I don't know how many of you live above sea level at all.. (if any) but I thought some of you may find this interesting. For those of you who do regular cardio especially.
When you live at an altitude above sea level your body will adapt to the lower amount of oxygen in the air. (hypoxia) When you do cardio training at an altitude several changes occur, one of these is what is used for energy. It was found that resting free fatty acid levels increase during stay (that is, when you live there) at altitude. Also, that blood glycerol concentration also increased, suggesting an increased use of fat for fuel at altitude. (Young & Young 1988) Also, Glycogen (stored form of carbs) use was decreased suggesting again that fat use was increased. However, Brooks (1991) found that the amount of glucose (blood sugar) that appeared in the blood during exercise increased. They found this to be linked to the increase in sympathetic nervous system (fight or flight) activity at altitude. In 1996, Roberts and colleagues found that depsoite elevated levels of blood free fatty acid, and glyceral at altitude compared to sea level, rates of fat uptake and glycerol release from the working muscles decreased after the body adapted to the higher altitude. They concluded that... adaptation to high altitude results in a decreased reliance on fat for fuel and increased reliance on glucose for fuel.