Use of fast neutrons for measuring muscle.

Elemental Partition Analysis (EPA) is a new approach to body composition. A major element of the body is measured and then, by means of other measurements, is partitioned to the contributing body compartments. We developed a model for measuring total body muscle by applying the EPA method to total body phosphorus (TBP). We measure TBP (head excluded) by in vivo fast neutron activation analysis using the reaction 31P(n, alpha)28 Al. The main contributors to TBP are the bone and skeletal muscle. Adipose tissue and the liver contribute less than 3%. We use dual-energy X-ray absorptiometry (DXA) to evaluate the contribution of bone to TBP. Corrections are applied for the small contributions of the liver and adiopose tissue to TBP to derive muscle phosphorous. The technique requires high precision measurements for both TBP (+/- 2%) and DXA (+/- 1%). The total body radiation exposure for measuring human subjects is 0.60 mSv. We observed that nitrogen can also be measured by fast neutron activation analysis through the (n,2n) reaction, simultaneously with TBP.