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. 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 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. 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. Since total carbohydrate intake is low because of the modest intake of total calories, muscle glycogen stores become depleted. Blood glucose levels are thus maintained by gluconeogenesis, a process 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.
How to Determine 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 who weighs 180 pounds with 15 percent body 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, 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. 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.
Research suggests that weight loss should proceed slowly at no more than 1 pound 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.
Runners with low to moderate training volumes can lose weight effectively via a combination of increased training and reduced caloric intake that produces an average daily energy deficit of about 500 calories. Higher-volume runners often have difficulty increasing volume and thus must rely on changes in energy intake to achieve goal weight. A reduction in the percentage of dietary fat consumed can be an effective weight-loss strategy as long as it is not extreme, healthy fats (e.g., omega-3) are retained in the diet, and the effect is a drop in total caloric intake. Find more information about successfully losing fat as an endurance athlete in Running Science, available in bookstores everywhere or online at www.HumanKinetics.com.race.