When you think of 'hybrids' you probably think of cars like the Toyota Prius or the Honda whatchamacallit. But did you realize that your body is a hybrid fuel machine? In fact, you have three different ways to provide energy for muscular contraction (and other body functions). You have some energy stores available at rest, but they are quickly depleted during exercise. As you exercise, you need more energy and your body relies on a mixture of two fuels, carbohydrates and fats. How you use these fuels is an indicator of your performance potential in endurance events, and has implications for fueling your body during exercise.
The human body's usable form of energy is a compound called adenosine triphosphate (ATP). Your body needs ATP for the energy to perform everything from life-sustaining cellular functions to muscle contractions. ATP is continually used and regenerated. There's even a little ATP stored in your body, which can last for about 5-10 seconds or so of all-out effort. The resting stores of ATP are also supplemented by other chemicals which can quickly help regenerate the ATP used. Unfortunately, those additional supplies can't make ATP fast enough to support exercise longer than a few seconds. So if you continue to exercise, your body will rely more on other processes for making energy, using two important fuels: carbohydrates and fat.

Once you've ingested carbohydrates, they are repackaged in your body as blood glucose or in their stored form, glycogen, in the muscles and liver. You may store between 1500 and 2000 kcals of carbohydrate. Your body breaks down carbohydrates and forms ATP in a process called glycolysis. Since oxygen is not needed for this process, it is said to be anaerobic. This process predominates during early stages of exercise because it happens quickly, without needing oxygen to make ATP. However, some oxygen may be present to act upon the end product of glycolysis, pyruvate. In that case, more ATP can be formed through a subsequent process. If no oxygen is present, another by-product, lactic acid, can be made from pyruvate. The terms lactic acid and lactate are often used interchangeably, but do in fact refer to slightly different compounds. If you rely too much on glycolysis for energy, you may accumulate lactate, which is associated with acidosis (accumulation of acid), which can inhibit muscle contraction. So there is a benefit to being able to rely less on carbohydrates and more on fats to fuel your exercise.

We generally have tens of thousands of kcals of fat stored in our bodies. That's a lot of potential energy, but this system is a little more sluggish at providing energy than glycolysis. First, the stored fat must be broken down into free fatty acids (FFA) so they can be acted upon to make ATP. Then the FFA themselves must be broken down, which requires oxygen, as do the next two major steps in the process of making ATP from fat stores. This process is said to be aerobic, since oxygen is required for it to occur. The yield of ATP from combusting fat is far greater than that from glycolysis. There are two additional benefits: no lactate is produced, and you will save your limited supplies of carbohydrates if you can meet your energy demand by using fat as a fuel. That's known as glycogen sparing.

There's always a mix

At rest and during exercise, you get energy from both aerobic and anaerobic metabolism. The mixture of fuels used will change with exercise intensity. The harder you work, the faster you need to restore ATP, and the harder both systems work to provide it. However, a gradually larger amount of the total will be provided by the anaerobic system since it can provide the ATP faster. As mentioned earlier, that will produce more lactate. That does not pose a problem with easier exercise because your body will re-use most of the lactate that gets produced. But if you keep working harder, you will eventually produce lactate too fast and overload your body's ability to remove it, and the amount of lactate will rise abruptly in the blood. The exercise intensity where this occurs is known as the lactate threshold. It has an important effect on exercise ability because, again, the change in acidity associated with this point will inhibit muscle contraction. You can continue to exercise at this intensity, but it is usually unpleasant, at least. A good runner may run a 10K race just faster than the lactate threshold speed, for example. The harder you work beyond the lactate threshold, the more urgent the need to stop or slow down becomes (and the more painful it gets!). Speed or power at lactate threshold is an important physiologic determinant of performance in endurance events. For an endurance athlete, the purpose of training is to increase the speed or power output generated at lactate threshold.

Changing the mix through training

An essential part of training is increasing your body's ability to make energy aerobically, using fat as a fuel. By doing this you will be able to meet larger energy demands through aerobic ATP generation, thus producing less lactate and sparing glycogen. You might even need to carry fewer energy bars with you on your long runs or rides!

Increasing your body's aerobic capacity is usually done by performing large volumes of training at low intensities. Adaptations take place over weeks and months; further adaptations take place with years of training. This is usually coupled with shorter bouts of higher intensity exercise. But be careful: high intensity bouts should only be done after you have developed an adequate base. A recent study showed that some sub-elite runners spend 70% of their training below their lactate threshold intensity! They trained about 21% at threshold, and only 8% above.

Measuring the improvements

How do you know if you've made changes in your aerobic ability and have improved your performance potential? If you've been training properly and long enough, it's almost a sure bet. You may even notice that your exercise feels easier than it used to (though that's subjective, and could be due to improving in several areas). An objective way is to undergo a submaximal (i.e., not to fatigue) laboratory test, where your calorie use is measured at a variety of exercise intensities. These tests can show how many calories you use at given workloads, as well as what portions come from fat and carbohydrates. If you repeat the test about ten to twelve weeks later, it could show, ideally, that you have trained and adapted to the point that you can now meet more of your energy demands by aerobic metabolism.

You could also perform an aerobic time trial at a goal heart rate - below your lactate threshold intensity. As your fitness improves, you should be able to run faster at the same heart rate. In the end, training is all about running faster, isn't it?

Author Paul Kammermeier, MS is the Clinical Exercise Physiologist at the Boulder Center for Sports Medicine, specializing in athlete testing and training. He has run several marathons and coaches with the Bolder Boulder Training Clubs.