Carbohydrate requirements for exercise Flashcards
amount of energy of carbs per gram
Approx 4 kilocalories of energy
per gram
Carbohydrate digestion process
- Mouth: Salivary amylase (action
enhanced by chewing) - Stomach: Highly acidic, amylase
activity reduces - Duodenum- amylase in
pancreatic juice - Ileum: Chyme
- Brush border- further enzymatic
breakdown - Fibre- progresses through tract
Carbohydrate absorption
- Monosaccharides: glucose,
fructose, galactose - Absorbed across brush
border - Carrier mediated
- SGLT (glucose, galactose)
- GLUT 5 (fructose)
- To GLUT 2 then first to liver
and onwards to circulation Absorption and transport of monosaccharides in enterocyte of small intestine by Sonabi.
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Glycaemic index
- Indicator of post-prandial
glycaemic response to food - Measures insulin response
relative to a reference food
(usually 50g) - Calculate glucose curves
- GI= area under the glucose
curve of test food/area under the
glucose curve of reference food - High 71+, Moderate 56-70, Low
55 or less
Glycaemic Load
- Factors in serving size
- GI/100 x carbs in serving=
glycaemic load - Low: 1-10
- Medium 11-19
- High 20 or more
Carb metabolism and exercise
- Carbohydrates utilised ultimately as glucose
- Stored in body as glycogen
- Although metabolism of fats yields more energy per gram (9kcal/g),
metabolism of carbohydrates is greater per unit of time - Hence, at higher intensities, carbohydrates fuel of choice
carbohydrate for physical athlete
- Carbohydrate primary fuel during higher intensity exercise
- Stored as glycogen (skeletal muscle and liver)
- GLYCOGEN STORES ARE LIMITED
- Glycogen stores can be used to fuel both aerobic and
anaerobic energy production - Majority of athletes train more frequently than they compete
and this training may deplete glycogen stores - Need nutritional strategies to consistently sustain training
programme and efficiently restore glycogen
Role of liver in glucose homeostasis
- Blood glucose levels need to be kept within narrow
range - Why? Because brain needs constant glucose supply, but
if concentration in blood too high over time- damage. - (Diabetes mellitus- hyperglycaemia due to either no
insulin produced in pancreas or resistant to insulin) - Hyperglycaemia
- Hypoglycaemia. Below a conc of approx. 3 will get
symptoms, (sweatiness, pallor, confusion, coma, death) - The liver is an important regulator of blood glucose homeostasis
- When blood glucose low: liver will either (i) break down and release liver
glycogen (glycogenolysis) or (ii) synthesise glucose (gluconeogenesis) - When blood glucose high: liver takes up glucose and stores as glycogen
(glycogenesis) - Regulated by hormones such as insulin, glucagon, adrenaline
Hormonal control of cellular uptake of glucose: Insulin
Insulin
* Beta cells of pancreatic islets
(stimulated by rise in plasma glucose)
* + cellular uptake of glucose
* - lipolysis
* + uptake of AA into cells & + protein
synthesis
* + glycogenesis
* - gluconeogenesis in liver
Overall, reduce blood glucose, promote
fat synthesis, inhibit release of fatty acids
Hormonal control of cellular uptake of glucose: Glucagon
Glucagon
* Alpha cells of pancreatic islets in
response to decrease in plasma blood
glucose
* Raises blood glucose level
* + glycogenolysis
* + gluconeogenesis
Why do we care about carbs
- Limited stores- Glycogen depletion- limiting factor
- Substrate for both aerobic and anaerobic energy systems
- At high intensities more ATP per unit volume of oxygen from carbohydrate
metabolism - Evidence suggests that improved prolonged endurance performance as well as high
intensity intermittent activities is improved by strategies to ensure adequate
carbohydrate availability - Carbohydrate strategies for recovery can maximise restoration of glycogen
Carb loading - traditional 7 day method
- Scandinavian researchers
- 1960s
- 1 week regimen before an event
- Day 1 exhaustive exercise
- 3 x days of carbohydrate depletion (less than 25% CHO)
- Day 4 exhaustive exercise
- Days 5-7 High carbohydrate diet (70% CHO)
- Result: Muscle glycogen stores increased above normal levels (80-
212 mmol/kg ww)
problems with the 7 day carb loading method
- Difficulty executing low CHO diet
- Athlete unease with training regime-
particularly days off! - Fatigue, nervousness, irritability
- Low blood glucose- muscle weakness,
disorientation - Poor recovery from exhaustive exercise
- GI discomfort (both on low CHO as well as
during CHO loading days) - Weight gain
- Not practical for sports requiring repeated
rounds/efforts etc.
