There is no need for overnight fasting of rats in regulatory toxicology studies

For many years overnight fasting of rats before the collection of clinical pathology blood samples and necropsy has been a common procedure in toxicological studies for regulatory purposes. The fasting was thought to minimize the intragroup variability for clinical pathology and organ weights. However, depriving rats of food overnight will impact animal welfare by interfering with the general metabolism and may result in physiological and behavioural changes. The effects of overnight fasting in comparison to rats that were not fasted prior to necropsy was investigated in lactating rats based on an evaluation of organ weights, haematological, and clinical biochemical parameters. The results of 92 OECD 422 studies were analysed (i.e., Combined Repeated Dose Toxicity Study with the Reproduction/Developmental Toxicity Screening Test) of which approximately half of the studies incorporated fasting prior to necropsy and the other half did not. Terminal body and organ weights from all 92 studies were evaluated. Clinical pathology was included in 78 of these 92 studies. Decreased glucose levels following fasting had been reported in the literature but were not observed when comparing 39 studies with fasted conditions versus 39 studies with non-fasted conditions. Both literature and the analysed database exhibited a reduction in liver weight, alanine aminotransferase, and alkaline phosphatase levels in overnight fasted groups. These differences between fasted and non-fasted states are considered of little account as study results are always interpreted based on the differences in parameter values between treated animals compared to control animals within a study. Contrarily to previously suggested, intragroup variability was lower in the majority of parameters in non-fasted animals. According to laboratory historical data, clinical pathology and organ weight parameters are found to be very similar in male and female rats, indicating that the results of this study may be extrapolatable to non-lactating female and male rats. Based on these comparisons, it is recommended not to fast rats prior to necropsy but to continue feeding all rats ad libitum, to minimize physiological changes in these animals, to reduce variability, improve animal welfare and thereby improve the scientific value of study results.