Burning fat during long-term running.

A question I often hear from beginner exercisers all the way to seasoned runners is, "Do I burn fat or carbohydrates when I run?" Unfortunately, there are no easy answers to this important exercise question. For an adequate scientific explanation, the issue is complicated because there are a variety of factors which influence whether fat or carbohydrate is used during running. Furthermore, many of the laboratory studies on this topic are difficult to interpret in order to adequately relate findings to those seeking answers.

How do we respond to the above question and those that follow such as "So when do I move from burning fat to carbohydrates?" The issue of fat or carbohydrate (CHO) fuel used during running is important to understand, even in a rudimentary way, because it can help guide training and food choices for improved performance and health.

Some of the factors that influence whether fat or CHO is being used as fuel during running, as delineated by various authors (1,2,3,4,5,6,7), are:

* Intensity or pace of the run

* Time or duration of the run

* Fitness level of the runner

* Diet immediately prior to the run

* Long-term diet for the runner

* Genetics (fast twitch or slow twitch muscle fibers)

* Factors of the environment (temperature, humidity, wind, incline, altitude, surface of run, etc.)

Pace or Intensity

The first critical factor is the pace or intensity of the run. Most track fans would say that the 100m is the fastest paced race. In fact, most running experts say that one accelerates out of the blocks to a top speed at about 60 meters, holds that speed until 85-95 meters, and then may start to slow down. Even the 200m race is raced at a slower pace than the 100. The 400m race is run at an even slower pace and the 800m slower still. This slowing of pace continues to the marathon but, even in the marathon, top speed at world class level can reach 13 miles per hour. For over two hours of running, this is still intense effort.

Whenever the runner starts a race, or races for one to two minutes all out, they burn nearly 100% CHO during this maximum effort. The percent of fat or CHO burned with less than 100% effort is difficult to determine. The issue becomes, "How do we accurately determine intensity?" Maximal effort can be measured by maximum heart rate or maximal oxygen consumption. Both estimates are generally measured in a percent; however, they do not equal each other. Fifty percent of maximum heart rate is not 50% maximum VO2 (see Table 1).

Some physiologists try to measure maximal effort by relating it to respiration. They tell the athlete to count their respirations and when it gets to about 40 per minute then they are about at maximum effort; however, it's extremely difficult--if not impossible--to count respirations at maximal effort.

When considering intensity of effort, we can say that at rest (zero percent intensity, as far as running goes), we may burn approximately 55% to 65% fat. In all out running, we quickly go to burning 100% CHO. In between rest and maximal effort, there is a percent of both fat and CHO burned; however, duration of the run can affect this percentage.

Time or Duration

The second factor affecting whether fat or CHO is used is the duration or total time of the run (continuous run at a steady pace). When looking at this factor, it's important to remember that when the run is longer, the pace or intensity must decrease for it to continue. Consequently, a higher percentage of fat will be burned. Another factor to consider in the runs of one hour or longer is that stored muscle glycogen rapidly depletes in the skeletal muscles, causing the pace to slow dramatically.

We burn approximately 55% to 65% fat and 45% to 35% CHO at rest. This can continue almost indefinitely while at rest but once activity is introduced, we start to burn a higher percentage of CHO until we are at 100% effort and burn 100% CHO.

The longer we run, the more our CHO stores are diminished and we must resort to using fat as a fuel; however, this is dependent on the fitness level of the runner and pace changes during the run.

Fitness Level or Training Level

The fitness level of the runner is the third, and I believe most important, factor in determining whether fat or CHO is used as fuel when running longer distances. This is easily shown by the world record in the men's marathon, which is 2:04:55. This supremely fit runner ran sub 4 minutes and 50 seconds per mile for 26 continuous miles. The unfit individual may not be able to run a single 9:40-mile, or complete the marathon in twice the time as the marathon record pace. The fit marathoner may race at about 80% to 90% max VO2 (or at 90% to 95% HR max) while the runner who is unfit cannot even finish a one-mile race at a given pace. By intelligently training, the extremely fit marathoner is capable of running faster and longer.

The fit runner is also an extremely efficient fat burning athlete. He or she has trained the body to do longer runs on depleted muscle and liver glycogen (1,2,4). The highly fit marathon runner develops his/her cardiovascular system and increases muscle function so that fat is the preferred fuel, while sparing the limited glycogen storage and circulating glucose reserves. The abundance of slow twitch fibers and increases in mitochondria and respiratory enzymes enable the lower leg muscles of an endurance athlete to burn fat on a flame of CHO in long-term aerobic exercise (1,2,3,4,7).