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Modified carb loading method
Tested 3 x protocols
* 1) Mixed diet (50% CHO)
* 2) Low CHO diet (25%) x 3 days + 3 days of High CHO diet (70%)
* 3) Mixed diet x 3 days (50%) + 3 days of high CHO diet (70%)
* Training wise- all reduced training gradually over 6 days, ending in a complete
rest day
* It was found that option 3 (moderate protocol) provided almost as much of an
increase in glycogen levels- just as effective without the disadvantages – deemed
preferable
Performance Benefits of high CHO
diets/CHO loading regimens
What type of athlete?
Athletes with endurance events lasting longer than 90 mins will
have most benefit
* CHO loading may improve endurance performance by 2-3% and time to
exhaustion by 20%
* Intermittent exercise events – high CHO diet
E.g. previous study in soccer players support
showed that high CHO diet increased glycogen stores and resulted in
players covering greater distances in two consecutive matches over a 3
day period
Recommended carb intake based off exercise type
Light 3-5 g/kg/d
Moderate
(approx. 1 hr)
5-7 g/kg/d
High Endurance programme (1-
3 hours per day at mod-
vigorous intensity)
6-10/g/kg/d
Very High 4-5 hrs per day of mod-
high intensity exercise
8-12/g/kg/d
Carb intake pre exercise
- Generally recommended to consume 3-5 hours in advance of exercise
- Increases liver and muscle glycogen stores.
- An especially important pre-event or pre-exercise meal is breakfast if event is later
in the day - This restores depleted liver glycogen following overnight fast – important to
maintain steady blood glucose levels (Jeukendrup & Gleeson, 2004). - 200-300g- may promote carbohydrate availability during the exercise session
Fuelling strategies
- To improve competition performance or performance in key training sessions
- ‘General fuelling up’ for events less than 90 mins: recommend approx. 7-12 g/kg
per 24 hours - Carbohydrate loading for events longer than 90 mins: 36-48 hrs of 10-12g/kg/day
(24 hours) - Specific pre-event fuelling before events of longer than 60 mins 1-4g/kg body
weight 1-4 hours before exercise- depending on athlete preferences - If athlete can’t consume carbs during event- consider low GI
Carbohydrates during endurance events
- Rationale: maintaining blood
glucose and rates of carbohydrate
oxidation when body’s stores
running low - Glycogen sparing effect on liver
- i.e. body will use the exogenous
carbohydrate first, before using
stores of liver glycogen - Research has examined
different mixtures of
carbohydrate types –
different transporters in
gut for absorption - Mouth rinses
carb intake post exercise
Main goal: Recovery and replace depleted glycogen stores
▪Some athletes carboloading between sessions to replenish
glycogen
Recap: GLUT 4 - glucose transporter
▪Normally in vesicles but moves to cell membrane (following
insulin and also during exercise)
▪2 phases of glycogen synthesis following exercise
▪ 1) Rapid phase- immediately after exercise. GLUT 4 still located at cell membrane- this will only last a few hours without insulin.
This phase may also be regulated by glycogen levels (if low, glucose uptake may last longer)
▪ 2) Slow phase- glycogen synthesis much slower during this phase. Will depend on insulin levels.
carb intake recommendations post exercise (numerical value)
- 1-1.2 g/kg/hour during first 4-6 hours (AND, DC & ACSM, 2016).
- Could take the carbohydrate at 30 min intervals (0.6g/kg every 30 mins) for about
4 hours
Train low concept
Manipulating the training conditions to cause adaptation
* Training in conditions of reduced CHO may enhance adaptations of skeletal muscle to
improve performance
* Increase activity of mitochondrial enzymes
* Cell signalling and gene transcription
* Increase lipid oxidation rates (i.e. improve ability to use fat stores)
* More research required to determine most effective protocols
* Disadvantages- athletes may struggle to maintain required intensity in CHO depleted state
* More evidence required.
