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Carbohydrates (CHO) C:H:O ratio of 1:2:1 • Sugars and starches • Functions – Predominant fuel in high-intensity exercise, intermittent-intensity and cause of fatigue (due to lack of CHO) in prolonged exercise – Fuel for CNS and blood cells • Intake – West 40-50% (300g/day) • 50% simple – Athletes 60% (up to 1000g/day) Monosaccharides (simple sugars) • 3 – 7 C atoms • The most important to humans are the hexose sugars eg. Glucose C6H12O6 • Straight chain or ring (more common in body) • Fructose same formula, diff structure (ie. Isomer) Dehydration vs hydrolysis Polysaccharides • Complex carbohydrates • Chains of sugars are straight or highly branched. Eg. – Cellulose (indigestible - fibre) or starch (digestible) in plants – Glycogen in animals (highly branched) • All must be broken down to monosaccharides before absorption Oligosaccharides • 3-15 monosaccharide units joined to form polysaccharides • Maltodextrins – partially hydrolysed starch – Include starch oligosaccharides and maltose – Less osmotically active than glucose and less sweet Glycogen Glycogen • Muscle – Rate of depletion relates to exercise intensity • High intensities glycogen is broken down v rapidly • Liver – Main role is to maintain blood glucose – stored as glycogen glucose and released – 80-100g, but reduced to <20g after overnight fast – Also produces glucose via gluconeogenesis fom lactate, glycerol , pyruvate, alanine, glutamine +other amino acids Function of CHO 1. Energy source 2. Metabolic primer ‘fat burns in a CHO flame’ 3. Protein sparer 4. Fuel for CNS CHO digestion • Saliva contains – α-amylase – breaks down starch into maltose, trisaccharides and small oligosaccharides • Amylase less active due to acid in stomach • Pancreatic juice contains – α-amylase • Disaccharides further digested by lactase, sucrase and maltase in brush border CHO Absorption • Monosaccharides absorbed by carriermediated transport • Glucose and galactose taken into epithelial cell with 2 x Na+ (SGLT) – Na is then actively pumped back into lumen • Fructose – Na-independent facilitated diffusion transporter (GLUT 5) • GLUT 2 transporter on contra-luminal side accepts all 3 monosaccharides hepatic portal vein. Hormones • Insulin – At rest increases glucose uptake by liver, muscle – Increases glycogen synthase activity, inhibits glycogen phosphorylase • Glucagon – Breakdown liver glycogen and release of glucose • Catecholamines – In exercise reduce release of insulin • Blood glucose kept fairly constant except: – High intensity exercise liver produces more glucose than taken up by muscle --. Elevated blood glucose – Prolonged exercise – rate of production less than utilisation hypoglycaemia Effect of exercise intensity • As intensity ↑ so do adrenaline/noradrenaline glucose release from liver and glycogen b’down in muscle • So post sprint levels are 40/50%+ • Muscle uses own glycogen stores before blood glucose in short intense exercise • Post exercise it enters muscle to replace glycogen stores Glycaemic Index (GI) & glycaemic load • Based on ingestion of food containing 50g CHO with reference food (usually 50g glucose or white bread) • Measure area under glucose curve over 2-hr period GI = area under curve for test food area under curve for ref food x 100 • Low GI <55, mod GI (56-70), high >71 • But GI does not take into account serving size e.g. 50g of CHO from carrots = 750g of carrots • Glycaemic Load (GL) is more practical and takes into account GI and serving size.