Final answer:
Less than 10% of the food we consume is not completely broken down, with most energy being converted to thermal energy or stored as fat.
Step-by-step explanation:
Approximately, less than 10% of the food we consume is NOT completely broken down. When considering energy utilization, very little of the energy released during the consumption of food is used to do actual physical work.
Most of the consumed energy is instead converted to thermal energy or stored as chemical energy in fat.
It's important to clarify that this question pertains to the efficiency of our body's use of food for energy and not the digestion process as such, where most of the food is digested and broken down unless there's some digestive disorder or unabsorbable compounds in the diet.