Running Science

by Owen Anderson

  Older Runners

  Older runners experience declines in muscle mass, muscle contractility, muscle elasticity, and bone density, along with drops in overall immune-system functioning. Such concerns are particularly relevant, given that masters and grand-masters participation in long-distance running events like the marathon has increased significantly in the last 10 years. Some studies have indeed shown that older runners have increased risks of leg injuries, compared with younger runners, especially including problems with the Achilles tendon, hamstrings, and calf muscles.

  However, running also provides many benefits for the older athlete, including decreased risks of heart disease, diabetes, high blood pressure, cancer, anxiety, depression, excess weight gain, and loss of mobility and coordination. If carried out properly, without the risk of overtraining, regular running can actually increase bone density in the feet, ankles, and legs, compared with a sedentary lifestyle. The use of running-specific strength training should further augment bone density and decrease the risk of injury in older runners.

  Older female runners are at higher risk for osteoporosis compared with their male peers and should be particularly conscious of maintaining sound nutritional practices, recovering well between workouts, and conducting running-specific strength training at least twice a week.

  Despite recent reports that running might in effect run a high-volume runner right into the grave, there is actually little evidence to suggest that a normal running program is damaging to the heart. There is a slight increase in risk for a cardiovascular event during running with aging, but a master runner’s heart responds to and recovers from training in a manner very similar to a younger runner’s.

  Conclusion

  Reasonable and progressive amounts of running appear to do no harm to children and can promote healthy levels of fitness and strength. Injury risk is actually lower in younger runners compared with older athletes, particularly if training is varied and does not focus entirely on running. Young female athletes are at increased risk of the female athlete triad, a condition requiring a multidisciplinary approach for proper intervention. The osteoporotic effects of the triad can be partially countered with a resistance-training regime supported by sound nutritional practices. Moderate levels of running appear to be safe during pregnancy especially after 18 weeks of gestation. Older runners can protect themselves from injury by enhancing recovery between workouts and conducting running-specific strength training on a regular basis.

  Part X

  Running Nutrition

  Chapter 43

  Energy Sources and Fuel Use for Runners

  According to the popular dictum, “You are what you eat.” This maxim is particularly appropriate for endurance runners, who depend on intakes of the appropriate amounts and types of nutrients to maintain their running performances. Failure to ingest adequate quantities of certain nutriments can lead to dramatic falloffs in performance times. Like all humans, runners rely on the carbohydrate, fat, and protein found in ingested foods to provide the energy and structural components necessary to maintain normal cellular activities.

  During actual running, the main nutrients used for energy are the carbohydrate and fat stored within a runner’s body. Protein contributes a very small portion of the energy that is needed.1 Carbohydrate, fat, and protein cannot directly provide the energy required for muscular contractions, however. During a workout or race, chemical pathways convert carbohydrate, fat, and a tiny amount of protein into a form of energy that muscles can use directly to shorten and thus create propulsive forces. The process by which the energy in carbohydrate and fat is converted into usable energy is called bioenergetics.

  Converting Carbohydrate to Energy

  Ingested carbohydrate stored in a runner’s body provides a quickly available form of energy. A single gram of carbohydrate (about .035 ounces) provides four kilocalories of energy—enough to run approximately 1/25th of a mile (1.6 km).2 As a point of reference, a banana furnishes 25 grams (.88 oz) of carbohydrate that—when stored in the muscles—would supply enough energy to run one mile.

  Carbohydrates exist in three forms: monosaccharides, disaccharides, and polysaccharides.3 Monosaccharides are simple sugars with just six or fewer carbon atoms. There are dozens of monosaccharides, including fructose, galactose, mannose, ribose, and xylose, but the most important monosaccharide for runners is glucose, aka blood sugar. The term blood sugar is a bit misleading from a functional standpoint because glucose plays its most critical role inside muscle cells where it can be broken down to produce adenosine triphosphate (ATP), which is the actual and immediate source of usable energy for muscle contraction.

