Primary, Secondary, and Tertiary Effects of Carbohydrate Ingestion During Exercise
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CURRENT OPINION
Primary, Secondary, and Tertiary Effects of Carbohydrate Ingestion During Exercise Ian Rollo1,2 · Javier T. Gonzalez3 · Cas J. Fuchs4 · Luc J. C. van Loon4 · Clyde Williams2
© Springer Nature Switzerland AG 2020
Abstract The purpose of this current opinion paper is to describe the journey of ingested carbohydrate from ‘mouth to mitochondria’ culminating in energy production in skeletal muscles during exercise. This journey is conveniently described as primary, secondary, and tertiary events. The primary stage is detection of ingested carbohydrate by receptors in the oral cavity and on the tongue that activate reward and other centers in the brain leading to insulin secretion. After digestion, the secondary stage is the transport of monosaccharides from the small intestine into the systemic circulation. The passage of these monosaccharides is facilitated by the presence of various transport proteins. The intestinal mucosa has carbohydrate sensors that stimulate the release of two ‘incretin’ hormones (GIP and GLP-1) whose actions range from the secretion of insulin to appetite regulation. Most of the ingested carbohydrate is taken up by the liver resulting in a transient inhibition of hepatic glucose release in a dose-dependent manner. Nonetheless, the subsequent increased hepatic glucose (and lactate) output can increase exogenous carbohydrate oxidation rates by 40–50%. The recognition and successful distribution of carbohydrate to the brain and skeletal muscles to maintain carbohydrate oxidation as well as prevent hypoglycaemia underpins the mechanisms to improve exercise performance.
Key Points Receptors in the oral cavity detect ingested carbohydrate and activate reward and other centers in the brain that can improve sports performance. Exogenous carbohydrate availability and subsequent carbohydrate oxidation rates can be increased by coingesting fructose with glucose (polymers). Ingested carbohydrate provides fuel for the brain as well as working skeletal muscle tissue. Its distribution via the liver is orchestrated, so that hypoglycaemia and fatigue are delayed. * Ian Rollo [email protected] 1
Gatorade Sports Science Institute, PepsiCo Life Sciences, Global R&D, Leicestershire, UK
2
School of Sports Exercise and Health Sciences, Loughborough University, Loughborough, UK
1 Introduction
3
Department for Health, University of Bath, Bath, UK
4
Department of Human Biology, NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Centre+, Maastricht, The Netherlands
Since the recognition that fatigue during prolonged exercise is accompanied by the depletion of muscle glycogen stores [1], nutritional studies have been directed at increasing
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pre-exercise carbohydrate (CHO) stores as well as providing additional CHO during exercise. Strategies to increase pre-exercise muscle glycogen stores include increasing daily dietary CHO intake [1–4] and ingesting CHO-rich meals prior to exercise. The consumption of easy-t
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