Chapter 6- Carbohydrates Flashcards
what is the most important fuel for the CNS ?
carbs
to optimize carb availability, what should an athlete do?
carb feeding before exercise and carb feeding during exercise
explain the Krogh and Lindhart experiment recognizing the importance of carbs as fuel for exercise.
high fat diet for three days (bacon, butter, eggs) and then high carb for 3 days (potatoes, flour, bread, sugar) w 2h exercise test. exercise reported as easy on carb diet, and as inducing fatigue on fat diet.
RER also decreases on a high fat diet
what did Levine, Gordon, and Derick observe when they measured blood glucose in some participants in the Boston Marathon
most runners, glucose concentration declined markedly after the race. then, recommended carb snacks to runners in next years marathon to reduce hypoglycemia and fatigue
what did Dill, Edwards, and Talbott do to their dogs ?
didn’t feed them carbs before running, dogs were fatigued after 4-6 hrs
when they were fed carbs, ran for 17-23 hrs
what did Christensen show about exercise intensity and carb utilization ?
that with increasing intensity, the proportion of carb used increases.
what did muscle biopsies in the 1960s show ?
that high carb diets (70 percent) increased endurance capacity and muscle glycogen
how is glycogen measured , in what units ?
mmol glucosyl units (glucose) / kg dry or wet mass
dry mass determined by freeze drying muscle sample
muscle contains how much water ? what does that mean ?
75-80% water, so dry mass will be 4.5 x wet mass concentrations
at rest, how much muscle glycogen do we have ?
300-400g CHO
how does hepatic glucose output depend on exercise intensity?
the higher the intensity, the more glucose output
what is the main determinant of the rate at which muscle glycogen is oxidized ?
exercise intensity (used a lot more at higher intensity) at lower intensity, utilizing more acetyl coA from carb or fat
what is the main role of the liver ?
regulation of blood glucose to maintain homeostasis
why can’t muscle glycogen be used to regulate blood glucose ?
because once glucose is inside the muscle, phosphorylated by hexokinase to G6P and muscle doesn’t have the enzyme that turns it back to glucose
which enzyme does muscle lack which would technically turn G6P back into glucose ?
glucose 6 phosphatase
which has the higher quantity of glycogen- liver or muscle ?
muscle
which has the higher concentration of glycogen- liver or muscle ?
liver
after an overnight fat, what is the mass of liver glycogen ?
<20g
what is the weight of the liver ?
1.5 kg
what is the rate of brain glucose usage in sleep
0.1g/min
during exercise, how does the rate of glucose utilization by tissues other than muscle change ?
doesn’t change much
at rest, what is the liver output of glucose and from which sources ?
150 mg/min
40% from gluconeogenesis
60% from glycogenolysis
what is the liver output of glucose during high intensity exercise and from which sources ?
1000 mg/min (almost 10-fold than at rest)
<10% from gluconeogenesis
>90% glycogenolysis
what will be the concentration of blood glucose desired ?
4-4.5 mmol/L
when is there less gluconeogenesis ?
with food intake
when is there more gluconeogenesis ?
starvation, and high duration of exercise
what is the most important glucoregulatory hormone ?
insulin
in exercise, how does insulin concentration change ?
decreases
what are 4 hormones that promote glycogen breakdown?
glucagon, cortisol, epinephrine, norepinephrine
in exercise, there is less insulin secretion but glucose uptake by muscle still occurs, enhanced by what ?
muscle contraction
when is it considered hypoglycemia ?
