December 2002/January 2003
This is not the first article we at ATI have ever written about bioelectrical impedance analysis (BIA), and we suspect that it will not be the last. BIA's continue to prove their importance in the clinical assessment and care of people living with HIV every day. They are being used more and more not only by dietitians but also by physicians and health clinics. In addition to estimating percentage of body fat, this test measures body cell mass (how much muscle and organ tissue you have) as well as extracellular mass (how much bone, collagen, and fluid you have). Undoubtedly, BIA has provided clinicians an invaluable tool for diagnosing and monitoring progress in HIV-related conditions such as wasting.
A common criticism of BIA has been its inability to detect or "see" lipodystrophy. The only types of BIA's that have been extensively researched and commonly accepted by healthcare providers have been total body BIA's. That means that -- as of now -- a BIA will only estimate the total amount of fat that is present in a person's body. It is not able to tell the clinician if the majority of that fat is located viscerally in the abdomen (the kind of central fat accumulation seen in lipodystrophy), or if it is more evenly distributed throughout the body. It cannot tell us whether the fat is subcutaneous (underneath the skin), or visceral (behind the muscle).
In September the Association of Nutrition Services Agencies (ANSA, formerly the AIDS Nutrition Services Alliance) held its ninth annual conference for nutrition services providers. Among the various nutrition workshops was a pre-conference institute led by Cade Fields-Gardner, M.S., R.D., L.D., C.D., focusing on BIA. This pre-conference institute was sponsored by AIDS Treatment Initiatives (ATI). Ms. Fields-Gardner included in her presentation a look into what the future of BIA might hold.
Currently, most of the studies investigating segmental BIA's are not focused on lipodystrophy or HIV. Researchers have been focusing primarily on obesity and diabetes. Their objective is to see if BIA can detect sarcopenic obesity, a condition in which fat mass is very high but muscle mass is low. In most obese patients abdominal fat should be subcutaneous, or directly under the skin. The problem that has researchers confused is that people with high visceral fat (under the muscle) are skewing the data. In other words, subcutaneous fat produces very different BIA readings than visceral fat.
While this seems to be a major point of conflict with researchers studying obesity, HIV clinicians see these results as potentially very useful. If future studies show that this phenomenon is consistent throughout populations, we may have a new tool that could easily tell us whether a patient is simply obese or developing lipodystrophy.
The unexpected results seen in the obesity and diabetes studies are exactly what HIV and lipodystrophy researchers are looking for. When compared with magnetic resonance imaging (MRI) and dual energy x-ray absorptiometer (DEXA) scans, these outlying BIA results are conclusively measuring an accumulation of abdominal visceral fat associated with lipodystrophy. Segmental BIA's done on the arms and legs are showing respective losses of subcutaneous fat in the extremities. Like total body BIA's, periodic segmental BIA's will show definitive changes in fat; and, it is these trends and changes which provides the most useful information. By repeating BIA tests over a period of time, it should be possible for clinicians to track trends in fat accumulation and loss; and, these trends may be useful in detecting lipodystrophy in its early stages before such fat changes are visible to the naked eye.
This theory drawn from the obesity and diabetes studies is speculative at this point, but it is a starting point for HIV-associated lipodystrophy research. We predict that over the next two years, segmental BIA's will become one of the most widely discussed fields of lipodystrophy study. As testing methods and formulas emerge for interpreting segmental BIA's, look for ATI and other places which perform BIA's to begin performing segmental BIA's as part of ongoing research in this emerging field.
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