  Disaccharides are formed by combining two monosaccharides and are also important for runners. Table sugar, or sucrose, is the most common disaccharide found in runners’ diets. Table sugar is formed from two monosaccharides: glucose and fructose, which is sometimes called fruit sugar because of its ubiquitous presence in fruits. Sucrose can supply runners with large fractions of their daily energy needs. For example, research has shown that about 10 to 20 percent of the daily caloric intake of elite Kenyan runners comes from table sugar.4

  Polysaccharides, complex carbohydrates that contain three or more monosaccharides, are enormously important in endurance running. The two most common forms of polysaccharides found in plants are starch and cellulose; the starch found in vegetables, grains, and beans is an essential source of carbohydrate in a runner’s daily eating plan. After they are eaten, polysaccharides are broken down by a runner’s digestive system to form monosaccharides such as glucose that can be used immediately for energy (i.e., ATP production). If the glucose is not needed right away for energy, it can be linked together in long chains to form glycogen, which is the key polysaccharide found in runners’ and all animals’ muscle fibers.

  Glycogen provides most of the energy required for runners to run at their best-possible speeds in competitions lasting from 2 to 180 minutes. Glycogen depletion, or the reduction of glycogen concentrations within muscle cells to low levels, is linked with strong sensations of fatigue; it has also been found to be an important signal that stimulates muscles to adapt to the training being conducted. Glycogen molecules are stored inside muscles and are usually quite large, consisting of hundreds to thousands of glucose molecules.5

  Consuming carbohydrate boosts intramuscular glycogen synthesis.

  McPHOTO/BLE/Insandco/age fotostock

  When runners conduct workouts or participate in 5Ks, 10Ks, or marathons, their muscle cells break down stored glycogen to release glucose molecules in a process called glycogenolysis. The glucose can then be used to produce ATP, providing the energy needed for muscle contractions. This process of splitting off glucose molecules from glycogen and then using glucose to generate ATP is the main source of energy for endurance runners in events lasting up to three hours. The liver joins the muscles in glycogenolysis by producing glucose and releasing it into the bloodstream where it can be picked up by muscle cells to provide an additional source of energy.5

  Despite its importance, glycogen is stored inside a runner’s body in rather small amounts, and glycogen can be nearly totally depleted from the muscles and liver after just a couple of hours of steady running. Furthermore, intramuscular glycogen concentrations can fall far enough in just one hour of running to reduce the quality of performance because the process of breaking glucose off from glycogen to generate ATP is hindered. For that reason, glycogen synthesis and storage are processes that should be optimized by competitive endurance runners. The greater the amount of glycogen stored in muscles and the liver, the longer an endurance runner can sustain a quality running pace. Not surprisingly, high-carbohydrate diets tend to enhance glycogen storage while low-carbohydrate diets reduce muscle glycogen. Guidelines for carbohydrate ingestion are provided in chapter 44.

  Converting Fat to Energy

  Fat molecules contain the same three elements found in carbohydrate—carbon, oxygen, and hydrogen—but the rat
io of carbon to oxygen is much greater in fats.5 This has an important consequence that will be explained later in the chapter. A gram of fat also provides more than two times as much energy as an equivalent amount of carbohydrate: Each gram of fat yields nine kilocalories, enough energy to run about 1/11th of a mile (.15 km).6 (Recall that a gram of carbohydrate provides adequate energy to run just 1/25th of a mile [.06 km].) An interesting comparison is that a whole banana, which is all carbohydrate, is needed to provide adequate fuel to run a mile (1.6 km), whereas a single tablespoon of olive oil, which is all fat, can supply the energy needed to complete that same mile.

