Lab IV
Part 1: Exercise Metabolism: Fuel substrate selection
Part 2: Thermoregulation & exercise
Introduction
Exercise is one of the greatest energy stresses that an organism is likely to encounter. Like
other stresses, severe exercise has the potential to disrupt homeostasis. Every cell requires a
certain amount of energy for life-sustaining processes. Thus, any disruption of cellular energy
homeostasis is potentially fatal for the cell. Fortunately, we have evolved several potent
mechanisms to maintain cellular energy homeostasis. Nearly every change in our body's
physiology during exercise is centered on maintaining cellular energy homeostasis. For example,
the increases in ventilation and cardiac output are meant to increase oxygen delivery to the
mitochondria in active tissues, where oxygen serves as the final electron acceptor in the electron
transport chain. The importance of energy metabolism during exercise is also illustrated by the
fact that many of the possible causes of fatigue are related to cellular energy metabolism (lab 4).
The energy that allows us to perform life-sustaining processes, accomplish work, grow,
and maintain body temperature ultimately comes from our food stuffs. However, the energy
contained in our food stuffs is not readily accessible as a source of energy for the energy requiring
processes of the cell. The energy from our food must first be converted to a common energy
currency; ATP (Adenosine triphosphate). It is ATP that serves as the immediate energy source for
muscular work. Because muscle ATP concentrations are very low, the production of ATP,
sometimes at very high rates, is of the highest priority during exercise.
If exercise physiology were a story, the plot would center on energy and work. It is
therefore extremely important for exercise physiology students to understand the basic principals
and concepts related to exercise metabolism. By this time students have already learned in their
introductory biology, human physiology, and exercise physiology courses about the following
basic cellular metabolic processes: the ATP-PC system, glycolysis, glycogenolysis,
gluconeogenesis, lipolysis, beta oxidation, the tricarboxylic acid cycle, and the electron transport
chain. It is recommended that students review these processes before coming to lab. These
processes will only be discussed briefly in order to help students understand the “big picture” of
exercise metabolism; a complete review of these processes is beyond the scope of this lab.
Today's laboratory experiments are meant to help students better understand those factors (e.g.
exercise intensity, duration, etc) that determine what fuel sources and metabolic pathways are
contributing to an exercise bout.
Exercise as a metabolic stress
At rest, almost all of our energy is coming from aerobic metabolism, and results in an
oxygen consumption of around 3.5 ml/kg/min, or about 0.25 L/min in an average human.
Assuming a caloric equivalent of 5 kcal/L, this means that, at rest most of us expend about 1 to 1.5
kcal/minute. An average college age male can reach energy expenditures of 15-20 kcal/minute at
maximal exercise. Olympic rowers average 36 kcal/min during a simulated 2000m race lasting 57 minutes, and can attain energy expenditures up to 40 kcal/min during training and competition –
a 40-fold increase in energy expenditure above rest! Of course these high rates of energy
expenditure can not be sustained for long periods of time. Among the highest measured daily
energy budgets to be maintained for more than a few days were obtained from cyclists
participating in the Tour de France. In these cyclists daily energy expenditure was around 7,000
kcal/day for 22 days. This is almost 4.5 times the typical basal metabolic rate (~1,600 kcal/day)
for these athletes. Exercise clearly places extreme demands on the body’s energy systems. For
example, it has been demonstrated that flux through the tricarboxylic acid cycle (also called the
citric acid cycle or Krebs cycle) can increase over 100-fold during moderate to high intensity
exercise.
Lab IV - 1
Fuel substrates & stores in the human body
Overall, the body has about 180,000 kcal of stored energy of which 70-80% (~140,000
kcal) are fats, ~20-30% are proteins (about 40,000 kcal), and less than 1% carbohydrates (usually
< 1,500 kcal). The contribution of these various fuel sources to exercise energy expenditure is
highly dependent upon diet, training, exercise intensity, and exercise duration.
Most of body's energy stores are in the form of fats. Triglycerides (TG) are the primary
storage form of fats in the body. Almost all of the body's TG stores are found in adipose tissue,
but a small amount (2-3%) of the fat stores are found within skeletal muscle (intramuscular TG).
