Cyclic volatile methyl siloxanes (cVMS) are commonly used down-the-drain manufactured substances that were assessed for their bioaccumulation (B) potential in Canada and the European Union, which resulted in different regulations. Discrepancies between regulations resulted in part from the unique intrinsic characteristics of cVMS, their log KOW values, which suggests that cVMS partition out of water and into lipids. Modeled simulations and laboratory-based bioconcentration (BCF) values exceed CEPA 1999 B criteria, while field-based analysis of biomagnification factors (BMFs) can produce values that are below CEPA 1999’s B threshold (BCF and/or BMF < 5000) and resulted in biodilution. The current research addressed discrepancies between B values by exploring in vivo dietary somatic biotransformation rate constants (kBM) of two cVMS, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5), in rainbow trout (Oncorhynchus mykiss). The kBMs were derived using a co-exposure of non-metabolized reference chemicals. Plotting the concentration of fish at time (CF) shows significant but slow somatic depuration rate constants (kBT, d-1) for D4 and D5 (0.045 ± 0.018 (SE); 0.042 ± 0.008 (SE)) and kBM (d-1) (0.039 ± 0.019 (SE); 0.037 ± 0.009 (SE)). The CF also produced significant, but relatively slow dietary uptake absorption efficiencies (ED) for D4 and D5 (0.322 ± 0.034 (SE); 0.079 ± 0.008 (SE)). CF was entered into an Absorption, Distribution, Metabolism, and Excretion Fish Bioaccumulation Calculator (ADME-B) to predict the biotransformation rate constant in the gut (kGM,d-1), resulting in relatively fast rate constants of 1.26 ± 0.61 (SE) and 6.46 ± 1.12 (SE) d-1 for D4 and D5. Results indicate that the exposure route effects the proportional contribution of somatic (φBM) and gut (φGM) biotransformation rate constants on the depuration of D4 and D5. For instance, dietary uptake results in high φGMs for D4 and D5 (61.34 and 93.02%). Furthermore, the ADME-B calculator indicates that D4 and D5 BCF values are below B criteria (2864.6 ± 1112.1 (SE); 2289.6 532.5 (SE)). Results are consistent with measured environmental concentrations that show biodilution.
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