Background

Chronic gastrointestinal diseases have a substantial impact on healthcare costs and utilisation.1 In 2016, they affected 3.7% of the population worldwide (a 23% increase compared with 1990), with a reported prevalence of 17% in older Australians.2 These conditions also affect productivity, through lost working days and reduced health-related quality of life (HRQoL).1 3–7 Some chronic gastrointestinal diseases, such as irritable bowel syndrome (IBS) and gastro-oesophageal reflux disease (GORD), are relatively common but do not necessarily require ongoing medical review. Others, such as Crohn’s disease, ulcerative colitis or coeliac disease, are less prevalent, but due to their more serious physical and mental health consequences, usually need ongoing treatment and regular follow-up with a healthcare provider.8–10 Dietary change is often recommended as part of appropriate management for these conditions. For instance, a gluten-free diet to treat coeliac disease, and more recently the ‘Fermentable Oligo-, Di-, Monosaccharides And Polyols’ (FODMAPs) diet has been found to be effective for some people with IBS.10–15 Interestingly, perhaps as a consequence of increased health knowledge in the community or presumed health benefits, many people without diagnosed chronic gastrointestinal conditions are restricting their dietary intake of gluten, milk products and/or foods containing FODMAPs.14 16–18 According to two recent surveys, 21% of Australian adults avoid wheat and/or dairy products, but 25% of them have no clinical condition or symptom to justify that decision.19 20 Little is known about the long-term consequences of these dietary choices among those without diagnosed gastrointestinal conditions.

Although much has been published about the impact of gastrointestinal conditions on HRQoL,3–7 21 only some studies have examined the added effect of dietary restrictions on this outcome. A recent review of dietary adherence in coeliac disease concluded that diet could improve but did not normalise HRQoL.12 While for IBS, very few studies have examined the impact of dietary changes on patients' HRQoL.22 23 Similarly, very little is known about dietary restriction and changes in HRQoL for those with GORD, Crohn’s disease or ulcerative colitis. Therefore, with the noted exceptions of restriction in the context of obesity24 and/or eating disorders,25 the impact of dietary restrictions on HRQoL has been little studied.

Additionally, psychiatric comorbidity among patients with gastrointestinal conditions, either IBS, GORD, inflammatory bowel disease or coeliac disease, has been found to be associated with increased symptomatology, worse prognosis and reduced HRQoL.26–32 In fact, in recent years, there has been an emerging interest in understanding the role of the gut–brain axis, as the complex interaction between nutrition, chronic inflammation and neurotransmitter concentrations seem to affect psychological processes and patient-centred outcomes.26 27 29 33 34

Finally, to our knowledge, no one has investigated the role of restrictive diets on HRQoL among individuals with or without gastrointestinal conditions using a population-based sample, or whether having a mental health condition changes the association between gastrointestinal conditions and HRQoL. Such information could help us develop better policies about health and food education. Therefore, this study aimed to explore the complex relationship between self-reported gastrointestinal conditions, restrictive diets, mental health status and HRQoL in a representative sample of the South Australian population. Our hypotheses were (1) that people reporting having a restricted diet, with or without a gastrointestinal condition, would have better HRQoL than those who did not and (2) having a mental health condition moderates the relationship between a gastrointestinal condition and/or restrictive diet and HRQoL.

Methods

This study used data from a cross-sectional, face-to-face survey (Health Omnibus Survey) including a representative sample of adults living in South Australia (SA). In 2015, a complex sampling process was used to select participants. Details of the methodology have been published elsewhere.35 Briefly, participants aged 15+ years were randomly chosen from a selection of 10 residences in each of the 530/3939 level 1 statistical areas selected for this study.36 Individuals terminally ill/mental incapacitated (n=104) or unable to speak English (n=87) were excluded. The final sample included 3005 individuals (71.1% of the 4226 eligible), but only 2912 individuals aged 20+ years were included in the analysis.

Results

A total of 2912 individuals aged 20+ years were included (mean age 48.9±18.1 years; 50.9% females). The most common gastrointestinal conditions were GORD (8.4%) and IBS (7.9%), while ulcerative colitis (0.9%), Crohn’s disease (0.8%) and coeliac disease (0.7%) were less frequent. Food avoidance because of pain, bloating or diarrhoea was reported by 23.3%, while 19.8% followed a lactose-restricted diet, and 9.2% avoided gluten products.

Table 1 shows the prevalence of gastrointestinal conditions (16.4%; 95% CI 14.6 to 18.3) was higher among females, individuals aged 50+ years, or retired people, but was not different according to marital status, residence area, educational level or socioeconomic position. The prevalence of individuals on any restrictive diet (36.1%; 95% CI 33.9 to 38.3) was higher among those with a gastrointestinal condition (60.7%) versus those without a gastrointestinal condition (31.3%). The difference was statistically significant (p<0.001). Females or those with a certificate/diploma or higher educational level were more likely to be on a restrictive diet, independent of whether they had or not a gastrointestinal condition.

