Vitamin D3 and K2 Taken May Benefit Health and Performance

Vitamin D3 and K2 Taken May Benefit Health and Performance

Vitamin D has been praised for its ability to improve bone health, immune function, and hormone balance. Learn how taking vitamin D3 and K2 may benefit health and performance.

What are D3 and K2 Vitamins

Vitamin D3 and K2 are essential nutrients needed for optimal health.

Vitamin D

Vitamin D is a fat-soluble vitamin also known as calciferol. It comes in two main forms D2 ergocalciferol (D2) and cholecalciferol (D3) [R].

Ergocalciferol (D2) vs Cholecalciferol (D3)

Ergocalciferol (D2) is derived mostly from plants, mushrooms, and yeasts or synthetically made. [R, R, R, R ].

Cholecalciferol (D3) is made in mammals and fish. Humans make D3 from absorbing the sun's UV rays. When the sun touches human skin it produces D3

[R, R, R].

D2 and D3 Need Convert to Active Vitamin D

Both D2 and D3 need to be activated for vitamin D levels to increase blood levels. First the liver metabolizes D2 and D3 into 25-hydroxyvitamin D (25OHD). The kidneys then convert 25OHD into the active form 1,25-dihydroxy vitamin D also known as calcitriol.

Often vitamin D status will be measured as 25OHD but can also be in the active calcitriol form of the vitamin [R]. Calcitriol is needed for hormone production, muscle function, blood sugar regulation, and immune function [R].

Vitamin K2

Vitamin K2 is a subset of the fat-soluble vitamin K.

K2 is found naturally occurring in some foods and also produced by the fermentation of bacteria. It is needed for bone and cardiovascular health [R, R].

MK-7 vs. MK-4

There are about 13 types of K2 but the most common forms are the menaquinone-7 (MK-7) and menaquinone-4 (MK-4).

The term menaquinone refers to how many structures are in the chemical formula. [R, R].

MK-7 appears to be the more active form. MK-7 has been detected within 48 hours of consumption while MK-7 supplementation did not increase serum MK-4 levels [R].

Vitamin D3 and K2 Health Benefits

Vitamin D3 and K2 have been studied for their possible improvements on health.

Vitamin D3 Health Benefits

Vitamin D3 may improve sleep along with immune function, cardiovascular, blood sugar regulation, and bone health.

Cardiovascular Health

Low and insufficient levels of vitamin D have been associated with a higher risk of heart disease [R].

Some research shows benefits of 1,000 IU daily supplementation for cardiovascular health while studies showed no change in markers for heart disease [R, R, R].

Vitamin D has been found to regulate blood pressure and prevent cardiac enlargement in mice [R].

Low vitamin D has been associated with calcium deposits within arteries. This calcification can lead to poor heart hearth. The right balance of just enough vitamin D may be needed to reverse this process [R].

Vitamin D has been found to prevent atherosclerosis in mice by preventing oxidation of immune cells [R].

Blood Sugar Regulation

Higher levels of vitamin D were associated with better blood sugar regulation [R].

Supplementation has also been shown to improve blood sugar levels in those who are deficient [R].

Those with higher vitamin D levels had a lower risk for diabetes [R].

Bone Health

Vitamin D is needed for calcium absorption to improve the health of bone.

It has been found to restore bone tissue. Bone forming cells are also able to convert 25OD into the active form [R, R].

Low vitamin D may cause bone fractures even in healthy young men and women. Stress fractures of the tibia and fibula may be prevented with blood levels of 40 ng/mL or greater. This can be accomplished with intakes of 2,000 to 4,000 IU of vitamin D3 [R, R].

Improved Sleep

Sleep disorders can be cause by vitamin D deficiency. Supplementation may help elsseon these condtions [R].

Veterans deficient in vitamin D trouble were found to have longer sleep durations after taking 50,000 IU per week for six months [R].

Those with sleeping disorders had longer and better quality sleep when supplementing with 50,000 IU once every two weeks for two months [R].

Immune Health

Increased risk of illness and infection has been seen with low levels of vitamin D [R].

Immune cells need vitamin D to function. Autoimmunity and inflammation may be prevented with vitamin D3 [R].

A supplement of 1,200 IU of vitamin D per day for four months reduced the flu infection in children when compared with a placebo [R].

K2 Health Benefits

Vitamin K2 health benefits include cardiovascular health, bone health, and immune function

Cardiovascular Health

Vitamin K2 may protect from heart disease risk.

Higher intake of K2 was associated with a lower risk of coronary heart disease, death, and arterial calcifications [R].

Bone Health

Vitamin has been associated with better bone health and reduced fractures.

MK-7 from fermented soybeans stimulates bone growth and remineralizes bone [R].

