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Feeding practices and the use of supplements for dogs kept by owners in different socioeconomic groups
  1. R. M. Thomson1,
  2. J. Hammond, MA, VetMB, BSc, MRCVS1,
  3. H. E. Ternent, MSc2 and
  4. P. S. Yam, BVM&S, BSc, PhD, CertSAM, MRCVS1
  1. 1 Division of Companion Animal Studies
  2. 2 Division of Animal Production and Public Health, Faculty of Veterinary Medicine, University of Glasgow, Bearsden, Glasgow G61 1QH


The prevalence of feeding practices and supplements for dogs used in private practice (pp) and the non-profit-making People's Dispensary for Sick Animals (pdsa) was evaluated. Questionnaires were completed by 400 pp clients and 400 pdsa clients, of which 27·2 per cent and 29·8 per cent, respectively, gave supplements to their dogs. Fatty acids/oils were given by 10·3 per cent of pp clients and 11·5 per cent of pdsa clients, glucosamine and/or chondroitin by 10·5 per cent and 5·8 per cent, and vitamins by 6·8 per cent and 19·3 per cent, respectively. The supplements were provided daily by 17·8 per cent of the pp clients and 14·3 per cent of the pdsa clients, and the pdsa clients were 50 per cent more likely to provide the supplements only weekly or monthly than the pp clients. A commercially available maintenance or dietetic diet was fed by 98·8 per cent of the pp clients and 94·2 per cent of the pdsa clients.

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AN animal's health and physiology can be maintained only if its intake of essential nutrients meets its minimal requirements. Complete pet foods are usually manufactured to meet these minimal requirements on the basis of guidelines published by the National Research Council (nrc) (2006), which also publishes values for the safe upper limit (sul) of nutrients. Unfortunately, the sul for many nutrients, especially minerals and vitamins, is not known. However, even when the minimal requirements are met, they may not provide the ‘optimal’ daily amount of a nutrient. In human medicine, there is an increased awareness that dietary supplements may provide a variety of health benefits ranging from cardioprotective effects to reducing the risk of cancer. A supplement can be defined as a pill, coated tablet, drops or liquid with a potentially health enhancing function. According to the Food Standards Agency (fsa) (2006), Britons spent £220 million on vitamin and mineral supplements in 2006, and 43 per cent of uk adults had taken vitamin or mineral supplements in the previous 12 months.

The increased use of dietary supplements by people may be mirrored by the administration of supplements to their pets. There are a number of products suitable for dogs that are either promoted in veterinary practices or available from health food shops. However, there is little information about the frequency and types of supplements being administered to dogs, or the differences between different demographic groups.

This paper describes the use of nutritional supplements for their dogs by 400 owners attending private veterinary practices and 400 attending People's Dispensary for Sick Animals (pdsa) clinics.


A questionnaire was designed to obtain information about the dietary supplements given to pet dogs by their owners. It was laid out in clear sections dealing with the lifestyle, income and demographic characteristics of the household, the nutritional management of the dog and the use of the most popular supplements available. These were categorised as vitamins, fatty acids or oils, glucosamine and/or chondroitin, and other types of supplements. The objectives and methods of the study were carefully explained to the practices taking part, and the questionnaire was approved by the University of Glasgow Ethics and Welfare Committee.

The questionnaire was distributed to two different types of practice. First, the pdsa in Glasgow, to include animals from households with a low income. Eligibility to be a pdsa client involves receiving financial help with housing costs through either housing benefit or council tax benefit (pdsa 2007). Secondly, three private first-opinion veterinary practices (pp) in and around the Glasgow area.

The questionnaire was distributed to clients in the waiting area during normal consultation hours and completed at that time. Each dog was included in the survey only once and only one dog per household could be included in the study. The survey ran for five weeks. The data collected were coded and analysed using Microsoft Access and Microsoft Excel. Chi-squared tests performed in Minitab version 15 were used where appropriate. Significance was set at P≤0·05.


Four hundred questionnaires completed by dog owners attending the pdsa and 400 by those attending pp were available for analysis. The median age of the pdsa dogs was seven years and 10 months and of the pp dogs five years and two months. The sexes of the pdsa and pp dogs were, respectively, 26·5 per cent and 17 per cent entire female, 17 per cent and 31·25 per cent neutered female, 42·75 per cent and 32·5 per cent male, and 11·75 per cent and 18·75 per cent neutered male. The sex of 2 per cent of the pdsa dogs and 0·5 per cent of the pp dogs was not given.

