Pigeon Racing and Nutrition of the Muscle

Part 2

By: Gordon A Chalmers, DVM

As a reserve fuel for flight, fat has definite advantages over carbohydrate and protein. For example, one unit of fat liberates as much energy as equal amounts of carbohydrate and protein put together. One gram ( about 1/30th of an ounce) provides about 9,300 calories of energy, whereas the same amount of carbohydrate yields only 4,200 calories of energy, and one gram of protein, only 4,100 calories of energy. Although carbohydrates are used by the bird to provide energy for certain functions of flight as discussed earlier, there is no real evidence that, under normal conditions, protein is utilized as a source of fuel for muscular work. Only under extreme conditions, such as the complete depletion of fat and carbohydrate reserves, would pigeons be expected to use protein as fuel. What other supportive evidence do we have to conclude that fat is THE major fuel for racing ? Experimental work on the muscles of racing pigeons has shown that after as little as 30 minutes of muscular activity, the amount of fat circulating in the bloodstream of birds during exercise increases by almost 18% compared with that of non-exercised birds. These experiments, also showed that the amount of fat in the major pectoral muscles of exercised birds increases by 25 to 40%, and in the liver by almost 30%. Clinching these observations was the finding that the amount of fat in body fat depots located under the skin, within the abdomen, ect., actually decreases by almost 25% during this time. Therefore, as fuel in the from of fat is required by working muscle, This fat is mobilized from these various body depots and is transported in the bloodstream to the liver and to working muscles. Thus, this mobilizing process decreases the supplies of fat in storage areas while increasing the fat in the bloodstream and then in the liver and in working muscle. Within the red muscle fibers, the mobilized fat is transported to the area immediately next to the mitochondria where it is readily available to be metabolized ( or burned, in the biological sense) to supply the energy necessary for on-going flight. In the breast muscles, the presence of an extensive network of tiny blood vessels encircling each of the re fibers has some obvious purposes besides being an elaborate pipeline for replenishing supplies of fuel for these muscles, these vessels also bring an abundant supply of oxygen that is as necessary in the fat burning process (known as aerobic metabolism, that is, a process that requires oxygen) in muscle as it is in the burning of oil, wood or natural gas in a home furnace. As well, because of the increased production of heat by working muscle, this meshwork of vessels is able to remove and help disperse this heat by carrying it to the lungs, mouth etc., and to remove carbon dioxide and other waste products of the metabolic process. One of the beneficial by-products of the metabolism of is water. During metabolism, the burning of one unit of fat produces nine units of water that, obviously, is of tremendous benefit to the bird during a long flight.

In looking now at the white fibers, we see that, by contrast, they contain very few mitochondria and consequently, almost no fat. In the absence of fat as a reserve fuel, what fuel do the white fibers require? Close inspection shows that the major fuel for white fibers is glycogen, seen under the electron microscope as many dark granules distributed throughout these fibers. Remember that we said earlier that glycogen is comprised of many units of the sugar glucose linked together. Because white fibers are believed to be necessary for the execution of split-second movements they must have a readily-utilizable fuel to provide energy almost instantly, and glycogen is that fuel. Since the metabolism of glycogen to glucose by these fibers doesn't require oxygen, it is therefore anaerobic metabolism ( without oxygen), and there is no need for a great network of blood vessels around each fiber to supply oxygen, nor is there a need for many mitochondria, since fat is not utilized.

All this is fine, you say, but how can we put this information together so that it translates into something more comprehensible and practical? Based on the foregoing information and some closely related investigations into the breast muscles of pigeons, we can indicate the likely sequence of events in these working muscles following liberation at a training or race point. In turn, these facts provide us with substantial practical clues in the formulation of rations for racing. Let's look at these events as they might transpire from liberation at the race point, over the long miles of the race, say, a distance race.

As the birds await the time of release, we see that the white fibers are loaded with glycogen and that the red fibers are well fortified with both glycogen and abundant reserves of droplets of fat, their primary fuel. Storage depots of fat in various areas of the body have sufficient reserves for the long demanding hours of the race, but most of the birds are not overweight. There is a good balance between the amount of body fat and the physical condition of the birds. The liver also has adequate reserves of glycogen and fat that can be mobilized and transported in the blood to working muscles as they are needed. The birds have been well prepared for a distance event. Those that are slightly heavy may be at a disadvantage if it is a normal race, but if it is a tough one, their extra reserves of fat may save the day. All is ready.

