Phosphorus is an absolutely critical dietary element. Metabolically, it is involved in the structural composition of bone, is a vital part of genetic messaging in phosphodiester linkages of DNA and RNA nucleotides, is involved with transport of energy through high-energy phosphate bonds (ATP), plays a role in systemic acid-base balance and is involved in fat and protein utilization via phospholipids and phosphoproteins.
In the diet, nutritionally available phosphorus (P) is higher in animal/avian/marine-derived protein and mineral sources and lower in vegetative sources due to the presence of phytate (an organic compound that tenaciously binds phosphorus). Further, an imbalance resulting from elevated levels of calcium (Ca) in the diet can antagonize phosphorus availability.
So, for proper dietary phosphorus nutrition, it is important to get the amount in the diet and the ratio to calcium just right. Unfortunately, the base ingredients of most pet diets often fail to meet all the nutritional requirements, so supplementation with concentrated mineral sources is necessary.
These supplemental dietary phosphorus sources are commonly derived from refined minerals. The challenge is that most pet owners do not recognize these ingredients, and many petfood companies are less than positive about their origin or utility. So, maybe shining a bit of light on the topic would be beneficial to both parties.
Wide range of sources
The more popular dietary sources of phosphorus found in dog and cat diets are dicalcium phosphate (commonly called Dical and sold at 18.5% P and 22% Ca) and monosodium phosphate (MSP, 21.8% P and 32.3% Na). Periodically, other sources are used for special nutritional applications, such as:
- Tricalcium phosphate (TCP, 18.5% P and 39.9% Ca, more commonly used as a calcium source in canned foods);
- Monocalcium phosphate (commonly called Monocal, 21% P and 16% Ca, more commonly used as a leavening agent); and
- Ammonium phosphate (20% P and 0.5% Ca, found more in urinary pH control diets and for research purposes).
These are not to be confused with the functional/acidulant food-phosphates such as phosphoric acid (23.8% P, used to stabilize digests and as a cat palatant), sodium tripolyphosphate (25% P, an acidulant and texture enhancer), hexametaphosphate (30.4%P, a dental sequestrant) and sodium acid pyrophosphate (27.9% P, a cat palatant aid). While these latter sources do contribute phosphrous to the diet, they are used judiciously because of their cost and functionality.
The dietary phosphorus source selected depends on cost, amount of supplemental phosphorus needed in the diet, potential interactions with other elements in the formula and biological availability.
All paths go through phosphoric acid
The two principal dietary phosphorus sources, Dical and MSP, are derived from a similar starting point. Each comes from phosphoric acid refined from mined phosphate rock (the so-called apatite group). The more commercially viable mines pull from ancient oceanic deposits (phosphorite) rich in calcium phosphate.
The mined rock phosphate is refined to remove unwanted impurities and concentrate and standardize the phosphorus content by a process of acidulation known as wet type (using hydrochloric acid, sulfuric acid or nitric acid) or through a high-temperature electric furnace (dry type process). Most phosphoric acid for petfood purposes is derived from the wet type process.
The majority of the resulting phosphoric acid is used in the soft drink industry, fertilizer production or other industrial applications. A relatively small fraction is further reacted with calcium carbonate (limestone) or calcium hydroxide (lime) to produce dicalcium phosphate or with sodium carbonate to form mono- or disodium phosphate.
Calcium and sodium phosphates
In the US, Dical and Monocal are commonly sold in the granular form. These granular products are a hybrid mixture of dicalcium and monocalcium phosphate in the ratio of 1.2 to 1.8:1, formed during dry bed reactions on recycled phosphate materials.
In Europe and Asia, the principal phosphorus products are precipitated salts formed as a result of the reaction between dilute phosphoric acid and a base. The result is a fine crystalline powder of a single composition phosphate.
The granular form is more stable and feed-mill-friendly due to its comparable particle size and flow characteristic relative to other ingredients in a compound feed. The precipitated forms are slightly higher in mineral concentration (in the anhydrous or monohydrate forms) and have a finer texture.
Like the precipitated calcium phosphates, MSP is usually produced by crystallization, then granulated in a rotary drum. The MSP and disodium phosphates produced by this technique are slightly hygroscopic and somewhat challenging to handle but work well when the goal is to bring phosphorus into the diet without added calcium
Depending on origin, the level of fluoride can be problematic. By law, dietary phosphates must be de-fluorinated to contain less than 1 part of fluorine (F) to 100 parts phosphorus (AAFCO, 2006). The fluorine is precipitated and removed during the production of phosphoric acid. The phosphate rock starting material also contains a host of other nutritionally beneficial (trace) minerals. For example, Dical contains 300 ppm manganese, 10,000 ppm iron, 80 ppm copper, 220 ppm zinc and 0.60 ppm selenium (Batal and Dale, 2009).
Animal considerations
No research could be found in which popular dietary phosphorus sources were evaluated for relative bioavailability in dogs or cats. Disodium phosphate and mono-basic ammonium phosphate were used in some of the early dog and cat phosphorus research, but the more common sources lack rigorous evaluation (NRC, 2006).
That's not to say the products are suspect, given there is more than ample research with surrogates such as swine, poultry and rodents. The net result from these other species is substantial and consistent and leads us to conclude that Dical, Monocal and MSP are nearly 100% biologically available sources of phosphorus (Soares, 1995). No reports of adverse reactions, flavor or palatability issues or other concerns have appeared.
So, while the principal dietary phosphorus sources used in dog and cat diets go through a pretty rigorous process, the starting materials are of natural origin. The steps deployed in their production are elementary thermal and acid-base processes to the extent that they are compliant even with the US National Organic Program standards. Finally, they have been demonstrated as safe and effective sources of phosphorus for the purpose of bridging the gap in base dietary ingredients used to produce modern dog and cat foods.
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