  Fatty acids represent the primary type of fat used by muscle cells to create the energy necessary for running. Fatty acids are stored in the muscles, fat cells, and other tissues as triglycerides. Each triglyceride consists of three molecules of fatty acids and one molecule of glycerol linked together. During running, triglycerides can be broken down into their component fatty acids and glycerol, and the fatty acids can be used to create ATP; this overall process is called lipolysis. Muscle fibers cannot use glycerol directly for energy, but the liver can convert glycerol to glucose, the key ATP-generating monosaccharide.

  While the storage of glycogen is rather tightly capped inside a runner’s body, fat can be stored to an almost unlimited degree. In this sense, fat represents a productive and reliable source of fuel for endurance runners. Even the skinniest runner ordinarily has enough body fat to run for extraordinarily long distances. For example, a slender, 120-pound (54 kg) male runner with just 4 percent body fat and thus 4.8 pounds (2.2 kg) of fat mass, could run for nearly 170 miles (274 km) using fat as the source of energy assuming that all his body fat could be broken down to create ATP. In contrast, the same athlete could ordinarily run no farther than 18 to 20 miles (29-32 km) when relying solely on carbohydrate.

  This might suggest that endurance runners should eat or train in ways that would ultimately produce less reliance on carbohydrate for energy and promote greater fat usage during running. However, because fat has a higher ration of carbon to oxygen compared with carbohydrate, the use of fat rather than carbohydrate as the primary fuel source during running raises the oxygen consumption rate associated with a particular speed. This means that the chosen velocity is carried out at a higher percentage of O2max. Since perceived effort and percentage of O2max are fairly tightly linked, the selected speed will feel much more difficult to sustain when fat is the primary source of energy even though total fuel availability with fat is greater than with carbohydrate.

  Converting Protein to Energy

  Proteins are composed of carbon, oxygen, hydrogen, and an additional element: nitrogen. All proteins are composed of subunits called amino acids. At least 20 types of amino acids are needed by a runner’s body for normal functioning. As is the case with carbohydrate, a gram of protein provides four kilocalories of energy. Protein can provide energy for running in two ways.5 First, an amino acid called alanine can be converted in the liver to glucose, which can then move through the blood to the muscles where it can be used for immediate energy or to create glycogen. Second, many amino acids can be converted inside muscle cells into compounds called metabolic intermediates, which can then be broken down directly to create ATP.7

  If a running workout or competition lasts less than an hour, protein generally provides less than 2 percent of the energy required to complete the exertion. When exercise is more prolonged, protein can contribute from 5 to 15 percent of the needed energy during the final minutes of the effort.8 It is clear that carbohydrate and fat furnish the largest share of energy during running even when a run is quite extended.

  Which Fuel the Body Prefers and When

  The relative balance between carbohydrate and fat breakdown during running is influenced by a number of factors, including diet, running intensity (i.e., speed), and the duration of the effort. Science tells us that high-fat, low-carbohydrate diets tend to increase the rate of fat breakdown during running while high-carbohydrate, low-fat diets tends to heighten the use of carbohydrate for fuel during workouts and races. Fats are the primary source of fuel for muscles during low-intensity running carried out at less than 30 percent of O2max (i.e., less than 55 percent of maximal heart rate) while carbohydrates are the preferred fuel during running carried out at intensities higher than 70 percent of O2max (i.e., greater than 80 percent of maximal heart rate).9, 10

  A general rule is that carbohydrate increases its relative contribution to the needed energy as running speed increases. The result is that carbohydrate furnishes all of the energy required for running at intensities of 100 percent of O2max and greater. There appear to be two mechanisms underlying the increased role of carbohydrate as a fuel for muscles at higher running speeds. One is that higher speeds increase the recruitment of fast-twitch muscle fibers within the leg muscles.1 These fast-twitch cells generally have a poor ability to oxidize fat and can be considered carbohydrate specialists.

  A second factor is that higher running velocities are linked with the greater production and release of a key hormone called epinephrine. Higher levels of epinephrine, aka adrenaline, in the blood increase breakdown rates of glycogen in the muscles and spur carbohydrate metabolism in general. During high-intensity running, epinephrine may also indirectly block the availability of fat as a substrate for energy production.11 As a result, the maintenance of high running speeds (i.e., at 10K speed and faster) is almost entirely dependent on carbohydrate as the fuel source.

