Short-chain fatty acids as a link between diet and cardiometabolic risk: a narrative review

AIM

Diet has a profound impact on cardiometabolic health outcomes such as obesity, blood glucose, blood lipids and blood pressure. In recent years, the gut microbiota has emerged as one of several potential key players explaining dietary effects on these outcomes. In this review we aim to summarise current knowledge of interaction between diet and gut microbiota focusing on the gut-derived microbial metabolites short-chain fatty acids and their role in modulating cardiometabolic risk.

FINDINGS

Many observational and interventional studies in humans have found that diets rich in fibre or supplemented with prebiotic fibres have a favourable effect on the gut microbiota composition, with increased diversity accompanied by enhancement in short-chain fatty acids and bacteria producing them. High-fat diets, particularly diets high in saturated fatty acids, have shown the opposite effect. Several recent studies indicate that the gut microbiota modulates metabolic responses to diet in, e.g., postprandial blood glucose and blood lipid levels. However, the metabolic responses to dietary interventions, seem to vary depending on individual traits such as age, sex, ethnicity, and existing gut microbiota, as well as genetics. Studies mainly in animal models and cell lines have shown possible pathways through which short-chain fatty acids may mediate these dietary effects on metabolic regulation. Human intervention studies appear to support the favourable effect of short-chain fatty acid in animal studies, but the effects may be modest and vary depending on which cofactors were taken into consideration.

CONCLUSION

This is an expanding and active field of research that in the near future is likely to broaden our understanding of the role of the gut microbiota and short-chain fatty acids in modulating metabolic responses to diet. Nevertheless, the findings so far seem to support current dietary guidelines encouraging the intake of fibre rich plant-based foods and discouraging the intake of animal foods rich in saturated fatty acids.

Effects of prebiotics on postprandial GLP-1, GLP-2 and glucose regulation in patients with type 2 diabetes: A randomised, double-blind, placebo-controlled crossover trial

AIMS

We aimed to investigate the effect of prebiotic inulin-type fructans (ITF) versus a control supplement on postprandial levels of glucagon-like peptide-1 and -2 (GLP-1 and -2), glucose and insulin in people with type 2 diabetes.

METHODS

Adult men and women with type 2 diabetes were randomised in a double-blind, placebo-controlled crossover study. The study participants received 16 g/d ITF and 16 g/d control supplement (maltodextrin) for 6 weeks each in two phases separated by a 4-week washout. A standardised mixed-meal test was performed before and after each intake period. The primary end point was changes in the GLP-1 response, and secondary end points were GLP-2, glucose and insulin responses. Data were analysed using mixed-model analysis.

RESULTS

A total of 29 participants were included in the study. Differences between and within the two treatments in estimated area under the curves were not significant. Yet, the predicted means for meal-induced GLP-1 response in plasma showed a 4.8% decline after the prebiotic treatment and an 8.6% increase after the control treatment (difference in changes between the treatments, p < 0.001). Fasting or postprandial glucose, insulin or GLP-2 levels were not changed.

CONCLUSIONS

Our findings do not support that ITF improve incretin responses or glucose regulations in this population. Clinicaltrials.gov (NCT02569684).

Prebiotic effect of inulin-type fructans on faecal microbiota and short-chain fatty acids in type 2 diabetes: a randomised controlled trial

PURPOSE

Compared to a healthy population, the gut microbiota in type 2 diabetes presents with several unfavourable features that may impair glucose regulation. The aim of this study was to evaluate the prebiotic effect of inulin-type fructans on the faecal microbiota and short-chain fatty acids (SCFA) in patients with type 2 diabetes.

METHODS

The study was a placebo controlled crossover study, where 25 patients (15 men) aged 41-71 years consumed 16 g of inulin-type fructans (a mixture of oligofructose and inulin) and 16-g placebo (maltodextrin) for 6 weeks in randomised order. A 4-week washout separated the 6 weeks treatments. The faecal microbiota was analysed by high-throughput 16S rRNA amplicon sequencing and SCFA in faeces were analysed using vacuum distillation followed by gas chromatography.

RESULTS

Treatment with inulin-type fructans induced moderate changes in the faecal microbiota composition (1.5%, p = 0.045). A bifidogenic effect was most prominent, with highest positive effect on operational taxonomic units (OTUs) of Bifidobacterium adolescentis, followed by OTUs of Bacteroides. Significantly higher faecal concentrations of total SCFA, acetic acid and propionic acid were detected after prebiotic consumption compared to placebo. The prebiotic fibre had no effects on the concentration of butyric acid or on the overall microbial diversity.

CONCLUSION

Six weeks supplementation with inulin-type fructans had a significant bifidogenic effect and induced increased concentrations of faecal SCFA, without changing faecal microbial diversity. Our findings suggest a moderate potential of inulin-type fructans to improve gut microbiota composition and to increase microbial fermentation in type 2 diabetes.

TRIAL REGISTRATION

The trial is registered at clinicaltrials.gov (NCT02569684).

