Vitamin K2, also known as menaquinone, is a fat-soluble vitamin often taken as a dietary supplement. It is chemically distinct from Vitamin K1 (phylloquinone), which is more commonly found in plant sources. K2 supports bone mineralization and promotes cardiovascular health by helping to manage calcium deposits in the body. This focus on K2’s systemic actions often leads people to question its overall safety profile, including whether it might negatively affect sensitive organs like the eyes.
Vitamin K2 and Ocular Health
Current clinical evidence does not support a link between Vitamin K2 supplementation and adverse ocular outcomes such as blurred vision, floaters, or dry eyes. Scientific literature indicates that K2 is not a recognized cause of eye toxicity at typical supplemental doses.
The mechanism by which K2 works involves preventing the calcification of soft tissues, which drives interest in its ocular benefits. K2 is necessary to activate Matrix Gla Protein (MGP), a potent inhibitor of calcium buildup in the vasculature. By keeping calcium out of the arteries, K2 may help maintain the integrity and health of the fine micro-vessels in the retina. These vessels are susceptible to age-related hardening and plaque formation. Some studies suggest that the status of inactive MGP is a predictor of smaller retinal arteriolar diameter, highlighting K2’s potential to promote retinal health, though direct studies on this link are limited.
Defining Vitamin K2 and Its Forms
Vitamin K2 is not a single compound but rather a group of molecules called menaquinones. These are differentiated by the length of their side chains and designated by the abbreviation MK followed by a number. The two most widely studied and commercially available forms are MK-4 and MK-7.
MK-4, or menaquinone-4, is a short-chain form primarily found in animal products like meat, eggs, and dairy. It is also produced in the body from Vitamin K1. MK-7, or menaquinone-7, is a long-chain form largely found in fermented foods, such as the Japanese soybean dish natto. MK-7 is valued for its superior bioavailability and longer half-life in the bloodstream. The overall function of K2 is to facilitate the activation of specific proteins that regulate calcium.
Common Side Effects and Safety Profile
Vitamin K2 is generally regarded as nontoxic, even at high doses. The United States Food and Nutrition Board has not established a Tolerable Upper Intake Level (UL) for Vitamin K in the US due to a lack of reported adverse effects from high oral intake of K1 or K2.
The most common adverse effects reported by individuals taking Vitamin K2 supplements are mild and involve the gastrointestinal tract. These effects can include minor issues like nausea, diarrhea, or an upset stomach. In rare cases, allergic reactions such as skin rashes have been reported, and these symptoms typically resolve quickly after discontinuing the supplement. Because K2 is a fat-soluble vitamin, it is recommended to take supplements with a meal containing some fat to optimize its absorption.
Important Drug Interactions
The most significant safety concern regarding Vitamin K2 involves its interference with anticoagulant medications, particularly warfarin (Coumadin). Warfarin works by blocking the vitamin K-dependent enzymes responsible for recycling Vitamin K, thereby reducing the production of clotting factors. Since Vitamin K2 directly contributes to the blood clotting cascade, an inconsistent or high intake of K2 can counteract the intended blood-thinning effect of warfarin.
Patients taking warfarin must maintain a consistent daily intake of Vitamin K to ensure the medication’s dose remains stable and effective.
Other Medication Interactions
In addition to anticoagulants, other medications can affect K2 absorption and status. Bile acid sequestrants, used to lower cholesterol, can bind to fat-soluble vitamins in the gut, reducing K2 absorption. Similarly, long-term use of certain antibiotics can disrupt the gut bacteria responsible for producing some forms of menaquinone, potentially leading to lower K2 levels.