For what it's worth, this (see below) was in my newsletter from Jason Karp this morning. I know, I know, he gets a bad rap. But, it still adds to this discussion.
Fat Burning Zone
People often assume that low-intensity exercise is best for burning fat. During exercise at a very low intensity (e.g., walking), fat does account for most of the energy expenditure, while at a moderate intensity (e.g., 80% maximum heart rate or about 70-75% VO2max), fat accounts for only about half of the energy used. While you use both fat and carbohydrates for energy during exercise, these two fuels provide that energy on a sliding scale-as you increase your intensity up to your lactate threshold, the contribution from fat decreases while the contribution from carbohydrates increases. When you exercise at an intensity above your lactate threshold, you use only carbohydrates. While there is only a minimal amount of fat used when exercising just below your lactate threshold, the number of calories used per minute and the total number of calories expended are much greater than when exercising at a lower intensity, so the amount of fat used is also greater. What matters is the rate of energy expenditure, rather than simply the percentage of energy expenditure derived from fat. Since you use only carbohydrates when exercising at a high intensity, does that mean that if you run fast or take a high-intensity Spinning class, you won't get rid of that flabby belly? Of course not.
Despite what most people think, you don't have to use fat during exercise to lose fat from your waistline. After all, have you ever seen a fat sprinter? Sprinters primarily train anaerobically, never using fat during their workouts. Carbohydrates are actually the muscles' preferred fuel during exercise. The little amount of fat that is used in combination with carbohydrates during exercise below the lactate threshold is in the form of intramuscular triglycerides-tiny droplets of fat within your muscles. Your adipose fat (the fat on your waistline and thighs) is burned during the hours before and after your workout. Since fat is oxidized inside your muscles' mitochondria, it is more efficient to use fat during exercise that is physically closer to the mitochondria, when you need to regenerate ATP quickly for muscle contraction. To use adipose fat, it has to be transported to the mitochondria where it can be oxidized.
To become a better fat burning machine, you must enhance the metabolic profile of the muscles. Endurance training enhances fat oxidation by increasing skeletal muscle mitochondrial content and cellular respiratory capacity, allowing for a greater use of fat and the sparing of muscle glycogen. This steering in fuel use to a greater reliance on fat at the same exercise intensity is one of the hallmark adaptations to endurance training. Since a metabolic priority of recovering muscle is to replenish glycogen stores, the way to not gain excess fat is to constantly use your glycogen stores during exercise so that the carbohydrates you ingest will be used to replenish the glycogen stores rather than be stored as fat. If your glycogen stores are already full, as they are in most of the population that doesn't exercise, any extra calories you consume are stored as fat. So when people say that sugar makes you fat, that's only true if that sugar is not used to replace your muscle glycogen stores.