Proteins are the next largest reserve of calories in the body. However, protein catabolism
generally contributes very little to exercise energy expenditure (typically less than 2% of all
energy expended, although it can reach 5-15% of energy expended during exercise lasting over 34 hours). Proteins can, on the other hand, play an important role as a "reserve" of energy stores
during prolonged fasting. The body also stores carbohydrates in the form of glycogen. Most of
the body's glycogen stores are in the muscles, with a smaller amount of glycogen stored in the
liver. While small, the liver glycogen stores are very important for maintaining blood glucose
concentrations between meals; glucose liberated from glycogen breakdown in the liver can enter
the circulation. The breakdown of glycogen is called glycogenolysis, and is catalyzed by the
enzyme phosphorylase. The making of glycogen from glucose and is called glycogenesis, and the
final step of this process is catalyzed by the enzyme glycogen synthase.
Certain tissues, for example nervous tissue, are very dependent upon glucose from the
circulation. The maintenance of blood glucose is therefore extremely important at rest and during
exercise. How can we maintain blood glucose with such limited supplies of carbohydrates? First,
we consume carbohydrates on a regular basis, and second, certain tissues in the body (liver,
kidney) are able to make glucose out of non-carbohydrate precursors (the making of glucose from
amino acids, glycerol, and lactate is called gluconeogenesis). Liver glycogen stores are used
primarily to maintain blood glucose concentrations between meals or when they are absolutely
necessary (e.g. exercise). The regulation of blood glucose and glycogen breakdown is coordinated
by the endocrine system. Several hormones can stimulate glycogen breakdown and increase blood
glucose while insulin, on the other hand, can decrease blood glucose and stimulate storage of
glycogen. In summary, the liver plays a central role in regulating blood glucose – it stores a small
amount of glycogen that can be broken down to increase blood glucose when necessary, and has
the ability to make glucose by gluconeogenesis. This regulation is mediated by several hormones.
The skeletal muscle glycogen stores are larger than liver glycogen stores, so why doesn't
skeletal muscle glycogen play a central role in regulation of blood glucose? In order to answer
this question, students must first understand a few things about glucose metabolism in the cell and
glucose transport into and out of the cell. Glucose is transported across the cell membrane by
glucose transporters, called GLUTs. In skeletal muscle, the GLUT that is involved in both
exercise-stimulated and insulin-stimulated glucose uptake is GLUT4. Once glucose enters a cell,
it is rapidly phosphorylated (a phosphate group is attached to it), making glucose-6-phosphate.
GLUT molecules are unable to transport glucose-6-phosphate. Liver cells have an enzyme
(glucose-6-phosphatase) that can remove this phosphate group, but skeletal muscle cells do not.
Thus, once glucose enters a skeletal muscle cell it can not leave. In summary, when glycogen is
broken down in the liver, glucose can re-enter the circulation, but when glycogen is broken down
in muscle, the resulting glucose can not re-enter the circulation. Whereas the primary role of liver
glycogen is the maintenance of blood glucose, the primary role of skeletal muscle glycogen is to
serve as an immediate source of energy during muscle activity. In fact, carbohydrate metabolism
during moderate or high intensity exercise relies more on muscle glycogen as a substrate than
glucose (from the circulation).
Muscle glycogen depletion is a possible cause of fatigue during exercise lasting over 90
minutes. Thus, sparing muscle glycogen would benefit individuals during endurance exercise.
Lab IV - 2
Exercise training tends to increase the use of fatty acids as a fuel source, thus at any given
intensity a trained individual has to rely less on glycogen; sparing their glycogen stores. There is
some evidence that caffeine may spare glycogen stores by increasing reliance on fat metabolism.
However, there is also evidence that caffeine has no effect on sparing muscle glycogen.
Carbohydrate ingestion during exercise is another strategy that has been used to help spare
glycogen (by providing an alternative source of glucose for glycolysis). Blood glucose
concentrations are usually well maintained during exercise, but during prolonged exercise lasting
hours, blood glucose may eventually drop. Carbohydrate ingestion would also help avoid an
undesired drop in blood glucose.
The fatty acids used to produce energy by skeletal muscle are derived primarily from
adipose tissue triglycerides, and to a lesser extent from intramuscular triglycerides. It should be
noted that triglycerides (TG) must be broken down before they can be used as a fuel substrate.
