The present study explores how IBS affects ratings of physical and mental health, and influences perceptions of hindrance of daily activity by physical or mental health. Further, we investigate whether the relationships between IBS and self-ratings of health are best explained by a model of causality or rather reflect common underlying familial and/or genetic factors. To our knowledge, this is the first study to explore family effect, genetic or shared environmental effects, underlying the relationship between IBS and SRH measures.
Our main findings reveal that IBS is predictable for poor ratings of physical health, and further, that mental health, and not physical health, are perceived to interfere with daily activities. The co-twin analyses suggest that causal mechanisms best explained the relationships between IBS with self-rated physical health and with the perception that health interferes with daily activity. Further, both the co-twin control design and the correlation analyses provide evidence that genetic effects are shared between IBS and self-rated mental health.
IBS prevalence
The prevalence of IBS was 10.9%, which is similar to the rates in most European countries42. The prevalence was almost twice as high among females than males (13.5% versus 7.5%), consistent with most studies44,45. Further, 39.5% of the IBS cases were doctor-diagnosed, similar to rates reported in some previous studies46,47.
The impact of IBS on ratings of self-rating health measures
The present study assessed two dimensions of SRH, physical and mental, and inquired about the extent to which physical or mental health interfered with daily activities.
Our findings revealed that individuals with IBS reported worse physical health compared to those without IBS (OR=1.83, 95% CI [1.42;2.35]).
Studies comparing self-reported quality of life among individuals reporting IBS with those who report other chronic somatic conditions are scarce. One study from California University compared the health-related quality of life (HRQOL) of 877 IBS patients enrolled from a large tertiary referral centre with previously published data from the general population and from an observational study including individuals with chronic diseases such as diabetes, end-stage renal disease and depression16.
Individuals with IBS scored lower on all the HRQOL measures: energy/fatigue, bodily pain, emotional and social function, except for physical function, compared to those who suffered from the above mentioned chronic somatic conditions. Further, those who suffered from depression had worse physical functioning caused by emotional and psychosocial health problems than IBS-sufferers. However, it is worth noting that it was not possible to control for age, gender, race, education or comorbidity in these analyses, because the IBS patients and those who suffer from chronic somatic diseases or depression, stem from different studies.
Results from the hierarchical regression analyses in our population-based study, revealed that the association between IBS and poor p_SRH was comparable with the association between somatic conditions and poor p_SRH (OR=2.25, CI 95% [1.87; 2.72], which underscores the potential debilitating effects of IBS.
The association between IBS and self-rated mental health was confounded only by PSS and depression. The confounding effects are explained by the strong association between m_SRH and depression or PSS, and between IBS and depression48 or PSS49. Stress is an important environmental factor in the pathophysiology of IBS which affects various aspects of IBS, including disease onset or exacerbation of abdominal symptoms among individuals already suffering from IBS.
Environmental experiences common to IBS, PSS and depression such as restricted uterine growth, traumatic events or chronic stressors in early life and /or adulthood50–53 are all triggers of the central stress pathways, the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis. Twin studies have provided evidence that shared genetic effects help to explain the covariation between IBS and depression52 and between IBS and social stressors54, suggesting that these disorders share genes involved in central stress mechanisms. Results from candidate studies also suggest that depression55,56 and perceived stress57,58 share in common gene variants with IBS.
Only two population-based studies22,24 have explored the impact of IBS on self-rated health. Both studies used a single question inquiring about the individual’s perception of their general health and one for functional limitations. The studies demonstrated that participants with IBS rate their overall health and their functional capacity worse compared to the general population. Tang et al.22 also included self-rated mental health and the prevalence of other chronic conditions among those with IBS. Their findings22 were consistent with our results showing that mental disorders affect IBS ratings of mental health among those with IBS. They report that that the odds for poor ratings of mental health were significantly higher among individuals with IBS and comorbid mental disorders (anxiety, mood disorder and other) than those who suffer from only IBS.
