Today there are a number of dog foods and treats promoted as beneficial for dental health. Some sport the Veterinary Oral Health Council seal (VOHC Accepted) for having passed a rigorous product test. Others use the limited claims that are allowed: "cleanse, freshen or whiten teeth." Regardless of the claim or labeling, a "dental product" is limited to the foods' "mechanical (e.g., abrasive) action" on the teeth (AAFCO, 2012). No references to chemical or antimicrobial additives are permitted without prior Food and Drug Administration approval. Despite this, most of the products with a dental message depend on food additives to enhance the "mechanical action." Generally these are mineral sequestering agents-a leading one being sodium hexametaphosphate (SHMP).
This salt of phosphoric acid is the same active dentifrice found in common toothpastes. It is generally recognized as safe (GRAS) and has been used in our food and feed supplies for years as a nutritional element (phosphorus source) and alkalinizing agent (increase pH). In "dental" targeted petfoods and treats, the use of SHMP has been protected by a number of patents for several years, but many of the first patents have now expired and others will soon. This could open up the landscape for other companies to enter this market. With that in mind, some background on its application, chemistry, utilization and safety might be helpful.
Incidence of periodontal disease in dogs and cats is on the order of 60%â€“80%, with dental calculus and gingivitis being the most reported abnormalities in veterinary physical exams. The purpose of these dental products, therefore, is to retard the buildup of tartar leading to periodontal disease or periodontitis. In an oversimplified description, salivary proteins adhere to the surface (acquired pellicle) of the tooth, and to this bacteria attach and begin to create a biofilm.
To this biofilm, calcium and phosphorus from saliva attach and harden into a plaque. This salivary calcium is primarily dissolved calcium carbonate appearing as calcite or apatite. As the biofilm becomes increasingly calcified it forms dental calculus or tartar on the surface of the tooth. The tartar can agitate the gum, incite inflammation, and create an entry point for bacterial infection, gum recession and root decay. Ultimately it can lead to tooth loss and secondary infections.
Preventing or decreasing the accumulation of plaque and calculus benefits tooth retention and oral health. Mechanical abrasion is promoted as a way to slow this plaque accumulation. This can be improved with the addition of mineral sequestering agents that tie up or chelate the calcium and perhaps other minerals like iron which are used by the biofilm bacteria. Thus, if we bind the calcium we slow the calcification of plaque, which in turn slows the calculus/tartar formation. There are several chelating compounds that have been proposed and evaluated. They include soluble pyrophosphates, sodium tripolyphosphates, soluble diphosphonates and soluble zincs such as zinc chloride. Sodium hexametaphosphate is the most prominent and commonly known polyphosphate for pet applications.
Sodium hexametaphosphate is also known as Grahms Salt, Glassy sodium, hexasodium metaphosphate and most accurately described as sodium polymetaphosphate. It is produced from phosphoric acid, soda ash and caustic soda that are mixed into a slurry of monosodium orthophosphate. This mixture is fed into a furnace and heated to 800-1,100ÂºC to drive off water and cause the reaction in which SHMP forms. In its molten form it is removed from the furnace, solidifies on cooling and then is milled to the appropriate particle size and concentration. It is sold on the basis of four characteristics: grade, chain length, phosphate content, and particle size.
Its two grades are technical and food; the food grade must comply with FCC guidelines. The chemical composition is (NaPO3)x, where x is the chain length; the pure form would have a chain length of 6 (hexa). But, shorter and longer chain lengths occur in the manufacturing process. As an ingredient, most suppliers claim 67%-69% phosphate and 99% ash with miniscule traces of other metals. Based on these specifications, the sodium and phosphorus content should be ~20% and ~27%, respectively. The final product is a translucent solid material available in "glass" needles, granular and powder forms, and for some applications an aqueous solution is available.
Sodium hexametaphosphate is used in a multitude of consumer goods, foods and industrial processes. It is slowly soluble in water and is used as a coagulant, a dispersant, a deflocculant and a crystal inhibitor of calcium carbonate and calcium sulfate. It is commonly found in water softeners and detergents, used in leather tanning and dyeing and in laundry works and textile processing. Its use as a sequestrant works by binding metal ions such as calcium, magnesium and iron. This reaction is very pH, temperature and ion concentration dependent.
At lower pH free ions are bound, whereas at higher pH the ions are released. The mode of action of SHMP in a dental product is to bind calcium and iron in the mouth. As bound calcium, and other minerals such as dietary iron, transit with the food and saliva through the gastrointestinal tract, the minerals are released when they reach the higher pH of the small intestines. At this point the sodium and phosphorus are absorbed and utilized accordingly.
Sodium hexametaphosphate is defined by the Association of American Feed Control Officials (2012; 57.132) as "the sodium salt of Phosphoric acid." It is further identified by Reg. 582.6760 as a "sequestrant â€¦ tartar control agent coated on dry food products for reducing the accumulation of dental tartar in dogs and cats." So, while only mechanical action is allowed for dental claims, it is clear this ingredient was expected to be used in these products. Studies with dogs have reported a reduction in dental calculus formation by up to 80% when coated on foods, biscuits and rawhide chews. Effective levels of SHMP reported have ranged from 0.6%-1.8%. These are levels well below the LD50 for oral consumption by rats at 3.7 g/kg body weight and for the no-effect level in dogs of more than 4 g/kg body weight/day.
Thus, sodium hexametaphosphate is a safe and effective ingredient to use in "dental" products for dogs and cats. It can provide a sequestering effect on calcium and other minerals in the dog or cat mouth and thereby retard dental plaque formation and periodontal disease and do so at low levels when coated onto the surface of the petfood or treat.