Vitamin K2 may improve bone quality and reduce fractures in conjunction with calcium and vitamin D. One fracture is reduced by 25% with the daily intake of 800 IU of vitamin D, 45 micrograms vitamin K2, and 1200 mg calcium [R].

K2 is thought to increase bone strength. Taking 45 mg of K2 per day significantly reduced hip, vertebral, and non-vertebral fractures [R].

Immune Function

K2 is thought to reduce markers for inflammation related to disease and aging

When added to cell cultures MK-7 had immunoprotective effects by producing immune fighting cells and preventing the production of inflammatory markers [R].

Vitamin D3 As a Performance Vitamin

Muscle mass is critical for performance. Adequate levels of vitamin D are needed for optimal muscle function [R].

Levels between 30 and 50 ng/mL have been shown to increase muscle protein synthesis, ATP concentration, strength, jump height, jump velocity, jump power, exercise capacity, and physical performance. Inflammation, pain and weakness are also reduced at this level [R].

Supplementation of vitamin D improves blood levels of 25OHD in deficient athletes and trainers. Especially those living in northern climates and practice indoors during winter months [R].

Doses of 2,000 and 5,000 IUs of vitamin D3 given over two and four month periods were found to improve muscle function and prevent injury in vitamin D deficient athletes when compared with a placebo [R, R, R].

Other studies found doses between 20,000 and 40,000 IUs were not effective for improving muscle function. [R].

Athletes and trainers may benefit from a D3 supplement if they are deficient in this vitamin.

Vitamin K2 As a Performance Vitamin

Endurance relies on cardiac output [R]. Vitamin K2 has been shown to improve cardiovascular function and prevent injury in active individuals.

Skeletal and cardiac muscle function may be improved with K2 supplements. Male and female athletes given K2 supplement over an 8 week period (300 mg per day for the first four weeks, then 150 mg for the last four weeks) had a 12% increase in cardiac output than those receiving a placebo [R].

Other research on vitamin K2 and performance has been focused on the correlation between injury and low vitamin levels.

Lower intakes of nutrients important for bone health like K2 were found to be lower in all but one of the 16 collegiate athletes who sustained sports fractures [R].

Nine out of 71 professional Japanese baseball players aged 18 to 39 year old were found to have low levels of phylloquinone despite getting an adequate daily intake of vitamin K [R].

The full benefits of Vitamin K2 and perfrance are still unknown.

Benefits of Taking Vitamin D3 and K2 Together

Separately vitamin D3 and K2 provide an abundance of nutrition and benefits for health. Together their benefits may be enhanced due to the synergy of their effects on both cardiovascular and bone health [R].

The combined strength of these nutrients may help enhance performance and health [R, R, R].

What to Look for in a Vitamin D3 and K2 supplement

Vitamin supplements are not regulated so looking for a supplement that is from a reputable company and is third-party tested. Look for a product that is free of contaminants, additives, and fillers.

D3 (cholecalciferol) is better elevating and maintaining 25-hydroxyvitamin D levels than D2. So look for a supplement that has D3 and not D2 [R, R, R].

Sources of Vitamin D3 and K2

If you are looking for other ways to amp up your intake of vitamin D3 and K2 you can also get these natural sources.

Vitamin D3 Sources

Vitamin D can be found in food and also can be made by the sun.

Vitamin D3 and the Sun

The sun is the best source of vitamin D. Getting 10 to 15 minutes of sunlight with no sunscreen will give you a day’s worth of Vitamin D. Thanks to our bodies’ ability to convert it into its active form.

Vitamin D3 In Food

Food is also where you will find vitamin D. Recently a lot of foods have been fortified with vitamin D to address the problem of deficiency within the population.

Take caution with these foods. Often they contain D2 and may not be as effective. They can also be overly processed and high in sugar or unhealthy fats.

The best foods to get vitamin D include the following:

  • Beef liver
  • Cod and cod liver oil
  • Egg yolk
  • Salmon
  • Sardines
  • Mushrooms
  • Swiss cheese.
  • Swordfish
  • Tuna
  • Herring

Vitamin K2 Sources

Vitamin K2 can be made in the body from K1 but it may not be enough to meet the needs of this nutrient. Luckily, some foods contain this vitamin that can be added to the diet. They include animal products and fermented foods:

  • High-fat dairy products from grass-fed cows
  • Egg yolks
  • Liver
  • Organ meats
  • Natto
  • Hard cheeses
  • Soft cheeses
  • Butter
  • Dark Chicken Meat

How to Take Vitamin D3 and K2

Vitamin D3 Dosage

Research shows a dose of 1,000 to 2,000 International Units (IU) of D3 per day should meet the needs of most people. Vitamin D is better absorbed when taken with a source of dietary fat [R].

