Range of Body Composition Among Runners
Given the four negative effects of fat, it is not surprising that the best endurance runners in the world are very trim. Studies have revealed that male Olympic marathon runners have just 3 to 4 percent body fat (i.e., fat accounts for only 3 to 4 percent of total body mass).
The running community at large displays a broad range of body compositions, however. Scientific research suggests that genes can account for 25 to 40 percent of such differences in amount of fat between endurance runners.2 This information has emerged from studies of monozygotic twins in which individuals experienced an energy surplus or deficit over an extended period of time.
In one investigation, identical twin pairs were underfed to produce a negative energy balance of 1,000 calories per day over a 93-day period.3 Since the negative caloric balance was the same for all individuals, everyone should have lost about the same amount of weight, but actual weight loss ranged from 2 to 18 pounds per person! Within each twin pair, however, body mass changes were extremely similar, illustrating the significant role played by genes in weight change. Such research reveals that because of genetic differences, endurance runners will not lose weight at the same rate even when their diets have exactly the same negative energy balance. At least 250 genes are known to have an impact on body fat.4
Since genes account for 25 to 40 percent of the variation in body fat between endurance runners, the other 60 to 75 percent must be due to environmental factors: disparities in energy (food) intake and energy expenditure. The latter includes the energy expended during routine metabolism, daily activity, and endurance training and competition. The large role played by environmental factors suggests that endurance runners can put major dents in their levels of body fat by manipulating diet or training.
Pitfalls Associated With Trimming Fat and Weight
Endurance running is considered to be a lean-body sport, and many endurance runners make a concerted effort to trim body fat. The effects of body fat on performance are relatively well known in the running community, and runners also pursue fat loss for aesthetic and cultural reasons. Such efforts can limit the four negative effects of excess fat and thus lead to considerably improved performances, but they can sometimes have disastrous effects on health and competitive ability.
One pitfall associated with weight loss is that a sizable amount of body fat is essential: The human body cannot function optimally without it. Essential body fat is present throughout the nervous system, in the bone marrow, and around all organs in the body, where it provides a form of protective cushioning. Loss of this essential fat disturbs physiological functioning and harms overall health. In male endurance runners, essential body fat is believed to be approximately 3 percent of total body weight; for female runners, the percentage is believed to be about 12 percent, although this may vary from woman to woman.1
The remainder of an endurance-runner’s body fat is storage fat, an energy depot composed primarily of triglyceride-containing adipose cells that gets bigger when energy intake, or calories consumed, is consistently greater than energy expenditure and shrinks when energy expenditure crests above intake. In the United States, total body fat—essential plus storage—averages about 12 to 15 percent for young men and 25 to 28 percent for young women.5
Although from a performance standpoint it might appear that an appropriate goal would be to eliminate almost all storage fat and leave essential fat untouched, many endurance athletes engaged in weight-loss processes encounter performance and health problems when their storage-fat levels are still considerable. An inescapable truth is that there is no accepted percentage body fat standard for endurance runners. A 6 percent level of body fat might be optimal for one male endurance runner—associated with the highest-possible levels of performance and excellent overall health—but the same percentage of body fat, once achieved, could actually be linked with increased fatigue and poorer running in another male athlete. Endurance runners should approach weight loss cautiously. The attempt to achieve an ideal, leaner body composition is a trial-and-error process during which an athlete gradually develops an understanding of what levels of body fat and overall mass are best for his or her performances and health.
Losing the Fat
Endurance runners can lose body fat by increasing daily energy expenditure through increasing running volume or upgrading the intensity of training sessions; by trimming total food consumption; by changing the quality of the diet and moving from high-calorie foods to those that are less calorie dense such as fruits, vegetables, and grains; or by employing a combination of these strategies.
Experienced runners with significant training volumes often find it impractical to lose weight by expanding training. A realistic, relatively safe rate of weight loss is believed to be about 1 pound (0.45 kg) per week,1 which would require an increase in energy expenditure through additional running or cross-training of about 500 calories a day. If eating habits remain the same, 4 to 5 miles (6.4-8.1 km) of additional daily running would be required to induce this energy shortfall, or about 28 to 35 extra miles (45-56 km) per week. Such an increase in training might represent a reasonable goal for low-volume runners, but it would be a near impossibility for busy runners who are already logging about 35 miles (56 km) of running per week.
This means that a change in dietary intake will be the key factor in weight loss for many runners who train on a regular basis. Such runners have already come close to optimizing their rates of energy expenditure and thus need to explore ways to reduce energy intake. Reducing the percentage of fat in the diet can be an effective way to enhance leanness.6 Compared with carbohydrate and protein, dietary fat has an increased potential for preserving weight or stimulating weight gain in runners for the following reasons:7
Each gram of fat in the diet has more than twice as many calories as a gram of carbohydrate or protein.
Fat is digested and assimilated quite efficiently compared with carbohydrate and protein. Thus, little energy is expended to process fat. Almost all of fat’s energy can be used for metabolic requirements, training, or for the storage of new fat within the body.
