Based on similar studies performed in dogs, it was expected that cats would have intakes of essential nutrients below NRC requirements with energy restriction for weight loss, even when fed a commercial therapeutic weight loss food. The findings of the current study showed that choline and arginine are of particular interest in obese cats undergoing energy restriction.
During energy restriction, all cats were below the NRC RA for choline at minimum intake and all except one were below RA at maximum and average intake. Moreover, two cats were below NRC MR at minimum intake and one cat also at maximum and average intake. Choline intake was not only below the NRC requirements during energy restriction; during the maintenance period there were lean and obese cats that were below NRC RA and NRC MR for choline, though only at minimum and average intakes. This was not surprising given similar findings in canine research [3, 5]. Choline is a vitamin like nutrient that has important bodily functions related to lipid metabolism. Firstly, it is part of phosphatidylcholine which makes up the membrane of very low-density lipoproteins, which are the vehicle for fat transport out of the liver . Secondly, choline has a role as a methyl-donor which again is important for lipoprotein production but also for production of L-carnitine . L-carnitine has a crucial role in fatty acid oxidation because it is necessary for entrance of fatty acids into the mitochondria – the site of oxidation in the cell. Obese cats already have an increased risk of hepatic lipidosis, which is characterized by fat accumulation in the liver and is the most common liver disease in cats . This is due to a larger quantity of fatty acids that can be released from peripheral fat stores and pre-existing insulin resistance related to obesity [10, 11], which diminishes the insulin-induced inhibition of lipolysis . The concern is that in obese cats energy restriction may induce fatty acid mobilization, but if choline is not present in enough quantity, there could be impaired transport of fat out of the liver as well as reduced fatty acid oxidation in the liver, leading to an accumulation of fat and subsequent hepatic lipidosis . It is important to note that all cats in the present study remained healthy for the duration of the maintenance and energy restriction periods and no cat demonstrated any clinical signs of hepatic lipidosis. Weight loss was monitored closely to ensure cats were not losing weight too rapidly. Performing liver biopsies would have allowed for interpretation of liver fat content to assess for potential underlying, subclinical hepatic lipidosis, however, this was outside the scope of the study [9, 13].
Arginine is an indispensable amino acid and must be provided in the diet for cats . In the present study, one lean and one obese cat fed at maintenance had an average arginine intake below NRC AI. However, during energy restriction, all obese cats had a minimum, average and even maximum intake of arginine less than the NRC AI. Arginine is involved in protein synthesis and is the key intermediate in the urea cycle . Consequences of arginine deficiency can include ammonia intoxication due to a limitation of the urea cycle being able to function properly to excrete nitrogen as urea . Ammonia intoxication has been demonstrated in the literature when young cats were fed an arginine-free diet . The onset of clinical symptoms, including lethargy, emesis, vocalization, mouth frothing, hyperactivity, hyperesthesia, ataxia and extended limbs, occurred rapidly – within a few hours. It is important to note that all cats in the present study remained clinically healthy for the duration of the energy restriction period and no cat exhibited any clinical signs of arginine deficiency. It is also important to consider that in the study where ammonia intoxication has been reported, the diet fed was completely devoid of arginine, whereas in contrast, the diet fed to the cats in the present study was a complete and balanced diet which included arginine.
Intakes of nutrients in the present study were compared to NRC recommendations for adult maintenance. It is not known whether these requirements reflect the true nutrient requirements for obese cats or for obese cats being energy restricted. Adipose tissue used to be thought of as benign tissue that did not require any additional energy and so fat mass was not taken into consideration when determining a cat’s energy requirement. In recent years however, studies have demonstrated that in fact adipose tissue is metabolically active and could therefore require energy [16, 17]. This makes it challenging to estimate the true nutrient requirement of obese cats and raises questions regarding comparisons of requirements for maintenance of lean adult cats versus maintenance of obese adult cats. Furthermore, in order to achieve weight loss, calories must be restricted, but in doing so, intake of all nutrients will also be restricted. This is the rationale behind purpose formulated veterinary therapeutic weight loss foods – the calories are reduced but essential nutrient concentrations are enhanced, aiming at meeting recommendations for adult maintenance. What is not well understood is what the intake of those essential nutrients should be – are they required in the same amounts as for lean cats at maintenance or are there some nutrients that are required in less – or even greater – amounts. Given the uncertainty and lack of published requirements for obese cats fed at maintenance and obese cats undergoing energy restriction, the comparisons made in the present study were to the NRC requirements (MR, AI and RA) of cats at adult maintenance as those are currently available, but intakes of nutrients below these requirements need to be interpreted cautiously. There is a possibility that obese cats undergoing energy restriction could benefit from additional supplementation of some essential nutrients. As discussed above, choline has an important role in utilization of fat for energy and for transport of fat. It could be hypothesized that increasing the availability of dietary choline during energy restriction would enhance weight loss and reduce the risk of hepatic lipidosis. In humans, choline’s potential role in non-alcoholic fatty liver disease has been reviewed by Sherrif et al., 2016 and in one study, choline supplementation lead to a reversal of fat accumulation in the liver [18, 19]. Such studies in obese cats have not yet been performed and so this leaves room for future research in this area.
