Oxygen Consumption

As the body works to generate increasing amounts of power, more energy in the form of adenosine
triphosphate (ATP) is required to fuel the increasing muscle activity. The body has several energy
systems to generate the ATP. These systems fall into two broad categories; those that require the
use of oxygen (aerobic) and those that do not (anaerobic). The aerobic energy systems generate
ATP in the muscles mitochondria at the end of the electron transport chain where the oxygen binds
with hydrogen ions and electrons to form H2O and ATP is generated in the process. Thus when we
measure oxygen consumption we are measuring the amount of oxygen used in these aerobic
metabolic processes. These measured absolute values of oxygen are typically expressed in liters per
minute (L/min). At rest, most humans consume roughly 3.5ml of oxygen per minute for every kg of
body mass (3.5ml/kg/min). This resting metabolic rate of 3.5ml/kg/min is relative oxygen
consumption, since we express the oxygen being consumed relative to the individual body mass, and
it is referred to as a MET (1 MET = 3.5ml/kg/min). Larger people tend to have larger absolute VO2
values at rest and during exercise (large parts require more oxygen), but we account for these
differences due to body size by expressing the absolute volume of oxygen consumption (L/min)
relative to their own body mass (ml/kg/min).
In addition to providing insights into a subject’s aerobic fitness through VO2max assessment, oxygen
consumption data can be used to assess energy expenditure. Accurately measuring energy
expenditure involves measuring the heat energy produced by the body and is termed Direct
Calorimetry. This process, although accurate at measuring energy expenditure, is expensive and not
logistically practical in most situations. Measuring gas exchange of a subject on the other hand is a
more feasible proposition. Measuring gas exchange allows you to estimate energy expenditure
through a process referred to as Indirect Calorimetry.
Indirect calorimetry relies on the knowledge that for every liter of oxygen consumed, 4.67 to 5.05 kcal
of energy will be liberated. The range of values relates to the specific fuel substrates being consumed
by the body. If you recall from lab 9, the metabolism of 1molecule of Palmitic Acid requires 23
molecules of oxygen, while the metabolism of one molecule of Glucose requires only 6 molecules of
oxygen.
1 molecule of a Fatty Acid (Palmitic Acid) C16H32O2 + 23O2 → 16CO2 + H2O + 129ATP
RER = 16CO2 / 23O2 = .70
1 molecule of Carbohydrate (Glucose) C6H12O6 + 6O2 → 6CO2 + 6H2O + 33ATP
RER = 6CO2 / 6O2 = 1.0
The advantage of course with fats is that they release more energy when they are consumed versus
carbohydrates (129 vs. 33 ATP/molecule), the disadvantage being they require more oxygen and the
metabolic process is much slower. Since oxygen supply is not finite during exercise, at intensities
above approximately 65% VO2max, the body begins to rely predominately on carbohydrate to fuel
muscle contractions. Table 1 shows the differences in Energy per Liter of Oxygen consumed.
Table 1. Oxygen Consumption and Energy Derived from Different Fuel Mixtures
Carbohydrate Fats RER Energy per L O2
(kcal)
0 100 .71 4.67
16 84 .75 4.74
33 67 .80 4.80
51 49 .85 4.86
68 32 .90 4.92
84 16 .95 4.99
100 0 1.0 5.05
During higher intensity exercise, the body relies predominantly on carbohydrate as the energy
substrate to fuel skeletal muscle contractions, therefore the RER is closer to 1.0 and the Energy per
Liter of Oxygen consumed approaches 5.0. To simplify basic metabolic calculations during exercise,
the value of 5kcal/LO2 is used with an acceptable amount of error. If one knows the amount of oxygen
being consumed per minute (absolute in L/min), they can determine the energy expenditure based on
the previously stated relationship. In fact, one can make many useful calculations and conversion if
they know the common conversion factors for the metabolic relationships between energy, oxygen
consumption (absolute), relative oxygen consumption, and metabolic equivalents (METS).
For example; a 75kg person exercising at 5 METS burns how many Kcal per minute?
If you remember that 1 MET is the resting metabolic rate of an average human, and 1 MET is equal to
approximately 3.5ml/kg/min. So the first step is to express 5 METS as relative oxygen consumption
by multiplying 5 METs by 3.5, since for every 1 MET you consume 3.5 ml of oxygen for every kg of
body mass. Doing this yields 17.5ml/kg/min, and represents the relative oxygen consumption of this
person at this workload.
5 METS x 3.5ml/kg/min/MET = 17.5ml/kg/min (Relative)
Next we need to express this relative oxygen consumption in absolute terms. We do this by
multiplying our relative oxygen consumption by the body mass of the subject, 75kg in this case.
17.5ml/kg/min multiplied by 75kg equals an absolute oxygen consumption of 1315.5 ml/min of oxygen
being consumed.
17.5ml/kg/min x 75kg mass = 1315.5ml/min (Absolute)
The next step is to convert this measure to how we typically express absolute oxygen consumption,
and that is in Liters per minute. We do this simply be dividing by 1000 since there are 1000ml in one
L. This results in an absolute measure of 1.3155 L of oxygen consumed per minute.
1315.5ml/min 1000ml/L = 1.3155L/min (Absolute)
Now we that we know the oxygen consumption; we can determine caloric expenditure per minute
based on the 5kcal per Liter of O2 consumed relationship. So we multiply our 1.3155LO2/min by
5kcal/LO2 and get a caloric expenditure of 6.5775kcal/min.
1.3155L/min x 5kcal/LO2 = 6.577kcal/min
Assuming the workload is maintained, you could determine the total caloric expenditure simply by
multiplying the rate of caloric expenditure per minute (6.577) by the total number of minutes.
Below is a useful tool called the Metabolic Ladder that you can use to become familiar with
converting these measures of metabolism. You can enter the ladder at any point depending on the
information that is given to you. You work your way up or down the ladder depending on where you
are trying to get to, and simply perform the proper conversion at each step along the way. With
practice, the conversion steps will become more automatic to you and you will not need the tool to
perform metabolic calculations. In reference to our previous example, we simply started at the bottom
of the ladder at METs (step 1) and worked our way to the top of the ladder Kcal//min(step 5).
Metabolic Ladder
 5 x 5
x 1000  1000
 Kg x Kg
 3.5 x 3.5
References
Brooks G., Fahey T., & Baldwin K. (2005). Exercise Physiology: Human Bioenergetics and Its
Applications. 4th Ed. New
York, NY: McGraw Hill.
Beam, W.C. & Adams, G.M. (2011). Exercise Physiology Laboratory Manual (6th
ed.). New York: McGraw Hill.
Maud, & Foster (2006). Physiological Assessment of Human Fitness. 2nd
Ed. Champaign, IL: Human Kinetics.
Kcal of Energy per min (Kcal/min)
L of O2 per min (Absolute O2 in L/min)
Ml of O2 per min (Absolute in ml/min)
Ml of O2 per Kg per min (Relative O2 in ml/kg/min)
Metabolic Equivalents (1 MET = 3.5ml/kg/min)