The dogs were fed twice a day by 61·5 per cent of the pdsa clients and 60·6 per cent of the pp clients, and once a day by 14·5 per cent of the pdsa clients and 24·8 per cent of pp clients; 6·3 per cent of the pdsa clients fed their dog ad libitum compared with 3·5 per cent of pp clients. The remaining clients fed three times a day or did not answer the question. The diet fed by 94·2 per cent of pdsa clients and 98·8 per cent of pp clients was either a commercial maintenance or dietetic diet; the other clients fed a homemade diet exclusively. The pdsa clients were almost twice as likely to feed tinned food as the pp clients. Approximately one-third of clients from both types of practice fed a combination of meat with a dry food or mixer. Dietetic diets were fed by only 2·0 per cent of pdsa clients compared with 13·5 per cent of pp clients. In addition, 40 per cent of the pdsa clients and 51 per cent of the pp clients fed snacks or treats daily, and 40 per cent and 29 per cent of them, respectively, provided them weekly. Only 6 per cent of the pdsa clients and 7 per cent of the pp clients said that they never fed snacks or treats to their dogs.

The difference in annual income between the pp and the pdsa clients was significant (P<0·001), with 82·3 per cent of the pdsa clients having an annual household income of less than £10,000, compared with 8·2 per cent of the pp clients; 28·3 per cent of the pp clients had a household income of more than £40,000, compared with only 0·6 per cent of the pdsa clients. Most (39·5 per cent) of the pp clients had a household income of between £20,000 and £40,000 per annum (Fig 1).


Annual household incomes of clients of the PDSA and three private practices in Glasgow

Supplements were fed by 29·8 per cent of the pdsa clients and 27·2 per cent of the pp clients but the choice of supplements by the two groups was significantly different (P<0·001) (Fig 2). A supplement of glucosamine and/or chondroitin was used by 10·5 per cent of pp clients compared with 5·8 per cent of pdsa clients, but vitamins were given by 19·3 per cent of the pdsa clients compared with 6·8 per cent of the pp clients. Supplements of fatty acids or oils were given by 11·5 per cent of pdsa clients and 10·3 per cent of pp clients. Other supplements used included yoghurts/probiotics, given by 8·75 per cent of pdsa clients and 5·75 per cent of pp clients, and others such as aloe vera or milk thistle (2·1 per cent).


Percentages of the clients of the PDSA and three private practices in Glasgow who gave their dogs different types of nutritional supplement

There was a significant difference between the frequency with which the pdsa and pp clients gave the supplements (P=0·017). At the pdsa 14·3 per cent of the clients provided the supplements daily, compared with 17·8 per cent of the pp clients. The pdsa clients were more likely to provide them weekly or monthly, with 50 per cent more of the pdsa clients than pp clients providing supplements monthly.


As would be expected, the income of the clients of the pdsa was significantly lower than that of the pp clients (P<0·001). In order to attend the pdsa, clients are obliged to qualify for free treatment, at present by receiving help either in the form of housing benefit or with the payment of council tax (pdsa 2007). Consultations and treatment are completely free, but clients are requested to make a donation. In contrast, clients attending the pp clinics would have to pay either for pet insurance or for any consultations and investigations or treatments received. The distribution of questionnaires to clients attending the pdsa and pp allowed comparisons to be made between dogs owned by people in two different demographic groups. Studies in the human field have shown that there are clear differences in relation to the purchase of food and in the use of nutritional supplements between people in different socioeconomic positions. Shoppers with low levels of education, and those residing in low-income households, were least likely to purchase foods that were comparatively high in fibre and low in fat, salt and sugar (Turrell and Kavanagh 2006). It has also been shown that there were correlations between the use of dietary supplements, a healthy lifestyle, lower body mass indices, active lifestyles, non-smokers and higher educational level (Lyle and others 1998, Kirk and others 1999, Reinart and others 2007). Conversely, in other studies, people with poorer health were more likely to use dietary supplements (Knudsen and others 2002), perhaps as a form of self-medication. Other research has suggested that people with specific medical conditions use particular supplements (Gunther and others 2004).