Essentially, the extremely fit and properly trained endurance runner can burn "fat on a flame of CHO," and learns to do this with exhaustive training in terms of mileage and intense runs on hard training days.

Diet

The next factor to consider in maximal running is the diet. Thanks to the elegant studies of Dr. David Costill, the late Dr. Philip Gollnick, and Dr. Timothy Noakes, we have learned of the important role CHO plays in the daily diet, as well as the diet immediately prior to running. What these researchers have informed us of is that the muscles used in running prefer to use CHO for moderate exercise (55% to 85% of effort). And when the diet is high in CHO on a regular basis, the body can store about two and a half times the amount of glycogen in the muscle than if the diet is high in protein or fat (1,2,3,4,7).

Glycogen storage turns out to be important not only in the initial time frame prior to exercise but also in the long-term--that is in the weeks, months, and even years of eating a high CHO diet prior to training and running events. Runners who run long on successive days need to continually have a large percentage of the calories in the diet from CHO (1,2,3,4,5,6,7). These experts suggest that the diet consist of as much as 60% to 65% of the calories from CHO. This would leave 15% from protein and the remaining 20% to 25% or so from fat (1,2,3,4,5,7). A diet high in CHO ends up being mostly vegetarian with lots of whole grains and fruits which, in turn, promotes good health and longevity (5,6).

Genetics

Many prominent exercise physiologists believe maximum aerobic capacity is largely dependent on one's parents. Discussion of genetics or heredity with distance running often refer to the fiber types found in marathon runners (2,3,4,5,7). It's reported that in great distance runners, slow twitch fibers make up more than 65% to 85% of their lower leg muscles (2,3,4,7). Slow twitch fibers are known for their superior aerobic capacity, extensive vasculature, and extremely high oxidative enzymes within the muscle fiber. Later research has shown that fast twitch fibers are sub-divided into two groups, A and B. In endurance training, the athlete can develop the fast twitch A fibers to behave similar in function to the slow twitch fibers. The twitch contraction characteristics seem to remain the same but the fuels used by fast twitch A are oxidative while the fast twitch B muscle fibers generate energy through anaerobic pathways (2,3,4,7).

Environmental Issues

Costill has reported that flat, steady state running is the most economical marathon running. That is why marathon courses like Rotterdam, Chicago, Berlin, and even London seem to be the fastest--they are the flattest. Uphill running makes the runner expend approximately one-third more energy than level running while downhill uses almost one-fifth less energy.

Hot, humid weather results in more glycogen usage. Because severe marathon running causes a rapid rise in body temperature, the ideal outdoor temperature seems to be about 50[degrees] F (10[degrees] C). Wind is also a factor. A tail wind of 13 MPH essentially causes a vacuum for the runner. The best situation is a crosswind from the rear or a stronger than 13 MPH tail wind.

Summary

In this review article we have seen that there may be at least seven factors affecting the use of fat or CHO as fuel during distance running. Considering each factor individually and then trying to consider the combination of all factors may enable us to alter training to get the most out of it. And a diet designed for better performance will, in turn, provide the benefits of a longer, healthier life.

REFERENCES

1. Costill DL. A scientific approach to distance running. Palo Alto, CA Track & Field News, 1979.

2. Costill DL. Inside Running: Basics of Sports Physiology. Indianapolis, IN: Benchmark Press, 1986.

3. Costill DL. 40 Years of Progress in Applied Exercise Physiology, ACSM, 1994.

4. Gollnick PD. Energy Metabolism and Prolonged Exercise. In: Perspectives in Exercise Science and Sports Medicine. Editors: DR Lamb and R Murray. New York, NY: Academic Press, 1988.

5. Morris AF. Sports Medicine. Dubuque, IA: William C. Brown, 1984.

6. McArdle WD, Katch FI, Katch VL. Essentials of Exercise Physiology, 2nd Ed. Baltimore, MD: Lippincott Williams & Wilkins, 2000.

7. Noakes T. Lore of Running, 4th Ed. Cape Town, South Africa: Oxford Univ. Press, 2003.

Dr. Al Morris is Director of Health Improvement and Physical Fitness at the U.S. Border Patrol in Washington, DC. He is also head of coaching education for PVA-USATF and a member of the Sports Medicine Committee of the Road Runners Club of America.

By Alfred F. Morris, PhD, FACSM 

Table 1 HR AND VO2 Max are related (but NOT exactly linearly so)

%HR  % VO2 MAX

 50   28
 60   42
 70   56
 80   70
 90   83
100  100