Carbohydrate-restricted diets in diabetes and the irritable bowel syndrome. Effects on symptoms, inflammation and microbiota

Abstract: This doctoral thesis explores the effects of carbohydrate(CHO)-restricted diets on symptoms and disease-related variables in type 2 diabetes mellitus (T2DM) and the irritable bowel syndrome (IBS). Aims were to examine the effects of (I) the moderately-low CHO Okinawan-based Nordic diet on anthropometry and metabolism and microbiota-related, inflammatory, and cognitive parameters in T2DM, and (II) the low-CHO starch- and sucrose reduced diet on gastrointestinal (GI) symptoms, dietary intakes, inflammatory parameters, microbiota and microribonucleic acid (miRNA) expression in IBS. (I) A prospective 12-week Okinawan-based Nordic dietary intervention with 30 T2DM patients was performed. Questionnaires and fecal and blood samples were collected and analyzed before and after the study. Fecal samples were analyzed for Enterobacteriaceae abundance by polymerase chain reaction (PCR) assay and alpha diversity by terminal restriction fragment length polymerization (T-RFLP). From blood, metabolic parameters were measured according to clinical routine by the Clinical Chemistry department, short-chain fatty acid concentrations were measured by gas-liquid chromatography, circulating cytokine concentrations by immunoassay and neurofilament levels by an in-house assay. (II) A randomized clinical trial with a 4-week starch- and sucrose-reduced diet (SSRD) was performed in 105 IBS patients. Questionnaires, 4-day food diaries and fecal and blood samples were collected before and at the end the study. GI symptoms were measured from questionnaires, i.e., the irritable bowel syndrome symptom severity score (IBS-SSS), the visual analog scale for IBS (VAS-IBS) and the Rome IV questionnaires. Dietary intakes were calculated by the dietician using the AIVO diet computer program. From plasma, C-reactive protein (CRP) was analyzed by the Clinical Chemistry department, cytokine concentrations by multiplex immunoassay and miRNA by RNA sequencing. Microbiota (taxonomy at phylum, genus and ASV level, alpha- and beta diversity) was analyzed from fecal samples by 16S ribosomal RNA (rRNA) gene sequencing. Results from study I showed that T2DM participants experienced significant weight loss (mean decrease of 6.2 kg), decreased waist circumference and body-mass index (BMI) after the intervention (p < 0.001 for all). Cortisol, fasting glucose, insulin, glycated hemoglobin (HbA1c) and lipid parameters decreased significantly (p < 0.001 for all). Microbiota-related parameters, i.e., Enterobacteriaceae abundance, microbial diversity measures and SCFA concentrations, were largely unaltered after the 12-week trial. There was an overall trend towards decreased proinflammatory parameters, significant for interleukin 18 (IL-18) (median value [IQR] at baseline [pg/mL]: 226 [173-280] and 12 weeks: 189 [136-242], p = 0.001). In study II, there was a 74% responder rate (RR=ΔTotal IBS-SSS≥-50 points) to the SSRD among IBS patients in the intervention group (RR = 24% for controls), with primarily decreased abdominal pain and bloating. The intakes of starch, sucrose, disaccharides and CHO decreased in the intervention group (p <0.001 for all). At 4 weeks, fat and protein energy percentages (E%) were significantly higher in the intervention compared to the control group. Sugar- and starch reductions correlated weakly-moderately with the improved GI symptom scores. CRP and cytokines levels were mainly unaltered in both groups. Microbiota analyses showed increased Proteobacteria (p=0.0036), decreased Bacteroidetes (p<0.001), as well as changes in gut microbiota composition (β-diversity: p<0.001) but not alpha diversity. There were 20 genus-level changes in the intervention group, compared to 2 in the control group. MiRNA was unaffected by the SSRD intervention. Conclusion: CHO-restricted diets improve key disease aspects with improved anthropometry and metabolism in T2DM and markedly reduced GI symptoms in IBS. Systemic inflammation was reduced and microbiota unaltered with the moderately-low CHO Okinawan-based Nordic diet in T2DM, whereas systemic inflammation was unaltered and gut microbiota composition significantly impacted with the low-CHO starch- and sucrose-reduced diet in IBS.