METABOLISM
AND
BODY COMPOSITION
Energy Metabolism
Energy metabolism involves all the physiological processes required to take in air and food molecules, and then extract energy from the food molecules that can be stored and used by cells of the body. The energy is used to power all the physical and chemical processes performed by the cells and the body as a whole.
Basically, enzymes in the cells break the chemical bonds between the atoms in the food molecules (carbon and hydrogen mostly) and combine them with oxygen (O2) in the process of extracting energy. The food atoms end up as carbon dioxide (CO2) and water (H2O). There is a set amount of energy taken from each broken bond, most of which is lost as heat (60%), and some (40%) is stored in a phosphate bond of the molecule Adenosine Tri-phosphate (ATP). The energy unit most often used in science is the kilocalorie (kcal), the amount of heat energy that would increase the temperature of 1000 g of water one degree centigrade. The kcal is the same as the value for Calories given on food package labels. So, when a particular type of food molecule is metabolized there is a specific amount of energy extracted from it. The three types of food-energy molecules are carbohydrates (CHO), proteins (PRO), and lipids (FAT).
glucose = C6H12O6 = carbohydrate
C6H12O6 + 6 O2 ® 6 CO2 + 6 H2O + 38ATP + heat
6CO2/6O2 = 1.0
fatty acid = C16H32O2 = fat
C16H32O2 + 23 O2 ® 16 CO2 + 6 H2O + 129 ATP + heat
16CO2/23O2 = 0.70
Proteins and lipids are metabolized in a similar way, but since they contain different proportions of C, H, and O, they use different amounts of O2 and produce different amounts of CO2, H2O, and energy. The volume of CO2 produced divided by the volume of O2 consumed each minute is called the Respiratory Quotient (RQ= VCO2/VO2, also called Respiratory Exchange Ration = RER), which indicates the type of food molecule being metabolized (or combination). The type and amount of each food molecule metabolized determines the exact amount of energy that is produced.
Food Molecule / RQ / kcal/l O2 / g/ l O2 / kcal/gCHO / 1.00 / 5.1 / 1.23 / 4.15
PRO / 0.82 / 4.5 / 1.05 / 4.30
FAT / 0.71 / 4.7 / 0.50 / 9.40
For the metabolism of a common “mixed diet” (a combination of carbohydrate, protein, and fat) the RQ is given as 0.82, and the energy per volume of O2 is given as 4.8kcal/lO2. These values are assumed when only the volume of O2 consumed is measured (like done with the Collins Respirometer). Unfortunately this does not reveal how much energy is actually being produced or what food molecules are actually being metabolized. But, it is close enough for an estimate.
The unit of energy, the kcal, stands for kilocalorie, which is the amount of heat energy (calories) that will increase the temperature of 1000 grams (kilo = 1000)(1000g = 1.000 kg) of water, 1.0°Centigrade. The kcal is the scientific unit that is the same as the Calorie (big C) on the nutrition label of food packages (1.0 kcal = 1.0 C).
Metabolic Analyzer
In professional circumstances it is important to measure the actual amounts and proportions of O2 and CO2 , so that the amounts and proportions of food molecules metabolized can be determined, and then the amount of energy produced can be calculated. An example is seen during critical care in a hospital, where knowing the amount of energy and type of food molecules to give an unconscious patient intravenously is essential for health. The exact amount of energy measured for Basal Metabolic Rate (BMR) is important because it is the largest percent of energy used every day (55-75%). The most important index of cardiovascular, aerobic fitness is the measurement of the maximum amount of energy that can be used by a unit of body weight, per minute (VO2max, kcal/kg min). The other major index of fitness is the level of exercise intensity at which fat can be metabolized for energy (related to the “lactate threshold”).
The metabolic analyzer measures the concentration (%) of O2 and CO2 in the exhaled air and the flow rate (l/min) of the air. The change in the concentration (%) of O2 or CO2 as a volume of air goes through a person and then flows out of the lungs shows the amount of O2 used or CO2 produced by metabolism. Room air that is inhaled is about 21% (0.2094) oxygen (O2) and .03% (0.0003) CO2, and exhaled air contains less oxygen (~17%,0.1686) and more carbon dioxide (~5%,0.05). The flow of air in and out of a person at rest may be about 6 l/min. All the correction and conversion factors are applied to the gas volumes when the data is integrated by the computer. The following example is for O2 , and the same calculation is used for CO2 also.