It was therefore surprising that very similar proportions, nearly 30 per cent, of both socioeconomic groups, and slightly more of the pdsa clients, gave supplements to their dogs. The results of a telephone survey of members of the public in the usa and Australia indicated that only 13·1 per cent of 624 dog owners who were not feeding a dietetic diet administered a dietary supplement (Freeman and others 2006). Although there was some bias by not selecting owners attending a veterinary clinic, this is considerably lower than observed here, suggesting either that supplementation may be on the increase or that supplements are used more commonly in the uk than in the usa or Australia. It has been shown in the usa that vitamin supplements for people have increased in popularity from 23·2 per cent of people taking them in 1987 to 33·9 per cent in 2000 (Millen and others 2004). It is probable that there is a similar trend for pet dogs.

Both demographic groups used supplements to a similar extent, but the proportions of the different supplement provided and the frequency of supplementation were significantly different (P<0·001). As in the study by Freeman and others (2006), multivitamins, chondroprotective agents and fatty acids were the most common supplements used.

The routine use of vitamin supplements is questionable when dogs are fed a commercially complete diet. A limitation of this study was that the dogs' owners were not asked specifically whether they were feeding commercially complete diets, and it has been assumed that this was the case. However, the owners were asked about the feeding of snacks and treats, and it is possible that some of them were only feeding complementary foods to their dogs inadvertently; it could therefore be argued that the supplementation of these dogs may have been necessary. Nevertheless, given that 94 per cent of the pdsa clients were feeding commercially prepared diets, the fact that over 19 per cent of them were spending money on vitamin supplements when it could be argued that they are at best unnecessary, or at worst harmful (Kallfelz and Dzanis 1989) is cause for concern. For example, the potential deleterious effects of an excess intake of vitamin D were demonstrated in two dogs when human error resulted in the oversupplementation of a dietetic pet food (Mellanby and others 2005). Toxic effects may also occur as a result of the presence of inappropriate components in complete pet foods as well as by oversupplementation. Although vitamin supplements are often recommended when clients feed a homemade diet to prevent deficiencies developing, only three of the 19 owners who fed exclusively homemade diets gave vitamin supplements and all three were clients of the pdsa. The reasons for vitamin supplementation by clients can only be speculated. It may be due to clients perception that the quality of the commercial pet food was not as good as they would like, they took vitamin supplements themselves, or as a result of veterinary recommendation or a positive feeling they experienced as a result of supplementing their dog's diet.

Approximately one-third of the supplements given by the pp clients were glucosamine and/or chondroitin. Chondroitin sulphate and other glycosaminoglycans have alleged benefits for arthritic conditions and degenerative joint diseases. Glucosamine salts are precursors of hyaluronic acid and other glycosaminoglycans and are key upregulators of glycos aminoglycan synthesis. They are available as oral preparations and in combination with chondroitin sulphate, the major glycosaminoglycan found in cartilage (Gross and others 2000). In human beings, the oral bioavailability of glucosamine hydrochloride is 84 per cent, almost double the 44 per cent availability of glucosamine sulphate (44 per cent) (Setnikar and others 1986, Setnikar and Rovati 2001, Boothe 2004); consequently, the oral dose of the sulphate should be twice that of the hydrochloride. In dogs, the oral bioavailability of glucosamine hydrochloride is only 12 per cent (Adebowale and others 2002). The cheaper glucosamine sulphate commonly bought in health food shops by owners to give to their pets is likely to have considerably less effect at the indicated dose, although it is uncertain whether the oral bioavailability data can be extrapolated between species. There is contradictory evidence about the efficacy of glucosamine and chondroitin in relieving joint inflammation and pain. A large double-blind placebo-controlled study in dogs reported no significant efficacy for glucosamine/chondroitin in the treatment of osteoarthritis (Moreau and others 2003) and a similar study in human beings also reported no significant effect (Clegg and others 2006). However, in another study, dogs treated with glucosamine hydrochloride and chondroitin sulphate showed statistically significant improvements in scores for pain, weight-bearing and severity of osteoarthritis (McCarthy and others 2007). However, the lack of a placebo control group, the subjective nature of the assessment and the wide inclusion criteria in that study suggest that the positive results should be treated with caution (Pettitt 2007). Regardless of whether there is good evidence for the efficacy of glucosamine supplementation, it was a commonly used supplement by the pp clients.

