Energy Gels
The issue of carbohydrate concentration makes the use of energy gels problematic during sustained endurance running. Although the gels are attractive from a weight standpoint, and it’s much easier to carry them during a run than lugging heavy bottles of sport drinks, their use can easily create hyperconcentrated stomach solutions that can increase the risks of gastric distress and diarrhea. A typical small pack of energy gel contains 25 grams of carbohydrate. If this were ingested with a 6-ounce portion of water on a relatively empty stomach, the result would be a 14 percent gastric solution that would increase the risk of gastrointestinal discomfort.
Topping off a steady stream of sport drink intake with gel could be worse yet, as the carbs in the gel would mix with those from the sport drink in the stomach to make a molasses-like mixture. Runners who insist on using gels should calculate appropriate amounts of gel and water before running. One half of a gel pack, with 12.5 grams of carbohydrate, could be consumed with 175 ml (6 oz) of plain water to create a 7 percent sport drink–like solution in the stomach. To be effective, this exact intake would have to be repeated approximately every 15 minutes. Individuals who are attracted to gels should be aware that such products do not provide an energy boost at mile 20 of the marathon as many runners believe. The gel carbohydrate taken into the stomach at that late stage of a marathon will usually not even reach the muscles to be oxidized until the runner is wrapped in a heat sheet after finishing the race.
Calculating Carbohydrate Concentration
Given the potential negative effects of too high a concentration of carbohydrate, runners need to know how to determine the concentration of a sport drink. This is an important consideration: Some races feature sport drinks with carb concentrations outside the optimal range; runners should be aware of this and plan accordingly by bringing their own sport beverage. Sometimes, an attractive new sport drink looks appealing but does not clearly state its carbohydrate potency in percentage fashion; a runner then may wonder if the drink is actually worth buying.
Fortunately, it is easy to figure the carbohydrate concentration in any sport drink. Carbohydrate concentration in a sport beverage is always reckoned as grams of carbohydrate per 100 milliliters (3.38 oz) of fluid. For example, if a sport drink declares that it has 7 grams of carbohydrate in each 100 milliliters, it is a 7 percent sport drink. But sometimes it takes a little bit of reckoning to figure things out. For example, a liter (1.06 qt) of sport drink found on a store shelf might state its carbohydrate content at 80 grams (320 calories). A liter has 10 100-milliliter components, and thus by dividing 80 grams by the number of components, a runner can determine that the sport drink has 8 grams per 100 ml. The carb content of this drink is 8 grams per 100 ml, and so the beverage has an acceptable 8 percent carbohydrate concentration.
At other times, the math can get a bit trickier. As an example, a sport drink product might state its carb concentration at 27 grams per 12 ounces. In this case, the ounces need to be converted to liters. One liter is 33.9 ounces so the following equation converts the ounces to liters:
12 ounces/33.9 ounces = .35 liters or 350 mL
Thus the sport drink contains 27 grams of carbohydrate per 350 milliliters. There are 3.5 100-milliliter units within 350 milliliters, so the following equation can be used to reach grams per 100 milliliters:
27/3.5 = 7.7 grams per 100 mL
Thus, the drink in question is a 7.7 percent sport beverage. It is within the optimal range of 6 to 10 percent and can be used successfully during prolonged runs lasting an hour or more and during very intense sessions as well.
Carbohydrate Type
Exercise scientists have carried out research to determine the best type of carbohydrate for sport drinks. Asker Jeukendrup and his colleagues in the Human Performance Laboratory at the School of Sport and Exercise Sciences at the University of Birmingham in the United Kingdom have discovered that the presence of varied intestinal transport mechanisms in the small intestine means that a mix of carbohydrates is preferable over a single carb source. Mixing carbohydrates together can actually raise carbohydrate oxidation during exercise above the gold standard of 1 gram (.04 oz) per minute.
Transport mechanisms come into play because glucose, fructose, sucrose, and other carbohydrates cannot move freely across the wall of the small intestine. Their movement from inside the hollow small intestine into the small blood capillaries that will carry the carbs into general circulation and thus to the muscles depends on transport proteins embedded in the walls of the small intestine. These proteins help give carbohydrate molecules an inward-directed ride through the wall of the gut and thus into the circulatory system, one example of how a transport mechanism works.