<3 mmol/L
what is hypoglycemia ?
insufficient uptake of glucose by brain
what kind of breath one may have in hypoglycemia ?
acidosis (fruity)
what can happen to HR in hypoglycemia
increased HR
what kind of food do you give someone with hypoglycemia ?
high GI and fast absorption, like juice
explain in more detail what insulin does ?
promotes uptake and storage of glucose
what two enzymes does insulin regulate
glycogen synthase increases and glycogen phosphorylase decreases
what happens at about 80%Vo2MAX when it comes to the mismatch glucose uptake and production ?
the liver produces more glucose than is uptaken
(increased hepatic release), probably due to feedforward mechanism resulting in elevated high blood glucose
what happens in later stages of exercise when it comes to the mismatch of glucose uptake and production ?
rate of glucose production may become insufficient as glycogen is depleted
what does pre-exercise carb consumption induce ?
reactive/rebound hypoglycemia
what are the two reasons for reactive hypoglycemia ?
1) preexercise CHO feeding, with increased insulin levels and decreased glucose output
2) muscle contraction leads to GLUT4 translocate and glucose uptake, meaning less in blood
2 ways to reduce reactive hypoglycemia
low GI preexercise CHO feeding
warm up exercise
how will repeated running bouts influence muscle glycogen concentration if you are on a high CHO diet vs a low CHO diet
if you are on a low CHO diet, there will be a marked decreased and you will slowly deplete your stores
in what exercise type do carbs help ?
every type: endurance, intermittent, short-duration high-intensity
how will muscle glycogen relate to fatigue during an activity??
on a graph, it will look like an x with the two lines intersecting as muscle glycogen decreases and fatigue increases
after 1.5 hours, liver glycogen starts being converted into glucose
what is the crossover point ?
when exercise intensity increases and more carbs start being used than fats
with training the crossover point between carb and fats usage changes in what manner ?
you start sparing more glycogen and using more fats
SNS stimulation will promote usage of which energy source ?
fats
what are the Olympic recommendations for carb intake per day?
5-12g/kg bw/d
up to 10-13 g/kg bw/d if you exercise for 3+ hours daily
during training what kind of food intake should you have ?
sports drink within the first hours
what kind of carbs should athletes be consuming ?
high GI
why is it important to eat nutrient rich carb foods ?
to provide a good source of protein (promoting muscle recovery) and other things
for recovery, what are the recommendations for food when the period between exericse sessions are less than 8 hours?
athlete should begin carb intake as soon as practical after the workout, to maximize the recovery time between sessions
for recovery, what are the recommendations for food when the period of recovery is 24 hours or more
organize pattern and timing of CHO rich meals and snacks
what is the difference in glycogen synthesis when you ingest a solid or a liquid ?
no difference
which foods have high GI? (>85)
potato, pancakes, bread, cereal, watermelon, gatorade
which foods have moderate GI? (60-85)
rice, ice cream, candy bars
which foods have low GI? (<60)
honey, carrots, apples, bananas, grapes, beans
what is GI?
area under curve of blood glucose over 2h after 50g of food
how do you calculate GI?
AUC test food/ AUC ref food (usually white bread) x100
what is glycemic load?
GI of food or meal x g available CHO in the meal / 100
meaning food affects other foods’ absorptin
the glycemic load
what is a low, med, high GL ?
low: 0-10
med: 11-19
high: 20+
what are the 6 things that may alter the glycemic index?
type of CHO fiber content protein and fat content timing of meal other foods total amount eaten
how will fiber content affect GI ?
high fiber decreases GI
how will protein and fat content affect GI?
increased protein and fat decreases GI
how will the timing of the meal affect GI ?
empty stomach means there will be increased absorption
what kind of GI for pre exercise meal ?
low GI
what kind of GI during high intensity or long duration exercise
med-high GI
what kind of GI after exercise ?
high GI
as exercise intensity increases and muscle glycogen gets depleted, hepatic glucose output ______
increases
what is the main glycogen that maintains blood glucose concentrations ?
liver
as exercise intensity increases, how will the contribution of maintaining blood glucose change between glycogenolysis relative to gluconeogenesis ?
more glycogenolysis will happen relative to gluconeogenesis
what are the three variables of CHO availability
timing, amount, GI of food
what are the goals of carb intake 24h before training or right before ?