The degradation of TG into fatty acids and glycerol is called lipolysis. Inside the cells, lipolysis is
catalyzed by the enzyme hormone sensitive lipase (HSL). A different lipase, lipoprotein lipase
(LPL), catalyzes the breakdown of TG in the circulation and allows adipose cells to take up fatty
acids from these circulating TG. Once fatty acids have been taken up by adipose cells they can be
stored as TG, this is called lipogenesis. While both LPL and HSL are involved in triglyceride
breakdown, they play very different roles in fat metabolism. LPL tends to increase storage of fatty
acids in adipose tissue, whereas HSL mobilizes fatty acids so they can be used as an energy source
by other tissues.
Summary of the storage and breakdown of carbohydrate and lipid fuel stores:
Glycogenolysis, catalyzed by phosphorylase
Glycogen
Glucose
Glycogenesis, catalyzed by glycogen synthase
Lipolysis, catalyzed by hormone sensitive lipase &
lipoprotein lipase
Free fatty acids
& glycerol
Triglycerides
Lipogenesis, catalyzed by several enzymes
Energy Systems and Metabolic Pathways
As previously stated, ATP is the immediate source of energy for muscular activity, but is
stored in only low concentrations in the cell. Thus, ATP must be rapidly resynthesized during
exercise. There are multiple pathways involved in the production of ATP. The simplest and
fastest mechanism of ATP production is the ATP-PC system (also called the phosphagen or
phosphocreatine system). Phosphocreatine (usually abbreviated PC or PCr) is a high energy
compound that can readily "donate" its phosphate group to ADP in order to rapidly produce ATP.
This reaction, which is catalyzed by the enzyme creatine kinase, is summarized below.
ADP & PCr
Creatine Kinase
ATP + Cr
This reaction is reversible and does not require oxygen. During exercise, when ATP is being used
rapidly and ADP concentrations increase, this reaction favors production of ATP at the expense of
PCr. During recovery, the PCr stores must be replenished (which, of course requires ATP). The
ATP-PC system is used at the beginning of any exercise bout, and because it can produce ATP so
quickly it is especially important for high intensity exercise lasting less than 10 seconds in
duration (see figure 1). Conversely, during long duration exercise, the contribution of the ATP-PC
system to overall energy production is relatively small.
Glycolysis is the breakdown of six-carbon glucose molecules into two three-carbon
pyruvate molecules in the cytoplasm of the cell. Pyruvate produced during glycolysis has two
Lab IV - 3
possible fates. One fate of pyruvate, which requires oxygen results in the formation of acetyl
CoA, which can then enter the citric acid cycle. The other major fate of pyruvate in exercising
skeletal muscle, which does not require oxygen, is the formation of lactate.
Glycogen
Glucose
Aerobic
Glycolysis
Pyruvate
Anaerobic
Mitochondria
Citric Acid
Acetyl CoA
Cycle &
Electron
Transport
Chain
Lactate
Figure 2. Highly simplified summary of the aerobic and
anaerobic metabolism of carbohydrates
Thus, glycolysis concludes with either 1) the formation of lactate, which does not require
oxygen, or 2) the formation of acetyl CoA under aerobic conditions. Therefore, glycolysis is
sometimes referred to as aerobic or anaerobic glycolysis depending on which product is formed;
acetyl CoA or lactate. Many students tend to think of lactate as just a by-product of anaerobic
metabolism, but it is not just a by-product. Lactate formed in skeletal muscle can enter the
circulation, be taken up by the liver, and converted back to glucose via gluconeogenesis. This
glucose can then re-enter the circulation and be returned to the muscle, which can use it as a fuel
substrate. This cycle (muscle  lactate  liver  glucose  muscle) is called the Cori cycle.
The anaerobic energy systems include the ATP-PC system and anaerobic glycolysis. The
term anaerobic means that these systems do not require oxygen. It is a common student
misconception that these systems are only used when the cells are lacking oxygen. This is false.
It is true that if a cell lacks oxygen it will have to rely on anaerobic energy systems to produce
ATP. However, most of the cells in our body typically are able to maintain oxygen concentrations
high enough for normal mitochondrial function; even during high intensity exercise. If the cells
have sufficient oxygen for aerobic metabolism, why are anaerobic energy systems called upon to
create ATP during high intensity exercise? The answer is that during high intensity exercise
aerobic energy systems can not produce ATP fast enough to meet the muscle cell’s very high ATP
demand. The anaerobic energy systems, which can produce ATP at high rates, are therefore
necessary to meet the cell's ATP demands. A well known exercise physiologist, George Brooks,
suggests that aerobic glycolysis be referred to as slow glycolysis and anaerobic glycolysis be
referred to as fast glycolysis, in order to avoid this confusion.