Glucose absorption depends on a sodium-dependent glucose transporter called SGLT1. Sodium-dependent means that sodium must be present for SGLT1 to do its job, which is a key reason why sport drinks contain sodium. Fructose, another simple, six-carbon sugar, seems to depend entirely for its absorption on a transporter called GLUT5 that is quite different from SGLT1. The mechanism underlying the absorption of sucrose, aka table sugar, which is a disaccharide composed of one part glucose and one part fructose, is controversial. Some scientists argue that sucrose is simply hydrolyzed to glucose and fructose at the small intestine’s inner membrane, followed by absorption of the two constituents using the SGLT1 and GLUT5 transporters. However, there is some evidence that disaccharides like sucrose are actually absorbed by specific disaccharidase-related transporters that are independent of SGLT1 and GLUT5.19
There are not an infinite number of SGLT1 transporters in the inner walls of the small intestine, nor is there an overwhelming quantity of GLUT5 carriers. The densities of these carriers appear to be rather moderate—good enough for a sedentary person but not ample enough for the endurance athlete who wants to maximize the carbohydrate exit rate from the small intestine and subsequent carbohydrate entry into the circulatory system during exercise. What may happen if a runner’s sport drink contains only glucose is that all of the SGLT1 carriers may become busy (i.e., attached to glucose molecules) as the runner moves along during his or her half marathon or marathon. Other glucose molecules wait impatiently in the small intestine, looking forward to their speedy passage to the muscles, but they can’t move into the blood because all of the transport vans, or carriers, in the intestinal wall are fully booked.
If this is the case, a drink that contains glucose plus an additional carbohydrate, thus relying on both SGLT1 and a second type of transporter, should provide a speedier passage of carbs into the blood. In theory, a beverage with three types of carbohydrate would be better still as long as there were three separate transporter mechanisms.
To see if combinations of carbs were really absorbed and oxidized more quickly than single carbs during exercise and to get a feeling for which specific carbs might be optimal, Jeukendrup and his colleagues carried out a definitive study.20 Eight well-trained male cyclists or triathletes carried out three exercise trials consisting of 150 minutes of sustained cycling at an intensity of about 62 percent of O2max (~75 percent of maximal heart rate). During one of the tests, the athletes ingested plain water, during a second trial they took in a drink that contained only glucose, and during a third trial they drank a beverage with glucose, sucrose, and fructose in a 2:1:1 ratio. The average rate of glucose intake with the pure-glucose drink was 2.4 grams per minute during the trial. For the mixed-source beverage, the mean intake rate of glucose was 1.2 grams per minute while sucrose and fructose each checked in with .6 grams per minute. Thus, the total carb intake rates were equivalent in those two trials (2.4 grams per minute).
Although carb intake rates were identical in the trials using the glucose beverage and the mixture of glucose, sucrose, and fructose, the rate of actual oxidation of exogenous carbohydrate peaked at 1.70 grams per minute for the mixed-source concoction versus just 1.18 grams per minute for pure glucose, about a 44 percent difference. Total exogenous carbohydrate oxidat
ion for the entire trial was 50 percent higher for the mixed-source drink compared with the glucose beverage—and was 70 percent higher than the traditional standard of 1 gram per minute.
This innovative research indicates that muscles can break down exogenous carbohydrate for energy at extremely high rates: 1.70 grams of carbohydrate per minute when 2.4 grams per minute are ingested if sport drinks contain a mix of carbohydrates. Previous work in Jeukendrup’s laboratory had revealed that an intake of 1.8 grams per minute of mixed carbs led to an exogenous carbohydrate oxidation rate of 1.3 grams per minute, also well above the benchmark of 1 gram per minute.
