Vitamin K3: is it unnecessary and toxic?
Supplementation with vitamin K3 doesn't appear to be a smoking gun
Vitamin K is one of those nutrients that we learn of early in nutrition training as an essential fat soluble vitamin that is important in clotting. Beyond that, we seldom discuss it. Recently, however, there has been a concerted effort by a few "pet enthusiasts" to spook petfood manufacturers and well-meaning pet owners into the notion that vitamin K supplementation in the diet, specifically with vitamin K3 (menadione), is unnecessary and potentially toxic to pets. Determining whether they have a valid point warrants a bit of investigation.
Vitamin K is known principally for its role in blood clotting, but has also been reported to be involved with osteocalcin and bone formation, along with a number of other biomedical functions. By definition, vitamin K is any of several compounds that are based on 2-methyl-1,4-napthoquinone (also known as menadione) that express anti-hemorrhagic properties (Suttie, 2007). The amount of vitamin K recommended for dogs and cats is approximately 1 part per million of the diet (NRC, 2006).
Vitamin K can be supplied to the animal from a combination of sources: vitamins K1, K2 and K3. Vitamin K1, also known as phylloquionone, is found in green leafy vegetables and vegetable oils. Vitamin K2 is produced by gut bacteria and vitamin K3 is chemically synthesized. Vitamins K1 and K2 are "active" upon absorption. However, vitamin K3 must be "alkylated" by gut bacteria or tissue enzymes to become active. This activation involves the addition of isoprenoid side chains, and in some literature this activated form is called menaquinone or designated as MK-4.
While it is true that most of the dietary vitamin K can be provided by vitamin K1 from green leafy plants and vegetable oils, or through stable gut fermentation (supplying vitamin K2), the variability in these sources, effects from processing and gut health of the animal make them less than 100% reliable. Further, not every petfood company considers green leafy vegetables or vegetable oils to be an essential part of a dog or cat diet. So, many petfood companies choose to supplement with a commercial source. These commercial sources of vitamin K3 are produced through industrial synthetic chemistry. The AAFCO Official Publication (2007) lists only vitamin K3 sources as approved for use. They include menadione dimethylpyrimidinol bisulfite (MDPB), menadione nicotinamide bisulfite (MNBS) and menadione sodium bisulfite complex (MSBC).
None of these are explicitly identified for use in petfoods, but no objections have been forthcoming if MSBC is the vitamin K3 source of choice. No forms of vitamin K1 or K2 are approved as feed ingredients, although there is an understanding that no regulatory action will be taken for pre-market approved use of vitamin K1. So, even if a petfood company gave in to these unfounded concerns over vitamin K3, they don't really have any other option besides vitamin K3 to use as the supplement unless they choose to completely remove it from the formula altogether.
As for the toxicity concerns, there are no reports of nutritional toxicity of any vitamin K sources in dogs or cats readily available in the literature, and certainly no descriptions that could be found showing vitamin K3 (menadione) was harmful to dogs, cats or other domestic animals. The recent Nutrient Requirements of Dogs and Cats (NRC, 2006) and the Vitamin Tolerances of Animals (1987) texts are authoritative reviews on the topic, and they state that toxicity of menadione by nutritional routes is in excess of 1,000 times the requirement. Further, vitamin K3 has been fed to poultry, swine and companion animals for more than 50 years without incident. So, nutritional supplementation with vitamin K3 doesn't appear to be a smoking gun.
However, the pharmacological or medical use of vitamin K to combat acute ingestion of anti-clotting agents (e.g., warfarin or coumarin) and bleeding disorders of neonates (human) associated with vitamin K deficiencies is a different story. Under these circumstances and dosages, vitamin K1 (phylloquinone) is the preferred intravenous (parenteral) source of vitamin K. Further, it has been demonstrated that intravenous administration of vitamin K3 at dosages of 100 mg/kg (which is around 100 times the nutritional requirement) may be toxic. One can speculate that this could be due to the lack of "alkylation" that occurs when vitamin K3 is administered through a route other than via the gut.
Necessary and nontoxic
While small amounts of vitamin K are required in the diet and might be provided by whole ingredients or healthy gut fermentation, the uncertainty of these sources leads many petfood companies to supplement with commercial vitamin K3 (menadione). To provide this wee bit of "insurance" in the petfood, there is only one form availablethe water soluble, stabilized menadione (MSBC). Judicious use of nutritional vitamin K3 is clearly not toxic, so this notion that vitamin K3 as an ingredient in petfoods should not be used is unfounded and should be reversed. Further, it is hoped that through education and communication, consumers can be made aware that not all that is printed on enthusiasts' websites is correct. Also, radical nutrition positions should be compared and contrasted with current and comprehensive research literature and not just a few, potentially unrelated, experiments.