The daily consumption of fruit/vegetables was not associated with having a gastrointestinal condition (table 2). However, gastrointestinal conditions were more frequent among those reporting physical activity 0–1 days/week, low alcohol consumption, ex-smokers, obesity or with a mental health problem. Table 2 also shows that individuals with higher fruit/vegetable consumption were also more likely to be on a restrictive diet, independent of their gastrointestinal status. None of the other lifestyle variables, BMI or mental health status was associated with the prevalence of restrictive diets in either group.

Table 3 summarises the association between gastrointestinal conditions and restrictive diets with HRQoL. Except for the relationship between gastrointestinal conditions and PCS, results adjusted for sociodemographic variables showed small differences compared with crude results. All gastrointestinal conditions and restrictive diets showed negative β coefficients (ie, negative values for the mean differences, indicating a lower HRQoL). However, for PCS, only the associations with GORD, IBS, restriction of food related to pain, bloating, or diarrhoea, and lactose-restricted diets were statistically significant. For MCS, the significant associations were observed with GORD, IBS, ulcerative colitis and restriction of food related to pain, bloating or diarrhoea. Consequently, having any gastrointestinal condition, or being on any restrictive diet, were both associated with lower HRQoL, especially with PCS. The adjusted r² for PCS increased from 21.0% (including all sociodemographic variables) to 22.4% and 21.7% when the combined gastrointestinal or restrictive diet variables were included in the model, respectively. For MCS, the adjusted r² remained steady at 10.0% even after any of the variables were included in the regression models. No evidence of multicollinearity between the independent variables was identified (mean VIF=1.9 for both outcomes). Further adjustment for mental health status (model 2) or lifestyle variables and BMI (model 3) had no substantial effect on the observed associations (online supplementary table S1) .

Being on a restrictive diet was not an effect modifier of the association between having a gastrointestinal condition and HRQOL (p value for interaction >0.4 for PCS or MCS). Nonetheless, mental health problems were not only twice as frequent among those with a gastrointestinal condition (22.5% vs 10.9% for those without these diseases), but having a mental health problem was also identified as a moderator of the association between gastrointestinal conditions and PCS (p value for interaction 0.02; figure 1). Among those without a mental health problem, having a gastrointestinal conditions was associated with a 2.4 points lower PCS (95% CI −3.5 to −1.3), while for those with a mental health problem this reduction was greater (−5.9 points; 95% CI −8.7 to −3.1). For MCS, having a mental health problem was associated with a lower HRQoL score (−9.5 points; 95% CI −10.9 to −8.1), but there was little evidence of interaction with the presence of gastrointestinal conditions (p value for interaction 0.09). A summary of the study findings can be found in online supplementary figure 1.

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Figure 1

Association between the presence of gastrointestinal conditions and the physical component of health-related quality of life among individuals ≥20 years in South Australia in 2015 (n=2912), stratified by the presence of mental health problems. Gastrointestinal (GI) conditions include those who had irritable bowel syndrome, celiac disease, Crohn’s disease, ulcerative colitis and/or gastrointestinal reflux. Mental health problems include those on medication for depression, anxiety or other mental condition. Results are adjusted for sex, age, marital status, area of residence, educational level, working status and socioeconomic position. Vertical lines represent the 95% CI in each category. 

Discussion

In this population-based cross-sectional study, almost a third of the sample was on some form of restricted diet. Those with a gastrointestinal condition were twice as likely as those without a gastrointestinal condition to be restricting their diet. Women, those with a higher education level, and people in higher socioeconomic groups were also more likely than others to restrict their diet. Having any gastrointestinal condition or being on a restricted diet were both associated with lower HRQoL, but the relationship was stronger with PCS. Moreover, being on a restrictive diet was not associated with a better HRQoL among individuals with a gastrointestinal condition. Having a mental problem was associated with a stronger adverse relationship between gastrointestinal conditions and PCS.