Suddenly, all baskets are opened simultaneously, and in an explosion of sound reflecting a tremendous burst of muscular effort, the birds launch into the air at an attack angle of about 30 ( although they are capable of almost sheer vertical flight for a few seconds) and at a maximal climbing velocity of about 5 miles per hour, wings beating on the average of 9.4 times per second, the tips creating a figure 8 pattern in relation to the body as they sweep through an angle of 142 in one beat. To illustrate the tremendous power of the breast muscles, it is know for example, that one of the great pectoral muscles alone is capable of exerting a force about ten times the weight of the bird. It is also know that the downstroke occupies about one third of each beat, and the upstroke, about two thirds of the beat at this time. The explosive power needed to launch a bird into the air requires a massive effort which means that all of the force the pectoral muscles are capable of developing must be brought to bear at this time. The launch is a rapid, explosive action, and although we would expect the white fibers alone to operate here, it is important to recall that they constitute only 6% of the total fibers in the breast muscles, whereas the red fibers make up the vast majority or about 94% of the total number of fibers present. It does not seem logical then, that only 6% of the fibers could handle most of the tremendous amount of work involved in such a powerful action. Therefore, it seems very likely that all of the red and the white fibers working co-operatively together contract rapidly, propelling the birds upward, allowing them to gain height and to reach cruising speed.

It has been shown experimentally that within minutes after release, all of the glycogen reserves in the white fibers are completely exhausted, and for all practical purposes, their activity virtually stops for the moment, as a result of this depletion of fuel. In fact, these experiments showed that glycogen stores in the white fibers are completely depleted after the first 10 minutes of effort. By the time the birds have reached cruising speed, the number of wing beats has decreased from an initial explosive rate of 9.4 to about 5.5 beats per second. The red fibers, which are now doing all of the work, continue to be loaded with glycogen ( note the important difference from the white fibers), and very significantly, with large reserves of fat, present as microscopic droplets. These fat droplets, located adjacent to the mitochondria where they are utilized, are metabolized ( or are burned chemically) in the mitochondria in the presence of oxygen, in a process known as oxidation. One important by product of the oxidation of fat is a very high energy compound called adenosine triphosphate or ATP for short, that can be likened to the steam generated by a locomotive. In one case, the steam provides the energy to drive the locomotive; comparably, the ATP produced from the burning of fat provides the energy to power the wings to beat on the average of an estimated 5.5 times per second for many hours on end during flight. Recall that at the time of liberation, the angle created by the sweep of the wings to launch the birds into the air was about 142. At cruising speed, however, this angle decreases to about 85 for the duration of the flight. For fast flight, the powerful downstroke provides both lift and strong forward propulsion, and for this reason, a powerful upstroke is not needed, and the angle formed by the sweep of the wings can be decreased to about 85.

This efficient system operates continually over the few to many hours of the race, and provided that the muscles have been sufficiently conditioned before hand to handle the distance and the weather conditions, they operate rhythmically throughout. On this point, there is evidence to suggest that once cruising speed is reached, the wings continue to beat rhythmically and automatically by reflex action that is centered in a small area of the spinal cord. This means that the basic rhythm of the wing beat in flight likely operates automatically, without any conscious effort or will on the part of the birds. It is worth noting also that not all of the red fibers in the great pectoral muscles are likely to be working at any one time. Instead , there is evidence that they work in shifts, thus allowing some fibers to rest and replenish fuel supplies from body depots by way of the bloodstream, whereas the great majority continue working.

Recall that within 30 minutes after the birds were released, fat in the great pectoral muscles had increased in amount by up to 40% compared with the amount of fat present in the muscles of resting birds. By 2 hours after release, the amount of fat in these flight muscles has increased even more and is about 85% greater than the amount of fat in the breast muscles of resting birds. By 5 hours, the amount of fat has increased by almost 170%, over four times compared with that found in the muscles of resting birds. These findings point up once again, the very great importance of fat as a primary nutrient in fueling working muscles and the reliance placed by the bird on this key source of energy. The facts speak for themselves ! The day wears on and the hours and miles pass. The very fit birds are in front as individuals or in varying sized flocks, fat continually mobilized from the body depots and picked up by the bloodstream, to be delivered to the massive pectoral muscles which are working rapidly and efficiently. Birds that are less fit are trailing for miles behind in differing sized flocks; the hours on the wing will take their toll, and it is inevitable that some birds will not be home by dark.

