In the current study, we have assessed the intake of essential nutrients for obese dogs during a period of controlled weight loss using a purpose-formulated diet. The intake of most essential nutrients exceeded NRC 2006 RA cut-offs. Although intake was less than RA or MR for some nutrients, all dogs remained healthy, showing no clinical signs of nutrient deficiency. It is important to note that RAs include a safety margin to take into account predicted nutrient bioavailability, and the actual bioavailability of individual nutrients might be higher than assumed in NRC 2006. The significance of dogs having intakes less than MR for some nutrients during weight loss is not known. Firstly, it is possible that requirements for essential nutrients might actually change when dogs are subjected to energy restriction, although the exact effect may well differ amongst nutrients. For example, requirements of many amino acids and B vitamins are directly related to energy metabolism [15, 23]. Given that MER declines during weight loss [13], the requirement of some essential nutrients might also decline. In contrast, requirements for other essential nutrients (i.e. minerals) are more directly related to bodyweight, or another exponent, rather than to energy metabolism [15]. For these essential nutrients, requirements might not change during weight loss despite the decrease in MER. A second issue is the fact that for loss of lean tissue mass to be minimised during weight loss, intake of dietary protein must be adequate [14]. However, it is not known which essential amino acids are most limiting in a purpose-formulated weight management diet. Therefore, additional studies are now required to assess adequate intake of essential nutrients during weight loss.
The majority of essential nutrients were fed at intakes (per kg BW0.75/day) above the guidelines recommended by NRC, throughout the weight loss period. However, there were some important exceptions to this, most notably selenium, choline, methionine and cysteine. At first glance, results were most concerning for selenium, since daily intakes for this mineral were less than AI in all cases. Selenium is involved in antioxidant pathways as well as both thyroid and immune system function, with deficiency reportedly causing anorexia, depression, dyspnoea, and coma [24]. None of these signs were evident in any of the dogs of the current study. Excessive dietary selenium intake can also have adverse effects in dogs [24]; further, both AAFCO and FEDIAF have set legal limits for selenium supplementation. Thus, it can be immensely challenging to formulate a diet appropriately for selenium content. A second challenge with selenium is the fact that optimal intake is not as easily determined as for other nutrients. Most notably, NRC does not report MR for selenium in dogs and, to the authors’ knowledge, no studies have examined this, partly because it can be difficult to measure selenium status accurately in animals. Nonetheless, a recent in vivo study in obese dogs, that used the same diet as for the current work, did not reveal any decrease in selenium status during weight loss. In fact, urinary selenium excretion was greater after weight loss, compared with before, perhaps suggesting that requirements for this essential nutrient might actually decline during weight loss [18]. Therefore, although less than NRC AI, the intake of selenium in all dogs was probably sufficient.
Choline is a vitamin-like substance that is reportedly involved in neurotransmission, hepatic lipid metabolism, coagulation, as well as acting as a methyl donor [15]. In dogs, deficiency of choline causes hypocholesterolaemia, vomiting, fatty liver disease, and death [25]. A previous theoretical study suggested that intake of choline might be at risk of deficiency if marked caloric restriction was required during weight loss [19]. In the current study, the daily choline intake of most dogs (24/27, 89 %) was less than the NRC 2006 RA for some of their period of weight loss, although intake was less than the suggested AI in only two dogs (7 %). However, requirements for choline have not been well established in dogs, and the current AI for choline is based on data from studies conducted over 50 years ago [16]. Therefore, it is unclear as to whether daily choline intake during the current study was actually deficient. That said, a recent study, that used the same diet as for the current work, demonstrated a 16 % decrease in plasma choline concentrations during weight loss in obese dogs [18]. Given that choline is a relatively easy nutrient to supplement and there are minimal toxicity concerns, increasing the choline content of canine weight management diets would seem be sensible until more data on choline requirements are available.
