We also don't have the ability (or understanding) to control the conversion of energy-in-food into energy-our-body-can-use + energy-our-body-can-store + waste-products. Food is not gasoline, and our bodies are not combustion engines.

Even that's a bad analogy; gasoline's quality and impurities can vary quite a bit, which effects how much energy an engine can extract from it, and engine efficiency can also vary greatly. Still, our bodies are much more complex.