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(Lesson adapted from http://btc.montana.edu/olympics/nutrition/default.htm).
Energy: How and where is it stored?
This tables displays the location and approximate amounts of energy stored as ATP,
carbohydrate, fat, and protein in an average young adult male. The total amount of energy
stored in the body varies greatly among individuals and is dependent on body size, body
composition, physical fitness level, and diet. In the table each energy source is documented as
to its gram weight within the body. Carbohydrate, protein, and fat yield 4, 4, and 9 calories per
gram, respectively.
Calculate the number of calories provided by each source and put it in the table.
Carbohydrate is available as glucose in the blood, glycogen within the liver, and glycogen
within muscle fibers. Note that total carbohydrate stores comprise about 2,000 calories. While a
similar amount of calories (2,500) are stored in muscle as intramuscular triglycerides, the total
caloric value of fat stores in this person is greater than 80,000 calories. Energy stored in adipose
tissue is the most variable component of total body energy stores and ranges from
approximately 50,000-100,000 calories in men and women with body compositions of 10-30%
body fat. Fatty acids and triglycerides circulating in the blood represent the smallest
component of fat stores.
While protein stores in muscle account for 30,000 calories, protein is infrequently used as an
energy source. Although protein may supply up to 5-10% of the energy needs of an athlete
engaged in endurance exercise, all body protein is functional, used for building and repairing
cells and tissues. Thus, protein is not specifically stored for the purpose of generating energy.
Calculate the percent of total energy stores that are available from each of the three major
energy sources (carbohydrate, protein, fat).
Total energy stores: _______________________
% carbohydrates: ____________________
% lipids: ____________________
% proteins: ____________________
What Fuels Are Used For Exercise?
While carbohydrate and fat are the two main sources of energy for muscular work, each of
these fuels is comprised of two components, one within skeletal muscles and one in the blood.
Thus, four primary energy substrates fuel muscle contraction and physical activity. Values for the
average person are listed below.
While you probably do not think of your muscles as being fatty, fat exists as droplets of
triglyceride within skeletal muscles. Muscle is the primary tissue that oxidizes fat, and
intramuscular fat is readily available as a fuel source for muscle work. Additionally, as from the
previous exercise, the total amount of calories available from intramuscular triglycerides is
greater than that available from muscle glycogen. Remember that while fat is more dense in
calories than either carbohydrate or protein, fat is light in weight. Thus, 2,500 calories of fat
weighs only about nine ounces.
What about protein as a fuel source? Amino acids from protein breakdown can undergo
oxidative metabolism to be used to fuel muscle contraction. Although not primary energy
substrates, amino acids are used as auxiliary fuels during muscular work. Exercise duration,
carbohydrate content of the diet, and muscle glycogen levels affect protein use for fuel.
Oxidation of branched-chain amino acids (leucine, isoleucine, and valine) within skeletal
muscles is increased during prolonged exercise and when muscle glycogen stores are low.
Protein may contribute up to 10-15% of the energy needs in endurance exercise, particularly
when the athlete is carbohydrate depleted.
So far, the focus has been on providing energy for muscle contraction. However, energy to fuel
the brain is essential for alertness and mental concentration in any sport. Normally, the brain is
fueled almost exclusively by carbohydrates, glucose from the blood or from the breakdown of
liver glycogen.
How would you respond to an endurance athlete (cross-country skier, cyclist) who believes that
carbohydrates are the only source of energy during a two-hour training session?
How would you respond to a strength-trained athlete (power-lifter, wrestler) who believes that
protein fuels his/her sport?
When Is Each Fuel Used During Exercise?
Training, nutrition, gender, intensity, and duration are the five factors which interact to provide a
balance of fuels to supply energy for working muscles. Although the most extensive research on
fuel use during exercise has been done on endurance athletes (distance runners, cross country
skiers, cyclists, and swimmers), data is also available for athletes in strength and power sports
such as ice hockey and for resistance exercise (weight training).
Intensity: Fast Action Demands High-Octane Fuel
Of all the factors, exercise intensity matters the most in determining which fuel is used for muscle
contraction. As the table below shows, carbohydrate is the preferred fuel for high intensity
exercise (performed at over 70% of aerobic capacity or VO2max). Side note: VO2max is
generally considered the best indicator of an athlete's cardiovascular fitness and aerobic
endurance. Theoretically, the more oxygen you can use during high level exercise, the
more ATP you can produce.
In this graph, the bread slices represent carbohydrate stores (glucose and glycogen) and the
butter pats represent fat stores (fatty acids and triglycerides). At rest and at low exercise
intensities (expressed as %VO2max), fat is the predominant fuel. During moderate intensity
exercise (you feel the effort but can carry on a conversation while you are moving) fat and
carbohydrate contribute equally to fuel muscle contraction. However, as exercise intensity
increases to moderately hard or hard (talking while moving is difficult or impossible), a point is
reached (ranging from 65-75% VO2max) where carbohydrate becomes the predominant, and
then the exclusive, fuel for working muscles. Why? Look below for the answers.
Carbohydrate: High Power Fuel
 Fast ATP production
 Carbohydrate is the only energy substrate that can produce ATP anaerobically
 More calories (kcal) provided per liter (L) of oxygen consumed (5.01 kcal/L)
 Primary use by fast twitch muscle fibers (recruited during high intensity exercise)
Fat: Low Power Fuel
 Slow ATP production -- Fat can only produce ATP via aerobic metabolism
 Fewer calories (kcal) provided per liter (L) of oxygen consumed (4.65 kcal/L)
 Primary use by slow twitch muscle fibers (recruited during low-moderate intensity
exercise)
Remember the four major fuels for exercise: muscle glycogen, plasma glucose, muscle
triglyceride, and plasma fatty acids. This graph shows how the use of these fuels changes as
energy intensity increases. Subjects for this study (Romijn, 1993) were endurance-trained men
who had fasted overnight. These data were derived after the individuals exercised for thirty
minutes at the given intensities. Note the following changes in substrate contribution to the total
energy supply as energy intensity increases from 25% to 65% to 85% of VO2max:
Make three observations about the graph above:
Halfway through the middle of the last period in an ice hockey game, a player has significantly
depleted the muscle glycogen in his leg muscles. How will his skating speed be affected for the
remainder of the game? Assume that he has not eaten in several hours and that he has
consumed only water during the game.
A competitive female cross country skier wants to improve her skiing performance. Although
she is within range of body composition (%body fat) as other competitors, she has the idea tha
eating a very low fat diet to reduce her percent body fat will improve her performance. To be
competitive she must ski at a pace abover 65% VO2max during her distance races. Based on
the previous graphic, why may her fat loss strategy be ineffective?