The consensus for treatment of obesity among veterinary professionals is to induce energy restriction [20, 21]. What is not agreed upon is the starting point for energy restriction. Various equations have been reported in literature [4, 5, 22,23,24], including the equation that was used in the present study [4, 22]. This equation is less conservative than other equations and provides energy allocation that is about 25% less than what is recommended by manufacturer of the food used for weight loss. One could argue that if a different equation was used, the obese cats would potentially have intakes of all essential nutrients within NRC requirements. While these theoretical calculations were not performed for the present study, it does leave room for future studies to investigate the effect of various levels of energy restriction on intake of essential nutrients. Still, in clinical practice, regardless of which energy equation is used, it is paramount in the successful weight loss plan to consider this as a starting point only and to evaluate success with frequent monitoring of the patient including measuring body weight and assessing BCS and MCS . Safe weight loss rate for cats is recommended at 0.5 to 2% of starting body weight per week [4, 23] and caloric requirement should be adjusted based on whether or not this target is reached. Thus, an initial equation could be more conservative, but based on the cat’s response to this calorie amount, stricter restriction could be indicated. Furthermore, the degree of energy restriction reported in literature needed to induce hepatic lipidosis in cats is between 50 and 75% restriction of maintenance energy requirements [8, 26, 27]. In the present study, cats were fed 60% of maintenance energy requirements and were therefore not restricted to the same degree. Cats’ body weight, BCS and MCS were monitored. The average weight loss rate over the 10-week energy restriction period was 0.94 (+/− 0.28) % of initial body weight per week, well within the 0.5 to 2% range. Therefore, though the energy equation used may have been more restrictive than others, it appeared to be appropriate for cats to lose an adequate percent of body weight.
On average, cats lost an appropriate amount of BW, however, by the end of the 10-week period of energy restriction, no cat had reached its calculated ideal BW. In the present study, ideal BW was calculated based on BCS and current BW . For obese cats with a BCS of 9/9, morphometry was used to determine body fat  and then ideal BW was calculated. While BCS and morphometric measurements have been validated against DEXA, the gold standard for body composition assessment [29,30,31], as acceptable methods for assessing body composition, there is room for human error with these methods. Performing DEXA before, during and after weight loss would have been ideal for not only determining percent body fat and calculating ideal BW, but also for measuring lean body mass and monitoring during the study period. This would have allowed for differentiation between a decrease in fat mass and a decrease in lean body mass.
The food used in the present study was a purpose formulated veterinary therapeutic weight loss food. All cats were transitioned onto this food for a period of 7 days before the study began. Owners were using gram scales to measure the food and were recording daily intake in a food log. Given that the owners were responsible for following instructions and recording accurately, using client owned cats for this study could be considered a limitation. It is possible that owners could have fed a food other than the weight loss food, fed treats, or measured or recorded imprecisely. However, the weight loss plan was successful during the restriction period based on the calculated weight loss rate and so it is thought that owners were compliant. Nevertheless, using client owned cats for this study was beneficial because it mimicked what a typical weight loss plan in a clinical setting could look like. Recommendations in a clinical setting are not standard. As discussed above, there is opportunity for variation when determining energy requirements. There is also a variety of cat foods available to owners of obese cats. For one, in contrast to dogs, fewer cat owners take their cats to a veterinarian  and so may be attempting a weight loss plan without veterinary assistance. A cat owner could decide to continue feeding a maintenance food but restrict the amounts of food fed. Alternatively, they may select an over the counter food at a pet store marketed as light or low calorie and think that they were selecting a weight loss food. Some veterinarians will also not recommend a therapeutic weight loss food at the start of a weight loss plan and may instead recommend portion control on the current diet or will base feeding recommendations off of label instructions [5, 21]. The present study only evaluated essential nutrient intake for one diet. Since intake of some essentials nutrients was below NRC recommendations on a purpose formulated weight loss food, it would be interesting to explore intakes of essential nutrients when foods not intended for weight loss are fed for energy restriction. Even though these foods are not meant to be restricted, because veterinarians and pet owners are restricting cats on these foods, it warrants investigation.