Why is it more ethical to test for the waste products of respiration rather than the reactants

The knowledge of simple biological reactions tells us that the product of reactions results from the complete or partial use of the reactants. In some reversible reactions, products can be converted to reactants in equal volume. Hence, in reactions that cannot be reversed, as evident in respiration, its waste products can be used to indicate that indeed respiration did take place. When glucose is consumed it is broken down through a series of reactions to give carbon dioxide and water as waste products.