Monday, May 23, 2016

Small animal nutrition - what's the big deal?

How much do you know about small animal nutrition?

Do you know enough about companion animal nutrition? Professor David Fraser AM is a giant in the field of nutrition and a world-expert on vitamin D (sometimes we don’t know how important these things are til we have a relative excess or deficiency). Nutrition impacts animal health and welfare, and is always the subject of vigorous, heated debate within and outside our profession. Professor Fraser answered our questions about nutrition.

In what key ways does small animal nutrition differ from human nutrition?

In general human and dog and cat nutrition are rather similar. However, there are differences and in particular there are differences between dog and cat nutrition.

A major difference between nutrition of these animals and the human is in calcium and phosphorus requirements.  All animals (including the human) have the ability to adapt to a shortfall in the calcium content of the food they eat.  Provided that they have adequate vitamin D status the capacity for absorbing calcium from food can be increased so that an absolute percentage of calcium in food is not needed. Flexibility in absorption capacity can compensate for variation in calcium content over quite a wide range.

However, and here is the big difference with humans, the calcium and phosphorus in the food of cats and dogs (and other domestic animals) has to be in the range of 1:1 to 2:1 calcium to phosphorus on a weight basis.  If the ratio is outside that range, particularly during growth, then severe bone undermineralisation and deformity will occur.  In contrast, such a relationship between the need for calcium and phosphorus in human diets does not seem to exist.  The typical western diet is high in phosphorus and low in calcium and no harmful consequences seem to follow. 

If such a diet were fed as the only food to dogs and cats they would develop bone disease, which in the jaws would lead to loss of teeth and gingivitis.  Before commercial pet foods became widely used in the feeding of dogs and cats, these animals were fed essentially on meat alone – high in phosphorus, very low in calcium.  As a results defective bones and teeth were frequently seen in veterinary practice. 

Another thing to note is that the cat most certainly and the dog, probably, are unable to obtain vitamin D from solar UV irradiation of skin.  They therefore need a dietary supply of this substance. The human on the other hand, eats foods that contain trivial amounts of vitamin D and depends almost entirely on exposure of skin to sunlight in order to acquire vitamin D.

The other thing to note is that the dog is a carnivorous omnivore, whereas the cat is an obligate carnivore.  This imposes differences on their nutrient requirements.  The cat has a higher protein requirement than the dog as it needs dietary amino acids to convert to glucose to maintain blood glucose concentration.  The cat also has a whole host of micronutrients that it has to get from food, whereas the dog is able to make a lot of these.  E.g. the cat requires a dietary source of preformed vitamin A, taurine, nicotinic acid, and gamma linolenic acid and arachidonic acid.  The cat also has higher requirements than dogs for sulphur containing amino acids and arginine.  These are all related to changes in metabolic processes associated with being entirely dependent on food from eating other animals.

What are the consequences of feeding a poor diet to dogs and cats?

This depends really on what is a poor diet.  The points in the above answer cover a lot of what could be considered poor diets.  Perhaps the major types of poor diets are those which provide insufficient protein or too much or too little metabolisable energy. 

Too little protein, particularly for cats will result in muscle wasting as non-essential proteins are broken down to supply amino acids for proteins essential for survival. The energy density of food for dogs and cats should ideally range from 4 to 4.5 kcal metabolisable energy (ME) per gram of dry matter.  If the energy concentration is below that range, dogs in particular may not be able to increase their food intake to provide them with enough ME and so they will lose weight. 

A more significant problem is if the energy density is well above 4.5 kcal ME/g, particularly if the food is highly palatable.  Dogs and cats will be inclined to eat more of such food than is needed to meet their energy expenditure and they will gain weight and eventually become obese. I suspect that the increase in prevalence of obesity of dogs and cats is a combination of the excess consumption of very palatable foods and lack of physical exercise.  Commercial foods compete with each other mainly on the basis of palatability.  Hence overeating is likely to result when a dog or cat likes a particular brand of food and it is provided by its owner in ad libitum amounts.

Has the advent of premium pet foods solved nutritional problems in animals? If not, why not?

I suspect that all pet foods on the market are formulated to match the AAFCO feeding standards for each species.  There may be differences in the digestibility of some food components – an increase in the amount of vegetable or cereal matter in a food may decrease the digestibility. 

Premium pet foods probably have higher proportions of the type of foodstuffs that dogs and cats would be eating in the wild.  They almost certainly have very high palatability that would help to confirm that the dog or cat eating them considers they are close to their ideal food. 

However, I think that analysis for all the essential nutrients would show that the premium foods are quite similar to those that are cheaper.

Why do you think it is important for vets to learn more about nutrition?

Nutritional knowledge can be used not just to advise on how to feed a particular species of animal but also it can help in the diagnosis and treatment of disease.  In the early 1980’s a survey was done by, I think the Australian Veterinary Association, of vets who had graduated five years earlier.  They were asked what aspect of their veterinary science course could have been improved, in the light of their experience in veterinary practice. 

A frequent response was that they felt that nutrition was appallingly taught because they found that they frequently needed nutritional knowledge and it wasn’t there.  In actual fact, nutrition had been well taught in the BVSc degree at that time, but it was a second year subject.  By the time students were learning clinical veterinary medicine, their memory of nutrition was fading and by the time they graduated it had virtually disappeared [Ed: That was my experience too].  Nutrition is really a subject that should be taught in an integrated way with clinical veterinary medicine so that its relevance can be more easily understood and thus, more likely to be part of the mental framework of being a veterinarian.

Do you have any advice for vets or future vets about nutrition?

Wow! I wouldn’t presume to provide any advice other than the general point that veterinary medicine is applied comparative biology.  By having a good understanding of the knowledge of how healthy animals function in all aspects of their lives and also the differences between species, the detection and correction of abnormalities in those functions would be more readily done.  Understanding the principles of nutrition and how malnutrition can occur should be part of the mental framework of a vet, regardless of the career path they are following.

Professor Fraser is teaching a short, online course on small animal nutrition through the Centre for Veterinary Education. For more information visit here