ATP-PC
Rate of ATP resynthesis during
maximal exercise
Anaerobic Glycolysis
Oxidative Metabolism
0
10sec
30 sec 60sec 90 sec 180 sec
Duration
Figure 1. Relative role of the different energy systems in
making ATP during maximal exercise of different durations.
Lab IV - 4
Hours
Anaerobic (fast) glycolysis is much faster than aerobic glycolysis partly owing to the fact
that it is a simpler process (it requires fewer steps to produce ATP) and aerobic glycolysis is
slower partly because the production of ATP from acetyl CoA requires the citric acid cycle and the
electron transport chain. It should, however, be noted that while anaerobic production of ATP
during glycolysis is much faster, the total amount of ATP produced from each molecule of glucose
is far greater during aerobic than during anaerobic glycolysis. It should also be noted that while
the anaerobic energy systems can produce ATP fairly quickly, they can not maintain these high
rates of ATP production for very long; sustained ATP production by the muscle cells requires
aerobic ATP production.
Before we discuss aerobic metabolism in greater detail we should recap anaerobic energy
systems. The two anaerobic energy systems are the ATP-PC system and anaerobic glycolysis.
The ATP-PC system is the fastest mechanism our cells have for producing ATP and is especially
important for very high intensity, short duration exercise. Anaerobic glycolysis does not produce
ATP as fast as the ATP-PC system, but is substantially faster than aerobic energy systems. As you
can see in figure 1, the ATP-PC system is extremely important for exercise lasting less than 10
seconds and anaerobic glycolysis is extremely important during high intensity exercises lasting
30-90 seconds. Students should realize, however, that exercise lasting longer than a few seconds
requires a significant amount of aerobic ATP production (see figure 1 and appendix page 77). For
example, when a physiologist refers to "anaerobic exercise" they mean that most of the energy
needed to perform the exercise comes from anaerobic sources; it does not mean that all of the
energy comes from anaerobic energy sources.
Glycogen
& Glucose
Glycolysis
NADH + H+
Pyruvate
NADH + H+
& FADH2
NADH + H+ & FADH2
Fatty acids
Beta Oxidation
Acetyl Co A
Citric Acid
Cycle
NADH + H+ & FADH2
from all of these processes,
along with oxygen, are used
to form ATP via the
Electron Transport Chain
Mitochondria
Figure 3. Highly simplified figure depicting the aerobic
production of ATP from fats and carbohydrates.
Whereas fats can not be used anaerobically to produce ATP, aerobic energy systems can
use both fats and carbohydrates as substrates to produce ATP. The mitochondria are where the
important aerobic metabolic processes(-oxidation, the citric acid cycle, and the electron transport
chain) take place. In general fat metabolism is increased by: exercise training, increasing the
duration of the exercise, decreasing the intensity of the exercise, or eating a diet rich in fats. On
the other hand, high intensity exercise, inactivity, carbohydrate ingestion, and heat stress tend to
increase reliance on carbohydrates.
Fatty acids are broken down to form acetyl CoA by a process called -oxidation (betaoxidation). The acetyl groups from Acetyl CoA, formed either from glycolysis or from oxidation, can then enter the citric acid cycle (also called the tricarboxylic acid cycle or the Krebs
cycle). The citric acid cycle, glycolysis, and -oxidation all result in the formation of reducing
Lab IV - 5
equivalents (either NADH + H+ or FADH2, or both). The electron transport chain uses the
electrons and hydrogen ions from these reducing equivalents to create a large concentration
gradient for H+ across the inner mitochondrial membrane. The energy created by this large
concentration gradient can then be harnessed to drive the formation of ATP. Oxygen serves as the
final electron acceptor in the electron transport chain such that oxygen, the electrons, and the
hydrogen ions ultimately form water (H2O) in the process of making ATP. This aerobic
production of ATP in the mitochondria is referred to as oxidative phosphorylation. It should also
be noted that one ATP is also indirectly produced during each round of the citric acid cycle. It is
beyond the scope of this class to cover the details of glycolysis, beta oxidation, the citric acid
cycle, and the electron transport chain. However, students are encouraged to review these
processes, which were covered in their introductory biology, human physiology, and exercise
physiology lecture courses.
