• to estimate the amount of energy use by the body, it is necessary to know the type of food substrate being oxidize.
• The carbon and oxygen content of glucose, free fatty acids and amino acids differ dramatically
• as a result, the amount of oxygen used during metabolism depends on the type of fuel being oxidize
• remember indirect, calorimeter measures the amount of carbon dioxide released, and oxygen consumed
• the ratio between these two values is termed the respiratory exchange ratio
• the ratio of the volume of carbon dioxide released (Vco2) divided by the volume of oxygen consumed(Vo2) on a total body level
the respiratory exchange ratio
• is the carbon dioxide produced divided by the oxygen consumed
the ratio varies due to difference in the chemical composition in substrate.
In general the amount of oxygen needed to completely oxidize a molecule of, carbohydrates or fat is proportional to the amount of carbon in that fule; for example, glucose (C6H1206) contains six carbon atoms.
During glucose combustion, six molecules of oxygen are used to produce six carbon dioxide molecule. Six H2O molecules, and 30 - 32 ATP molecules
(when we oxidize the glucose what we gonna get is six molecules of carbon dioxide, six waters and 30 to 30 ATPs)
(so really what we're going to see is where the oxygen and the carbon dioxide produced from that oxygen is really1 to 1 for carbohydrates)
• so by evaluated how much CO2 is released compared with the amount of O2 consumed. We find that the RER(respiratory exchange ratio) is 1.0
• RER - VC02/Co2= 6co2/6o2=1.0 (six carbohydrate divided by six oxygen equals one)
• as mentioned, the RER value varies with the type of fuels being used for energy.
• Free fatty acids have considerable more carbon and hydrogen, but they have less oxygen. Than glucose.
• Therefore the combustion of a fat molecule requires significantly more oxygen than combustion of a carbohydrate molecule
• although fat provides more energy than carbohydrates, more oxygen is needed to oxidize that fat than carbohydrate
• this means that the RER value for fat is substantially lower than for carbohydrate
• for palmite acid
• RER = VCO 2/VCO2 = 16co2/23o2=0.70
Once the RER value is determined from the calculated respiratory gas volume. The value can be compared with a table to determine the food mixture being oxidize(table 4.1)
if, for example, the RER value is 1.0 then the cells are using only glucose or glycogen and each liter of oxygen consumed would generate 5.05kcal (kilocalorie per liter)(if you know the RER, you can go to the table and figure out the kcal expenditure )
the oxidation of only fat would yield 4.69kcal L of o2 consumed
Individuals do not often use only one fuel. Thus, the classic values are rarely seen
the RER value at rest is usually 0.78 -0.80
(which is indicating 33 carbohydrates and 67 of fat, but we are using mostly fat, but some carbohydrates. We have to exercise because it's not utilizing enough calories because fat stores a lot of calories)