Zone In: Are You Going Hard or Going Backwards?

Dear Speed Lab,
I have a question about training and racing in different zones (aerobic and anaerobic). As I understand it, when you are in your anaerobic zone you are predominantly using glycogen (sugars).

Is it true that once you have entered the anaerobic (sugar) zone you cannot revert to burning fats (aerobic zone)? Let's say that during a triathlon you hit a hill hard and enter into the anaerobic zone; is it at all possible to revert to the aerobic zone (fat burning) after the hill without bonking because of the hill climb? Can you move between these zones and still be able to compete to your potential?

Tempe, Arizona

Thanks for the interesting question. A brief explanation of the interaction of carbohydrate and fat metabolism during exercise is required here.

During rest and low-intensity exercise, plasma fatty acids (available from fat-tissue stores) and muscle triglycerides are our dominant source of energy. However, bear in mind that carbohydrates (what you call sugars) also contribute energy, available as glycogen (storage form of glucose) in muscle and glucose in blood.
Think of fats as the main burner of a gas stove, and carbohydrates (sugars) as the pilot light. When a person goes from walking to running a marathon and then to racing a 10K, there is a steady increase in the total rate of energy expenditure from both fats and carbohydrates, but there is also a dramatic change in the relative contributions of each fuel source.

For example, a marathon may typically be run between 65 to 85 percent of VO2 max. At 65 percent of VO2 max, there is a greater relative energy contribution from glucose (sugars) and muscle triglycerides than from plasma fatty acids. There is negligible muscle glycogen contribution at this exercise intensity.

As the exercise intensity increases from 65 to 85 percent of VO2 max, the increasing contribution from plasma glucose (sugars) continues, and the energy contribution from muscle triglycerides and plasma fatty acids declines. The majority of additional required energy comes from muscle-glycogen stores.

Beyond approximately 85 percent of VO2 max there is increased oxygen demand for anaerobic metabolism (what you call the anaerobic zone) for which only carbohydrates (sugars) can be utilized. Performance at such high-intensity levels is limited by the accumulation of blood and tissue lactic acid. Lactic acid is produced during anaerobic metabolism, resulting in fatigue.
However, to directly answer your question, you can shift between energy substrate utilization zones. For example, if you slow your running pace to 65 percent of VO2 max (aerobic) after running at 85 percent of VO2 max (anaerobic) for a short period of time, you will start to use more muscle triglycerides and blood glucose and muscle-glycogen usage will decline.

But it takes time to lower your blood-lactate levels, and accumulated blood lactate will continue to affect your exercise performance until it has dropped sufficiently. In addition, lactate can be processed in the liver to resynthesize glucose, which can then be considered an additional fuel source.

So, what happens if you run in your anaerobic zone (e.g., 85 percent of VO2 max) for too long and thus build up high levels of lactic acid and deplete your muscle-glycogen stores, before dropping back into the aerobic zone (e.g., 65 percent of VO2 max)? Your high blood-lactate levels will be detrimental to continued exercise performance and your low carbohydrate stores will impede efficient fat metabolism.

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