In much the same way that an engine runs on diesel fuel, the human body runs on what?

The human body primarily runs on glucose, which is a simple sugar and an essential energy source for cells. Glucose is derived from the carbohydrates we consume, which are broken down during digestion. When carbohydrates are ingested, they are converted into glucose, making it a critical fuel for the body's metabolic processes. This glucose is then used by cells to generate adenosine triphosphate (ATP), the energy currency of the cell, allowing us to perform all bodily functions, from movement to cellular repair.

While carbohydrates, fluids, and insulin play significant roles in overall health and the metabolic process, glucose is the direct energy source that the body's cells utilize for their immediate needs. Fluids are vital for hydration and maintaining bodily functions, but they do not provide energy in the same way that glucose does. Insulin is a hormone that helps regulate blood glucose levels but is not an energy source itself. Thus, glucose stands out as the correct answer in this context.