Alternative respiratory substrates are other substances, such as lipids and proteins, that can be oxidised by cells to release energy.

1. Hydrolysed to glycerol and fatty acids
2. Glycerol is converted to triose phosphate and enters the glycolysis pathway
3. Fatty acids are broken down into 2-carbon fragments and converted into acetyl coenzyme A, which enters the Krebs cycle

Lipids release more than double the energy of the same mass of carbohydrate.

1. Proteins are hydrolysed into amino acids
2. The amino group is removed (deamination)
3. 3-carbon compounds are converted to pyruvate, while 4- and 5-carbon compounds are converted to intermediates in the Krebs cycle

Lipids contain a greater proportion of C-H bonds in their molecules, which release energy when the lipids are oxidised in aerobic respiration.