Barley β-glucan reduces blood cholesterol levels via interrupting bile acid metabolism
Wang, Y., Harding, S.V., Thandapilly, S.J., Tosh, S.M., Jones, P.J.H., Ames, N.P. (2017). Barley β-glucan reduces blood cholesterol levels via interrupting bile acid metabolism. British Journal of Nutrition, [online] 118(10), 822-829. http://dx.doi.org/10.1017/S0007114517002835
Plain language summary
Barley grain is a valuable source of a unique type of fibre called beta-glucan. It has been proven that eating three grams of barley beta-glucan per day can help lower cholesterol levels in humans; however the mechanism by which beta-glucan acts in the body to achieve this health benefit is not yet fully understood. In this study researchers conducted a comprehensive human feeding trial where 30 participants consumed breakfast meals containing barley beta-glucan or a non-barley control food for five weeks. Blood and stool samples were collected and analysed to examine how the beta-glucan meals impacted metabolic pathways that control cholesterol levels in the body, including the body’s own cholesterol production (synthesis), uptake from food consumed (absorption) and interactions with bile acids (compounds that aid in fat digestion). While study results confirmed that blood cholesterol levels were reduced with the consumption of barley beta-glucan, they revealed that cholesterol synthesis and absorption were not affected as previously thought. Rather, researchers found that beta-glucan resulted in more bile acids being excreted from the body through the stool, in turn triggering more bile acid production in the body to restore the bile acid pool. Since cholesterol is used in the body’s production of bile acids, levels of circulating cholesterol in the blood were reduced. An additional novel finding of this research was that, based on the genetic background of an individual, some participants showed a more pronounced increase in bile acid synthesis and subsequent reduction in cholesterol levels than others. This study is the first to show clinical evidence supporting the mechanism for how barley beta-glucan lowers cholesterol.
Underlying mechanisms responsible for the cholesterol-lowering effect of β-glucan have been proposed, yet have not been fully demonstrated. The primary aim of this study was to determine whether the consumption of barley β-glucan lowers cholesterol by affecting the cholesterol absorption, cholesterol synthesis or bile acid synthesis. In addition, this study was aimed to assess whether the underlying mechanisms are related to cholesterol 7α hydroxylase (CYP7A1) SNP rs3808607 as proposed by us earlier. In a controlled, randomised, cross-over study, participants with mild hypercholesterolaemia (n 30) were randomly assigned to receive breakfast containing 3 g high-molecular weight (HMW), 5 g low-molecular weight (LMW), 3 g LMW barley β-glucan or a control diet, each for 5 weeks. Cholesterol absorption was determined by assessing the enrichment of circulating 13C-cholesterol over 96 h following oral administration; fractional rate of synthesis for cholesterol was assessed by measuring the incorporation rate of 2H derived from deuterium oxide within the body water pool into the erythrocyte cholesterol pool over 24 h; bile acid synthesis was determined by measuring serum 7α-hydroxy-4-cholesten-3-one concentrations. Consumption of 3 g HMW β-glucan decreased total cholesterol (TC) levels (P=0·029), but did not affect cholesterol absorption (P=0·25) or cholesterol synthesis (P=0·14). Increased bile acid synthesis after consumption of 3 g HMW β-glucan was observed in all participants (P=0·049), and more pronounced in individuals carrying homozygous G of rs3808607 (P=0·033). In addition, a linear relationship between log (viscosity) of β-glucan and serum 7α-HC concentration was observed in homozygous G allele carriers. Results indicate that increased bile acid synthesis rather than inhibition of cholesterol absorption or synthesis may be responsible for the cholesterol-lowering effect of barley β-glucan. The pronounced TC reduction in G allele carriers of rs3808607 observed in the previous study may be due to enhanced bile acid synthesis in response to high-viscosity β-glucan consumption in those individuals.