Celia Bowles and Dr. Larry Tucker, Physical Education
The problem of obesity has become a pressing issue for the general population. Currently, about one-in-two adults is overweight or obese.1 Obesity is a predecessor of various health problems such as diabetes, heart disease, hypertension, hypercholesterol-emia, and some cancers.2 Annually, almost 300,000 deaths in the U.S. can be attributed to obesity.3
Years ago, obesity was typically defined as excess body weight for a given height. Now obesity is viewed as excess body fat. The most commonly used methods to measure body fat are bioelectrical impedance, skinfolds, hydrostatic weighing, and the Bod Pod, all of which vary in accuracy.
Previously, hydrostatic weighing was considered the gold standard for measuring body fat, but because of factors such as time, cost, subject compatibility, and difficulties measuring residual volume (air trapped in the subject’s lungs while being weighed under water) other methods have been developed. Dual Energy X-ray Absorptiometry (DEXA) is one such method that has recently been developed. Although its primary purpose has been to measure bone density to determine risk of osteoporosis, DEXA is becoming more commonly used as a measure of body fat, also. DEXA has few shortcomings as a measure of body fat.
Because the use of DEXA to measure body fat is relatively new, few studies have been conducted to assess the test-retest reliability of the technique. Hence, the purpose of the present study was to determine the extent to which DEXA gives consistent body fat results over time, or in other words, the objective was to measure the amount of measurement error in repeated measures of body fat using DEXA.
Methods
Subjects
A total of 100 middle-age women were used as subjects. Subjects were recruited from the Utah Valley area. All participants signed an informed consent form, approved by the University IRB, before being tested.
Procedure
Using DEXA to assess body fat was straight forward. To begin, the DEXA machine was calibrated using two different standard samples to test density. Then each subject changed into a swimming suit and removed any jewelry or metal she was wearing. Next, three questions were asked to ensure safety in performing the test, which involves the use of X-rays: Is there a possibility that you may be pregnant? Have you been injected with any radioactive substance in the last two weeks? Have you had any major surgery in the past 6 months? If all questions were answered appropriately, the subject was then placed laying on here back on the DEXA table with her head one inch from the top of the table, arms down along her sides, palms facing down. The subject’s body was aligned with a laser cross-hair to ensure that her body was centered and straight on the table. The lower legs and feet of the subject were rotated 25 degrees inward and her feet were taped together in a dorsiflexed position.
With the subject positioned, scanning began. Each whole-body scan took approximately 6 minutes, during which the X-ray arm of the machine completed a series of 7 passes over the body. Once the scan was finished, the subject exited the table and was repositioned for a second scan. Repositioning between the DEXA scans allowed any error associated with positioning subjects on the table to be included. Once the scans were collected, they were analyzed by the same trained research assistant to eliminate inter-tester error.
Results
Using DEXA, the test and retest means (+SD) were statistically equal (Mean 1 = 30.29 + 7.15 and Mean 2 = 30.41 + 7.33; Mean difference: 0.12 + 0.72, p > 0.05). The mean of the sum of the absolute values of each test-retest difference was 0.41. The intraclass correlation between the two DEXA tests was 0.999. Of the 100 subjects, 95 (95%) had less than 1% point difference between their first and second DEXA tests. Similarly, 75 subjects had less than 0.5% point difference between the test and retest using DEXA.
Conclusion
Clearly, the Dual Energy X-ray Absorptiometry (DEXA) method of assessing body fat is extremely reliable, with the DEXA method showing remarkable test-retest stability. Given these findings and the results of other research supporting the validity of the DEXA method, it appears that DEXA will be an excellent method to assess changes in body fat in middle-age women.
References
- Flegal MD, Carroll RJ, Duczmarski RJ, Johnson CL. Overweight and obesity in the United States: prevalence and trends, 1960-1994. Int J Obes Relat Metab Disord. 1998; 22:39-47.
- Must A, Spadano J, Coakley EH, Field AE, Colditz G, Dietz WH. The disease burden associated with overweight and obesity. JAMA. 1999; 282:1523-1529.
- Allison DB, Fontaine KR, Manson JE, Stevens J, VanItallie TB. Annual deaths attributable to obesity in the United States. JAMA. 1999; 282:1530-1538.