Is it a disadvantage that the aerobic energy systems, which can produce ATP for prolonged
periods of time can not produce ATP at high rates, or that anaerobic energy systems can produce
ATP very fast but not for very long? No, because there is no single energy system that could allow
for both very high rates of ATP production and long-term sustainable ATP production. It is the
presence of these multiple energy systems that allows humans to perform exercise at such varying
intensities and durations. The contrasting properties of our energy systems allows us to exercise at
very high intensities when necessary and maintain moderately high rates of energy expenditure for
prolonged periods of time when necessary. These energy systems and metabolic pathways have
evolved over a long time (unicellular eukaryote fossils have been traced back to 1.8 billion years
ago!). In our more recent evolutionary past (10,000 years ago), it would have been necessary for
our hunter-gatherer ancestors to be able to move both very quickly (to evade predators or capture
prey) and be able to travel long distances (for gathering food stuffs). Thus, it is no accident that
humans have retained energy systems for both quick and for prolonged physical activity.
In summary, while the anaerobic energy systems can produce ATP much faster than the
aerobic systems, they can not produce ATP at these very high rates for very long. Thus, when
high power outputs are required, the anaerobic systems are absolutely necessary in order to meet
the cell's ATP demands (although aerobic energy systems contribute significantly to any exercise
lasting more than a few seconds). On the other hand, while aerobic energy systems can not
produce ATP as fast, they can produce ATP at moderately high rates for long periods of time and
can use both fats and carbohydrates.
Intracellular Regulation of Energy Metabolism During Exercise
The cell's metabolic pathways are regulated by multiple complex mechanisms, including
both intracellular and endocrine regulation. Dozens of enzymes catalyze the important steps
involved in the cell's metabolic pathways. The most important sites of energy system regulation
are at the rate limiting steps of these energy systems. A rate limiting step is the slowest step in a
series of reactions. For example, consider a series of reaction in which A is converted to D in a
series of steps:
ABCD
If the rate limiting step in the following series of reactions is the formation of C from B, then
speeding up, or slowing down, the transition of A to B or C to D will have no effect on the overall
process. The only reaction that has the potential to speed up or slow down the conversion of A to
D is the rate limiting step (B to C). The enzymes that catalyze the rate limiting steps of metabolic
pathways are therefore referred to as rate limiting enzymes. For example, the rate limiting enzyme
for glycolysis is phosphofructokinase (PFK). If PFK is stimulated then glycolysis proceeds more
quickly, if PFK is inhibited then glycolysis proceeds more slowly.
Fortunately for students, the intracellular factors that help to regulate cellular metabolic
pathways generally make sense. For example, an increase in ATP tends to inhibit the rate limiting
enzymes of glycolysis (PFK), glycogenolysis (phosphorylase), and the citric acid cycle (citrate
Lab IV - 6
synthase and isocitrate dehydrogenase). On the other hand, an increase in ADP or AMP tends to
stimulate glycolysis, glycogenolysis, the citric acid cycle, as well as the electron transport chain.
This makes sense; if there is a sufficient quantity of ATP available to meet the cell's needs, why
make more? On the other hand, an increase in ADP or AMP suggests that the cell's energy
demands are not being met, and therefore the processes that make ATP need to speed up. Thus,
the ATP/ADP ratio is a very potent regulator of the cell's energy producing processes.
Calcium and hydrogen ions also play important roles in intracellular regulation of the
metabolic pathways. The increase in cytoplasmic calcium during muscle contraction is also an
important stimulus for some of these metabolic pathways. For example, calcium stimulates both
glycogenolysis (by stimulating phosphorylase) and the citric acid cycle (by stimulating isocitrate
dehydrogenase). A drop in pH (an increase in H+) inhibits PFK activity and thus slows down
glycolysis. This means that during high intensity exercise, if lactate and H+ start to accumulate,
glycolysis will likely slow down.
