Final answer:
Undigested and unabsorbed food cannot be used in metabolic processes due to the inability of large molecules to cross the digestive epithelial barrier and enter the bloodstream, where they can be used for energy production.
Step-by-step explanation:
All digested and absorbed food cannot be used in metabolic processes because some substances in food cannot be broken down into nutrients and are removed as solid waste, a process known as elimination. Specifically, large molecules in food can't cross cell membranes and must be broken down into smaller, absorbable nutrients to pass through the digestive epithelium. Even after absorption, certain elements like dietary fats cannot be released directly into the bloodstream due to their size and are instead transported via lymphatic vessels before they enter the bloodstream.
Moreover, the absorption process requires that nutrients be small enough to be taken up by the mucosa of the small intestine. Substances like fiber, which provide no energy, or foods that contain complex molecules like cellulase, which cannot be digested by humans, are excreted as waste. It's important for the diet to be balanced to ensure that nutrients can be adequately absorbed and used for energy production, where ATP is the primary energy currency used by cells for metabolic processes.