Start
Part 1
JBJ normally jogs for an hour before breakfast. One morning he measured his oxygen consumption and carbon dioxide production for 10 minutes while he was in bed drinking a cup of unsweetened black coffee, again during his jog, and for 10 minutes quarter of an hour after a breakfast of cereal with milk, a boiled egg, toast and butter.
The main metabolic fuels are fatty acids and glucose.
How much carbon dioxide will be produced, and how much oxygen will be consumed, by the total oxidation to carbon dioxide and water of:
- the fatty acid palmitate (C15H31COOH)
- glucose (C6H12O6)
What is the ratio of carbon dioxide produced / oxygen consumed for each?
for palmitate:
C15H31COOH+
23 O2 =
16 CO2
+ 16 H2O
ratio CO2
/ O2 =
0.7
for glucose:
C6H12O6
+ 6 O2 =
6 CO2 +
6 H2O
ratio CO2
/ O2
= 1.0
How can you use this information to determine the approximate mixture of metabolic fuels being oxidised?
By
measuring the consumption of oxygen and production of carbon dioxide, using
a Douglas bag or similar device to sample inhaled and exhaled air. This ratio
of carbon dioxide produced / oxygen consumed is the respiratory quotient (RQ).
When mainly fatty acids are being metabolised the RQ is near to 0.707; when
mainly glucose is being metabolised the RQ is close to 1.0.
For the present exercise you can assume that only fat and glucose are being metabolised, but normally some amino acids arising from protein are also metabolised as metabolic fuel. The RQ is for protein oxidation is 0.8. You will see in later exercises that it is possible to estimate the amount of protein being oxidised by measuring the urinary excretion of urea, the main product of nitrogen metabolism.
Part 2
JBJ normally jogs for an hour before breakfast. One morning he measured his oxygen consumption and carbon dioxide production for 10 minutes while he was in bed drinking a cup of unsweetened black coffee, again during his jog, and for 10 minutes quarter of an hour after a breakfast of cereal with milk, a boiled egg, toast and butter.
The results he obtained were as follows:
- RQ sitting up in bed = 0.8
- average RQ while jogging = 0.71
- RQ after breakfast = 0.98
What are the main fuels being metabolised at each of these times?
When fat is being metabolised, RQ=0.7
When glucose is being metabolised, RQ = 1.0
Therefore:
- RQ = 0.8 suggests approximately equal amounts of fat and carbohydrate are being metabolised after an overnight fast
- RQ = 0.71 suggests mainly fat is being metabolised in jogging after an overnight fast
- RQ = 0.98 suggests mainly carbohydrate is being metabolised after breakfast
Part 3
JBJ also had blood samples taken before he went jogging, when he returned and 15 minutes after breakfast. Unfortunately, although he labelled the blood tubes, his research assistant in the laboratory mixed them up, and did not know which sample was which. The results obtained were:
glucose (mmol /L) |
free fatty acids (mmol
/L) |
|
| sample 1 | 6.5 |
0.1 |
| sample 2 | 4.5 |
0.8 |
| sample 3 | 3.5 |
2.4 |
Which sample is which?
- Sample 1 has moderately high glucose and very low fatty acids, so is likely to be the sample taken after breakfast
- Sample 2 has moderately low glucose and moderately high fatty acids, so is likely to be the sample taken while he was sitting up in bed
- Sample 3 has lower glucose and high fatty acids, so is likely to be the sample taken after jogging and before breakfast
When the samples were centrifuged, it was noticed that the plasma in sample 1 was milky in appearance, whereas it was clear in samples 2 and 3. What caused the milky appearance of the plasma in sample 1?
The cloudy appearance of the plasma in sample 1, taken after a moderately high fat meal, is due to the presence of chylomicrons, the lipoprotein particles containing newly absorbed lipids. Chylomicrons are large enough to scatter light, so causing the milky appearance of the plasma. If a sample is taken more than about an hour after the meal the plasma will be clear, as the lipid in the chylomicrons has been taken up into adipose tissue and muscle for storage, so that they shrink to a size small enough not to scatter light. The chylomicron remnants are taken up by the liver.
What do you think is the main source of the glucose and free fatty acids in his plasma (a) after an overnight fast and (b) after jogging?
The
main source of free fatty acids in the bloodstream will be adipose tissue triacylglycerol
(three mol of fatty acids esterified to glycerol, as shown on the right). Triacylglycerol
is hydrolysed by the enzyme hormone-sensitive lipase in adipose tissue cells.
The fatty acids and glycerol are released into the bloodstream. (The
fatty acids are bound non-covalently to serum albumin, to prevent them lysing
cell membranes or forming insoluble soaps with calcium ions).
After an overnight fast the main source of glucose will
be liver reserves of the storage carbohydrate glycogen, which is a branched
polymer of glucose. 
There will also be significant production of glucose in the liver from the glycerol liberated from adipose tissue - the process of gluconeogenesis Gluconeogenesis will increase while he is jogging, not only because more glycerol is available from triacylglycerol breakdown, but also using amino acids released from protein breakdown as substrates.
The problem in the fasting state is that glucose must be provided for the brain (which is largely reliant on glucose) and red blood cells (which are wholly reliant on glucose). Neither tissue can use fatty acids as a metabolic fuel. For reasons that will become apparent later on, fatty acids can never be a substrate for gluconeogenesis.
Key points from this exercise:
- The ratio of carbon dioxide formed : oxygen consumed (the respiratory quotient,
RQ) is:
- 0.7 when fatty acids are being metabolised
- 1.0 when glucose is being metabolised
- Measurement of RQ thus permits determination of the relative amounts of fat and carbohydrate being metabolised
- In the fasting state blood glucose is maintained by:
- breakdown of liver reserves of glycogen
- gluconeogenesis from glycerol and amino acids
- provision of fatty acids to tissues as an alternative fuel
- In the fasting state fatty acids are provided to tissues from the hydrolysis of adipose tissue triacylglycerol
- In the fed state fatty acids are provided to tissues (mainly for storage) as triacylglycerol in chylomicrons (and other plasma lipoproteins)