24h before: replenish muscle glycogen
right before: optimize liver stores
describe the original supercompensation protocol ?
days -7 to -4 no carbs, high protein high fat followed by days -3 to competition high carb (75%)
glycogen-depleting exercise bout on -7 and -4 and then competition
describe the modified supercompensation protocol ?
days -6 to competition slowly increasing carb intake, and slowly decreasing training
what were the gender differences in supercompensation protocol ?
if you abide by the protocol there aren’t supposed to be any gender differences
why is it important to eat familiar foods ?
because you can tolerate them well
24h prior to exercise, what should you eat ?
high CHO meal (60-70% total kcal), familiar food `
3-5h prior to competition, what should you eat ?
1-4.5g CHO/kg body weight
easily digestible food with low to moderate in fiber, and low GI
what are CHO-rich protein sources that can be eaten 3-5 h before an event ?
yogurt, soy products, small amount of legumes
is it ok to eat food 1h before an event ?
yes!
what are 3 factors determining the glycemic response in exercise ?
- insulin+ contractile activity increases glucose uptake
- balance of inhibitory and stimulatory effects of insulin and catecholamines on liver glucose output
- magnitude of intestinal absorption ongoing of ingested CHO
intake of CHO 1h before exercise could lead to what ?
increase in CHO oxidation and glycogenlosys, suppression of fat metabolism as energy source.
when is reactive hypoglycemia more prevalent ?
when you eat 75 min prior to exercise
with ingestion 45 min or even 15 min prior to exercise, how does the risk of reactive hypoglycemia change ?
decreases
how can reactive hypoglycemia be prevented ?
ingestion 5 min prior and warm up
if a high GI food is ingested before exercise, what kind of effect does a CHO ingested during exercise have ?
it means that there will be little to no effect on metabolism- potential harmful effect of high GI food is reduced.
what is the convincing evidence that carb feeding during exercise improves endurance capacity?
if you eat in exercise >45 min, it helps maintain blood glucose and CHO oxidation, spares glycogen in the liver and possibly in muscle, promotes glycogen synthesis, and affects the CNS
does the frequency of CHO ingestion during exercise >45 min matter?
no, it doesn’t matter if it’s a bolus or several small amounts
what is the optimal amount of exercise CHO feeding ?
needs to be the amount that brings maximal exogenous oxidation rates without any GI disturbance
up to 70g/h
how many gCHO/l do sport drinks have
60-80 g/L
solutions up to what % of CHO in sport drinks are tolerated during exercise w/o any GI disturbance ?
8% CHO
70g CHO is in what quantity of sport drink ?
1 L
70g CHO is in what quantity of bananas ?
3 bananas
70g CHO is in what quantity of wine gum ?
120-150g
70g CHO is in what quantity of powerbars with high protein and high CHO
1.5 bars
how do you spare endogenous sources of CHO ?
by ingesting exogenous CHO
true or false: rate of oxidation depends on whether it is a mono or disaccharides
false
which monosaccharide is oxidized at a high rate
glucose
what is a high rate of CHO oxidation ?
1g/min
high rate of oxidation: 1M, 2D, 2P
glucose maltose sucrose maltodextrin amylopectin (branch type)
which will be broken down faster: branched CHO or single line ?
branched
what will be broken down faster? amylopectin or amylose ?
amylopectin bc it is branched
low rate of oxidation: 2M, 2D, 1P
fructose galactose isomaltose trehalose amylose
why are monosaccharides not all rapidly oxidated ?
galactose and fructose have to first convert to glucose
where would you find trehalose ?
fungi, plants, invertebrate animals
where would you find isomaltose
honey and sugarcane
where would you find maltodextrin ?
starch breakdown
how is exogenous carb oxidation related to exercise intensity
exogenous carb oxidation rate will increase with increasing exercise intensity to up to 50% VO2max
after that, may decrease
why is it a good idea to combine 2-3 different CHO?
because if there is a lot of glucose, the transporters are saturated but fructose has a different transport mechanism. this enhances the rate of oxidation up to more than 1g/min
what is a light solution if you’re likely to get hungry while exercising ?
gels slowing gastric emptying with 2+ CHO in them
what could limit exogenous CHO oxidation in general ? (6) what is the main one ?
