Effects of Dual-Source Carbohydrate Intake and Liver Glycogen Repletion After Overnight Fasting.

Overview

This study is looking at whether eating a breakfast which has two different sources of carbohydrates, glucose and fructose (found in foods like honey and fruits), can increase how much glycogen can be stored in the liver. Glucose is a type of sugar that the body uses to provide energy during exercise. When it is not circulating in the blood, it is stored in the muscles and liver. The stored version of glucose is often referred to as glycogen. When the body needs energy, for example, it will break down glycogen into glucose so that it can be used as fuel. Muscle and liver glycogen stores are vital in providing energy during prolonged exercise, and strenuous activity can rapidly deplete these stores, leading to increased fatigue and a decline in performance. Liver glycogen, however, is particularly important because it controls blood glucose levels. This is important because the brain and other organs are constantly relying on the supply of glucose to function properly. When sleeping, the body goes through a natural period of fasting. During this period, the liver gradually breaks down its glycogen stores to release glucose into the bloodstream. Because of this, following sleep, liver glycogen stores are automatically low (which is why having breakfast is important). There is research to suggest that eating a high-carbohydrate breakfast can prevent further declines in liver glycogen; however, it is not known if eating different types of carbohydrates within the breakfast (glucose and fructose together) will affect the liver's ability to store glycogen. This research will aid in understanding optimal ways to increase liver glycogen stores before performing exercise, which may influence exercise performance. Therefore, the main aim of this study is: 1\. Investigate whether a high fructose breakfast will increase liver glycogen storage To achieve this, participants will be recruited to complete a randomised crossover study where they will undertake three different conditions. All laboratory trials will take place at the Manchester Metropolitan University Institute of Sport. 1. No breakfast (Control) 2. 3 g/kg of body mass of carbohydrate (of which contains 0% fructose) 3. 3 g/kg of body mass of carbohydrate (of which contains50% fructose) Liver glycogen stores will be measured using magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS). The investigators will measure liver glycogen content, liver volume, and stomach volume. Blood samples will also be taken to measure different metabolic hormone responses.

Glycogen stores in the muscle and liver play a crucial role in providing fuel during prolonged exercise and strenuous activities, which can rapidly deplete these stores, leading to increased fatigue and reduced performance. During prolonged exercise, liver glycogen is broken down to maintain blood glucose (sugar) levels and provide an important fuel source for the exercising muscles. Accordingly, liver glycogen is important for our ability to perform prolonged endurance exercise. Following sleep, the body is placed under a natural period of fasting; because of this, the body's main source of fuel derives from liver glycogen stores, meaning hepatic glycogen stores decline sufficiently during this period. Accordingly, without sufficient pre-exercise carbohydrate intake, athletes begin training or competition with reduced liver glycogen availability, which can impair their ability to maintain blood glucose during prolonged exercise and may result in hypoglycaemia and subsequent fatigue. Whilst current guidelines suggest that pre-exercise carbohydrate intake may support the replenishment of depleted liver glycogen stores, it is currently unclear whether the composition of carbohydrates contained within the breakfast meal may impact the ability of the liver to store glycogen. Fructose is primarily metabolised in the liver and enters hepatic carbohydrate metabolism downstream to key regulatory steps in glycolysis. Research has shown that fructose preferentially contributes to hepatic glycogen storage and, when co-ingested with glucose, enhances post-exercise hepatic glycogen synthesis when compared with glucose alone. Despite this, the effects of fructose within a high-carbohydrate breakfast following overnight fasting (where the metabolic and hormonal environment is markedly different) remain unknown. As such, the present study aims to assess 1) the effect of a high-carbohydrate breakfast diet on liver glycogen storage and 2) the effect of fructose content on liver glycogen storage. To achieve our aims, participants will complete three experimental conditions in a randomised, counterbalanced order: 1) no breakfast (control), 2) high carbohydrate breakfast (3 g/kg of body mass) (0% fructose), and 3) high carbohydrate breakfast (3 g/kg of body mass) (50% fructose). Hepatic glycogen concentrations will be quantified using non-invasive 13C magnetic resonance spectroscopy (MRS), with magnetic resonance imaging (MRI) used to assess liver volume and gastric volume. Secondary measures will include circulating metabolic and hormonal responses assessed via venous blood sampling. Primary and secondary outcomes will be compared across conditions to determine whether fructose co-ingestion alters hepatic glycogen storage relative to glucose intake and fasting. This study will provide mechanistic insight into the role of fructose in promoting hepatic glycogen synthesis.