Download Carbohydrates – the High Octane Fuel for Exercise

Carbohydrates – the High Octane Fuel for Exercise
Adequate carbohydrate stores (muscle and liver glycogen and blood glucose) are
critical for optimum performance during both intermittent high-intensity exercise (e.g.
basketball and soccer) and prolonged endurance exercise (e.g. marathon, Olympic
distance triathlon). During high-intensity training, athletes should consume enough
carbohydrate to meet the energy requirements of their training programs and replenish
muscle and liver glycogen between training sessions and competitive events (1).
Muscle glycogen represents the major source of carbohydrate in the body (300 to 400
g or 1,200 to 1,600 kcal), followed by liver glycogen (75 to 100 g or 300 to 400 kcal),
and, lastly, blood glucose (25 g or 100 kcal). These amounts vary substantially among
individuals, depending on factors such as dietary intake and state of training. Untrained
individuals have muscle glycogen stores that are roughly 80 to 90 mmol/kg of wet muscle
weight. Endurance athletes have muscle glycogen stores of 130 to 135 mmol/kg of wet
muscle weight. Carbohydrate loading increases muscle glycogen stores to 210 to 230
mmol/kg of wet muscle weight (2).
Carbohydrate is the predominant fuel for most types of exercise (3). Muscle
glycogen and blood glucose provide about half of the energy for moderate-intensity
exercise (65% of VO2max) and two-thirds of the energy for high-intensity exercise (85%
of VO2max). It is impossible to meet the energy requirements for high intensity, high
power output exercise when these carbohydrate fuels are depleted (3). The utilization of
muscle glycogen is most rapid during the early stages of exercise and is exponentially
related to exercise intensity (2,4).
Exercise Fuel Use
As the intensity of exercise increases, muscle glycogen constitutes
a greater portion of the energy source
Different Exercise Intensities
(Weightlifting,
sprinting, etc)
300
(Soccer,
dancing, etc)
Muscle glycogen
200
Energy Expended
(cal/kg/min)
100
0
Muscle triglyceride
(Leisurely walking,
slow cycling)
Plasma FFA
Plasma glucose
25
65
85
Maximal Oxygen Consumption, %
Abbreviations FFA, free fatty acid.
Romijn JA, et al. Am J Physiol. 1993;265(Part 1):E380-E391.
Liver glycogen stores maintain blood glucose levels both at rest and during
exercise. At rest, the brain and central nervous system (CNS) utilize most of the blood
glucose, and the muscle accounts for less than 20% of blood glucose utilization. During
exercise, however, muscle glucose uptake can increase 30-fold, depending on exercise
intensity and duration. Initially, the majority of hepatic glucose output comes from
glycogenolysis; however, as the exercise duration increases and liver glycogen decreases,
the contribution of glucose from gluconeogenesis increases (2,4).
At the beginning of exercise, hepatic glucose output matches the increased muscle
glucose uptake so that blood glucose levels remain near resting levels (4). Although
muscle glycogen is the primary source of carbohydrate during exercise intensities
between 65 and 75% of VO2max, blood glucose becomes an increasingly important source
of carbohydrate as muscle glycogen stores decline (3). Hypoglycemia occurs when the
hepatic glucose output can no longer keep up with muscle glucose uptake during
prolonged exercise (4).
Liver glycogen stores can be depleted by a 15-hour fast and can decrease from a
typical level of 490 mmol on a mixed diet to 60 mmol on a low-carbohydrate diet. A
high-carbohydrate diet can increase liver glycogen content to approximately 900 mmol
(2).
Targets for Carbohydrate Intake
Carbohydrate recommendations for athletes range from 3 to 12 grams of
carbohydrate/kg per day (1). Athletes with very light training programs (low intensity
exercise or skill-based exercise) should consume 3 to 5 g of carbohydrate/kg per day.
These targets may be particularly suitable for athletes with large body mass or a need to
reduce energy intake to lose weight. Athletes engaged in moderate-intensity training
programs for 60 minutes per day should consume 5 to 7 g of carbohydrate/kg per day.
During moderate- to high-intensity endurance exercise for one to three hours, athletes
should consume 6 to 10 g of carbohydrate/kg/day. Athletes participating in moderate- to
high-intensity endurance exercise for 4 to 5 hours per day (e.g. Tour de France) should
consume 8 to 12 g of carbohydrate/kg per day. These are general recommendations and
should be adjusted with consideration of the athlete’s total energy needs, specific training
needs and feedback from their training performance (see Tables 1 and 2 for lists of
carbohydrate-containing foods and for examples of diets containing 5 and 12 g
carbohydrate/kg body weight/day). Carbohydrate intake should be spread over the day to
promote fuel availability for key training sessions – before, during and after exercise (1).
