HIIT has been shown to increase both fat and carbohydrate metabolism. In under 20 minutes, HIIT generates increased fat metabolism that extends well beyond the period of workout. To achieve the same effect doing steady state cardio, it will take upwards of 60 minutes.
To understand why HIIT is so effective we need to look at how the human body generates energy during exercise. There are basically three methods:
1) ATP-PC System, 2) Glycolitic System, 3) Aerobic Systems.
The details are complicated and best saved for another more detailed post, but for the sake of the question at hand its sufficient to know the function of these systems during exercise at a high level. First thing to note is that ATP (Adenosine Triphosphate) is the energy currency of the body and to supply energy during an activity, the body relies on creation of ATP molecules. These ATP molecules are acted on by the enzyme ATPase to break it down into ADP and Inorganic Phosphate molecule and release 7.3 Kcal per ATP molecule. Before ATP can release additional energy again, it must add back another phosphate group to ADP.
When you start an exercise, the ATP-PC system is triggered first and is capable of supplying energy for about 15-30 seconds. This system is anaerobic (does not rely on oxygen) and is fast, so the body prefers it at the onset of an activity. In this process, ADP (Adenosine Diphosphate) combines with a Creatine Phosphate molecule to form ATP and Creatine.
If the exercise continues beyond that time, the Anaerobic glycolytic system is triggered next, which uses Glucose (sugar) or Glycogen (stored carbohydrate) to release ATP. This leads to creation of Lactic Acid as a byproduct, which gives the burn feeling in the muscles. This system can provide energy for 30-50 seconds. Most traditional workouts with 6-12 reps fall within this time period.
At this point the Oxygen intake catches up to the energy demands and Aerobic processes for energy generation are initiated.
In the presence of Oxygen, instead of Lactic Acid, the byproduct is Pyruvic Acid which is broken down into Acetyl CoA. Acetyl CoA enters what is called the Krebs Cycle where it undergoes Oxidation and produces ATP molecules and byproducts Carbon Dioxide and Hydrogen. These Hydrogen ions work with Enzymes and lead to phosphorylation of ADP to form ATP.
Complete metabolization of 1 glucose molecule produces 35-40 ATP molecules. After about 90 minutes, muscle glycogen stores are fully depleted. Athletes can with training store up to 50% more glycogen for longer sustained activity.
When glycogen metabolization stops, the exercise intensity begins to sow as the primary energy supply switches from glycogen to fats.
Fat metabolization is also an Aerobic process. Triglycerides stored in fat cells are converted to Free Fatty Acids (FFAs), which is converted to Acetyl-CoA through the beta-oxidation process. This then enters the Krebs cycle described above to result in ATP molecules.
Different fat molecules from our diet may result in different number of ATP molecules but its typically 3-4 times as much as that produced from glucose.