Using the data below to perform the following calculations.
Subject # 1 2 3
Male Male
Elite Pro
Male Cyclist
Body Mass (kg) 100 57 70
Absolute VO2 max (L/min) 3.5 3.6 6.1
Ventilatory Threshold (as % of VO2max) 47 65 88
Watts @ VO2 Max 220 330 450
Watts @ Threshold 85 215 400
Power to Wt. Ratio (Watts/Kg) 0.85 3.5 5.7
At their Ventilatory Threshold

1. Calculate the relative VO2max for each subject and categorize the aerobic fitness of each.
2. Calculate the MET max for each subject.
3. If subject # 1 exercises within the ACSM guidelines of 30min a day, 5 days a week, at say 5 METS
   (moderate intensity), how long will it take him to lose 10 pounds of fat if it takes roughly a 3500kcal
   deficit to lose 1 pound of fat? (Assume he is not dieting also)
4. If subject #1 lost 70 lbs. through diet but no exercise, and hence his absolute VO2max remained
   unaltered, what would his new relative VO2 max be and how would you categorize his aerobic fitness?
5. After the 70 pound weight loss through dieting alone, subject #1 bought a bicycle and trained 4-6
   days/week for 12 months and improved his absolute VO2max by 20%, what would his new relative VO2
   max be?
6. What would his relative VO2max be if he also lost another 10 pounds in the process of training?
7. After all the weight loss how many calories would subject#1 burn in a week using the same exercise
   regimen as in problem # 1?
8. What if after all the weight loss subject # 1 exercises 60 minutes a day at 10 METS (vigorous) 6 days a
   week, how many calories a week will he burn then in a week?
9. How man calories are in the following?
   a. In-n-Out Double Double, fries, and reg. 16oz coke
   b. Venti Mocha Frap with Whip from Starbucks
10. Do the answers to number 8 and 7 combined seem fair?
11. If subject number 2 goes on a 5-hour training ride and maintains a average effort equal to 50% of his
    VO2 max, roughly how many total calories would he use?
12. If during a 5-hour race the Elite Pro maintains an average effort equal to 75% of his VO2max, roughly
    how many total calories would he use?