Fat is the most energetic hydrocarbon, gram for gram, but glucose (and many other sugars) are metabolized more efficiently, requiring less oxygen for the same energy payoff. This is because glucose can enter into glycolysis, which generates two molecules of ATP (the energy "currency " of the cell) for every glucose molecule metabolized, and it doesn't need oxygen. The result of glycolysis is (in simplified terms), energy and acetate. The acetate then goes on to the TCA (tricarboxylic acid), or Krebs cycle, where it is broken down into carbon dioxide and water, and it releases more energy in the form of ATP. The TCA cycle requires oxygen to function. Fat, on the other hand, is broken down directly into acetate, which is fed into the TCA cycle, without the initial payoff of glycolysis. So technically glucose is more "efficient", but fat is more "energetic".