Unlike carbohydrate intake, which leads to heightened carbohydrate metabolism, the ingestion of fat does not stimulate increased fat oxidation.
Science suggests that a reduction in dietary fat is not a magical way to improve body composition, however. It promotes weight loss only when it diminishes average daily energy intake.8 In most cases, a runner consuming 1,500 calories per day and 25 grams (225 calories) of daily fat will not necessarily lose more weight than another runner who takes in 1,500 calories with 50 grams (450 calories) of lipids, provided that genetic factors do not play a significant role. Fortunately, for runners desiring to lose weight, the removal of fat-rich foods from the diet often leads to a reduced daily caloric intake and thus a higher probability of weight loss because the lower-fat foods that replace the high-fat products have lower energy densities (i.e., fewer calories per unit mass).
This does not mean that fat should be eliminated from a runner’s diet. Extremely low-fat diets can lead to vitamin and mineral deficiencies and are unnecessary for weight loss.7 A reasonable recommendation is for fat to make up 15 to 20 percent of daily energy intake, with omega-3 and monounsaturated fats making up the bulk of this lipid consumption.
In an effort to lose body fat, some endurance runners are tempted to follow extremely low-calorie diets, in some cases ingesting as few as 800 to 1,000 calories per day while still maintaining a regular training program. Such dietary plans can lead to fairly rapid weight loss, but the initial decline in mass is almost entirely accounted for by decreases in internal glycogen concentrations and water levels.1 Since total carbohydrate intake is low because of the modest intake of total calories, muscle glycogen stores become depleted, and blood glucose levels are maintained by a process called gluconeogenesis, in which glycerol from triglycerides and an amino acid called alanine are used to create blood sugar. Because
of the glycogen drain from the muscles, endurance runners engaged in low-calorie dieting experience an inability to conduct high-quality workouts, greater fatigue, and a loss of competitive ability. Disturbing potential consequences of such diets also include the loss of body protein, electrolyte imbalances, and dehydration.
Determining Weight Goals
Determining a body weight goal can be a relatively straightforward process. After body fat is estimated by a competent professional, current lean body weight can be calculated. For example, a male runner with a weight of 180 pounds (82 kg) and a determined body-fat percent of 15 percent has a lean mass of 153 pounds (69 kg) (.85 × 180 = 153). In consultation with a health professional, he may decide that he wants to lower his percent body fat to 10 percent. To determine his body weight goal, he would then divide his current lean body weight (153) by the desired percent lean body weight (90) and multiply the resulting value by 100. In this case 153/90 × 100 = 170 pounds (7 kg), which is the goal weight.
There are potential pitfalls involved in this process, however. Body weight history should be considered: If a runner has never weighed less than goal mass in his or her adult life, the desired weight may be very difficult to achieve.7 Furthermore, the changes in diet that are undertaken to achieve reductions in percent body fat, especially if they produce very rapid weight loss, can lead to a number of problems, including an inadequate intake of vitamins and minerals, a lack of energy, negative changes in mood, a loss of muscle mass, reduced endurance, and even depressed immune function.9
Research suggests that weight loss should proceed slowly at no more than 1 pound (0.45 kg) per week. Runners should monitor themselves closely for negative health or performance effects associated with reduced weight. As mentioned, it is impossible to prescribe in advance an ideal percent body fat for an individual runner.
Strategies for Losing Weight While Training
Popular dietary plans such as the Zone Diet, the Atkins Diet, the Sugar-Busters Diet, and various high-protein plans present particular problems for endurance runners engaged in regular training. One of the key difficulties is that these eating strategies are simply too low in carbohydrate. In the Zone Diet, for example, carbohydrate accounts for only 40 percent of daily caloric intake, and relative carbohydrate input can be even lower in the Atkins, Sugar-Buster, and high-protein plans. Such eating patterns can lead to the exhaustion of glycogen stores in the muscles and liver and thus a reduced ability to perform during prolonged or intense workouts.10 The plans can produce rather quick initial weight loss, but the consequent reductions in training volume and intensity can make the maintenance of weight loss difficult.
Best Intensity for Burning Body Fat
Endurance runners are sometimes told that specific running intensities are optimal for burning body fat and thus producing the greatest improvement in body composition. Such recommendations are based on an inescapable fact about fat metabolism: The rate at which fat is metabolized for energy during running does depend heavily on the intensity of the running being conducted. During high-intensity efforts—a 400-meter sprint or a 5K race, for example—almost no fat is broken down to provide the needed energy. However, a jog carried out at a very easy pace will rely to a larger extent on stored fat to furnish the necessary energy, especially if the exertion continues for longer than 30 to 60 minutes. The slower the pace, the greater the contribution made by fat to the required energy pie.
