There are many more other respiratory substances other than glucose. Triglycerides are hydrolysed to fatty acids, which enter the Krebs cycle via acetyl CoA and glycerol.

Glycerol is first converted to pyruvate before undergoing oxidative decarboxylation, producing a acetyl group which is picked up by coenzyme A, forming acetyl CoA. The fatty acids in a triglyceride molecule can lead to the formation of as many as 50 acetyl CoA molecules, resulting in the synthesis of up to 500 ATP’s.

• Lipids stores and release about twice as much energy as carbohydrates

• Alcohol contains more energy than carbohydrates but less than lipids

• Proteins are roughly equivalent to carbohydrates

Proteins first have to be hydrolysed to amino acids and then amino acids have to be deaminated (removal of the amine group) before they enter the respiratory pathway, usually via pyruvate.

These steps require ATP, thus reducing the net production of ATP

The respiratory quotient (RQ) of a substrate is calculated by dividing the volume of CO2 released by the volume of O2 taken in during respiration of that particular substrate. This is measured using a simple piece of apparatus called a respirometer.

RQ = CO2 produced / O2 consumed

It takes 6 oxygen molecules to completely respire one molecule of glucose and this results in the production of six molecules of carbon dioxide. This results in an RQ of 1.0.

Lipids contains a greater proportion of C-H bonds than carbohydrates which is why they produce so much more ATP in respiration.

Due to the greater C-H bonds, lipids require relatively more oxygen to break down and release relatively less carbon dioxide. This results in the RQs of less than one for lipids.

The structure of amino acids leads to RQs somewhere between carbohydrates and lipids.

Carbohydrates = 1.0

Protein = 0.9

Lipids = 0.7

During normal activity, the RQ is in the range of 0.8 to 0.9, showing that carbohydrates and lipids and probably some proteins are being used as respiratory substrates.

During anaerobic respiration, the RQ increases above 1.0 although this is not easy to measure as the point at which anaerobic respiration begins is not easy to pinpoint.

Low carbohydrates diets

• Triglycerides are hydrolysed into fatty acids and glycerol. The fatty acids are broke down in the mitochondria to give many two carbon acetyl groups that combine with coenzyme A molecules and enter the Krebs cycle.

• Triglycerides cannot act as the only respiratory substrate. Carbohydrates are needed to keep the Krebs cycle going so that acetyl groups from the breakdown of fatty acids can be fed in. If the carbohydrates are in short supply the body will make them using a process called gluconeogenesis. This process often uses glycerol, but it may also use pyruvate from glycolysis.

• Oxaloacetate from the Krebs cycle can be used to make glucose when carbohydrates levels are low. Reducing the number of oxaloacetate molecules in the Krebs cycle reduces the rate at which the acetyl groups produced during the breakdown of lipids can be fed into the cycle and produce ATP.

• Oxaloacetate can be replaced by the conversion of pyruvate from the carbohydrate breakdown in the mitochondria. Pyruvate is also synthesised using glycerol from the breakdown of lipids. However, the breakdown of a lipid molecule provides a relatively small quantity of glycerol and so a relatively small amount of pyruvate. This means that carbohydrates are still needed to ensure the continued respiration of fat.

• Proteins can be hydrolysed into amino acids which are then deaminated in the liver. The remaining keto acids can be converted into glucose molecules. Lean muscle is the protein of choice in this process, so a low carbohydrate diet can lead to the breakdown of muscle tissue. The liver and kidney also have to remove nitrogenous waste.

• If the level of acetyl CoA increases because it is not being taken into the Krebs cycle, the liver starts converting it into ketone bodies. Brain cells normally require glucose as an energy source. They cannot use fatty acids as a respiratory substrate but they can use ketone bodies.

• When the body is producing more ketone bodies than usual, it is said to be in ketosis. This can lead to a dangerous condition known as ketoacidosis.

• Ketoacidosis is the result of an accumulation of ketone bodies which cause the pH level of the blood to drop to dangerous or even fatal levels.

• This condition is often seen in alcoholics, untreated diabetes and during starvation. It is often diagnosed by the fruity smell of propanone(acetone) a breakdown product of ketone bodies on the breath of an affected person.