The metabolic pathways also interact with each other. For example, when a lot of fatty
acids are being broken down (e.g. during low intensity exercise), it results in large quantities of
acetyl CoA, which speeds up the citric acid cycle, and increases cellular concentrations of citrate
(the first product produced in the citric acid cycle). High citrate concentrations can slow down
glycolysis and reduce the use of carbohydrates. On the other hand, when glycolysis is proceeding
very quickly (e.g. during high intensity exercise) H+ concentrations in the body increase and
inhibit lipolysis (by inhibiting hormone sensitive lipase), thus reducing fat utilization as a fuel
source. These types of interactions are partly responsible for the fact that at low intensities we
tend to rely more on fats as a fuel source and less on carbohydrates as a fuel source.
Endocrine Regulation of Energy Metabolism During Exercise
The endocrine system is also extremely important in regulating our metabolic pathways
(see table on appendix page 78). Insulin, which is secreted from the -cells of the pancreas, 1)
stimulates uptake of fuel sources (glucose, fatty acids, amino acids), 2) promotes storage of these
fuel sources (glycogenesis, lipogenesis, and protein synthesis), and 3) tends to reduce the
breakdown of glycogen and proteins. Glucagon (secreted by -cells of the pancreas), epinephrine
and cortisol (secreted by the adrenal gland), and growth hormone (GH, secreted by the pituitary
gland) have slightly different actions, but tend to promote mobilization of glucose and fatty acids
(by stimulating glycogenolysis and lipolysis); increasing the delivery of these important fuel
sources to the active skeletal muscle.
During exercise insulin secretion decreases and the secretion of glucagon, epinephrine,
cortisol, and growth hormone all increase. The combination of these changes in hormone
concentrations results in 1) an increased breakdown of fuel stores (e.g. glycogen and triglycerides)
and 2) a mobilization of fuel substrates (e.g. glucose and fatty acids) that can be used by the active
tissues.
When comparing trained and untrained individuals, trained individuals will usually have
less of a decrease in insulin and less of an increase in glucagon, epinephrine, cortisol, and GH at
any given exercise intensity. As a result, glycogen will not be broken down as quickly in trained
individuals and they will tend to rely more on fats than untrained individuals. High exercise
intensities result in a large increase in circulating epinephrine and a large decrease in insulin. This
results in a large increase in glycogen breakdown, and therefore increases carbohydrate
metabolism and decreases fat metabolism. As you can see, changes in circulating hormone
concentrations are partly responsible for the changes in fat and carbohydrate metabolism that
occur with exercise training or with varying exercise intensities.
Thermoregulation and Exercise
The human body and the physiological processes within it function best when the
temperature is maintained at around 37C. A significant increase or decrease in body temperature
can be fatal. The hypothalamus is responsible for regulation of body temperature. Heat is
Lab IV - 7
generated by the metabolic processes in the body, and is dissipated at the skin. At rest, most of the
body's heat is given off by radiation, whereas during exercise most of the body's heat is given off
by evaporation.
The body's core temperature is usually around 37C, but it is not uncommon for this to
increase somewhat during exercise. In order to dissipate heat from the core to the skin, the skin
temperature must be lower than the core temperature. That is, there must be a thermal gradient
between the core and the skin in order to transfer heat from the core to the skin. The body's
thermal gradient can be calculated as core temperature minus skin temperature. During exercise in
a normal environment (not too hot or too humid) the skin temperature decreases, which increases
the thermal gradient and prevents core temperature from increasing too much. However, if the
subject is exercising in a hot humid environment (or wearing too much clothing), evaporative
cooling of the skin will not be as effective, and the body will have a hard time dissipating heat. If
the body can not dissipate heat, skin temperature increases, decreasing the thermal gradient and
reducing the transfer of heat from the core to the skin. As a result, core temperature will now start
to increase more rapidly; increasing the danger of developing heat-related illnesses.
It also should be noted that in a hot, humid environment, more blood must be sent to the
skin in order to transfer the heat from the core to the skin; thus heart rate and cardiac output will
need to be higher at any given exercise intensity in a hot humid environment. Heat also effects
energy metabolism. Heat stress tends to increase the use of carbohydrates and by accelerating
glycogen breakdown and speeding up anaerobic glycolysis.
LABORATORY OBJECTIVES:
1. To help students better understand how exercise training, intensity, duration, thermal stress, and
carbohydrate ingestion influence exercise metabolism:
2. To help students understand how exercise training, intensity, duration, thermal stress, and carbohydrate
ingestion affect the relative contributions of the major metabolic pathways to exercise energy
expenditure.
