Body composition and body weight are two of the many factors that contribute to optimal exercise performance. Taken together, these two factors may affect an athlete's potential for success for a given sport. Body weight can influence an athlete's speed, endurance, and power, whereas body composition can affect an athlete's strength, agility, and appearance. A lean body, i.e., one with greater muscle/fat ratio, is often advantageous in sports where speed is involved.
Athletic performance cannot be accurately predicted based solely on body weight and composition given that many factors affect body composition. Some sports dictate that athletes make changes in body weight and composition that may not be best for the individual athlete. Athletes who participate in weight-class sports—such as wrestling or lightweight rowing—may be required to lose or gain weight to qualify for a specific weight category. Athletes who participate in body-conscious sports, such as dance, gymnastics, figure skating, or diving, may be pressured to lose weight and body fat to have a lean physique, although their current weight for health and performance is appropriate. With extreme energy restrictions, losses of both muscle and fat mass may adversely influence an athlete's performance.
Individualized assessment of an athlete's body composition and body weight or body image may be advantageous for the improvement of athletic performance. Age, sex, genetics, and the requirements of the sport are factors that impact the individual athlete's body composition. An optimal competitive body weight and relative body fatness should be determined when an athlete is healthy and performing at his or her best.
Methodology and equipment to perform body composition assessments must be accessible and cost-effective. Not all of the following methods meet these criteria for the practitioner. In addition, athletes and coaches should know that there are errors associated with all body composition techniques and that it is not appropriate to set a specific body fat percentage goal for an individual athlete. Rather, a range of target percentages of body fat values should be recommended.
Assessment Methodology. Three levels of assessment techniques are used to assess body composition. Direct assessment based on analysis of cadavers, although not used in clinical practice, is designated as a Level 1 technique. The other two technique levels are indirect assessments (Level II) and doubly indirect assessments (Level III). Hydrodensitometry, or underwater weighing, dual-energy x-ray absorptiometry (DXA), and air displacement plethysmography are Level II techniques, and skinfold measurements and bioelectrical impedance analysis (BIA) are Level III techniques. Levels II and III techniques are used in practice by sports dietitians.
Underwater weighing, once considered the criterion standard, is no longer common. DXA, originally developed to assess bone mineral, can be used for body composition analysis. Although DXA is fairly accurate, quick, and noninvasive, the cost of and access to the instrument limits its use in practice. Air displacement plethysmography (BodPod; Life Measurement, Inc, Concord, CA) is also used to determine body composition by body density, and body fat percentage is calculated using the equation of either Siri or Brozek. Although this method provides valid and reliable assessment of body composition, it may underestimate body fat in adults and children by 2%-3%.
Two of the most commonly used Level III methods are skinfold measurements and BIA. In addition, measures of body weight, height, wrist and girth circumferences, and skinfold measurements are routinely used by sports dietitians to assess body composition. Usually, seven skinfold sites are used including abdominal, biceps, front thigh, medial calf, subscapular, supraspinale, and triceps. The standard techniques and definitions of each of these sites are provided by Heymsfield et al. and Marfell-Jones et al.. Prediction equations using skinfold measurements to determine body fat content are numerous. Approximately 50%-70% of the variance in body density is accounted for by this measurement. In addition, population differences limit the ability to interchange the prediction equations and standardization of skinfold sites and skinfold measurement techniques vary from investigator to investigator. Even the skinfold caliper is a source of variability. Despite the inherent problems of skinfold measurement, this technique remains a method of choice because it is convenient and inexpensive. The US Olympic Committee (USOC) is using the International Society for Advances in Kinanthropometry (ISAK) techniques as efforts are underway to standardize measures worldwide. The USOC advocates using the sum of seven skinfolds (mm) based on ISAK landmarks, marking skinfold sites on the body, reporting duplicate measures, and communicating the results as a range, rather than percentage of body fat.
BIA is based on the principle that an electrical signal is more easily conducted through lean tissue than fat or bone. Fat mass is estimated by subtracting the BIA-determined estimate of FFM from total body mass. Whole body resistance to the flow of an electrical current conducted through the body by electrodes placed on wrists and ankles can provide fairly accurate estimates of total body water and FFM. Bioelectrical impedance analysis is dependent on several factors that can cause error in the measurement and must be taken into account to obtain a fairly accurate estimate. Hydration status is the most important factor that may alter the estimated percentage body fat. The prediction accuracy of BIA is similar to skinfold assessments, but BIA may be preferable because it does not require the technical skill associated with skinfold measurements. Currently, upper and lower body impedance devices have been developed but have not been evaluated in an athletic population.
Body Composition and Sports Performance. Body fat percentage of athletes varies depending on the sex of the athlete and the sport. The estimated minimal level of body fat compatible with health is 5% for males and 12% for females; however, optimal body fat percentages for an individual athlete may be much higher than these minimums and should be determined on an individual basis. The ISAK sum of seven skinfolds indicates that the range of values for the athletic population is 30-60 mm for males and 40-90 mm for females. Body composition analysis should not be used as a criterion for selection of athletes for athletic teams. Weight management interventions should be thoughtfully designed to avoid detrimental outcomes with specific regard for performance, as well as body composition (i.e., loss of lean body mass). See Figure 3 for practical guidelines for weight management of athletes.
Weight management strategies for athletes. Modified with permission from: Manore MM. Chronic dieting in active women: what are the health consequences? Womens Health Issues. 1996;6:332-41.
Conclusion Statement. Four studies have reported inconclusive findings related to the effects of energy and protein restriction on athletic performance, but carbohydrate restriction has been shown to be detrimental. For weight-class athletes, two studies show that weight loss preceding athletic competition may have no significant effect on measures of performance, depending on refeeding protocol. (Evidence Grade III = Limited). (www.adaevidencelibrary.com/conclusion.cfm?conclusion_statement_id=250448).
Cite this: Nutrition and Athletic Performance - Medscape - Mar 01, 2009.