LAUSR

Non-alcoholic fatty liver disease (NAFLD) has a prevalence of 7–10% among children in the developed world, making it the most common cause of chronic liver disease in this population. Its prevalence is rising concomitant with the rise in obesity. NAFLD incorporates a spectrum of liver abnormalities, ranging from simple steatosis, to, non-alcoholic steatohepatitis (NASH), involving various degrees of inflammation, ballooning and fibrosis. NASH may evolve into cirrhosis, accounting for its place as the second most common listing indication for adult liver transplantation (LT) in the United States. It is vital to intervene in the paediatric population to avert this outcome. Dietary and lifestyle management in NAFLD remains commonplace, but poor compliance hinders this otherwise effective strategy. Pharmacological therapies, such as insulin sensitizers, antioxidants and hepato-protective agents, have shown mixed results and varying side effect profiles. There remains a desire to identify other interventional strategies. Given increasing focus on the gut-liver axis, the gut microbiome (GM) is emerging as the new kid on the block, with revolutionary potential for hepatological conditions, including NAFLD. We therefore, welcome Stanislawski et al.’s large crosssectional study on the role of GM in paediatric fatty liver. The authors selected magnetic resonance imaging to evaluate steatosis in a paediatric population cohort (n= 107), in which one-quarter were born to gestational diabetic mothers. GM was evaluated in this cohort using stool samples. The study highlights the technical challenges in the recruitment of asymptomatic children; with less than half of all children approached being able to provide a suitable stool specimen. The authors demonstrated a negative association of GM biodiversity with hepatic fat fraction (HFF). Qualitatively, genera, including Bilophila, Paraprevotella, Oscillospira and Bacteroides, are correlated with HFF. The authors defined NAFLD, based on HFF ≥ 5%, in 8 children (7.8%). However, as highlighted in our recent review, caution must be exercised when labelling children with ‘NAFLD’, based on hepatic steatosis only, given that steatosis can be a sign of other diseases. Also, as steatosis per se, is not an accurate prognostic factor for disease outcome, clinical applicability of the study remains uncertain. Lipopolysaccharide (LPS), the main component of gram-negative bacteria, through activation of toll-like receptor 4 (TLR-4) propagates an inflammatory signalling cascade, which can contribute to insulin resistance and TNF-α mediated fibroinflammatory processes within the liver. The intricate interplay between the microbiome and the immune system, will likely play a role in NAFLD, with increased LPS concentrations being reported in NASH children. Future researchers should consider routinely evaluating the microbiome in steatosis, in combination with inflammatory and fibrotic biomarkers, to understand the microbiome in the context of disease pathogenesis. Increased intestinal permeability (IP), and small intestinal bacterial overgrowth (SIBO), have been consistently described in paediatric obesity, with increasing evidence to suggest their potential involvement in fatty liver phenotypes. Although altered GM composition, known as dysbiosis, is described, inconsistent results across studies exist. Initially, a decreased abundance of Bacteroidetes and increased Firmicutes, in obesity, was found at a phylum level. However, subsequent studies have shown different, and often, opposite results. Similarly, in paediatrics, contradictory results at phylum, genus and species level exist, not only between disease and healthy states, but also within disease states. In addition to differences in study populations (e.g. obesity, NAFLD, NASH, and in this study, hepatic steatosis), the inherent complexities and limitations in conducting any GM study will largely contribute to inter-study variation. Antimicrobials, medications, diet, age, genetics and ethnicity can all influence GM. An ideal microbiome study would account for all of these, which is, understandably, challenging. We commend Stanislawski et al., for their efforts to account for diet. Dietary analysis, is particular relevant in GM-NAFLD studies, due to the bidirectional influence of diet on both GM, and NAFLD. As an example, a high fructose diet, is associated with hepatic steatosis, and, has also been correlated with SIBO and increased LPS. Stanislawski et al. used food questionnaire tools to demonstrate a weak association between mono-saturated fat, carbohydrates and HFF, but the effect on GM was difficult to establish. Dietary assessment, although technically challenging, should be attempted in any GM-NAFLD study, in order to elucidate the complex relationship between GM, diet and NAFLD. Antimicrobials can have a significant effect on the microbiome; the Human Microbiome Project excludes cases exposed to antimicrobials within six months of stool sampling, whilst others suggest the microbiome never reaches its preantimicrobial state. In addition, commonly used medications, such as, histamine receptor antagonists and proton pump inhibitors, and of particular interest in NAFLD, metformin, can also alter the microbiome. Hence, relevant medications should be incorporated into the study design of microbiome projects. In addition to differences in study populations (e.g. obesity, NAFLD, NASH and in this study, hepatic steatosis), the inherent complexities and limitations in conducting any GM study, such