There are two main kinds of vitamin K: Phylloquinone (Vitamin K1) and Menaquinones (Vitamin K2). Vitamin K is mainly known as an agent involved in blood coagulation, maintaining the activity of coagulation factors in the liver. 

Vitamin K1 has a positive effect on bone health, such as maintaining bone mineral density and preventing fractures. Epidemiological studies suggested that a lack of vitamin K is associated with several diseases, including osteoporosis and vascular calcification. According to research that investigates the relationship between bone mineral density and serum vitamin K1 levels in post-menopausal women, vitamin K1 may contribute to maintaining bone mineral density. (1) The mean serum vitamin K1 level was significantly lower in the postmenopausal osteoporotic women group than in the normal control group. Serum vitamin k1 concentration was positively correlated with lumbar spine bone mineral density among postmenopausal osteoporotic women and in postmenopausal healthy control. Another study has been conducted to determine whether daily vitamin K1 supplementation safely reduces bone loss, bone turnover, and fractures. The results showed that daily 5 mg of vitamin K1 supplementation for 2 to 4 years does not protect against age-related decline in bone mineral density. But it may protect against fractures and cancers in postmenopausal women with osteopenia. (2) The incidence of clinical vertebral fractures among women in the Phylloquinone (Vitamin K1) group was significantly lower. Also, there was a synergistic effect on hip fractures observed between vitamin K1 and vitamin D. Torbergsen et al. found that serum concentrations of vitamin K1 and 25 (OH)-vitamin-D were lower and significantly related to the risk of fracture in aged patients admitted for hip fracture. (3) 

Vitamin K1 plays an important role in preventing heart disease. A study has shown that vitamin K1 supplements improve various measures of heart health. According to a study on the effect of phylloquinone (Vitamin K1) supplementation on the coronary artery calcification progression, Phylloquinone (Vitamin K1) supplementation slows the progression of coronary artery calcification in healthy older adults with preexisting coronary artery calcification, independent of its effect on total matrix Gla protein concentrations. (4) 

In addition, a recent review suggests that vitamin K supplementation may be used as a novel adjuvant therapy to improve glycemic control and quality of life. (5) Increased vitamin K1 intake in a cohort study (LOE = B) was shown to decrease the risk of developing diabetes by 51%. Vitamin K1 is also associated with cognitive and memory performances. Vitamin K dependent proteins such as Protein Gas6 have been shown to play a key role in the peripheral and central nervous system. (6) Vitamin K may have a role in the pathogenesis of Alzheimer's disease because of its regulatory role in sulfotransferase activity and growth factor/tyrosine kinase receptor activity in the brain. (7) There is evidence that vitamin K1 intake in the elderly with Alzheimer's disease is significantly lower than in controls in the community. (8) One study showed that vitamin K1 was associated with better verbal episodic memory performances, especially on recall tasks. (9) 

References: 

1. Sawsan Jaghsi, Taghrid Hammoud, Shaden Haddad. Relation Between Circulating Vitamin K1 and Osteoporosis in the Lumbar Spine in Syrian Post-Menopausal Women Open Rheumatol J. 2018; 12: 1–9. 

2. Cheung AM, Tile L, Lee Y, Tomlinson G, Hawker G, Scher J, Hu H, Vieth R, Thompson L, Jamal S, Josse R. Vitamin K supplementation in postmenopausal women with osteopenia (ECKO trial): a randomized controlled trial. PLoS Med. 2008 Oct 14 ; 5 (10): e196 

3. Torbergsen AC, Watne LO, Wyller TB, Frihagen F, Strømsøe K, Bøhmer T, Mowe M. Vitamin K1 and 25(OH)D are independently and synergistically associated with a risk for hip fracture in an elderly population: a case control study. Clin Nutr. 2015 Feb; 34 (1): 101-6. 

4. Shea MK, O'Donnell CJ, Hoffmann U, Dallal GE, Dawson-Hughes B, Ordovas JM, Price PA, Williamson MK, Booth SL. Vitamin K supplementation and progression of coronary artery calcium in older men and women. Am J Clin Nutr. 2009 Jun; 89 (6): 1799-807. 

5. Manna P, Kalita J. Beneficial role of vitamin K supplementation on insulin sensitivity, glucose metabolism, and the reduced risk of type 2 diabetes: A review. Nutrition. 2016 Jul-Aug; 32 (7-8): 732-9. 

6. Guylaine Ferland Vitamin K and the Nervous System: An Overview of its Actions. Adv Nutr. 2012 Mar; 3 (2): 204–212. 

7. Allison AC. The possible role of vitamin K deficiency in the pathogenesis of Alzheimer's disease and in augmenting brain damage associated with cardiovascular disease. Med Hypotheses. 2001 Aug; 57 (2):151-5. 

8. Presse N, Shatenstein B, Kergoat MJ, Ferland G. Low vitamin K intakes in community-dwelling elders at an early stage of Alzheimer's disease. J Am Diet Assoc. 2008 Dec;108(12):2095-9. doi: 10.1016/j.jada. 2008.09.013. 

9. Presse N, Belleville S, Gaudreau P, Greenwood CE, Kergoat MJ, Morais JA, Payette H, Shatenstein B, Ferland G. Vitamin K status and cognitive function in healthy older adults. Neurobiol Aging. 2013 Dec; 34 (12): 2777-83. doi: 10.1016/ j.neurobiolaging.2013.05.031. Epub 2013 Jul 11.