O2 consumed = (% change)(volume of air/min)
VO2 = (0.2094 - 0.1686)(6 l/min)
= (0.0408)(6 l/min)
= 0.2450 l O2 /min
When both VO2 and VCO2 have been determined, then the RQ (VCO2 /VO2) can be calculated. This indicates the proportion of CHO, PRT, and FAT that is being metabolized to produce energy. However, since there are three different food molecules and only information about two gasses the calculation cannot be done. Fortunately there is another way to determine how much protein (PRT) has been metabolized, so the associated amount of gas can be subtracted from the total, and the rest of the gas was used for CHO and FAT metabolism. The VCO2 /VO2 related to CHO and FAT metabolism is called a non-protein RQ, and can be used to figure out how much VO2 and VCO2 was used for CHO and FAT separately. From the table above the amount of O2 and CO2, and energy related to metabolism of each food molecule is known, and so the total amount of energy produced can be calculated. Fortunately, again, the metabolic analyzer does all these calculations and prints out a summary page with all the values (see attached copy of an Energy Report in Appendix A).
Standard, Temperature, and Pressure, Dry (STPD)
For a more complete discussion of STPD refer to APPENDIX C
In order to determine the amount of energy produced by metabolizing nutrient molecules it is necessary to know how many molecules of oxygen have been used. The number of oxygen molecules used indicates how many hydrogen atom bonds have been broken on the nutrient molecule. This is a known proportion, since two H atoms reform bonds with each oxygen atom you breath in and use for metabolism, to become H2O. Each H bond broken yields a specific amount of energy. The metabolic measurements quantify the volume (liters) of oxygen consumed per minute. There is only one condition in which the number of molecules of oxygen in a liter of oxygen is known, this is called Standard, Temperature, and Pressure, Dry (STPD). The reason is that each of the different components of STPD affect the volume of a set number of atoms or molecules of a gas, like oxygen or carbon dioxide.
The conditions in the room where the volume of oxygen and carbon dioxide are measured are often 25°C, 760 mmHg, and 65%RH. STPD conditions are 0°C, 760 mmHg, and 0%RH. So, generally under STPD conditions the temperature is lower, the %RH is lower, and the pressure is about the same. Therefore, under STPD conditions the volume of a gas would be smaller compared to room conditions. Since there are set physical relationships between a gas volume and changes in temperature, pressure, and %RH, a correction factor number can be used to calculate (correct) the volume measured at room conditions (uncorrected), to what the volume would be at STPD conditions (corrected). The correction factor number value for the stated conditions (first sentence of this paragraph) would be 0.9. So, the change in volume of oxygen measured at room conditions (Vuncorrected = VO2un) would be multiplied by 0.9, to calculate what its correct volume would be at STPD (Vcorrected = VO2c).
(VO2un)(0.9) = VO2c
(using our algebra, divide both sides by 0.9, VO2un can also be calculated from VO2c)
(VO2un) = VO2c
(0.9)
Energy Calculations
As mentioned previously, the energy related to the consumption of a one-liter volume of oxygen is 4.8 kcal (= 4.8 kcal/1.0 l O2), for a person metabolizing a set “mixed diet” of specific nutrients (carbohydrate, protein, fat). The volume of oxygen consumed (VO2un) is usually measured in ml O2/min. The usual way to denote the use of energy is in kcal/day. So, a few “unit” conversions are needed to calculate kcal/day from VO2un.
First, VO2un must be corrected to VO2c, by multiplying by 0.9 (ml O2c/ml O2un). Second, since the energy factor (4.8 kcal/1.0 l O2), has units of liters (l), and the VO2c has units of milliliters (ml O2/min), it is necessary to convert ml to l (only the same unit values can be used in one calculation). The conversion factor is 1.0 l/1000 ml. Next, since VO2c also has units of minutes (ml O2/min), and the final energy value has units of days (kcal/day), so minutes must be converted to hours (60 min/hr), and then hours converted to days (24 hrs/d). All the values and conversion factors can be lined up together in one equation that clearly shows how they are related, and how the units cancel-out, to produce only the proper value and units of the answer. Units that are both on the top of the equation (numerator), and bottom of the equation (denominator) cancel each other, as indicated by the strikethrough mark (ml/ml). Only the un-cancelled units are in the answer.