3. To help students understand how exercise training, intensity, duration, thermal stress, and carbohydrate
ingestion influence the active muscles' use of fats and carbohydrates as fuel substrates during exercise.
4. To help students understand the body's thermoregulatory systems and the physiological consequences of
thermal stress.
5. To be able to calculate the amount of energy coming from fats and carbohydrates during different
exercise bouts
LABORATORY PROCEDURES & CALCULATIONS
Assess VO2, RER, blood glucose, and blood lactate during the following.
Using the RER, determine what percent of kcal are coming from fat and what percent are
coming from carbohydrates. Calculate the total number of kcal coming from fat and from
carbohydrates.
Each group will be assigned one or more of the following experiments:
Experiment 1. Effects of Intensity
Same subject: Two 10min bouts of exercise at~50% and ~75% of max. Take measurements during
the last minute of exercise.
Experiment 2. Effects of Duration
Same subject: One continuous exercise bout at ~60-70% VO2max. Take measurements at ~5
minutes into the exercise bout and after 30-35minutes of exercise.
Experiment 3. Effects of Training:
Use two subjects of the same gender and who are approximately the same size. Use one subject
who does not run (or cycle) and one subject who does run (or cycle). Both subjects will exercise at
the same absolute exercise intensity (recommend ~90-120 Watts if subjects are female or ~120-150
Watts if male)
Lab IV - 8
Experiment 4. Effects of Thermal Stress:
Same subject: Two 10 min bouts of exercise at ~75% of max. The first bout should be performed
while wearing shorts and a T-shirt. The subject should put on multiple layers of clothing for the
next exercise bout. Take measurements during the last minute of exercise. Additional data to be
collected include heart rate, skin temperature, core temperature, and thermal gradient.
Experiment 5. Effects of Carbohydrate ingestion:
Same subject: Use a subject who has not eaten for at least 3-4 hours, preferably longer. Two 10
minute bouts of exercise, performed at a moderate intensity (approximately 60-70% of maximum)
before and then one hour after ingesting a drink containing carbohydrate.
1. Collect your subject's expired air during rest for 5 minutes and collect the necessary data to
calculate resting oxygen consumption in L/min.
2. Collect expired air during the final minute of exercise (except for the first collection in
experiment 2, which should be taken at 5 minutes) to insure steady state exercise. Following
analysis of collected air (as above) a second exercise bout will be performed. Your instructor
will assign you an ergometer and intensity.
3. Calculate resting VO2 and exercise VO2 as you have done in previous labs. Then calculate the
Net VO2 for each of the exercise bouts as you did in the previous lab.
VO2 = VEstpd (NF x 0.2093 - FEO2)
where NF = (1-FEO2+ FECO2)/.7904
Net VO2 (L/min) = Gross VO2 - Rest VO2 (both in L/min)
4. Calculate the RER during the exercise bouts, and determine the caloric equivalent from table
in the Appendix. Calculate energy expended as follows:
RER = VCO2 / Gross VO2
*look at RER table for Kcal/L
Net VO2 (L/min) x caloric equivalent (Kcal/L) = Energy expended/min (Kcal/min)
Energy expended (kcal/min) x minutes of exercise = Energy expended (kcal)
5. Using your RER table to determine what percent of Kcal came from fats and from
carbohydrates. Determine the total Kcal from fat and carbohydrates as follows (remember to
convert the percent values to decimals for your calculations). You do not need to perform these
two calculations for experiment # 2 (effects of duration).
Total kcal from fat = Energy expended (kcal) x % of kcal from fat
Total kcal from carbohydrate = Energy expended (kcal) x % of kcal from
carbohydrate
6. Alert your instructor at the end of each exercise for blood lactate & glucose determination.
7. For the thermal stress experiments, experiment #4, also collect heart rate, skin temperature, core
temperature, and thermal gradient information. Also collect environmental temperature and
relative humidity information using the sling psychrometer. Thermal gradient can be
calculated as follows:
Thermal gradient = core temp – skin temp
Lab IV - 9
Experiment # _______
Subject Weight(s)
__
Water Vapor Pressure
Kg Ambient Temp.
mmHg STPD Correction Factor _______________
Rest
a.
FEO2
b.
FECO2
c.
Sample Volume (L)
d.
Meter Volume (L)
e.
f.
ATPS Volume (L)
VE min. Vol. ATPS
(L/min)
g.