Toward the end of a very long or tough day, we see that he situation regarding the fuel supplies within the two types of muscle fibers has changed remarkably. If we look again at the red and white fibers, we see that the large white fibers that became depleted of glycogen soon after the birds were released at the race point, are now, surprisingly, refueled with glycogen! How did this happen and why? The "how" is answered by pointing out again that the liver has abundant reserves of glycogen, and it is likely at this source that glycogen was converted to glucose for transport in the blood stream to these muscle fibers, where in turn, it was re converted to glycogen, its storage form. The "why" may be explained by looking at the obvious advantages of this refueling process in the white fibers. After all, once the launch has been accomplished and the birds have reached cruising speed, there may be lightning fast aerial predators form which escape is critical. Power lines, telephone wires, cables, and the possibility of collisions with other birds in the flock, are all potential reasons for rapid escape or avoidance, and the bird must be ready for dodging bursts of speed at any time during the race. The white fibers must be refueled to take care of contingencies, and this replenishing process begins soon after these fibers were depleted following liberation. Even at the extreme time of 18 hours after release, these white fibers are found to contain abundant amounts of replenished glycogen.

However, the situation in the red fibers, the real work horses of the muscular system, has equally dramatically changed, but in the reverse direction. Once loaded with both fat and glycogen, the red fibers at 18 hours, in general, are seen to be severely depleted of fat and may be slightly to severely depleted of glycogen. Incidentally, the role of glycogen in the function of red fibers is not completely clear, but since fat is seriously depleted and glycogen reserves may be seriously depleted by 18 hours, one suggestion is that glycogen may be associated with providing a continuous supply of a substance called oxaloacetate which is one of several in the burning of fat and the liberation of energy in the red fibers. It is also possible that a significant role of glycogen in red fibers is to provide readily available fuel for the powerful muscular contractions that launch a bird into the air at the time of liberation. Although fat and glycogen continue to be evident in the red fibers, reserves of both are becoming low. Most of the red fibers, especially those in the center of a bundle of fibers, are particularly low in reserves of fat, but it is also interesting that others near the surface of a bundle have increased amounts of fat. These findings, among others, suggest that red fibers located close to the edge of a bundle are the first to begin work, and therefore, are the first to tire. As their "shift" is completed, they stop work to refuel, and their function is taken over by the next "shift" of fibers, which are located somewhat more deeply inside a bundle, and so on. Quite likely, long before these fuel supplies have decreased so drastically, the bird has begun to experience increasing signs of fatigue, as its all important major fuel reserves diminish steadily.

These facts related to the major fuel reserves can perhaps give us a significant clue as to why there are so relatively few racing pigeons willing or able to maintain sustained flight beyond perhaps 14 to 15 hours in one stretch, or why long races, or any very tough short or middle distance races may become second day races or more simply because necessary fuel reserves are running precariously low, and fatigue is setting in relentlessly. The bird is in dire need of some external sources of fuel such as cereal grains, and something we have not really touched upon so far: water, which next to air, is the most important requirement for life and something we all tend to take for granted. As noted previously, water becomes available to the bird as a by product of the burning of fat, but as fat reserves are consumed during flight, the amount of water supplied by this fat diminishes accordingly. Water and food may be available in some locales along the route and very scarce or non existent in others. Some body reserves of glycogen and fat may exist together in the liver and other sites, and during a rest period, fat may be mobilized from all available sites of storage in the body to allow the bird to carry on. It fat reserves fall too low, the bird simply has to stop to find water to combat dehydration, and grain to restore enough glycogen and fat to allow it to reach home. Depending on the availability of food and water, the possibility of concurrent injury, and the distance remaining to reach home, the process of restoring the bird to a reasonable flying state may take days or weeks. Those birds that reach home just before night fall from a long grueling race have almost reached the end of their fat and glycogen fuel reserves, much as your gas tank registers nearly empty when there are prolonged distances between gas stations.

Conclusion Next Month.