- gastric emptying rate
- digestion rate
- ABSORPTION RATE in small intestine (main)
- retention in liver
- muscle glucose uptake
- metabolism in muscle
in exercise, why would there be a reduced digestion and absorption of CHO ?
because of reduced blood flow to gut
how does exercise stimulate glucose uptake ?
for 1-2 hrs post exercise, due to increased insulin sensitivity
high insulin stimulates glycogen synthesis by which enzyme ?
glycogen synthase
what are the two post-exercise phases of glycogen synthesis ?
the initial insulin independent rapid phase
and then the insulin dependent slow phase
what is the rate limiting enzyme of the rapid phase of glycogen synthesis ? what are its forms ?
glycogen synthase, existing in inactive D-form and active I-form
when is glycogen synthase in what form ?
when glycogen concentrations are low - active I form
when glycogen is high- inactive D form
what does exercise do to glycogen synthase
excites it
after exercise, what proportion of glycogen is in the active I form ?
80%
what is the rate-limiting substrate of glycogen in the rapid phase ?
UDP-glucose (activated glucose) availaibility
what mineral enhances the rapid phase of glycogen synthesis
Ca2+ increases GLUT4 translocation
when does the slow phase of glycogen synthesis begin ?
when the effect of the exercise-induced increase in glucose transport wears off, approximately 2h after exercise
what does the glycogen synthesis rate in slow phase mainly depend on ?
insulin concentrations
why is insulin with a more significant role in the slow phase of glycogen synthesis ?
because at this pt it is no longer suppressed by catecholamines
muscle contraction enhances which phase of glycogen synthesis
both ! the effect lasts several hours
glycogen: branched chained or line ?
branched
what enzyme makes glucose into a branched chain ?
branching enzyme
what does glycogen synthase do ?
combines excess glucose residues one by one into a polymeric chain for storage as glycogen.
what does the branching enzyme do ?
adds branches to the growing glycogen molecule during the synthesis of glycogen
what 3 main things does glycogen synthesis depend on ?
glucose availability,
transport of glucose into cell (exercise stimulation, insulin, muscle glycogen)
enzyme activity
what is the effect of muscle glycogen on glycogen synthesis
when muscle glycogen concentrations are low, the enhanced glucose uptake has to last longer
why is it better to eat CHO immediately post exercise vs 2hrs post exercise ?
because there will be a much more significant rate of glycogen synthesis in the first two hours
what is the correlation between muscle glycogen resynthesis and carb intake ?
positive
why does fructose take more time to result in glycogen synthesis ?
bc it has to be converted to glucose in the liver first
what is the relative rate of glycogen synthesis post exercise from fructose, glucose, or sucrose ?
fructose takes more time
will a high GI meal or a low GI meal post exercise result in more glycogen synthesis ?
high GI (higher absorption rate and availaibility of glucose)
why can it be beneficial post exercise to include protein in the meal ? what is more important, though ?
some aa have an effect on insulin secretion
glycogen storage increases more when carbs ingested w protein, but adequate CHO is more important
could caffeine positively affect carb ingestion after exercise ?
maybe
what form of CHO post exercise is preferable
solid= liquid
before exercise, what will mostly happen to carbs eaten ?
some will go to muscle glycogen but most to liver
what is the best GI schedule for an athlete
low GI (balanced meal w protein) in morning high GI in competition and in evening
what is the rate of CHO ingestion post exercise per min ? per hour ?
- 4g/min
1. 2g/kg bw /h for 3-4 h