VE STPD (L/min)
h.
NF
i.
Gross VO2 (L/min)
j.
VCO2 (L/min)
k.
l.
RER
Caloric equivalent
(Kcal/L) (from RER chart)
m.
Net VO2 (L/min)
n.
= Gross VO2 - rest VO2
Energy Expended/min
(kcal/min)
= Net VO2 x Caloric
Equiv.
o.
p.
q.
°C Ambient Pressure
Energy Expended (kcal)
% of Kcal from fat
(from RER chart)
% of Kcal from carbs
(from RER chart)
Total Kcal from fat
= E. Exp x % from fat
Total Kcal from carbs
= E. Exp x % from carbs
Blood Lactate
Blood Glucose
r
Lab IV - 10
Exercise 1
Exercise 2
mmHg
Experiment # _______
Subject Weight(s)
__
Water Vapor Pressure
Kg Ambient Temp.
mmHg STPD Correction Factor _______________
Rest
a.
FEO2
b.
FECO2
c.
Sample Volume (L)
d.
Meter Volume (L)
e.
f.
ATPS Volume (L)
VE min. Vol. ATPS
(L/min)
g.
VE STPD (L/min)
h.
NF
i.
Gross VO2 (L/min)
j.
VCO2 (L/min)
k.
l.
RER
Caloric equivalent
(Kcal/L) (from RER chart)
m.
Net VO2 (L/min)
n.
= Gross VO2 - rest VO2
Energy Expended/min
(kcal/min)
= Net VO2 x Caloric
Equiv.
o.
p.
q.
°C Ambient Pressure
Energy Expended (kcal)
% of Kcal from fat
(from RER chart)
% of Kcal from carbs
(from RER chart)
Total Kcal from fat
= E. Exp x % from fat
Total Kcal from carbs
= E. Exp x % from carbs
Blood Lactate
Blood Glucose
r
Lab IV - 11
Exercise 1
Exercise 2
mmHg
Thermal stress – other data
1. determine % humidity using sling psychrometer
wet bulb temp:
_______
dry bulb temp:
_______
% humidity:
_______
2. Record heart rate, skin temperature, core temperature, and thermal gradient during each minute
of exercise:
Time
(min)
1
2
3
4
5
6
7
8
9
10
Heart
rate
Normal
Core Skin
temp temp
Thermal
gradient
Heart
rate
Thermal Stress
Core
Skin
temp
temp
Thermal
gradient
Class data
% Kcal
RER
fats
Experiment #1 - intensity
%Kcal
carbs
Energy
expended
~ 50%
~75%
Experiment #2 - duration
at 5 minutes
at ~30
minutes
Experiment #3 - training
trained
untrained
Experiment #4 - Thermal stress
normal
Heat stress
Experiment #5 - Carbohydrate ingestion
fasted
after
ingestion
Lab IV - 12
kcal from
fats
kcal from
carbs
Blood
glucose
Blood
lactate
In lab questions:
1. How (and why) were RER and the percent of kcal from fat and carbohydrate affected by each of
the following?
Exercise intensity
Exercise duration
Exercise training
Thermal stress
Carbohydrate ingestion
2. How (and why) were the total kcal from fat and carbohydrate affected by each of the following?
Exercise intensity
Exercise training
Thermal stress
Carbohydrate ingestion
3. How (and why) were blood glucose affected by each of the following?
Exercise intensity
Exercise duration
Exercise training
Thermal stress
Carbohydrate ingestion
Lab IV - 13
Study Questions
1. What happens to skin temperature, core temperature, and thermal gradient during normal exercise? What
happened to these variables (how did they change) during exercise in a hot humid environment (such as
when wearing lots of clothes)? Explain why these variables follow these trends.
2. Did heart rate stay that same between normal and thermal stress conditions? Explain any observed
differences.
3. Fill in the following table regarding the major hormones that influence exercise metabolism.
Hormone
secreted from
major metabolic
actions
effect of
acute exercise
effect of
chronic exercise
4. How do the hormonal changes that occur with acute exercise (from table above) influence exercise
metabolism?
5. How do the hormonal changes that occur with chronic exercise (from table above) influence exercise
metabolism?
6. If a person wanted to lower their percent body fat, would you recommend a high or a low intensity, or
does intensity matter?
Lab IV - 14