The 4 whole body liquid scintillation counting system at St. Luke's-Roosevelt Hospital in New York City was transferred from the Walter Reed Army Institute of Research in 1967. The whole body counter consists of a cylinder 72 inches long and 20 inches in interior diameter surrounded by a cylindrical tank containing 144 gallons of scintillator fluid. Gamma scintillations, primarily Compton interactions, in the annular volume of the detector, are viewed by 30 5-inch photomultiplier tubes in a 5x6 symmetrical array. The entire scintillation cylinder is shielded by a closed cylinder of 5-inch, pre-1945 steel lined with 1/2 inch of lead and 1/16 inch of copper to reduce low-energy back-scattered gamma radiation. The standardized counting time is 9 minutes for one subject measurement. This whole body counting system has been mainly used for measuring total body potassium (TBK) by counting the trace amount of naturally occurring 40K in the body but it can also be used to study metabolism of elements such as calcium by counting a trace amount of radioactive 47Ca given to the study subject to determine the turnover rate of calcium. There have been three major upgrades of the system which have improved the reproducibility of measurements to ±2.2% for human subjects weighing 40 to 250 lbs. Since the installation of the system, more than 10,000 subjects have been measured with ages 2 to 107 years, patient cohorts ranges from anorexia to severe obesity for a large number of research projects and clinical purposes for investigators working at this center and at other institutions in the New York area. The telephone number for this laboratory is 212 523-3390. Background and Applications of 40K Counting for Body Composition Research History Potassium in nature occurs as three isotopes, 39K (93.1%), 40K (0.0118%), and 41K (6.9%). The most abundant forms, 39K and 41K, are non-radioactive while the naturally-occurring radioactive potassium isotope 40K (t_=1.3 x 109 years) gives off a 1.46 MeV gamma-ray that can be counted using detectors such as crystalline sodium iodide. The proportion of total potassium found in human tissues as 40K is constant at 0.0118% of total potassium. Thus, by measuring 40K one can compute total-body potassium (TBK). In turn, potassium is distributed almost entirely within the intracellular compartment of fat-free mass. As the ratio of total-body potassium to fat-free mass is relatively stable in adult humans, one can compute fat-free mass and total-body fat if TBK is known. Forbes and his colleagues were the first to report in 1961 the measurement of 40K and thus TBK using a whole-body counter. From measured TBK Forbes and his colleagues proposed the use of the relatively stable TBK to fat-free mass ratio as a means of estimating in vivo fat-free mass and total-body fat. The whole body 40K counting method became the reference approach for evaluating total-body fat for several decades. Application The widespread use of whole body 40K counting as a means of estimating total-body fat proliferated for use in humans and animals because the method, once established, is relatively simple to carry out, safe, and inexpensive after instrument costs are considered. The measurement approach involves first shielding the subject from naturally occurring radiation in the environment using concrete, lead, or steel. Once external radiation is minimized, the subject’s natural radiation as 40K is measured using scintillation counters. The 40K counts are quantified over a specified time period and then, using an appropriate calibration standard, the subject’s 40K and ultimately TBK are estimated. The average adult human has approximately 80-150 g of total potassium and smaller amounts in regions such as the arms, legs, and trunk. While the most common counter type is for the whole body, it is also possible to construct small regional counters for the arms, legs, or even miniature counters for small animals. The technology for quantifying either regional or whole body potassium is well developed and measurements can be completed with a relatively small technical error in the range of 2-4%. While early investigators proposed the use of an assumed stable TBK to FFM ratio as a means of estimating fat-free mass and fat, it is now recognized that the TBK/FFM ratio varies as a function of age, sex, and other potential influencing factors. For this reason, investigators have sought other means of estimating total-body fat and these will be described in later sections. On the other hand, TBK offers an important opportunity to estimate two other components, body cell mass and skeletal muscle mass. Almost all of potassium is distributed in the intracellular compartment and the concentration of potassium in the intracellular fluid is stable in mammals at approximately 150-160 mmol/kg. Total-body potassium is thus frequently used as a measure of body cell mass and hence metabolically active tissue. Additionally, approximately 60% of total-body potassium is distributed in the skeletal muscle compartment. Accordingly, TBK is often used as a surrogate measure of cellular regional and whole-body muscle mass. Moreover, recent studies indicate that the relationship between TBK and skeletal muscle in adults is stable across age and sex groups. This stable relationship can be exploited as a means of predicting total-body skeletal muscle mass from measured total-body potassium. In summary, estimation of TBK is recognized as a classical method of quantifying total-body fat that has been replaced by newer more accurate approaches that will be described in later sections. While whole body counters are very costly instruments to install, their operational expenses are relatively small. Systems are simple to operate and there are no recognized health risks. As the regional and whole body counters are extremely heavy, they cannot be used outside of specialized research laboratories dedicated to the study of metabolic diseases and human body composition. Whole-body counters are not widely available, the measurements are sometimes time consuming, and with some systems the procedure may be difficult for subjects. Finally, the measurement of potassium offers a means of quantifying both regional and whole body cell mass and skeletal muscle mass.