Matrix metalloproteinases in subjects with type 1 diabetes

Background

Nephropathy is serious complication of diabetes. We have previously shown that level of the proteoglycan syndecan-1 in blood is associated with ultrastructural kidney changes in young persons with type 1 diabetes. Dysregulation of matrix metalloproteinases (MMPs) and tissue inhibitors of matrix metalloproteinases (TIMPs) may contribute to the development of nephropathy. The aim of this study was to investigate if the levels of MMPs in blood samples are potential markers of early nephropathy in type 1 diabetes.

Methods

Blood samples were collected from type 1 diabetes patients after 11 years of diabetes (n = 15) and healthy volunteers (n = 12) and stored at ÷80°C until measurement. Levels and activities of serum MMP-2, MMP-9, TIMP-1 and TIMP- 2 were analyzed and compared to those of control individuals using ELISA, SDS-PAGE gelatin zymography, and Western blot analysis.

Results

The serum levels of both MMP-9 and MMP-2 were significantly higher in subjects with type 1 diabetes, compared to controls (p = 0.016 and p = 0.008 respectively). Western blotting revealed no differences between the two groups in the levels of TIMP-1 or TIMP-2, respectively.

Conclusion

Our MMP analysis of serum from a limited number of patients with type 1 diabetes suggest that such analysis is potentially useful as markers in studies of people at risk of progression to chronic kidney disease.

Effect of hyperglycemic condition on proteoglycan secretion in cultured human endothelial cells

BACKGROUND

Proteoglycans (PGs) are important constituents of the plasma membrane and of the basement membrane supporting the endothelial cell layer. Changes in the amounts or the structures of PGs in the endothelium may affect important functions such as turnover of lipoproteins, filtration properties, and regulation of chemokines during inflammation, which are all relevant in diabetes.

AIM OF THE STUDY

The purpose of this study was to investigate if hyperglycemic conditions would affect the biosynthesis and secretion of PGs in cultured primary human endothelial cells.

METHODS

Primary human umbilical cord vein endothelial cells were established and cultured in vitro. The cells were cultured either in medium with low glucose (LG) (1 g/l) or high glucose (HG) (4.5 g/l). From day 3-4 cells were labeled with (35)S-sulfate for 24 h. (35)S-Labeled macromolecules (medium) were purified by gel chromatography, and isolated macromolecules were analyzed by gel chromatography after different types of treatment, electrophoresis, and immunoprecipitation.

RESULTS

Lower levels of secreted PGs were found in human endothelial cells exposed to HG. The major part of the PGs released was of the heparan sulfate (HS) type, and immunoprecipitation experiments showed that one such PG was syndecan-1. However, there was no difference in the ratio between HS and chondroitin sulfate (CS) under the different experimental conditions. Further, the PGs expressed neither differ with regard to molecular size of the glycosaminoglycan (GAG) chains, nor were their polyanionic properties affected by the different experimental conditions.

CONCLUSION

The results obtained suggest that treatment of primary human endothelial cells with hyperglycemia leads to a decrease in PG secretion in primary cultures of human endothelial cells.

High glucose and Nepsilon-(carboxymethyl) lysine bovine serum albumin modulate release of matrix metalloproteinases in cultured human endothelial cells

BACKGROUND

Hyperglycaemia may contribute to endothelial dysfunction. Disturbances in endothelial functions include changes in the extracellular matrix underneath the cells. This may result from altered biosynthesis of matrix molecules or from modified biosynthesis and secretion of enzymes involved in the turnover of extracellular matrix. One important class of such enzymes are the matrix metalloproteinases (MMPs).

AIM OF THE STUDY

The aim of this study was to investigate whether the condition of high glucose concentration relevant both to diabetes type 1 and 2 and metabolic syndrome, would affect the synthesis and release of MMPs in human umbilical cord endothelial cells (HUVEC) in vitro.

METHODS

The HUVEC were isolated and cultured in vitro. The cells were exposed to medium with either low glucose (LG, 1 g/l) or high glucose (HG, 4.5 g/l) or the advanced glycation end product (AGE) N(epsilon)-(carboxymethyl) lysine bovine serum albumin (CML-BSA), at a concentration of 10 microg/ml. The HUVEC-conditioned media were harvested and subjected to gelatin zymography and Western blotting.

RESULTS

When HUVEC were incubated with HG or CML-BSA under serum free conditions a decreased secretion of pro MMP-2 was observed, both with gelatin zymography and Western blotting. The HUVEC also secreted MMP-9, but at lower levels, and effects of HG treatment were not significant. When HUVEC were stimulated with phorbol 12-myristate 13-acetate (PMA) secretion of pro MMP-2 was not increased, but the activation of pro MMP-2 into lower molecular forms increased, irrespective of culturing in LG, HG or CML-BSA.

CONCLUSION

The HUVEC exposed to high glucose or AGE exhibit decreased secretion of MMP-2. These findings may be relevant in understanding the altered turnover of the endothelial extracellular matrix observed in the diabetic state.