Eggs are considered a ‘whole food’, that is, they contain all of the nutrients required for good health. They are a super convenient food in their handy little ‘package’, easy to transport, store and serve. They are also a food which is easy for cats and dogs to digest with no missing enzymes to trigger stomach upsets as with Dairy products or no cellulose walls to prevent nutrient absorption as you get with fruits and vegetables. Eggs are an excellent source of the highest quality protein available, suitable for dogs and cats of all ages. They contain all of the vitamins, minerals, amino and fatty acids dogs and cats need for good health which makes them an ideal addition to a balanced diet (1).
The nutritional or health benefits of eggs is reliant on feeding the whole egg, this is because the different components, shell, white, and yolk, all provide their own individual nutrient composition. Alone, each component is unbalanced, but together the components form a complete food (3)
Egg Whites
In many circles egg whites are the favoured portion of the egg, promoted as a low-fat protein, a great dietary addition for those concerned about fat in the diet. Just a few days ago a friend commented her vet had recommended egg whites as a nutritional treat 2-3 times a week for her senior girl. It’s easy to understand how this viewpoint has come about as egg whites do contain over half the eggs protein and only 1% of the egg's fat content. Unfortunately, there are a lot of other nutrients also missing from the egg white which substantially reduces its nutritional value. The egg white is very low in important minerals such as Iron, zinc, calcium and phosphorus. As there is almost no fat in the egg white there are none of the essential fatty acids present, fat soluble vitamins are also distinctly lacking (2)
In addition to the unbalanced nutrient profile of egg whites they also contain two inhibitors, one is Avidin, a protein which binds to biotin preventing its absorption (1). The other is an enzyme which inhibits Trypsin (4). Both Biotin and Trypsin are important nutrients for cats and dogs with Biotin playing a key role in converting nutrients to energy (1), and Trypsin an enzyme involved in the digestion of proteins (4).
Egg Yolks
Just as egg whites are a great protein source while being an unbalanced food item, so are egg yolks. Egg yolks contain the protein Phosvitin which has been shown to reduce compounds in the body which cause inflammation (3). With almost all of the egg's fat content present in the yolk this is where you’ll find the eggs nutritious supply of essential fatty acids as well as the fat-soluble vitamins A, D, E and K which makes them a great addition to the pets’ diet (2).
The egg yolk is also where you will find proportionately high levels of important minerals like calcium, Iron, Phosphorous and Zinc which are all lacking in the egg white (2). Additionally, egg yolks are a healthy source of B-vitamins including Folate, Thiamine and Biotin as well as containing important antioxidants, the Carotenoids Lutein and Zeaxanthin, which are both beneficial for eye health. (3)
In Summary
With each component having deficiencies and excesses of various nutrients as well as the presence of inhibitors in the egg whites, it’s difficult to find logic in feeding part of the egg to a dog or cat, especially when the whole egg is a complete nutritive package suitable for cats and dogs of all ages.
Eggs are a healthy addition to a pet's diet 2-3 times a week.
Sources
Billinghurst, I., 1993. Give Your Dog a Bone. Bathurst, New South Wales: Warrigal Publishing.
Derocha, D., 2011. The Nutritional Value of Egg Whites Versus Egg Yolks:What Do You Use? Retrieved October 16th, 2020, from A Healthier Michigan web site: https://www.ahealthiermichigan.org/2011/10/11/the-nurtional-value-of-egg-whites-versus-egg-yolks-what-do-you-use/
Olsen, N., 2019. All you need to know about egg yolk. Retrieved October 16th, 2020, from Medical news today web site: https://www.medicalnewstoday.com/articles/320445
Worthington, A., 2002. Molecular Biology Tutorials: Mutant rat trypsin. Retrieved October 16th, 2020, from Kenyon College Web site: http://biology.kenyon.edu/BMB/Chime2/2002/trypsin/frames
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