Hepatic Glycogen and Fat Oxidation

Overview

Excessive fat in the liver is associated with impairments in metabolic health. Low levels of DNL and high levels of hepatic fat oxidation are considered to be protective. A decrease in glycogen stores has been causally linked to improved whole body fat oxidation. Also on an organ level, it is suggested that hepatic fat oxidation is stimulated by low hepatic glycogen stores. Next to hepatic fat oxidation, DNL may be influenced by hepatic glycogen stores. Some studies have shown that prolongation of fasting time lowers hepatic glycogen content. It is therefore hypothesized that prolonging fasting time will lower glycogen content and thereby increases fat oxidation and decreases DNL in the liver. To this end, hepatic fat oxidation (plasma marker beta-hydroxybutyrate), de novo lipogenesis, hepatic glycogen content and intrahepatic fat content, will be measured upon a short overnight fast and an extended overnight fast in 13 overweight/obese subjects with hepatic steatosis.

Rationale: Excessive fat in the liver is associated with impairments in metabolic health. Hepatic lipid storage can originate from several metabolic pathways, including de novo lipogenesis (DNL). On the other hand, there are several metabolic pathways that can decrease hepatic fat storage, such as hepatic fat oxidation. Therefore, low levels of DNL and high levels of hepatic fat oxidation are considered to be protective. A decrease in glycogen stores has been causally linked to improved whole body fat oxidation. Also on an organ level, it is suggested that hepatic fat oxidation is stimulated by low hepatic glycogen stores. Furthermore, whole body glycogen levels were also linked to DNL and in situations with increased whole body glycogen levels in animal models rates of DNL were also high. Therefore, next to hepatic fat oxidation, DNL may be influenced by hepatic glycogen stores. Some studies have shown that prolongation of fasting time lowers hepatic glycogen content. It is therefore hypothesized that prolonging fasting time will lower glycogen content and thereby increases fat oxidation and decreases DNL in the liver. Objective: The primary objective of this study is to investigate to what extent hepatic fat oxidation can be stimulated by lowering hepatic glycogen content during a prolonged overnight fast. The secondary objective is to investigate whether reductions in hepatic glycogen content are also accompanied by a decrease in DNL. In addition, the metabolic response to a meal (metabolites related to energy metabolism and substrate oxidation) will be studied upon prolonged overnight fasting. Finally, it will be studied whether reducing hepatic glycogen content by prolonged overnight fasting during a few consecutive days lowers intrahepatic fat storage and changes hepatic fat composition. Study design: This is a randomized cross-over study. Study population: Thirteen healthy overweight/obese males and postmenopausal females, aged between 45-75 years and BMI between 27-38 kg/m2, with liver fat content ≥ 5% will participate in the study. To be able to include enough people with a liver fat content ≥5%, around 27 participants are expected to be screened for liver fat. When liver fat content is \<5%, participants will not be included in the study after determination of liver fat content. Main study parameters/endpoints: The primary study outcome is hepatic fat oxidation measured as plasma beta-hydroxybutyrate (BHB) levels, which will be measured before and after one night adherence to the fasting protocols. The secondary study outcome is DNL as determined by stable isotope techniques and will also be measured after one night adherence to the fasting protocols. Other study outcomes include hepatic glycogen content, substrate oxidation and plasma metabolites related to energy metabolism (measured before and after one night adherence to the fasting protocols) and intrahepatic fat content and composition (measured after 6 days of adherence to the fasting protocols).