Physiological effects of dietary moisture in cats (Felis catus) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science, Massey University, Palmerston North, New Zealand

Abstract

There is no system in the mammalian body that does not depend on water. Water is an essential nutrient and comprises approximately two thirds of a domestic cat’s body weight. It is recommended that sufficient water is provided to domestic cats to allow self-regulation of intake, encompassing both voluntary drinking and intake of dietary moisture. Understanding the role of dietary moisture in health and disease in domestic cats, however, has thus far been neglected, and any physiological differences between cats consuming either high moisture (HM) or low moisture (LM) diets, have been assumed unimportant. Therefore, to fill this gap in the literature this thesis investigates the effects of consuming HM and LM diets in healthy cats and cats diagnosed with chronic kidney disease. By feeding cats diets which were created to differ in dietary moisture content only, this work aimed to determine if single or multiple meal feeding of a LM food influenced total water intake (TWI), urine concentration and pH, plasma osmolality (POsm), blood pressure (BP), water balance, and activity in cats when compared to free access HM food feeding (Chapters 2 and 3). Then the effect of a single meal and a single simulated meal on post-prandial BP and heart rate using the same diets was investigated (Chapter 4). Further, using these diets, the molecular weight distribution of urinary proteins was quantified and described (Chapter 6), and the effects of a meal on post-prandial vasopressin (VP) and copeptin concentrations were investigated (Chapter 7). Lastly the diets were used to compare the effects of consuming a HM and LM diet on urine concentration and pH, POsm, plasma VP, and BP, in cats recently diagnosed with chronic kidney disease (Chapter 8). Additionally, due to the difficult and inaccurate nature of VP hormone analysis, several alternative methods to quantitate the VP proxy copeptin were investigated, however these were unsuccessful (Chapter 5). It was found that TWI and output were significantly lower when healthy cats were fed LM diets compared to HM diets and increasing meal frequency of LM diets did not increase TWI. This difference in TWI did not result in a difference in BP or total urinary protein content. However, lower TWI did lead to a decrease in water turnover rate, and increased urine concentration, POsm, and plasma VP levels. Therefore, varying dietary moisture has meaningful physiological effects in the cat. In addition, the results may indicate increasing adverse effects on health when TWI is low, with particularly detrimental effects to the kidney. Furthermore, there were modest indications that additional adverse effects such as increasing urine protein to creatinine ratio appeared when older cats were fed LM diets. Therefore, it is proposed that the role of dietary moisture is significant for the health of domestic cats, especially as they age, and that primarily feeding a LM diet and allowing self-regulation of water intake through voluntary drinking does potentially not secure a high enough TWI for their long-term health. Feeding a HM diet ensures a high TWI, which may have clinical implications for rehydration of ill cats, especially in cats diagnosed with chronic kidney disease, and potentially may even prevent or delay progression of disease in healthy cats. Feeding a HM diet should primarily be considered when a greater water intake is required to counter dehydration, or in the treatment of diseases that benefit from an increase in water intake.