Table 1. Carbohydrate Content of Selected Foods
Each provides about 25 grams of carbohydrate
Grains
2 slices whole wheat bread
½ deli-style bagel
1 2-ounce English muffin
1 cup oatmeal
1 cup ready-to-eat breakfast
cereal
1 package snack-type
cheese
crackers (6 to package)
2 fig cookie bars
½ cup rice
1 cup cooked pasta
5 cups popcorn
½ large soft pretzel
17 mini-pretzels
1 12-in. flour tortilla
1 oz tortilla chips and
½ to 1.5 cup salsa
Sports Drinks, Bars and
Gels
2 cups sport drink (6%-8%
carbohydrate-containing
sport drinks)
1 energy bar (average of
many energy bars)
1 carbohydrate gel
½-1 bottle of commercial
nutrition or weight loss
shake
Diary Products and Other
Beverages
Beans and Starchy
Vegetables
2 cups milk (low-fat or
fat-free)
1 cup low-fat chocolate
milk
1 4.5-oz container fruit
flavored yogurt
1½ 8-oz cartons sugar-free
yogurt
1 cup vanilla-flavored soy
milk
1 package instant hot
chocolate (made with
water)
½ cup black beans
½ cup baked beans
¾ cup kidney beans
1 cup lima beans
1 cup green peas
3/4 cup corn
¾ cup mashed potatoes
1 medium baked potato
with skin
Mixed Dishes
Fruit and Juice
1 slice thin-crust pizza with
meat or veggie toppings
½ slice thick crust pizza
with meat or veggie
toppings
1 small bean and rice
burrito
½ cup black beans and rice
1½ cups canned chicken
noodle soup
¾ cup tomato soup
1 cup cooked ramen
noodles
½ 6-in. sub sandwich
½ cup macaroni and cheese
2 cups fresh strawberries
1 large orange
¾ cup orange juice
½ cup cranberry-apple juice
1 medium apple
Table 2. Diets containing 5 g carbohydrate/kg/day (3000 kcal/day) and 12 g/kg/day
(5000 kcal).
5 g carbohydrate/kg/day; 3000 kcal
Breakfast
1 slice whole
wheat toast
Morning
Snack
1/2 med.
bagel
1 tsp.
1 T. cream
butter/margarine cheese
6 oz. orange
juice
1 cup bran
flakes
½ c.
cottage
cheese
(4.5% fat)
1 peach
half
1 cup 2% milk
Water
Lunch
Afternoon
Snack
1 c. tomato 1.5 oz trail
soup
mix
(made
w/water)
2 slices
1 cup 2%
whole
milk
wheat
bread
2 T. peanut
butter
Dinner
1-1/2 c.
mashed
potatoes
-1.5 tsp.
butter/margine
1 large
apple
¼ c. gravy
1 c. 2%
milk
2 c.
cantaloupe
pieces
1 c. 2%
milk
-1 oz.
American
cheese
Water
4 oz. roast
beef
1 c. green
beans
Approximate composition: 346 g carbohydrate, 129 g protein, 123 g fat
Evening
snack
Grilled cheese
sandwich
(ingredients
below)
- 2 slices
whole wheat
bread
Table 2, continued
12 g carbohydrate/kg/day; 5000 kcal
Breakfast
Pancakes (four
4-inch)
¼ c. pancake
syrup
Morning
snack
1 bagel
Lunch
2 T. peanut
butter
Turkey
sandwich
(ingredients
below)
-2 slices
whole wheat
bread
-2 oz. lean
turkey
1 tsp.
1 large
butter/margarine banana
12 oz. orange
juice
10 oz.
sports drink
2 c. tomato
soup (made
with water)
8 oz. skim milk
-1 T. lite
mayonnaise
1 oz. Canadian
bacon
2 c. apple
juice
2 c. mixed
green salad
4 T. reduced
fat salad
dressing
¼ c. raisins
(in salad)
Afternoon
snack
2 c. sports
drink
½ c. cottage
cheese
4 peach
halves
1 chewy
granola bar
Dinner
2 c.
spaghetti
and
meatballs
1 T. grated
Parmesan
cheese
1 c. cooked
peas and
carrots
2 bread
sticks (4
inch)
1 tsp. olive
oil (drizzle
on bread
sticks)
2 c.
lemonade
½ c.
applesauce
Evening
snack
12 oz.
chocolate
milk (1%)
2 oz.
graham
crackers
2 T.
hazelnut
spread
Approximate composition: 839 g carbohydrate, 137 g protein, 128 g fat
The absolute quantity of carbohydrate (grams of carbohydrate per kg of body
weight), rather than the percentage of energy from carbohydrate, is important for exercise
performance. An athlete’s estimated carbohydrate requirements should consider the
amount of carbohydrate required for optimal glycogen restoration or the amount
expended during training. These estimates should also be provided according to the
athlete’s body weight to account for the size of the athlete’s muscle mass. Carbohydrate
guidelines based on grams per kg are user-friendly and practical – it is relatively easy for
athletes to determine the carbohydrate content of meals and snacks to achieve their daily
carbohydrate goals (1).
Athletes should consume sufficient energy as well as carbohydrate to optimize
muscle and liver glycogen storage. The restrained eating practices of some athletes (often
in weight-restricted sports or aesthetically judged sports) can interfere with meeting
targets for carbohydrate intake and compromise glycogen storage (1).
It is important to choose nutrient-dense carbohydrates (whole grain products,
fruits, vegetables, and beans) and to add other foods to recovery meals and snacks to
provide a good source of protein and other nutrients (5). These nutrients may assist in
other recovery processes, and in the case of protein, may promote additional glycogen
recovery when carbohydrate intake is below targets or when frequent snacking is not
possible (1).
References
1. Burke LM, Hawley JA, Wong S, Jeukendrup AE. Carbohydrates for training and
competition. J Sports Sci 2011, in press.
2. Jacobs KA, Sherman WM. The efficacy of carbohydrate supplementation and chronic
high carbohydrate diets for improving endurance performance. Int J Sport Nutr.
1999;9:92-115.
3. Coyle EF. Substrate utilization during exercise in active people. Am J Clin Nutr. 61
(suppl): 1995; 968S-979S.
4. Hargreaves M. Exercise physiology and metabolism. In: Clinical Sports Nutrition. 3rd
ed., Burke, L. and Deakin, V. Eds. McGraw-Hill, Australia, 2006.
5. Position of the American Dietetic Association, Dietitians of Canada, and the American
College of Sports Medicine: Nutrition and athletic performance. Rodriguez NR, Di
Marco NM, Langley S; American Dietetic Association; Dietitians of Canada; American
College of Sports Medicine. J Am Diet Assoc. 2009; 109:509-27.