The molecular and physicochemical mechanisms by which mushroom functional foods affect obesity associated metabolic diseases : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy

Abstract

Obesity is rising around the world as a result of many factors, including an imbalance in food intake, an increased consumption of high-calorie diets, and a lack of physical exercise. Obesity not only leads to an increased risk for type 2 diabetes, cardiovascular disease and cancer, but also negatively affects longevity and quality of life. Anti-obesogenic approaches which are safe, widely available, and inexpensive are required to combat metabolic diseases. Mushrooms contain antioxidants, fibre and other phytochemicals, and have been shown to have anti-obesogenic and anti-diabetic effects through the modulation of diverse cellular and physiological pathways. In this thesis, three edible mushroom species (white button mushroom, shiitake mushroom and oyster mushroom) were selected as sustainable and functional food ingredients. Wheat-flour noodles were enriched with mushroom powder sourced from different parts (cap, stem, whole) at different levels (5%, 10%, 15%) to investigate the effects of mushroom addition on the nutritional composition, physical and textural properties. Noodles containing mushrooms had a nutritional advantage with higher protein and dietary fibre contents compared with wheat-flour noodles. The addition of 15% mushroom powder in the noodles resulted in a significant (p < 0.05) decrease in glycaemic glucose equivalents after in vitro digestion. The antioxidant levels of the samples increased following the in vitro digestion, and were positively correlated with the fibre content (p<0.05). The stems of mushrooms acted as fibre-rich by-products and were able to regulate the glycaemic response of foods and increase antioxidants intake. The noodles which contained 15% mushroom powder were used in a model food in a mice experiment. Male C57BL/6 mice were divided into four groups and fed with either one of the isocaloric diets as followed: standard chow diet group (CHOW), high-fat diet (HFD), HFD + 30% of wheat-flour noodles (HFD-FL), HFD + 30% shiitake mushroom noodles (HFD-SH), and HFD + 30% white button mushroom noodles (HFD-WB) and HFD + 30% oyster mushroom noodles (HFD-OY). After eight weeks of dietary intervention, all of the diets containing mushrooms were observed to ameliorate HFD-induced metabolic syndrome, with mice exhibiting decreased body weight gain, fat accumulation, glucose intolerance, and insulin resistance. Dietary mushrooms reduced non-alcoholic fatty liver disease (NAFLD) in mice. Histological studies showed that mushroom diets significantly diminished the liver steatosis of mice compared with those fed with wheat-flour noodles. Analysis of serum markers further revealed decreased contents of total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-C) and high-density lipoprotein-cholesterol (HDL-C). The dysbiosis of the gut microbiome was shifted to the normal level, and the synthesis of bile acids was modulated by mushroom diets. Shiitake enriched diets altered the hepatic and faeces bile acid profile and activated farnesoid X receptor (FXR) signalling via downregulating the FXR antagonists (tauro-α-muricholic acid, tauro-β-muricholic acid). These changes in bile acids were correlated with the expansion of genera (Bacteroides, Akkermansia, Alloprevotella) in the gut microbiome. Shiitake acted as an aryl hydrocarbon receptor (AhR) ligand and a potent inducer of interleukin-22 (IL-22), suggesting its potential protective role in intestinal immunity. These results elucidate the nutraceutical functionalities of mushroom fortified foods via the signalling of bile acids together with the gut microbiome in the liver-gut axis, and provide the evidence of the further application of food matrix in treating and preventing metabolic diseases.