Estimates of the community prevalence of gastrointestinal conditions vary considerably.2 Our figure for IBS (7.9%) is at the lower end of the prevalence identified in other studies, with published pooled prevalence of 11.2% (95% CI 9.8% to 12.8%). Prevalence estimates varied according to country (from 1.1% to 45.0%) and criteria used to define IBS.8 For GORD, the frequency we identified in SA (8.4%) was also lower than the Australian estimate of 11.6%.9 However, for coeliac disease, our results (0.7%) are comparable with the estimated prevalence among Australian adults (0.56%–0.96%).51 There do not appear to be any recent estimates for the prevalence of Crohn’s disease and ulcerative colitis in Australia, although the incidence is apparently increasing.52 Self-report of the investigated conditions may account for these discrepancies, considering the intermediate levels of sensitivity for such kind of information (ranging from 33% to 85%), even though reliability and specificity are considered satisfactory.53 54 Overall, our data are consistent with previous estimates for conditions that require ongoing medical follow-up and less for those that do not, such as IBS or GORD.8 9

By comparison few population estimates exist for dietary restriction (or food avoidance/intolerance) in Australia. The Commonwealth Scientific and Industrial Research Organisation undertook a national survey in 2013 that found 7.3% of their sample reported physiological effects, mainly gastrointestinal, that the individual believed was due to wheat.19 In the same survey, 11.8% of the sample reported avoiding dairy products because of adverse physiological effects.20 However, the National Health and Nutrition Survey 2011–2012 identified that 17% of Australians reported food avoidance due to allergy or intolerance with their estimates for cow’s milk/dairy (4.5%) and gluten (2.5%) avoidance being much lower than ours (19.8% and 9.2%, respectively).55 These differences may reflect an increase in the availability and accessibility to gluten-free and lactose-free products in recent years. This would make it easier to follow restrictive diets, not only for people with an established clinical diagnosis but also for those affected by some form of sensitivity, intolerance and/or allergic reaction to these diet components.14 17 18 Another explanation for the higher prevalence of restrictive diets in our study are changes in knowledge or beliefs about foods or terminology used, with people defining their behaviours as a restriction, avoidance, intolerance or elimination, which therefore results in different estimates of their frequency.18

While there are many cultural, religious, ethical or economic reasons for eating, or not eating, particular foods,56 there are potential health implications for dietary restriction (especially gluten-free diets), when not medically indicated or appropriately supervised.13 15 18 Furthermore, dietary restriction may be taken to extreme, as in the orthorexia nervosa syndrome,57 with subsequent adverse effects on health status and HRQoL.

Some studies have reported impaired HRQoL in those with chronic gastrointestinal conditions, including IBS, GORD, coeliac disease and inflammatory bowel disease.3–7 29 However, results regarding the benefits of following a restrictive diet on HRQoL are controversial, as some studies identified no associations with this outcome,42 while others report that individuals with a gastrointestinal condition that received support to modify their diet show improvements in their HRQoL.12 22 23 42 Contrary to our original hypothesis, individuals following a restrictive diet had a lower HRQoL (PCS and MCS), even among those without a history of gastrointestinal problems. Reverse causality could explain these results: individuals with a more severe condition and reduced HRQoL following a restrictive diet to ameliorate their symptoms. The same explanation is also plausible for the results identified among those without any of the investigated gastrointestinal diseases. This group could include a number of undiagnosed cases, individuals with other gastrointestinal conditions, or even those affected by a wider spectrum of sensitivity/allergy to different diet components.14 18 It is also possible that these individuals have other undiagnosed mental health or physical conditions affecting their HRQoL which we have not adjusted for (residual confounding). Therefore, a history of dietary restriction, or for that matter low quality of life in a person who is seemingly otherwise healthy, should alert health professionals for the need to search for underlying health problems. In any case, further studies in this field are necessary to elucidate the influence of other possible determinants of HRQoL, because after accounting for the variables under investigation, there is still a lot of unexplained variance in the outcomes.

Mental health status also seems to have a central role in the association between gastrointestinal diseases and HRQoL.42 44 45 The lower HRQoL score among those affected by a mental health problem may be a consequence of the ‘affective fallacy bias’. Individuals with mental health problems tend to report lower well-being and/or satisfaction with their life when compared with the assessment by an independent observer.46 47 However, this information bias does not explain the substantially lower PCS among those affected by a gastrointestinal condition in our study. There is evidence that the same immunological disturbances, oxidative stress, autonomous nervous system dysfunction and gut–brain axis interactions that play a role in the pathophysiology of gastrointestinal conditions, also increase the susceptibility for mental health problems, such as depression and anxiety.26 27 29 33 34 42 44 45 At the same time, the coexistence of psychiatric problems seem to alter the course of gastrointestinal disease, as they increase recurrence, the severity of symptoms, and interfere with the treatment of these conditions, thus leading to a reduction in HRQoL.21 44 45

Conclusions

In a large population-based survey, South Australians self-report frequencies of gastrointestinal conditions that are similar to previous estimates. One-third of Australians are restricting their diet, and this is associated with lower HRQoL, especially the physical domain. Women, those from higher socioeconomic groups and middle-aged people are more likely to restrict their diet. Being on a restrictive diet was not associated with a better HRQoL among individuals with a gastrointestinal condition, but having a mental health problem was associated with a stronger adverse relationship between gastrointestinal diseases and the physical HRQoL. Health professionals should be alert to these associations and interactions when trying to improve health outcomes for their patients.