If I had titled this article "Fuel Efficiency in Running" or "The Glucose Sparing Effect," I might not have gotten your attention. By the time you finish reading this article, though, you will know what both of those terms mean and how important they are to running. Fuel efficiency and sparing glucose are the name of the game for increasing speed and endurance for long distance running and also for feeling great along the way. Look at what makes (or breaks) fuel efficiency.

Our primary choices for fuel at any given time are carbohydrate and fat. There are times that your body prefers carbohydrate, such as during quick, high intensity bursts of exercise. Carbohydrate is available immediately. You have some already stored within your muscle cells, as well as additional glucose stored in your liver. There is no waiting necessary to access carbohydrates for fuel, but there is a catch. It is only available in limited quantities and if you use too much, too fast, you end up with a build up of lactic acid (waste product) that ultimately makes you quit. (A sprint would not be a sprint if you could go on forever, right?). Fat, as a fuel source, on the other hand, is available in practically unlimited quantities in your body. It is the preferred fuel source at rest and during low intensity exercise, when there is no rush to supply huge amounts of fuel in a short period of time. For times like these, the body has plenty of time and oxygen to mobilize and break down fat for fuel. And whether you are thin or heavy, you most likely have more than enough fat in your systems to fuel a run from here to Timbuktu.

The question is whether or not you can access it. You may have heard the old phrase: "Fat can only burn in the flame of carbohydrate." This means that if you do not have any carbohydrate, you cannot burn any fat either.

Furthermore, it takes time to mobilize fat--to get it released from your fat cells, delivered to the muscle cells that need it, and converted to glucose. A steady oxygen supply is necessary to meet the challenge, so if your exercise intensity is too high to deliver enough oxygen, your body will throw the fat usage idea out the window and slip into carbohydrate-only gear (remember; quick fuel but the clock will be ticking and you will have the lactic acid build up).

In addition to the oxygen levels necessary to utilize fat for fuel, there is variation from person to person in how readily (or stubbornly) your fat cells are willing to release fat. There is an enzyme in your body called hormone sensitive lipase (HSL) that sits along the border of your fat cells and releases fat from the fat cells into the bloodstream. As long as HSL keeps dumping fat into the bloodstream, you can continue using it for fuel. Studies show that HSL levels vary greatly from person to person. If you have a high HSL level, your body is capable of releasing a lot of fat into the bloodstream for fuel. If you have a low HSL level . . . then not so much. There are two factors that increase HSL: 1) Long term (meaning years) of consistent aerobic exercise and 2) high levels of daily activity on a regular basis. How quickly do HSL levels rise and fall? If you have a lifetime of exercise behind you, your HSL level will stay high even when you take a break from exercise due to sickness or vacation. Your long history of exercise has trained your HSL levels that they will be needed again soon, even when you take a break. If you are within the first few years of a new exercise habit, you will acquire more HSL, but they may not yet get the message that they are needed for life. Therefore, when new exercisers take a week or two off from exercise, HSL levels will decrease again quite quickly.

Here is an illustration of the concept. Compare a new marathon runner with a seasoned marathon runner. The seasoned runner may have been training for more than a decade and have an estimated HSL level of 30,000. He may be able to sustain a very fast race pace, and furthermore, as he has such a high level of HSL, he is able to access a ton of fat to get the job done. Also, since the seasoned marathoner uses so much fat for fuel, he has not needed to dip into his glucose stores as much. Want to know one organ that relies solely on glucose and cannot use fat as fuel? It is the brain. So, since his muscles did not have to steal his brain's glucose supply, the seasoned marathon runner finishes the marathon feeling great and celebrating with his friends.

Now take a novice marathon runner who may have been training for only a year. His HSL level might only be 500 (as compared to 30,000). He may still very well finish the marathon, but in addition to using fat as fuel, he will use up every drop of glucose in his system. Chances are, with his muscles fighting for all the available glucose in his system, his brain (which can only use glucose for fuel) has gotten shortchanged. As he crosses the finish line, he is stumbling, a little disoriented. He may be proud as can be but making very little sense!

As your body gets more efficient at using fat for fuel, it gets better at conserving glucose for when you really need it. You will be able to work out harder and still feel great. Do not forget, also, the other way you can increase your HSL levels-- consistent daily activity throughout the day. When you ask your body to move throughout the day, you are asking it over and over to mobilize fat and your body will get the message that it needs to get more efficient at releasing fat to get the job done. Your body will continue to make adaptations based on years of living your life this way.

Individuals vary in their fat usage efficiency during exercise, and testing can reveal interesting information. Remember, we have unlimited fat stores and limited carbohydrate stores. As exercise becomes more intense, the longer you can continue to use high levels of fat for fuel, the more carbohydrate you can spare (and save for later). For a short run, this may not make much of a difference. But for a long run? It is the difference between hitting the wall and finishing strong. A good analogy is comparing carbohydrate stores to the gas tank in your car. You do not need a full tank to drive from Germantown to Washington, DC, just as you do not need to "carbo load" the night before a 5K. Heading out on a long road trip is like preparing for a marathon; you will definitely want to tank up and also conserve fuel along the way.

You need ten calories of fuel per minute to sustain your normal running pace. Where are those ten calories coming from? If you are able to supply seven of them per minute from fat, then you will only need to use up three calories per minute from carbohydrate stores. What if you could only supply two calories per minute from fat and you needed to use up eight calories per minute from carbohydrate? You may not experience any problems in short runs, but long runs will become a problem. Your carbohydrate "tank" will be empty a lot more quickly and you will find yourself relying on fuel replacement gels, etc. Train to increase fat usage and you will have plenty of carbohydrates to pull you through without relying on supplements.

How and when does the shiftover from high fat to high carbohydrate utilization take place? As intensity increases, fat usage decreases. The technical term for the point where fat utilization drops off considerably is "anaerobic threshold." Your body is shifting from "aerobic" (with oxygen) to "anaerobic" (without oxygen). For some people, it is a gradual decrease, like sliding down a slope. For others, it is like falling off a cliff. Either way, it may or may not happen at the intensity that you would expect. In testing, I have found some people surprisingly continue onward with no fat to speak of and others who continue to burn a huge percentage of fat until the bitter end. The remarkable thing is the uniqueness of each individual's response to progressive exercise stages and how well the data lends itself to individualized heart rate prescriptions to maximize efficiency. For instance, I usually recommend that runners plan some of their runs at the heart rate zone with maximum fat usage in order to reinforce those pathways and build their "base" of high fat utilization. Other runs should include intervals above and below the heart rate at the point of the sharpest drop in fat utilization, which trains the body to bring up that fat usage at the higher heart rates. The end results are faster times and feeling better during your runs!

Cathy Moxley, M.A., CSCS is an exercise physiologist and fitness coach in Germantown, MD. She has a Master's Degree in Exercise Physiology and specialized training in metabolic physiology. She offers cardiovascular and metabolic fitness testing through her business, Fitness InSight. To find out more about metabolic testing and see sample tests, you can visit her at www.TestYourMetabolism.com. In addition, she is the author of The Busy Mom's Ultimate Fitness Guide, available at www.BusyMomSolutions.com, as well as Amazon.com and Barnes & Noble.