Similarly, approximately one-third of the supplements given by clients of the pdsa and pp were fatty acids or oils. Essential fatty acids play vital roles in the structure and function of the body, including the fluidity of cell membranes and the health of the skin. Dogs can synthesise the polyunsaturated omega-6 (n-6) fats gamma-linolenic acid and arachidonic acids from their dietary precursor linoleic acid. The n-6 fatty acid family is required for growth and reproduction and as precursors of eicasanoid and prostaglandin synthesis. Omega-3 (n-3) polyunsaturated acids have important functions in the body, including the function of the brain and retina. Although they can be synthesised from other dietary fatty acids (alpha-linolenic acid) and are not themselves essential components of a dog's diet, their addition may be beneficial because they are considered ‘conditionally essential’ under certain conditions, such as for puppy growth and reproduction (Bauer 2006). They can be found in fish oils as eicosapentaenoic acid and docosahexaenoic acid. It has been shown that the dose of n-3 fatty acids is more important than the dietary ratio of n-6 to n-3 fatty acids in affecting the plasma fatty acid profiles in dogs (Hall and others 2006). Most commercial foods do not have the concentrations of docosahexaenoic acid needed to maximise plasma levels, and studies are needed to determine the minimum and maximum intakes of n-3 fatty acids necessary to exert favourable results relevant to specific diseases (Hall and others 2006). Feeding the optional amounts of n-3 fatty acids to dogs may benefit several pathological conditions. They have cardioprotective effects due to their potent anti-inflammatory, anti-atherosclerotic and anti-arrhythmic properties, as a result of their ability to inhibit the accumulation of lipids (Billman and others 1997, 1999, Singh and others 1997, Anandan and others 2007). Other reported benefits include renoprotective effects due to their ability to delay the progression of early and chronic renal insufficiency (Brown and others 1998), positive neurological effects due to improved neuronal development during gestation, thereby enhancing learning and memory in puppies (Heinemann and Bauer 2006), enhanced visual acuity due to increased rod sensitivity (Bauer and others 2006), and antitumorigenic effects due to their ability to inhibit the growth and metastasis of tumours (Ogilvie and others 2000). Chondroprotective effects have been demonstrated in human beings, and in dogs reductions in the rate of production of metalloproteinases, enzymes thought to play a key role in joint destruction in arthritis, have been observed (Hansen and others 2008). Considering the potential beneficial effects of polyunsaturated fatty acids and their low cost, they may be too little used in veterinary medicine, being given to dogs by only approximately 10 per cent of all the clients in this study. There are few licensed veterinary fatty acid supplements and those available are very similar to the widely available human equivalents.

At the pdsa, 14·3 per cent of the clients provided supplements daily, compared with 17·8 per cent of pp clients. The pdsa clients were more likely to provide them weekly or monthly than the pp clients; 7·5 per cent of the pdsa clients provided them monthly, compared with 3·8 per cent of the pp clients. It is doubtful whether supplements provided monthly would provide any benefit, although this would depend on the metabolism of the supplement in question. For example, the fat-soluble vitamins are stored in the lipid deposits of tissues and a deficiency is less likely than toxicity due to oversupplementation. Conversely, the water-soluble vitamins have limited stores and are depleted more rapidly. Gomega (Vetplus), an omega-3 oil supplement for dogs, Seraquin (Boehringer Ingelheim) a glucosamine-chondroitin supplement, and Pet-Tabs (Pfizer), a vitamin and mineral supplement, are all recommended by the manufacturers to be administered daily.

The differences between the pdsa and pp clients with respect to supplementation were probably due to such factors as their socioeconomic status, the costs of the different supplements, their availability from the veterinary practices and veterinary recommendations. It is difficult to disentangle the effects of the socioeconomic differences and the differences in practice policy on the recommendation of supplements. The potential benefits of certain supplements are uncertain, despite the fact that many clients purchased them.

The results show that nearly 30 per cent of both groups of owners provided their dogs with dietary supplements, but that the proportions of them providing glucosamine, fatty acids and vitamins differed significantly, probably for several reasons. If clients seek veterinary advice about the use of supplements, it would be sensible to recommend those with demonstrable benefits. None has been proved conclusively to be effective in dogs, but the evidence that fatty acids provide benefits for a range of clinical complaints is strongest. It is questionable whether dogs fed a commercially balanced diet would benefit from vitamin supplements.


This work was funded by the Pet Food Manufacturers' Association. P. Y. would like to thank Hill's Pet Nutrition for funding her post at the University of Glasgow. The authors acknowledge the contribution of the owners and veterinary practices (Abbey Vet Group, Craigpark Vet Centre, The Veterinary Centre), without whom this work could not have been undertaken. They also thank the pdsa for their help and willingness to take part in the study.


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