【Introduction】
Barley β-glucan suppresses postprandial hyperglycemia and improves serum lipid concentrations. These mechanisms are considered the viscosity of high molecular weight barley β-glucan (hereafter referred to as high-molecular-weight β-glucan) to inhibit lipid absorption and promote lipid elimination. However, some previous studies have shown that barley β-glucan, whether low molecular weight or high molecular weight, lowers serum cholesterol levels *1. The authors also confirmed in their previous study that low molecular weight barley β-glucan (hereafter referred to as low molecular weight β-glucan) improves glucose and lipid metabolism *2.
In general, low molecular weight dietary fiber is more fermentable than high molecular weight dietary fiber. Low molecular weight β-glucan does not have a strong viscosity like high molecular weight β-glucan. However, short-chain fatty acids and other substances produced by prebiotic action might affect glucose and lipid metabolism.
【Materials and Methods】
Four-week-old male C57BL/6J mice were acclimated for one week. Then, they were divided into three groups (8 mice each) fed a medium fat diet (25% lipid energy ratio) with high molecular weight (500 kDa) barley β-glucan (HMG-BG), low molecular weight (12 kDa), barley β-glucan (LMW-BG), or cellulose (C) for 61 days. The HMW-BG and LMW-BG diets contained 4% barley β-glucan, and each group's total dietary fiber content was adjusted to be 5.48%.
Feed intake and body weight were measured three times a week, and feces were collected for the last five days of the study period. At the end of the rearing period, the animals were dissected, and various biochemical examinations included the weight of the cecum, adipose tissue, liver, amount of lipid and β-glucan in feces, and amount of short-chain fatty acids in cecal contents were measured. Analysis of bacterial flora in cecum by real-time PCR and mRNA expression analysis of liver and ileum was performed.
【Results】
The weight of HMW-BG was significantly lower than that of C, and feed intake was significantly lower than that of C in both BG groups. The weight of the cecum was significantly higher than that of C in both BG groups. Liver weight and adipose tissue weight in the total abdominal, retroperitoneal, epididymal, and mesenteric were significantly lower in HMW-BG than in C.
The amount of barley β-glucan in the feces was significantly higher in HMW-BG than in LMW-BG, and the fermentation efficiency of barley β-glucan in LMW-BG was almost 100%, which was significantly different from that in HMW-BG.
The amounts of total short-chain fatty acids, acetic acid, and propionic acid in the cecum contents were significantly higher in LMW-BG than in C. The same trend was observed in HMW-BG, but there was no significant relationship.
Bacteria from the Bifidobacterium group were significantly more abundant in LMW-BG than in C, and bacteria from the Bacteroides fragilis group were significantly more abundant in LMW-BG than in C and HMW-BG.
Serum concentrations of total cholesterol, LDL-cholesterol, and leptin were significantly lower in both BG groups compared to C. HDL-cholesterol was significantly lower in LMW-BG compared to C. Glucose concentration was significantly lower in LMW-BG compared to C. Insulin concentration was significantly lower in HMW-BG compared to C. Glucose concentration was significantly lower in LMW-BG than in C, and insulin concentration was significantly lower in HMW-BG than in C. Free fatty acid concentration was significantly lower in HMW-BG than in LMW-BG.
As for genes involved in lipid biosynthesis in the liver, the mRNA expression level of SREBP-1c was significantly lower in both BG groups than in C. The expression level of diacylglycerol o-acyltransferase-1 (DGAT1) was significantly lower in HMW-BG compared to C.
As for genes involved in lipid transport and oxidation, carnitine palmitoyltransferase-1 (CPT-1) mRNA expression level was significantly higher in HMW-BG than in C.
The mRNA expression level of neurogenin 3 (NGN3) was significantly higher in LMW-BG than in C. The mRNA expression level of NGN3 was significantly higher in LMW-BG than in C
【Discussion and Conclusion】
HMW-BG inhibited lipid absorption and the accumulation of abdominal fat, while LMW-BG increased the number of Bifidobacterium and Bacteroides fragilis group bacteria in the cecum, the amount of short-chain fatty acids, and the expression level of NGN3. These results suggest that the improvement of glucose and lipid metabolism by low molecular weight β-glucan is different from that by high molecular weight β-glucan and may be due to the prebiotic effect.
Although we did not measure the concentration of GLP-1 secreted by L cells in this study, NGN3 is an essential factor in the induction of GLP-1 secretion, suggesting the involvement of GLP-1 in the improvement of glucose metabolism in LMW-BG.
Serum insulin concentrations were significantly lower in HMW-BG only, but insulin-regulated mRNA expression levels of SREBP-1c in the liver were significantly reduced in both BG groups. Glucose concentration was lower in LMW-BG, suggesting that the modification of glucose metabolism by incretin secretion, such as GLP-1, enhanced by short-chain fatty acids, decreased SREBP-1c mRNA expression and affected lipid metabolism. We conclude that the mechanism of SREBP-1c-mediated regulation of lipid metabolism is different between high molecular weight β-glucans and low molecular weight β-glucans.
【Research institution】
Otsuma Women's University (Japan)
ADEKA CORPORATION (Japan)
*1 Br J Nutr 104, 364–73, 2010
*2 Cereal Chem 97, 1056–65, 2020
Low Molecular Weight Barley β-Glucan Affects Glucose and Lipid Metabolism by Prebiotic Effects
Nutrients 13, 1, 130, 2021