It should be noted that this research was carried out with cyclists, however, and the benefits for runners may not be as attainable because of the large fluid intakes required. A runner using an 8 percent sport drink in hopes of taking in 2.4 grams of carbohydrate per minute would have to consume 1.8 liters (1.9 qt) of beverage per hour, an enormous amount of fluid. For an intake of 1.8 grams per minute, the runner would have to take in 1.35 liters (1.43 qt) per hour, also a very heavy load. Such large intakes are much easier for cyclists to handle during exertion. This is a key reason why a sport drink with a higher carb concentration like Shaklee Performance Pure Hydration would be preferred for runners. At 10 percent, Shaklee Performance would provide 83 grams of carbohydrate per hour with a seven-swallow per 15-minute ingestion rate, 1.4 grams per minute. That’s not as much as cyclists are getting, but it is 25 percent more carbohydrate per hour, than one gets with an 8 percent sport drink.
Runners who are interested in trying mixed-carbohydrate sport drinks can make their own. The recipe in figure 45.1 is for a 10 percent drink. Powdered glucose and fructose can be purchased from various online sources, and sucrose is readily available.
The result will be a 10 percent sport drink with the exact relative composition of sugars used by Jeukendrup in his study, the combo that boosted exogenous carbohydrate oxidation to 1.7 grams per minute. To achieve a carbohydrate-intake rate of 1.8 grams per minute, a runner would have to ingest about 1.1 liters (1.16 qt) of this beverage per hour—or .26 liter (9 oz) every 15 minutes.
Runners who are concerned about the increasingly high cost of commercial sport beverages or who do not want to go to the trouble of mixing types of powders can make their own, perfectly workable 7.6 percent sport drink with the recipe in figure 45.2.
This mixture is not quite as sophisticated or effective as Jeukendrup’s triad of glucose, sucrose, and fructose, but it will support 1 gram per minute of carbohydrate oxidation when six swallows are ingested every 15 minutes and thus will enhance performance during races and workouts lasting longer than an hour.
Absorption Rate
A final obstacle to overcome with the use of sport drinks is their relatively slow passage from the stomach into the small intestine, where absorption actually occurs; the stomach is a kind of holding pouch with nonabsorptive walls. Since a typical stomach-emptying rate for fluid is about 10 milliliters (.34 oz) per minute, approximately 600 milliliters (20 oz) of water or sport drink can move into the intestine from the stomach each hour under average conditions. A runner ingesting about 25 ounces (.74 L) of sport drink per hour would thus accumulate 5 ounces (.15 L) of water in his or her gullet each hour unless emptying rate could be advanced.
The good news is that water movement from the stomach to the small intestine can be optimized with a minor intervention. Research carried out by Nancy Rehrer and her colleagues has determined that the rate at which water moves from the stomach into the intestine depends on how much water is actually in the stomach, with larger volumes of stomach water permitting greater emptying rates.
In her investigations, Rehrer asked nine endurance athletes to ingest about 584 milliliters (20 oz) of sport drink and then run at the moderate pace of 70 percent of O2max, 7:34 per mile (1.6 km), for 80 minutes while swallowing an additional 146 milliliters (5 oz) of sport drink every 20 minutes during the run. Two sport drinks were used: (a) Isostar, an 8.1 percent carbohydrate concoction containing sucrose, maltose, maltodextrin, glucose, and fructose as well as electrolytes, and (b) Perform, a 19.1 percent carbohydrate drink that also contained electrolytes. On another occasion, the nine athletes consumed the sport drinks without exercising, and then they drank an artificially sweetened drink containing no carbohydrate while exercising to see whether the carbohydrate in Isostar and Perform tended to slow the stomach-emptying process.21
Over 90 percent of the Isostar ingested by the runners moved from stomach to intestine during the 80 minutes of exercise. The key to such fast movement was the large, 584-milliliter (20 oz) bolus of fluid consumed by the athletes just before they began running. This large quantity of water forced about 400 milliliters (13.5 oz) of water to pass into the small intestine within only 20 minutes, a remarkable emptying rate of about 20 milliliters (.67 oz) per minute—twice the average.
This pattern—filling the stomach well before a long race or training run and then taking in more sport drink at regular intervals—keeps the stomach full enough to maximize gastric emptying without significantly heightening the risk of gastric distress. As a result, runners can optimize water and carbohydrate absorption during endurance running; carbohydrate absorption increases because more carbohydrate is carried into the small intestine along with the water.
