Download Carbohydrates (CHO)

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.