I understand that when you do cardio exercise in the aerobic zones (moderate heart rates) you are mostly burning fat to create energy. When you exercise in anaerobic zones (high heart rates) you are mostly burning sugar. What is happening in your body when you are burning sugar? Are you using up your energy supply?
You are always burning sugar and fat when you work out. When working out in an aerobic zone you burn mostly fat and when you work in an anaerobic zone you are burning mostly sugar (also known as glycogen). You store approximately 119 hours of stored fat but only about 90 minutes of glycogen or sugar. When you work out anaerobically for long periods of time you use up glycogen but do not tap into a lot of your fat stores. At the end of an anaerobic workout, you are fatigued and need to replace the glycogen stores you used up; however, you have fat stores that have still not been utilized. This is why you see a lot of people in the gym that beat themselves to a pulp but do not change their body shape.
When you workout in endurance zones or aerobic zones you burn mostly fat and the resulting weight loss changes the shape of your body. When you workout in zone 4 or anaerobic zones (without oxygen), you burn sugar and increase your cardiovascular limits. You should only work out in anaerobic zones about 20% of the time. That way you eventually increase your anaerobic threshold thus increasing your ability to burn fat at more intense and higher heart rates.
The terms "aerobic" and "anaerobic" refer to the presence and absence of oxygen, respectively. Most of our cells prefer to get their energy by using oxygen to fuel our metabolism. During aerobic exercise with adequate fuel in the form of stored fat, plus oxygen, muscle cells can contract repeatedly without fatigue. During higher intensity exercises that are anaerobic or non-oxygen fueled, muscle cells must rely on other reactions that do not require oxygen to fuel muscle contraction. Burning sugar does not require a lot of oxygen. This anaerobic metabolism in the cells produces waste molecules that can impair muscle contractions. This deterioration in performance is called fatigue.
Fatigue causes you to experience added discomfort and weakening muscles which forces you to slow down and lower your exercise intensity. Slowing down allows the muscles to once again rely solely on aerobic metabolism, burning mostly fat, and supports the removal or chemical conversion of waste molecules.
So how do these terms relate to you? It all depends on your exercise goals. Most of us are non-competitive or non-elite active individuals.We just want to exercise to gain health benefits, feel good and possibly lose weight. Aerobic exercise conditions enable you to exercise for long periods of time, potentially benefiting from the sustained energy expenditure or calories burned. Aerobic exercise tends to be less stressful to muscles, joints, and your heart, which may be important for individuals with arthritis, heart disease, or high blood pressure. However, to more rapidly improve your exercise capacities, tolerance, and performance, some anaerobic exercise training is a necessity.