(ml O2un) (0.9 ml O2c) (1.0 l) (4.8 kcal) (60 min) (24 hr) = kcal
(1.0 min) (1.0ml O2un) (1000 ml) (1.0 l O2c) (1.0 hr) (1.0 day) day
Since all the conversion factors have constant values, all the numbers on top (numerator) are multiplied together, and all the numbers on the bottom (denominator) are multiplied together, and then denominator is divided into the numerator to produce the overall value for the conversion factors.
(VO2un) (0.9) (1.0) (4.8) (60) (24) = (VO2un) (6221) = (VO2un)(6.221)
(1.0) (1.0) (1000) (1.0) (1.0) (1.0) (1000)
(VO2un) (6.221) = kcal
day
(ml O2) (6.221 kcal··min) = kcal
(min) (ml O2· day) day
These are the units of the value 6.221, which are used whenever the 6.221 conversion factor is applied in a calculation.
For example, if someone was measured to consume 272 ml O2/min (VO2un), then their energy expenditure for a whole day, for whatever activity they were doing during the measurement, could be calculated by multiplying by 6.221.
(VO2un)(6.221) = (272 ml O2)(6.221) = 1692 kcal
min day
Standard Energy Values for Basal Metabolic Rate (BMR)
Thousands of people have had their BMR measured under basal conditions (defined below), and two estimates of the BMR based on these measurements are commonly used.
The Metabolic Equivalent (MET)
The MET is the average value 3.5 ml O2/kg min (=VO2c), for a resting (~basal) metabolic rate, used regardless of gender, or body size. This values means that 3.5 ml O2 is use by each 1.0 kg (kilogram) of body mass, each 1.0 minute. The MET value can be used to compare to your measured value of resting metabolic rate (+ 2 ml O2/kg min) to get a rough idea if your value is close to a reasonable value.
MET = 3.5 ml O2
kg min
As an example, for a 70 kg person the value for the MET would be:
MET = (3.5 ml O2)(70 kg) = 245 ml O2 = VO2c
(min)(kg) min
VO2un = (VO2c) = 245 ml O2 = 272 ml O2
(0.9) (min) (0.9) min
The amount of energy this 70 kg person would expend, if resting for 24 hours, would be: (remember the units for 6.221)
(VO2un)(6.221) = (272 ml O2)(6.221) = 1692 kcal
(min) day
The Kleiber Equation for BMR Energy Expenditure
Thousands of people and other mammals, over a large range of body weights, have also had their BMR measured under basal conditions (shrews [0.01 kg] to whales [20,000 kg]). Almost all the different species and sizes have a BMR (kcal/day) within +15% of the value calculated by the “Kleiber equation” (70 x body mass to the 0.75 exponent). To perform this calculation, first convert your body mass (M) from pounds (lbs) to kg, by dividing lbs by 2.2 lbs/1.0 kg. Calculate BMR by raising M to the 0.75 exponent (need to use a calculator with this type of function), and then multiplying by 70, to obtain a value for kcal/day. This provides a better value for BMR, since it is based on the actual body mass of each person.
M = kg = ( lbs)(1.0 kg)
(2.2 lbs)
BMR = (70)(M 0.75) = kcal/day
As an example, for a person weighing 154 lbs, the BMR can be calculated as:
M = kg = (154 lbs) (1.0 kg) = 70 kg
(2.2 lbs)
BMR = (70)(70 0.75) = (70)(24.2) = 1692 kcal/day
Basal Metabolic Rate (BMR)
There is a special significance to BMR. BMR is, as the name implies, a basal level of metabolic rate, related to the lowest, awake, conditional level of metabolism. This is the energy expended by your body to just maintain itself (breathing, heart beating, and cellular operations like replacing old proteins and pumping sodium and potassium ion across the membrane). To a large extent the BMR is set by your genetics.
There are three major points of the significance of BMR. First, BMR is the largest part of your whole energy output, for the whole day (~55%-75%). Second, BMR can change by as much as 30%, in response to energy imbalances (dieting, starvation, cold environment). And third, BMR would be the minimum amount of food energy to take in to just maintain the body, without any other activities.
The normal range for BMR in response to the range of energy imbalance from gluttony (great excess of food energy intake) to starvation (zero food energy intake) is within +15% of the BMR at energy balance (food energy = output energy). So, the full range would be 2 times +15%, which equals a maximum change of as much as 30%.