Similar to the issues for choline, the intake of methionine and cysteine were less than RA in 12 (44 %) and less than MR in 2 (7 %) of the dogs undergoing weight loss in the current study. As with choline, these amino acids were also identified as ‘at risk’ in a recent theoretical study [19]. Methionine is a sulphur-containing amino acid that is not only required for protein synthesis, but also forms part of the coenzyme s-adenosyl methionine [26]. As a result, its deficiency can result in various metabolic aberrations [26]. Cysteine is a critical amino acid for maintaining the secondary structure of compounds such as glutathione and those required for hair synthesis. Cysteine is synthesised from methionine and, therefore, both amino acids are typically considered together when determining requirements. When methionine is deficient in the diet, there is an immediate decrease in food intake, and severe weight loss [27]. Whilst all dogs lost weight during the study, this was not likely to be due to methionine or cysteine deficiency because food intake was never affected and weight loss occurred in a controlled fashion and was never excessive (i.e. < 3 % per week). Puppies fed a methionine-deficient diet also develop dermatological lesions such as erythema, footpad necrosis and hyperkeratosis [28], while adult dogs fed a diet with borderline methionine and taurine content, develop gallstones [29, 30]. None of the dogs in the current study, or indeed from the larger cohort of dogs seen at our weight management clinic, developed any dermatological signs at any stage during or after their period of weight loss. Given that abdominal ultrasonography was not performed, it is unclear as to whether or not gallstones had developed. That said, this possibility is less likely, given that the diet was also supplemented with taurine (which can partially substitute for methionine and cysteine), and none of the dogs were taurine deficient (data not shown). Further, no signs were noted pertaining to liver disease, and clinicopathological markers of biliary system disturbance (e.g. bilirubin, and liver enzyme activity) were unchanged after weight loss. Finally, given the requirement of both methionine and cysteine for protein synthesis, the effect of daily intake on change in lean tissue mass and also serum albumin concentration were assessed, and no associations were seen with either. Nonetheless, as with choline, supplementing methionine and cysteine in the current diet by 15 % would ensure that daily intake is greater than NRC MR. Such a reformulation would be sensible until further data are available clarifying the safety of intakes of the level in the current study.
There were a number of other nutrients where intake was less than the RA suggested by the NRC [15]. These included tryptophan, magnesium, and potassium. The significance of these observations is not known since intake was always above MR throughout weight loss, there were no signs of deficiency for any of these nutrients, and no association was seen with changes in lean tissue or serum albumin.
As with all studies, a number of limitations exist. Firstly, the data were collected retrospectively, and dogs included were drawn from a larger population. Given the eligibility criteria used, and most notably the duration of the weight loss period, a number of dogs were excluded since they reached target weight within 6 months. Other dogs were excluded because different weight loss diets had been used, clinical data were missing, the dogs either had another disease, or they were on concurrent drug therapy. It is possible that, by excluding dogs in this way, the results might have been unfairly biased. That said, dogs were not excluded for becoming sick or ill, perhaps the most important outcome factor of interest for the study. Further, to the authors’ knowledge, no nutrient deficiencies have ever been recognised in any dog that has attended the weight management clinic where the study was conducted, and this includes the dogs from the current study. Nonetheless, it would be worth considering a prospective study, to confirm the current findings in a cohort of obese dogs undergoing weight loss.
Given that client-owned dogs were used, a second limitation was the diversity in the population of dogs used, which is arguably more marked than would have been the case for a study undertaken on dogs from a research colony. Not only was the population more diverse, but also the environment in which the dogs were kept was more variable, not least with regard to controlling food intake. To offset this, owners measured food out precisely using kitchen scales, maintained a diary record, and dogs were only included dogs if there was no evidence of poor compliance i.e. recorded feeding of additional foodstuffs. That said, under-reporting is a concern with the use of food questionnaires [11, 31] and, as a result, it is possible that some dogs received additional food, causing errors in estimation of adequate intake. Whilst this is a notable limitation, the use of client-owned dogs means that the results are arguably more representative for the target population of interest, than using dogs from a research colony.
A third limitation regarding the population was the fact that it was very variable in terms of breed, age gender, presence of concurrent diseases, and in outcome. For this reason we applied very strict eligibility criteria, for instance measurement of body composition by DEXA. As a result, the final population was closely monitored, well phenotyped, and all data were complete.
A fourth limitation was the fact that, since the study was conducted over a number of years and there was no actual analysis of the batches of food used. Instead, the nutrient intake for each dog was based upon the average nutrient content of the diet, based upon analysis. Thus, actual nutrient intake might have differed from that reported in the current study. Further, whilst we observed no obvious clinical signs of malnutrition, based upon physical examination, the outcomes measured might not have adequately assessed nutrient status or detected subclinical deficiencies. For instance, plasma nutrient status was not measured. However, such a study has recently been performed, in a similar cohort of dogs [18], which demonstrated no change in the plasma concentrations of most key nutrients during weight loss.
A final limitation relates to generalisability of the data to canine weight loss programmes in general. Given the use of client-owned dogs, results should be broadly representative of weight loss programmes using this or similar diets in practice although, since the final population was small, subtle individual issues with a particular breed or dog type might have been missed. Further, the population came from a referral clinic and so might not be typical of the usual dogs undergoing weight management. Moreover, only a single weight loss diet was used and, thus, findings might not be generalisable to other weight loss diets, especially those from other manufacturers. Therefore, it would be worth considering individual validation of other weight loss strategies in the future.
