An increased contribution of de novo lipogenesis (DNL) may play a role in cases of dyslipidemia and adipose accretion, this suggests that inhibition of fatty acid synthesis may affect clinical phenotypes. Since it is not clear whether modulation of one step in the lipogenic pathway is more important than another, the use of tracer methods can provide a deeper level of insight regarding the control of metabolic activity. Although [2H]water is generally considered a reliable tracer for quantifying DNL in vivo (it yields a homogenous and quantifiable precursor labeling), the relatively long half-life of body water is thought to limit the ability of performing repeat studies in the same subjects; this can create a bottleneck in the development and evaluation of novel therapeutics for inhibiting DNL. Herein, we demonstrate the ability to perform back-to-back studies of DNL using [2H]water. However, this work uncovered special circumstances that affect the data interpretation, i.e. it is possible to obtain seemingly negative values for DNL. Using a rodent model we have identified a physiological mechanism that explains the data. We show that one can use [2H]water to test inhibitors of DNL by performing back-to-back studies in higher species (i.e. treat non-human primates with Platensimycin "PTM", an inhibitor of fatty acid synthase), studies also demonstrate the unsuitability of [13C]acetate.