What's the problem with 'prey model' or 'ratio' raw diets?
Updated: Jun 10, 2019
Prey model (or ratio) raw diets for dogs and cats are undoubtedly popular. There are many websites dedicated to this raw feeding approach, with how-to guidelines that vary from very simple, to confusingly complex. Essentially, the idea is that the diet is meant to replicate the percentage or ratio of muscle meat, organs and bone found in a prey species that a wild dog or cat would be likely to catch and eat. The most common recommendation is to feed 80% muscle meat, 10% bone (from raw meaty bones), 5% liver and 5% other secretory organs. Some say slightly more liver, some say slightly more bone, but that's the basic concept. Most prey model raw (PMR) feeders typically aim to feed chunks of meat and bone (not ground or minced meat), no vegetables or fruit, no other carbohydrates or dairy products, and no vitamin or mineral supplements.
In this post, I don't want to discuss the numerous philosophical questions that arise from the practice of trying to feed a Maltese terrier in a similar way to an Alaskan Timber Wolf, or any of the other concerns that may relate to PMR feeding (such as fractured teeth from bone feeding). I want to focus on one thing, and that's whether these diets meet nutritional requirements. So let's take a look.
To start off with, let's formulate a simple PMR diet for my Labrador, Willow.
Willow needs around 1000 kcal per day to meet her maintenance energy requirement, so I know that's what I'm aiming for. However, most people calculate the amount to feed per day by using simple percentages: around 2-3% of body weight for a moderately active pet dog. Willow is not very active right now, so 2% of body weight would be about 500 grams of food per day. If I apply the ratio, that gives me 400 grams of muscle meat, 50 grams of bone, 25 grams of liver, and 25 grams of other organs. So here's the recipe I'm going to analyse:
100 grams raw pork shoulder (15% fat)
100 grams raw beef heart
110 grams raw lamb (5% fat)
140 grams raw chicken necks (36% bone, providing 50 grams bone and 90 grams meat/skin)
25 grams raw chicken liver
25 grams raw beef tripe
I've made an effort to include a reasonable variety of proteins, as per common recommendations, but also to select items that are accessible and affordable for most people. So what does the analysis look like? Here's the recipe above, compared with the National Research Council (NRC) requirements for an adult dog:
Some basics first. This diet provides 794.7 kcal per day, so I'm a little short of my 1000 kcal goal. If I had used fattier lamb, I might have come pretty close. 41.2% of calories are from protein and 58.4% of calories are from fat, so this is a high fat diet. Not surprisingly, you can see that essential amino acid requirements have all been met. Also more-than-adequate is linoleic acid (18:2 undifferentiated), which is an essential fatty acid for both dogs and cats. In this case, 61% of linoleic acid is from the chicken necks (the skin), and 26% is from the raw pork. What this means is that if you select skinless chicken, or very lean proteins to make up your muscle meat component, the diet is very likely to be deficient in linoleic acid. This can cause obvious clinical signs - a dry dull coat, greasiness, and scaling of the skin.
There are some obvious deficiencies worth focusing on. Vitamin E is a big one - you can see that my diet for Willow provides only 23.1% of the recommended minimum intake for an adult dog. In other words, this diet is severely deficient in vitamin E. Does this matter? Yes! Vitamin E is the main fat-soluble antioxidant and, together with selenium, is important for maintaining the stability of cell membranes. It protects cells from the potentially damaging effects of free radicals, which are partially derived from lipid metabolism. Unfortunately, vitamin E is almost always deficient in PMR diets, and this is particularly concerning, given that these diets are often quite high in fat. Vitamin E deficiency can cause a wide range of clinical signs in dogs and cats, such as muscular weakness, loss of appetite, depression, difficulty breathing and coma.
Vitamin D is also severely deficient in the diet, while calcium is excessive (13.71 grams per 1000 kcal) and phosphorus is moderate. The calcium to phosphorus ratio of this diet is 12.15 (!) – the recommended ratio is 0–2. If this diet was fed to a large or giant breed growing dog, it would potentially cause visible orthopaedic disease, like rickets. There are a few things to note though. It's very difficult to accurately predict the bioavailability of calcium from ingested bone. The true absorption rate of calcium is estimated at around 30%, but this varies depending on the life stage of the dog or cat, breed differences, the amount of calcium in the diet, and other dietary components. There is also great variation in how dogs eat bones, with some dogs (unfortunately) swallowing large chunks of bone, and some chewing them up more diligently - this will clearly affect digestibility and absorption. As if that wasn't complicated enough, PMR diets also rely on the owner to accurately calculate the amount of bone in the various raw meaty bones they are feeding, which can be quite tricky to do sometimes.
You can see that several other minerals are also mildly to severely deficient - copper, zinc, iodine and manganese. Copper and zinc (and also iron) can be much more deficient than this if the diet is predominantly chicken-based. Manganese is an interesting one - there isn't a lot of research on the effects of deficiency in dogs or cats. However, in other species, manganese deficiency can cause lameness and enlarged joints, and abnormal gait. Manganese is a component of several enzymes required for normal cartilage formation and turnover, so this isn't really surprising. Reproductive problems have also been reported: poor conception, increased abortion rates, stillbirths and low birth rates, so this diet would be a poor choice for breeding animals.
Feeding a wide range of proteins and many different types of offal is advised by many as the only strategy needed to prevent these deficiencies. There's some truth in this statement – it does take a large number of ingredients to produce a balanced raw-meat based diet, if minimal supplements are included. The problem is, though, that it takes a large number of carefully considered ingredients in specific amounts, and it is impossible to know if you have it right unless you analyse the diet. Will this salmon fillet provide enough vitamin D? Is there enough zinc in this beef heart? It's difficult to know without doing the maths.
So, what should you do, if you are feeding a PMR diet and you are concerned that it might be deficient? There are a few options. There are a growing number of complete and balanced AAFCO-formulated raw or 'raw-type' commercial diets – these are a much better option than continuing to feed a deficient diet. This is particularly important if you are feeding a growing dog or cat (less than one year of age). Some examples suitable for all life stages (available in Australia and NZ) include K9 Natural canned and freeze-dried diets, and Ziwi Peak canned and air-dried diets. Alternatively, if you want to feed a balanced, home-prepared raw diet to your dog or cat we can help with recipe formulation, just contact us: firstname.lastname@example.org
Disclaimer: This content is not intended to be a substitute for the professional recommendations of your pet's veterinarian. Always seek the advice of your veterinarian with any questions you may have regarding the medical condition of your pet. If you think your pet has a medical emergency, please call or visit your veterinarian or your local veterinary emergency hospital immediately.