The impact of IBS on the perception of hindrance of daily activity by health
Perceptions of health, are strongly associated with physical functioning and both measures are linked to chronic health conditions14. For most patients with chronic conditions, it is their ability to function in their daily activities that matters. Therefore, it is important to elucidate the factors that shape the experiences of and perceptions of IBS patients of how their health hinders their daily activities.
To our knowledge, our study is the first to investigate whether individuals with IBS experience that their physical or mental health hampers their daily activity.
Although IBS was strongly associated with poor ratings of physical health, individuals with IBS report interference of daily activities by their mental health, and not by their physical health, after accounting for PSS and depression.
These findings suggest that self-rated physical and mental health are multidimensional constructs, influenced by other factors than IBS symptoms and depression or PSS, respectively.
Lackner et al.20 emphasized the importance of fatigue, psychosocial and emotional factors when individuals with IBS rate their health.
In contrast to those with IBS only, those with other chronic somatic conditions reported interference of daily activity only by physical health (Figure 3).
The link between IBS and mental health interfering with daily activity is consistent with several studies20,21,23,59 showing that psychosocial, emotional and social factors were more important than severity of physical symptoms when IBS patients rate their quality of life. For instance, Weert et al.60 report that decreased severity of symptoms did not impact quality of life. About 30% of patients who no longer fulfil the Rome III criteria after a 5-year follow‐up period, did not have improved quality of life.
The relationship between IBS and self-reported health measures – causality or shared genetic pathways - Co-twin control analyses
The co-twin control design seeks to discriminate causal from non-causal relationships, in this study, between IBS as a predictor and all the SRH measures. Our findings suggest that a causal model best explained the relationships between IBS and self-rated physical health and between IBS and the extent to which physical or mental health hinders daily activity. In contrast, the relationship between IBS and self-rated mental health most likely reflects the effects of shared genetic factors (Figure 4) which might in part, explain the covariation between IBS and the extent to which mental health interferes with functional impairment.
Genetic influences in IBS imply a broad array of mechanisms from the brain to the gut, involving central processing, immune function and visceral sensitivity in interaction with the HPA-axis42. Psychosocial stressors, personality traits and emotional state are all factors that influence these mechanisms, in part through shared genetic pathways52,54,61,62.
Kutschke et al.54 demonstrated that genetic variation of IBS was fully shared with social stress factors, like social strain and low support in close relationships, suggesting that genes involved in central stress mechanisms are the main source of the genetic variation of IBS.
Twin studies have revealed that genetic effects contribute to the variation of IBS 29–31 as well as to SRH and functional limitation dependent on age and sex25,27. The study of Leionen et al.27 -demonstrated that SRH shared genetic effects with functional limitation, severity of disease and depression, which accounted for 64% of the genetic variation of SRH. Our study was underpowered to perform similar analyses, but the co-twin control analyses and analyses of alternative models testing for causality, shared genes (pleiotropy) or shared environment (Table 5), suggested that the relationship between IBS and self-rated mental health seems to be explained by common genes.
Limitations
The main limitation of this study is the lack of power, due to sample size. Although the analyses included 575 cases of IBS, the sample size restricts the ability of our analytical models to differentiate between shared environmental effects and genetic effects in the co-twin control analyses, especially for whether IBS predicts hindrance of daily activity by physical or mental health. However, additional analyses, the fit statistics of the co-twin control analyses, contributed to the final interpretation of the results. The fit statistics, the comparisons between alternative models in order to decide which model best described each relationship, indicate that the causality model is the best model explaining the associations between IBS and ph_hind, between IBS and mh_hind.
Another limitation is that most of the IBS diagnoses were self-reported. We used a short version of Rome IV criteria63: “Do you have or have you ever had IBS including abdominal pain and disturbed bowel functions, constipation and/or diarrhoea, at least once a week”. We also asked whether the diagnosis was doctor-diagnosed or not. Only 39.5% of the IBS cases were doctor-diagnosed, however, the coefficients of the associations between self-rated health measures and IBS, did not differ between doctor-diagnosed IBS or self-reported IBS (Table 1).