Respiratory substrates are the compounds that are broken down during respiration to derive energy. Since respiration involves breakdown of substrates, the respiratory process has traditionally been considered a catabolic process. In living organisms, respiratory substrates are often more than one; pure proteins or fats are never used as respiratory substrates alone.

The most common respiratory substrate is glucose. It is the favoured substrate for respiration. All carbohydrates are usually first converted into glucose before they are used for respiration. Other substrates like fats and proteins can also be respired, but they do not enter the respiratory pathway at the first step.