Why is Phase Angle referred to as "Body Age"?

Phase Angle Body AgeThe "Phase Angle" measurement found on more high-end BIA Body Composition Analyzers is sometimes referred to as a way of estimating "Body Age". Why is that the case?
A comprehensive study published in the American Journal of Clinical Nutrition (2005) helped demonstrates this phenomenon very clearly! 1,967 healthy adults between the age of 18-94 years conducted BIA measurements, and several variables were analyzed to evaluate their effect on phase angle.
Phase Angles according to age group and sex1
Age group Men (n=832) Women (n=1135) p2
18-20y 7.90±0.47 (6.97, 8.75) [17] 7.04±0.85 (5.90, 8.91) [20] 0.001
20-29y 8.02±0.75 (6.83, 9.17) [178] 6.98± 0.92 (5.64, 8.55) [171] 0.001
30-39y 8.01± 0.85 (6.64, 9.48) [178] 6.87±0.84 (5.57, 8.36) [242] 0.001
40-49y 7.76±0.85 (6.53, 9.00) [121] 6.91±0.85 (5.57, 8.33) [165] 0.001
50-59y 7.31±0.89 (6.12, 8.68) [106] 6.55±0.87 (5.48, 7.96) [205] 0.001
60-69y 6.96± 1.10 (5.40, 8.88) [111] 5.97±0.83 (4.69, 7.48) [180] 0.001
≧70y 6.19± 0.97 (4.77, 8.01) [121] 5.64±1.02 (4.22, 7.04) [152] 0.001
1 All values are x±SD; 5th and 95th percentiles in parentheses. n values in brackets.
2 t test.
Source: Gonzalez, Maria Cristina & J D Barros, Aluisio & Wang, Jack & Heymsfield, Steven & Pierson, Richard. (2005). Bioelectrical impedance analysis: Population reference values for phase angle by age and sex. The American journal of clinical nutrition. 82. 49-52. 10.1093/ajcn.82.1.49. 
To summarize what the researchers found: as we can see in the table, phase angle was significantly greater in men than women for all age categories. We can also observe a significant and decreasing linear trend with age in both sexes after 20 years old. The decrease in phase angle with increasing age may suggest that it is not only an indicator of body composition or nutritional status, but possibly an indicator of function and general health as well. (note that the raw phase angle numbers in this study will not necessarily be the same as that found on other BIA devices, as difference in raw measurements are a normal phenomenon when measuring with different hardware)
Other studies have found that phase angle tends to correlate with the health of cell membranes, and healthier subjects tend to have higher phase angles compared to subjects that have illnesses.
There are some other methods that are commonly used to calculate a "body age", such as metabolism, which theoretically also tends to decrease with age. Ultimately, all forms of "body age" should be used as general reference only at best, and any diagnosis should be conducted by a trained professional!
Click here to read another introductory article about Phase Angle if you're interested in learning more!

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