There are several reasons why body composition in IBD patients may differ from that of healthy individuals. The secretion of pro-inflammatory cytokines may alter energy metabolism, protein turnover and energy substrate utilisation, whereas the use of corticosteroids increased body fat with catabolic effects on lean mass (Ma et al., 2003). Physical activity, on the other hand, was reported as being low in adult IBD patients and correlated inversely with fat mass (FM) in one recent report (Sousa et al., 2007), although evidence in paediatric patients is scarce. Whether there is an association between body composition and physical activity patterns in paediatric IBD remains to be explored.
There are few studies that have assessed body composition in IBD children. Body lean mass has been consistently reported as being significantly lower than healthy control groups (Boot et al., 1998; Sentongo et al., 2000; Burnham et al., 2005; Thayu et al., 2007), whereas gender-specific associations with FM were observed in some studies (Sentongo et al., 2000; Burnham et al., 2005; Thayu et al., 2007). In a cross-sectional study, Sentongo et al. (2000) found that boys and girls with longstanding CD presented lower fat free mass (FFM) by an average of 3.5 and 2.9 kg, respectively, compared to a control sample, whereas FM, although not different from the control group in CD boys, was 3.7 kg higher in CD girls. Similarly, in a cross-sectional study, Burnham et al. (2005) found that children with longstanding CD had a 6% deficit in lean mass compared to healthy children after accounting for stature, age, pubertal stage and race; however, FM was normal, suggesting nutritional cachexia. Recently, Thayu et al. (2007) in a well-designed study of newly-diagnosed children with CD, also reported gender associated differences with body composition. Fat mass and lean mass for height (adjusted for age, race and pubertal stage) were lower in female than in male patients. Compared to a cohort of healthy controls, body composition in girls was more consistent with wasting (low lean and FM), whereas, in boys, there was mostly preservation of FM and deficits in lean mass consistent with cachexia. No associations were observed between body composition, clinical activity, disease location or diagnosis delay. Interestingly, normalisation of BMI at 2 years of follow-up was not associated with a significant increment in FFM in children with CD (Sylvester et al., 2009), which implies that changes in body weight or BMI for age are not good proxies for body composition changes in IBD and the introduction of simple bedside techniques of body composition assessment, such as bioelectrical impedance analysis, for routine clinical use is required.
Nevertheless, interpretation of body composition data in disease has to be approached with caution because the underlying assumptions about the composition of body compartments may be invalid (Wells & Fewtrell, 2006). Most in vivo body composition methods used in previous IBD studies [e.g. dual energy X-ray absorptiometry (DXA)] have been tested and validated in healthy individuals or animal cadavers and their applicability in chronic illness is questionable given the changes that may occur in the hydration level and distribution of fluids within the body compartments (Williams et al., 2006). These errors will affect lean mass composition estimation because of the assumptions made during the DXA calculations. Assessment of the validity of these techniques in an IBD population and replication of these results with the application of more sophisticated methods needs to be explored (Reilly et al., 2010). At the same time, the validity and reliability of simple bedside methods of body composition more suitable for routine clinical use should be evaluated in paediatric IBD population. Development of IBD-specific prediction equations could improve the validity and accuracy of bioelectrical impedance analysis techniques, (Dung et al., 2007). The use of functional tests (e.g. handgrip strength) has been proposed as a proxy estimate of FFM in adult IBD patients, although these techniques lack specificity. Moreover, Wiroth et al. (2005) found that adult patients with CD in clinical remission have overall lower muscle performance than healthy controls, although this was independent of FFM levels. Studies that have evaluated the use of functional tests in paediatric IBD patients are lacking.