When more isn’t more: nutrient excesses

Giving your pet an extra “boost” of nutrients or feeding a diet that “exceeds” the AAFCO requirements might sound amazing in theory, but did you know you could actually be doing your pet harm?

Nutrient excesses can be just as damaging as deficiencies. With the rise of supplementation and pet owners being encouraged to add multiple toppers or foods on their pets kibble to “boost” the nutrient content, hypervitaminosis and nutrient toxicities are becoming more common.

Nutrient excesses can occur if the delicate balance of nutrition is disturbed

So how does this actually occur? For vitamin excesses, these can happen in one of two ways, as vitamins come in two different forms; fat soluble and water soluble. Fat soluble vitamins are stored and absorbed through fatty tissue. They can be stored for very long periods of time, therefore if something is provided in excess it can quickly build up to a toxic level as it’s continuously stored and generally isn’t used up unless the pet burns a large amount of fat. With water soluble vitamins, they dissolve in water and as most of your pet’s body is water, all body tissues can absorb these vitamins – most excesses will be excreted in urine, but given the kidneys are filtering the excess, it can also be damaging to these organs where they reach a point where they cannot continue to filter out the excess that then builds up in blood and tissues. In addition to vitamins, macronutrients such as protein and fat can also be in excess and cause issues. With many diets boasting “high protein”, are pets actually getting a benefit from this? Or is it damaging?

Given AAFCO lays out minimums and maximums for certain nutrients, but these are simply just recommendations. Pet food manufacturers can formulate foods to meet the requirements, but they do not need to test them to ensure this is actually the case, through a feeding trial. Some diets can also claim to exceed AAFCO standards, but could this be harmful too? Let’s take a look.


First of all, it’s important to remember that AAFCO does NOT directly test, regulate, approve, or certify pet foods to make sure that they meet the standard requirements. Instead, they establish guidelines for ingredient definitions, product labels, feeding trials, and laboratory analyses of the nutrients that go into pet foods. So despite needing to follow an AAFCO guideline to claim a food is complete and balanced, AAFCO does not regulate if a food does or does not meet the guidelines. They are simply guidelines that manufacturers follow when formulating a pet food. To ensure the diet is safe to be fed to a pet, it needs to meet the minimums of the AAFCO guidelines. Feeding trials are not required but can be a method of substantiating that a pet food meets the guidelines and do not cause any adverse health effects when fed to an animal. After formulating, pet food companies then use third-party testing agencies to analyze their foods according to the AAFCO guidelines.

When a diet claims to exceed the guidelines, this is not very hard to do; not many nutrients have specified maximums in the AAFCO guidelines so as long as they exceed the minimums they can claim to exceed it. Again, as AAFCO doesn’t regulate or approve foods, even if a pet food exceeds a maximum, there’s no real recourse to the company (unless a pet gets sick and the owner or vet makes a complaint). The reason many nutrients don’t have maximums is because it hasn’t been tested or established what the safe upper limit is, so because of this, some may take this to mean they can put as much of that nutrient into the diet without an issue, however that doesn’t mean it won’t cause problems…it really means we don’t yet know what the problems will be by having it in excess.

With that out the way, lets talk about some of the effects of common nutrient excesses:

In homemade pet foods, it’s common for there to be calcium deficiencies, however in commercial foods, according to James Richards, PhD for PetFoodIndustry.com, excesses of calcium are common in pet food. Based on a survey of 58 pet foods, calcium averaged 1.2% which is double the AAFCO recommendations. Hypercalcaemia is a serious condition on its own, but calcium at high levels can block the absorption of other vitamins and minerals, such as Zinc, creating deficiencies. Which, a recent study showed feeding puppies a diet with excessive calcium, actively caused zinc deficiencies. Hypercalcemia can be toxic to all body tissues, but major deleterious effects occur in the kidneys, nervous system, and cardiovascular system. Symptoms can be silent all the way to severe seizures, depending on the level of excess calcium.

Vitamin A

Hypervitaminosis A is most commonly caused by feeding too much organ meat, particularly liver, as they are very high in Vitamin A. Consuming cod-liver oil is also a risk factor for developing hypervitaminosis A. This type of excess is usually associated with pets being fed a raw diet, due to the organ meat content. However, there appears to be varying levels of tolerance in dogs; some develop hypervitaminosis A quite rapidly, others do not appear to develop it at all. The main clinical signs are joint stiffness and pain; this type of vitamin excess causes new bone to form around the joint and can sometimes fuse the joint entirely. As vitamin A is stored in the liver, once hypervitaminosis has developed the high levels of vitamin A will stay stored in the liver but the symptoms generally improve once supplementation is stopped or the diet is changed.


Excess sodium, or hypernatremia can occur in a number of ways. The most common is a lack of water intake, or an excessive intake in sodium. It can also be caused by certain disease states such as renal disease, or diabetes. Feeding a diet high in sodium can cause hypernatremia, this is often from homecooking where salt can be added for flavour, or when the pet isn’t drinking enough water. Most healthy pets can handle excess sodium as long as there is water available at all times; so if the pet is a poor drinker and the diet is high in sodium, this is generally how hypernatremia occurs.

Excess protein

Excessive protein in the diet can have a few negative consequences. Protein is processed by the kidneys and excessive amounts of protein can place stress on these sensitive organs in older patients who often have some form of renal insufficiency. Although studies show it doesn’t induce kidney disease in healthy pets, it’s not recommended as we do not currently know the optimal levels of protein pets need – there is no maximum currently set by AAFCO, and the minimums (18%) are generally exceeded in most commercial pet foods. Excessive protein can also throw off the balance of the calcium: phosphorus ratio in the diet, so high protein diets can affect the levels in foods. In addition to kidney damage, excessive protein is usually just excreted as waste, or stored as fat. A recent presentation from Debbie Phillips for PetFoodIndustry.com discussed how the increasing consumer demand for high protein levels in food, is likely contributing to the rising obesity levels in pets. The amount of protein in a diet isn’t what is important, it’s actually the quality of the protein; the pet needs nutrients such as amino acids from the protein source, and these nutrients must be bioavailable to the pet, and highly digestible to ensure they receive a benefit from it. Protein can also be difficult to digest if it isn’t of high quality, and can cause digestive upsets if it’s excessive in the diet. Increasing the amount of protein doesn’t provide any additional benefit, especially if the nutrients in the protein aren’t usable by the patient.

It’s important to remember that there are limits in animal nutrition – adding more and more of anything is not always going to be safe or effective in improving your pet’s nutrition. This includes supplementing already balanced diets, or feeding diets that claim to exceed the AAFCO guidelines.

The take home message is, always look for diets that meet the AAFCO guidelines as balance is key, and never supplement a diet unless your veterinarian has advised you to do so.

Have you ever fed a diet that exceeds the guidelines? Did you learn something new about nutrient excesses? Let me know in the comments!

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