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.