Vitamin D3 supplements come in both liquid and pill form. Liquid D3 is often mixed with olive oil for better absorption. Those with digestive issues may benefit from taking a liquid form of this vitamin.

Vitamin D has been known to cause toxicity when taken in high doses. That is why an upper limit has been imposed. It is recommended not to exceed 4,000 IU a day.

Newer research indicates 10,0000 IU of vitamin D had no signs of toxicity in healthy individuals. Unless under the direction of a health professional it is best to not exceed the recommended upper-level intake. [R, R].

Vitamin K2 Dosage

The MK-7 appears to be more readily absorbed in humans when compared with the MK-4 form of K2.

MK-4 was found to be effective in healthy subjects at higher doses of at least 1500 micrograms per day [R].

Dosages of MK-7 increased K2 at levels between 45 and 90 micrograms a day [R].

Other research showed MK-7 doses should be 150 to 180 micrograms per day [R, R].

MK-7 may induce blood clotting in some individuals with excessive blood clotting at doses as little as 10 micrograms [R].

It may be best to start with a lower dose of MK-7 or consult with a specialist who can help you find the right dose.

Side Effects of D3 and K2 Supplements

Like any supplements there can be side effects when taking vitamin D3 and K2.

Side Effects of D3

Side effects are rare unless the intake is excessive. It usually occurs in those taking high dosing for long periods. Side effects include the following [R]:

  • Weakness
  • Fatigue
  • Sleepiness
  • Headache
  • Loss of appetite
  • Dry Mouth
  • Metallic taste
  • Nausea
  • Vomiting
  • High calcium levels
  • High blood pressure
  • Digestive issues (stomach pain, constipation, diarrhea)

Side Effects of K2

Upset stomach or diarrhea may occur but most people have not had any adverse side effects when taking the recommended doses.

No toxic effects were seen in mice given lethal doses of MK-7 (greater than 2000 mg per kg of body weight). Other research indicates there is no adverse effect on humans so no limit has been posed [R, R].

Contraindications of D3 and K2 Supplements

For some individuals, there are risks to taking vitamin D3 and K2 supplements. If you have medical issues it is best to check with your healthcare provider before taking these supplements.

Contraindications of D3 and K2 Supplements

Supplementation of vitamin D is not recommended for those with [R] :

  • Sarcoidosis
  • Elevated phosphate levels
  • High calcium levels
  • High vitamin D levels
  • Kidney stones or decreased kidney function
  • Artery blockages (arteriosclerosis obliterans)

Medications that have been found to interact with vitamin D3 and K2 include:

Antacids, calcipotriene, digoxin, diltiazem, verapamil, water pills (chlorothiazide, hydrochlorothiazide, indapamide metolazone, and chlorthalidone, orlistat, anticoagulants, Colesevelam [R, R].

Final Thoughts on Vitamin D3 and K2 Vitamins

Low levels of vitamins D and K2 can negatively affect overall health. Decreased levels have also been tied to poor muscle function which can impact performance. Athletes and high endurance trainers deficient in these vitamins may benefit from supplementation. Though side effects are rare it is best to start with a low dose to prevent toxicity and side effects. Those with health issues or on medications should ask their doctor before taking vitamin D3 and K2.

