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 carrier-
mediated 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.
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