Investigators, however, have chosen three basic aproaches to calculating the elimination rates. They have been calculated using the original two-point method 1,2 in which the isotopic enrichment is measured at the start and end of the metabolic period, a multi-point method in which the elimination rates are calculated by linear regression of natural log transformed isotopic enrichments 3,4, or a multi-point method in which elimination rates are calculated from a non-linear curve fit of untransformed data to a mono-exponential equation 5. At this meeting two further approaches were suggested, one in which an error structure intermediate between linear and nonlinear regression (i.e. Poisson) is assumed, and a fifth in which the elimination parameters are calculated from a regression analysis of the isotope enrichment products (COCD) and ratios (CO/CD). This method, however, leads to the identical CO2 production rate as the single isotope fits and only the estimate of precision is changed because covariance of 2H and 18O data is taken into account. In addition, each of these methods has two possible variants, one in which a single isotope dilution space is measured and the other space is assumed to be a fixed proportion of the first and the second variant in which both isotope dilution spaces are measured 3, 4. For the two-point method, the dilution spaces are typically measured by the plateau method in which the isotopic enrichment is measured at an equilibrium which is assumed to occur two to six hours after the dose. For the multi-point method, the dilution spaces are usually calculated from the zero-time isotope enrichments predicted by the regression line.