  Exercise duration also influences the relative rates at which carbohydrate and fat are used as fuel. As the length of a running workout or competition gradually increases beyond 30 minutes, there is a progressive shift in energy creation from carbohydrate metabolism to the breakdown of greater and greater amounts of fat.12 This pattern is related to running intensity since longer runs tend to be carried out at slower speeds. In addition, the shift to fat as duration expands can be partly offset by the ingestion of carbohydrate-containing sport drinks.

  ATP as the Body’s Primary Energy Currency

  As mentioned, the energy contained in the food that a runner eats, whether it is carbohydrate, protein, or fat, must first be converted to energy within ATP molecules before it can be used by muscle cells to provide propulsive forces. ATP is always present in muscle cells and indeed in all the living cells in a runner’s body; without ATP, cells would quickly stop working and die. ATP is often called the universal energy donor because it provides energy to all cells, not just a select few, but perhaps a better name would be the body’s primary energy currency. Carbohydrate, fat, and protein are not directly usable by cells for energy, but ATP is immediately available; it is the only energy molecule that is directly serviceable for muscle contraction and other cellular activities.

  Since ATP is so important, runners and coaches sometimes think that it might be possible and beneficial to increase intrinsic ATP concentrations inside muscle cells. However, science reveals that human cells refuse to stockpile the precious high-energy phosphate; they are much more interested in storing carbohydrate and fat. Since ATP is stored to a very limited degree, and since running depends on a steady, often expansive supply of ATP to provide the energy needed for muscular contractions, there is a strong need for muscles to have dependable metabolic pathways that can provide ATP at a rapid, reliable rate. If there is not going to be much ATP at the ready, then there has to be some way for runners to manufacture it in a quick, predictable manner. As it turns out, runners have three such ATP-creating pathways—that is, three unique and distinct series of chemical reactions whose sole purpose is creating ATP. These three pathways are discussed in turn in the following sections.

  ATP-PC System

  The simplest and most rapid pathway involves a chemical found inside muscle cells called phosphocreatine. Phosphocreatine cannot provide energy for muscle contractions directly—only ATP can do that—but it is very willing to donate a high-energy phosphate group to a chemical called ADP to form ATP. This cr
itically important reaction is catalyzed by an enzyme called creatine kinase, which is consequently found in significant concentrations inside muscle cells. The ubiquity of creatine kinase in muscle tissue explains why high levels of creatine kinase in the blood are associated with muscle breakdown, including the kinds of catastrophes that can occur in cardiac muscle tissue after a heart attack.

  One special feature of phosphocreatine is that its intramuscular levels are not as tightly capped as ATP concentrations. This means the muscles can allow phosphocreatine concentrations to soar quite dramatically, which helps explain why creatine supplementation has been so successful from a performance standpoint in high-power athletics. Creatine added to the diet is absorbed readily and makes it way to the muscles where it combines with the phosphates that are always present to make phosphocreatine. Runners who have a substantial amount of phosphocreatine in their muscle cells can generate a lot of ATP in a very short period of time. These runners will have the potential to improve sprinting ability, which depends on a readily available source of large amounts of ATP.

  All physiological systems have their limits, however. The system just described, which is sometimes called the ATP-PC system or even the phosphagen system, has definite limitations. A key factor is that the phosphagen system, even in athletes who have creatine-loaded their muscles, can probably provide energy for no more than about 8 to 10 seconds of intense muscular exertion, at which point phosphocreatine becomes depleted. This means that the system can work effectively for high jumpers, power weightlifters, 50-meter sprinters, elite 100-meter sprinters, pole vaulters, cricket bowlers, soccer players racing across the pitch during the opening moments of play, and other athletes whose sports call for short bursts of high-intensity exertion.