At a running intensity of 50 percent of O2max, which often corresponds with about 65 percent of maximal heart rate, fat can provide about half of the energy, or calories required to keep moving. If this kind of easy exertion is sustained for a long time, fat’s share of the energy pie increases. After two hours of jogging at 50 percent of O2max, fat will be contributing 70 percent or more of the total required energy.11
Such findings suggest that the use of easy running paces might be best for getting rid of unwanted body fat. Indeed, for purposes of upgrading body composition, some trainers, aerobics instructors, health professionals, and coaches recommend exercising within what is often called the fat-burning zone, generally thought to be the range of intensities between 50 and 60 percent of O2max (i.e., 65 to 73 percent of maximal heart rate). For many endurance runners, this would mean running at speeds ranging from 1.5 to 3 minutes per mile (1.6 km) slower than marathon pace.11
Scientific research reveals that such advice is misguided. While it is true that fat provides only 33 percent of the required calories when an endurance runner moves along at 75 percent of O2max (i.e., 84 percent of maximal heart rate), the total calories expended per minute are greater with the higher-intensity running and thus the potential for weight loss is greater. A moderately fit runner exercising at 50 percent of O2max burns about 220 calories during a 30-minute workout; the same runner, when working at 75 percent of O2max, would expend roughly 330 calories in the same time. Since 50 percent of 220 and 33 percent of 330 yield the identical number—110 calories—it is easy to see that total fat burning is identical in the two kinds of sessions, and that energy expenditure is greater in the latter. Training in the so-called fat-burning zone often provides no special increase in fat burning, and such training would certainly have a less potent effect on overall weight loss and fitness compared with higher training intensities. Greater fitness is an excellent stimulus for fat loss since it is generally associated with longer and higher-quality workouts.
Some endurance runners worry that high-intensity training will burn little fat during workouts and thus will not be conducive to chipping away at unwanted storage fat. Such concerns are ill-founded. High-quality training can induce a substantial caloric deficit and a significant decline in internal glycogen stores. During the 24 hours following a high-quality session, as glycogen depots are gradually being refilled, a runner’s body must turn to other internal stores of energy to keep metabolism going and stimulate recovery. The source of this energy will often be stored fat, and the greater energy deficits associated with high-quality training will lead to a dramatically increased draw on stored fat. Anecdotally, the leanest endurance runners in the world are the elite Kenyans, and they seldom train at low intensities or in the fat-burning zone.12
Impact of Strength Training
The overall training program that produces optimal changes in body composition for endurance runners has not yet been identified. However, it is clear that engaging in regular strength training can be beneficial.13, 14 Strength training can increase the amount of metabolically active tissue (i.e., muscle) in a runner’s body and therefore enhance daily energy expenditure, making it easier for a runner working toward leanness to keep his or her daily energy budget in the red on a frequent basis. Weight loss can lower resting metabolic rate (RMR) and thus make it difficult to preserve decreases in weight, but resistance training can counteract this effect by transforming the body into a more metabolically active collection of tissues.1
Effects of Resting Metabolic Rate on Weight Loss
A problem for runners who want to reduce weight is that losses in body mass become increasingly difficult to make as the number on the scale drops. One difficulty is that RMR diminishes as weight is lost.15 This lowers daily energy expenditure and thus potentially pushes a runner’s energy budget up into the black (i.e., out of deficit territory), and caloric intake exceeds the newly decreased expenditure. In humans and other animals, a kind of autoregulatory feedback mechanism appears to operate during periods of sustained weight loss and causes the body to become more efficient with energy expendature. This mechanism can block further weight loss even though an apparent net energy deficit is being maintained1 and can even lead to weight gain in situations in which caloric intakes appear to be quite low and strenuous training is being maintained.16
This accounts for the discovery that some endurance runners, particularly females, have unusually low energy intakes that would not appear to be able to satisfy the combined demands of resting metabolism, normal daily activity, and training. Research ha
s shown that some highly trained female runners have energy intakes comparable in magnitude to sedentary women of the same age despite the fact that the athletes are running from 30 to 90 kilometers (19-56 mi) each week!17, 18
Female runners with high-volume training programs and modest intakes of calories are more likely to have significantly depressed resting metabolic rates and to experience amenorrhea.19 To avoid amenorrhea—and thus protect bone mass—and to perform at a higher level by refilling depressed glycogen stores in the muscles, such female runners would have to increase food intake; however, increased eating would very likely lead to unwanted gains in body weight since the runners’ metabolic processes have become more efficient. The optimal strategy for emerging from this energy efficiency trap has yet to be determined. It would seem, however, that a gradual, inchmeal increase in food intake might be the best way to restore menstrual status, improve performance, and move away from energy superefficiency without piling on undesired mass.
Tracking Caloric and Carbohydrate Intake
Endurance runners are often unaware of how many calories they ingest on a daily basis. This lack of awareness can have significant consequences for weight loss, glycogen replenishment, and performance. Runners who are attempting to lose weight are at even more risk of consuming too few calories even when they think they are following a healthy diet.
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