In years past, we seldom pondered phosphorus availability in pet diets. That was because phosphorus was often found in excess (see Petfood Industry magazine, July 2021). With animal-based protein concentrate meals (rendered proteins), the level of phosphorus has been abundant and the bioavailability high — nearing 100%.
However, as we consider some of the new diets coming into the market which tout their intentional use of plant-based proteins for dietary claims such as vegetarian, flexitarian, limited animal proteins, sustainable and vegan, the nutritional considerations start to get more complicated. This is because these new diets, with their heavier reliance on plant-based proteins, bring along some differences in structural (fiber) and nonstructural (oligosaccharide) carbohydrates. They also possess differences in the essential fatty acids and vitamins. However, the most important difference is in the minerals, especially phosphorus. This is due to a phosphorus storage system that is unique to plants. Going forward, we need to get a better handle on how to manage plant-based phosphorus in pet foods.
More on phosphorus and phytate
In the plant, most of the phosphorus is bound to inositol, a 6 carbon ring sugar (polyol). Inositol can bind up to six phosphates and is referred to as phytic acid. As phytic acid, it can also chelate other minerals like calcium and trace elements like zinc, copper and iron. In this mineral salt form, it is known as phytate. That it binds these trace minerals in addition to phosphorus is important and can be a cause for compromised trace mineral bioavailability.
In addition, it has been reported that phytate can form a chelate with amino acids (e.g., methionine) and can limit their bioavailability too. Phytate from plant-based carbohydrates and proteins is considered an anti-nutritional compound. Not that it is harmful or toxic, it simply decreases the bioavailability of phosphorus and other minerals to the animal.
In essence, phytate is merely the storage form of phosphorus in plants and is concentrated in the seed coat (bran) and associated with their proteins. The amount of phytate phosphorus relative to total phosphorus is higher in cereal grains than in legume seeds by a few percentage points. Additionally, cereal byproducts like bran and wheat midds are more concentrated than the starchy flour.
Regardless, they all suffer from the same issue: Most of the phosphorus is bound and not biologically available to the animal. We have known of this phenomenon in livestock nutrition for decades. Researchers in swine and poultry nutrition have been exploring the topic extensively as it relates to production efficiency and for control of phosphorus runoff and pollution in some sensitive regions.
Human nutrition has also dealt with this topic, especially for the vegetarian and vegan populations and/or conditions when other sources of phosphorus may not be readily available. However, this is a relatively new area of concern for pet nutrition that accompanies the emerging popularity of plant-based products. It is important because if formulators only look at total phosphorus and do not consider the available phosphorus, then the diet could be deficient.
Solutions to the problem of availability
One approach to remedy this would be to supplement. Supplemental sources of phosphorus like phosphoric acid, calcium phosphates (mono-, di- or tri-calcium phosphate) or monosodium phosphate could be used. If one knows the available phosphorus levels for each of the plant-based ingredients, then fortification with supplemental phosphorus could fill the gap. However, if formulators presumed that the dietary phosphorus was completely unavailable and fortified wholly with these supplemental forms, then there is risk that the total phosphorus in the diet could exceed the maximum level permitted (even though available phosphorus was marginally met).
This could be problematic from a regulatory perspective and would certainly be costly, wasteful and environmentally unfriendly. This approach would also overlook some of the challenges that phytates exert on the bioavailability of the trace minerals and possibly the amino acids too.
Another option is to unlock the phosphorus in phytate. Livestock producers have been using phytase enzyme systems to cleave the phosphorus from the inositol ring, thereby liberating it for animal digestion and absorption. This has also had other beneficial effects that go well beyond just phosphorus nutrition. Namely, they have reported improvements in animal performance due to increases in overall diet/energy digestibility.
For pets, there are very few published studies (only two could be found) that have evaluated this approach, perhaps because it was not previously an issue. In these studies, the results were inconsistent as to whether the phytase enzyme addition helped. Maybe this is due to the very short retention time in the gut for our carnivore companions, or maybe other issues with the substrate or execution. The use of phytase enzyme should probably be reevaluated now that there is a greater need and bigger opportunity with the market. The first consideration should be determination of efficacy and dose for a given diet composition. Then, given phytase enzyme is a protein and subject to denaturation from high temperatures during processing, there is need to determine the best route to include it with the diet (e.g., coating or top-dressing) so that it retains efficacy.
The future of plant-based pet foods is here. This will require rethinking what we consider as minimums for some of our nutrients like phosphorus that we may have overlooked in the past. Developing nutritional datasets for pet food ingredients with an estimate of available phosphorus will be essential to formulators. Then, determining the best route to fulfill the animal’s requirement for phosphorus and other minerals will be necessary. This may involve adoption of technology that other industries have been using for decades.
In the end, changing the discussion to “available nutrients” as we turn the corner to the next era of modern humanized pet foods will be essential.
Briefly: top 5 takeaways
- As we consider some of the new diets coming into the market which tout their intentional use of plant-based proteins, the nutritional considerations start to get more complicated.
- Phytate can bind phosphorus and trace minerals and can be a cause for compromised trace mineral bioavailability.
- Phytate is the storage form of phosphorus in plants and is concentrated in the seed coat and associated with their proteins.
- Supplementing phosphorus could be a solution, but could be problematic from a regulatory standpoint as well as ignoring the challenges presented by limited trace mineral bioavailability.
- Another option is to unlock the phosphorus in phytate, but there have bee very few studies done evaluating this approach in pet food.
More “Ingredient Issues” by Dr. Greg Aldrich
