponds with the findings in this present study. This may also account for the higher overall reporting of bowel symptoms found in this study given the higher number of female respondents. The exact pathophysiologic mechanisms of this are unknown however previous studies suggest the female sex hormones play a role [35]. Previous studies have demonstrated the positive impact soluble fibre had on the IBS symptoms, although insoluble fibre did not appear to have the same effect [36, 37]. This contradicts the findings of the present study which revealed only vegetable fibre demonstrated protective effects against disordered bowel symptoms, despite its predominant composition of insoluble fibre.

Functional bowel disorders and their management can be associated with significant healthcare utilisation [12]. Furthermore, there is strong evidence that increasing fibre intake decreases diverticular disease [38] and colorectal cancer risk [39]. In this cohort, as in the rest of the country, the majority of individuals were consuming less than the recommended 25 g/day of fibre. Thus, public health campaigns to increase dietary fibre consumption could prove to be a cost-effective, safe and efficient way to reduce disease burden associated with bowel dysfunction in Ireland.