A description is given of the calibration and performance of the upgraded facility at Brookhaven National Laboratory (BNL) for measuring total body nitrogen using the technique of prompt gamma neutron activation analysis. With the improved calibration, total body nitrogen can be more accurately measured not only in normal subjects but also in obese and wasted patients. Body hydrogen is used as an internal standard. We examined the effect of a heavy-water premoderator on the uniformity of composite sensitivity, nitrogen and hydrogen measurement statistics, and dose to the subject. The calibration technique corrects the ratio of nitrogen-to-hydrogen counts measured from the subject for body size. Additionally, a correction for subcutaneous adipose tissue on the nitrogen-to-hydrogen count ratio is introduced. The newly upgraded BNL facility provides precision in counting statistics using a Remcal anthropomorphic phantom filled with a tissue-equivalent solution of 2.1% for a body dose of 0.35 mSv using a 2.5 cm D2O premoderator. The measurements were made at five 20 cm sections, counting for 200 s per section.