Many runners avoid Rehrer’s bolus prior to running out of ignorance or because they dislike running with a relatively full stomach. The latter effect is counteracted by repeated use of the prerunning bolus technique during training, which makes the presence of fluid in the gut increasingly more comfortable over time. Anecdotally, a 10-ounce (295 mL) prerun bolus, taken 10 minutes before the onset of a long run, also seems to enhance gastric-emptying rate and carbohydrate absorption and is much more comfortable than the 20-ounce (584 mL) sample originally tested by Rehrer.
Conclusion
Carbohydrate ingestion during intense interval workouts and sustained runs lasting longer than an hour enhances endurance and upgrades average running speed. The use of sport drinks provides an ideal way to ingest readily available, performance-enhancing carbohydrate. The optimal sport drink composition is from 6 to 10 percent, and runners should attempt to take in at least 60 grams of carbohydrate per hour, somewhat less for small runners, or more if a mixed-carbohydrate sport drink is employed since more carb can be absorbed with such a beverage. A 10-ounce bolus (295 mL) of sport drink, taken 10 minutes before running begins, speeds gastric emptying and thus stops the stomach from limiting the rate of carbohydrate absorption.
Chapter 46
Weight Control and Body Composition
For endurance runners, nonessential body fat can be a distinct disadvantage. Scientific research reveals that there is an inverse relationship between endurance-running performance and percent body fat. Runners who carry more body fat tend to run more slowly in competitive events for a variety of reasons.1 In general, female runners naturally have more body fat than male runners, and this is a key reason why women’s world records are roughly 8 to 10 percent slower than men’s marks. Gradual losses of body fat can have a positive effect on performance, but such drop-offs must be undertaken carefully and are not without health risks. A cautiously created training plan involving small but steady increases in volume and intensity in combination with an eating program that eliminates a moderate number of unnecessary calories without harming overall nutritional quality provides the best and safest path to improved body composition.
Fat has four key negative effects on running capacity:
Increases energy costs. Fat adds mass to a runner’s body without providing any propulsive force. Surplus fat thus increases the energy cost of running at a specific speed because there is more weight to be dragged along.
Hampers running economy. As percent body fat increases, the oxygen-consumption rate associated with a particular running velocity also rises because a greater mass must be moved at a specific speed. This hurts running economy, a key predicto
r of endurance performance.
Lessens ability to accelerate. Excess fat also makes it difficult for runners to accelerate and surge within races. During running, the ability to accelerate is inversely proportional to nonpropulsive body mass. As a result, extra fat mandates slower changes in running velocity for a given level of force production.1 For runners locked in a hard-fought competition, the outcome of the final sprint to the finish line can depend on which runner has the lowest percent body fat. Other factors being equal, the runner with the leanest body composition will often have the most suddenly initiated and most powerful kick at the end of the race.
Diminishes O2max. Upswings in body fat also reduce maximal aerobic capacity (O2max). A 70 kilogram (154 lb) male endurance runner with a O2max of 60 ml • kg-1 • min-1 uses 4,200 milliliters (142 oz) of oxygen each minute (70 × 60 = 4,200) when he is running at O2max intensity. If he adds just 1 kilogram (2.2 lb) of fat to his frame, increasing his mass to 71 kilograms (157 lb), his new O2max will be 59.15 ml • kg-1 • min-1 (4,200/71 = 59.15), a 1.4 percent fall-off that could hurt performance by a similar amount. (For these calculations, the logical and reasonable assumption is that the gain in body fat has no significant, positive effect on O2max. In other words, the extra fatty tissue has minimal metabolic demands and does not hike the oxygen burn rate significantly during running.) The performance-thwarting effects of unit gains in body fat are inversely proportional to initial body weight. A 50 kilogram (110 lb) female runner with a O2max of 60 ml • kg-1 • min-1 who augments body mass with one kilogram of fat would have a new O2max of 58.8 ml • kg-1 • min-1 (3,000/51 = 58.8), a 2 percent diminishment. Since female runners are generally smaller than males, unit increases in fat mass tend to have a more detrimental effect on aerobic capacity in females than with males. Percent body fat is usually higher in competitive female runners compared with competitive male runners, and this is one reason why top female athletes have lower values of O2max than males.
Running Science Page 63