Bioelectrical Impedance (BIA) Predictive Value and Validity

New challenges face personal trainers as the American population experiences an increase in both physical size and the consequent need for intervention assistance. Although most clients hire certified personal trainers for weight loss purposes reflecting vanity, the reality is the weight situation in the United States is now largely a medical issue, directly and indirectly accounting for 50% of health care costs. In 1990, no single State in the country had an obesity rate above 20%. Sadly in 2010, that number (20%) now represents the obesity rate in the healthiest State – Colorado. Overweight individuals are getting larger and obesity rates are continuing to climb. These rates are likely increasing faster than the predictive data suggests, because the nation uses Body Mass Index (BMI) as a prediction of obesity. New research suggests that the use of BMI as a national predictive measure is underestimating the actual number of obese Americans, but until DEXA scans are placed in physician offices the actual numbers will always be an estimate.

Personal trainers have tools at their disposal to assess body composition beyond those found in physician’s offices, but as people get larger the validity and reliability of field tests become further challenged. Secondary to this issue is the need of the personal trainer to assess visceral adiposity along with overall fatness and lean mass. The relationship between central girth and risk for disease is significant and should be considered by every personal trainer in the program decision-making process. Skin fold measures do not have the capacity to measure visceral fat levels and often underestimate body fatness. This leaves girth measurement and bioelectrical impedance (BIA) as viable options. Girth measurements that include central girth measures have been effectively used as a tool to track both visceral and subcutaneous adiposity and have acceptable validity and good reliability for body fat assessment in general populations. Girth measures though, have reduced effectiveness among athletic populations. BIA has been gaining popularity due to its easy implementation and affordable price tag and it may be able to provide personal trainers with better data regarding total fatness and predict visceral levels above that of skin fold.

Historically, clinical use of BIA was regarded as acceptably valid and reliable. The clinical method required a subject lie on a nonconductive surface with electrodes placed on the tops of hands and feet while a 50kHz electrical impulse was directed through the body and its rate assessed for level of impedance. Fat is approximately 45% water and will slow the impulse down compared to the more conductive muscle tissue. Today’s BIA devices commonly use a foot to foot or hand to hand analysis. Additionally, bioimpedance spectroscopy, which requires multi-frequency impedance meters, has been introduced but is preferred for fluid volume measurements, whereas bioimpedance analysis at 50 kHz is more widely used for measuring fat-free mass. Several clinical trials have evaluated BIA as a means to address the body mass of multiple populations, but with mixed reviews.

In a study published in the Internal Journal of Body Composition Research investigators evaluated foot to foot BIA for validity in overweight and obese children using the Tanita TBF-310 body composition analyzer. When averaged, the estimation range was not unreasonable for the group but demonstrated significant variation for individual children. Male children had measures which varied by as much 9.3 kg for fat free mass (FFM) and fat mass (FM) and 11.0% for percent body fat (PBF) compared to the reference measures. Female subjects did not range as dramatically but still demonstrated significant variations from the four compartment reference (up to +/-5.5 kg for FFM and FM and +/- 6.5% for PBF). Researchers concluded that BIA using this device with the manufacturer's prediction equations was not appropriate for measuring overweight or obese children. (The Tanita BC-532 demonstrated similar results in a 2008 study published in the Asia Pacific Journal of Clinical Nutrition)

A related trial published in the Journal of Clinical Densiometry analyzed the body fatness of obese postmenopausal females. The BIA values correlated with, but did not accurately reflect, the actual values found using DEXA scan. In this and other clinical trials, systematic disagreements at both ends of the range of values suggest new equations should be used if BIA is going to accurately predict body fatness in overweight and obese individuals. When researchers from the University of São Paulo Medical School investigated the limitations and validation of bioelectrical impedance analysis in morbidly obese patients they found that obese individuals have a relatively high amount of extracellular water and total body water, which may overestimate fat-free mass and underestimate fat mass. Additionally, researchers concluded overweight and obese adults with higher levels of central body fat will generally have an overestimation of percentage of fat-free mass along with an underestimation of percentage of fat mass using the prediction formulas developed in normal weight individuals. Supporting the researchers’ conclusion, the on-going consensus that new equations are needed to validate bioelectrical impedance analysis in obese individuals is well documented. Using normal weight equations for overweight and obese individuals does not seem appropriate at this time.

Interestingly, in an article published in Current Opinion in Clinical Nutrition and Metabolic Care, underweight individuals also experienced significant variations to the BIA assessment when equations for normal weight individuals were used. These researchers found BIA to be an acceptable measure for individuals with normal BMI, but concluded foot-to-foot impedance meters (body fat analyzers) were unacceptable for individuals with very low or high BMI values.

Personal trainers should recognize the relevant practical implications for using BIA measures for varied populations. Based on this information, BIA produces the same limitations as skin fold assessment for overweight and obese individuals. Until new equations are released for these populations, girth measurements continue to be a valued assessment tool for personal trainers, particularly when assessing clients with central obesity.