References

  1. “Vitamin D.” n.d. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/.
  2. Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D and Calcium, A. Catharine Ross, Christine L. Taylor, Ann L. Yaktine, and Heather B. Del Valle. 2011. Overview of Vitamin D. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK56061/
  3. Frank, Kurtis, Kamal Patel, Gregory Lopez, and Bill Willis. 2020. “Vitamin D Research Analysis,” May. https://examine.com/supplements/vitamin-d/research.
  4. “Vitamin D.” 2014. April 22, 2014. https://lpi.oregonstate.edu/mic/vitamins/vitamin-D.
  5. Akbulut, Asim Cengiz, Angelina Pavlic, Ploingarm Petsophonsakul, Maurice Halder, Katarzyna Maresz, Rafael Kramann, and Leon Schurgers. 2020. “Vitamin K2 Needs an RDI Separate from Vitamin K1.” Nutrients 12 (6). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7353270/.
  6. Conly, J. M., and K. Stein. 1992. “The Production of Menaquinones (vitamin K2) by Intestinal Bacteria and Their Role in Maintaining Coagulation Homeostasis.” Progress in Food & Nutrition Science 16 (4): 307–43. https://pubmed.ncbi.nlm.nih.gov/1492156/
  7. Usui, Y., N. Nishimura, N. Kobayashi, T. Okanoue, M. Kimoto, and K. Ozawa. 1989. “Measurement of Vitamin K in Human Liver by Gradient Elution High-Performance Liquid Chromatography Using Platinum-Black Catalyst Reduction and Fluorimetric Detection.” Journal of Chromatography 489 (2): 291–301. https://pubmed.ncbi.nlm.nih.gov/2753953/
  8. Shino, M. 1988. “Determination of Endogenous Vitamin K (phylloquinone and Menaquinone-N) in Plasma by High-Performance Liquid Chromatography Using Platinum Oxide Catalyst Reduction and Fluorescence Detection.” The Analyst 113 (3): 393–97. https://pubmed.ncbi.nlm.nih.gov/3377180/
  9. Sato, Toshiro, Leon J. Schurgers, and Kazuhiro Uenishi. 2012. “Comparison of Menaquinone-4 and Menaquinone-7 Bioavailability in Healthy Women.” Nutrition Journal 11 (November): 93. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3502319/
  10. Wang, Thomas J., Michael J. Pencina, Sarah L. Booth, Paul F. Jacques, Erik Ingelsson, Katherine Lanier, Emelia J. Benjamin, Ralph B. D’Agostino, Myles Wolf, and Ramachandran S. Vasan. 2008. “Vitamin D Deficiency and Risk of Cardiovascular Disease.” Circulation 117 (4): 503–11. https://pubmed.ncbi.nlm.nih.gov/18180395/
  11. Wang, Lu, Joann E. Manson, Yiqing Song, and Howard D. Sesso. 2010. “Systematic Review: Vitamin D and Calcium Supplementation in Prevention of Cardiovascular Events.” Annals of Internal Medicine 152 (5): 315–23. https://pubmed.ncbi.nlm.nih.gov/20194238/
  12. Wood, Adrian D., Karen R. Secombes, Frank Thies, Lorna Aucott, Alison J. Black, Alexandra Mavroeidi, William G. Simpson, William D. Fraser, David M. Reid, and Helen M. Macdonald. 2012. “Vitamin D3 Supplementation Has No Effect on Conventional Cardiovascular Risk Factors: A Parallel-Group, Double-Blind, Placebo-Controlled RCT.” The Journal of Clinical Endocrinology and Metabolism 97 (10): 3557–68. https://pubmed.ncbi.nlm.nih.gov/22865902/
  13. Seibert, Eric, Ulrike Lehmann, Annett Riedel, Christof Ulrich, Frank Hirche, Corinna Brandsch, Jutta Dierkes, Matthias Girndt, and Gabriele I. Stangl. 2017. “Vitamin D3 Supplementation Does Not Modify Cardiovascular Risk Profile of Adults with Inadequate Vitamin D Status.” European Journal of Nutrition 56 (2): 621–34. https://www.ncbi.nlm.nih.gov/pubmed/26621634
  14. Xiang, Wei, Juan Kong, Songcang Chen, Li-Ping Cao, Guilin Qiao, Wei Zheng, Wenhua Liu, Xinmin Li, David G. Gardner, and Yan Chun Li. 2005. “Cardiac Hypertrophy in Vitamin D Receptor Knockout Mice: Role of the Systemic and Cardiac Renin-Angiotensin Systems.” American Journal of Physiology. Endocrinology and Metabolism 288 (1): E125–32. https://pubmed.ncbi.nlm.nih.gov/15367398/
  15. Zittermann, Armin, Stefanie S. Schleithoff, and Reiner Koerfer. 2007. “Vitamin D and Vascular Calcification.” Current Opinion in Lipidology 18 (1): 41–46. https://pubmed.ncbi.nlm.nih.gov/17218831/
  16. Weng, Sherry, Jennifer E. Sprague, Jisu Oh, Amy E. Riek, Kathleen Chin, Miguel Garcia, and Carlos Bernal-Mizrachi. 2013. “Vitamin D Deficiency Induces High Blood Pressure and Accelerates Atherosclerosis in Mice.” PloS One 8 (1): e54625. https://pubmed.ncbi.nlm.nih.gov/23349943/
  17. Liu, Enju, James B. Meigs, Anastassios G. Pittas, Nicola M. McKeown, Christina D. Economos, Sarah L. Booth, and Paul F. Jacques. 2009. “Plasma 25-Hydroxyvitamin D Is Associated with Markers of the Insulin Resistant Phenotype in Nondiabetic Adults.” The Journal of Nutrition 139 (2): 329–34. https://pubmed.ncbi.nlm.nih.gov/19106328/
  18. Knekt, Paul, Maarit Laaksonen, Catharina Mattila, Tommi Härkänen, Jukka Marniemi, Markku Heliövaara, Harri Rissanen, Jukka Montonen, and Antti Reunanen. 2008. “Serum Vitamin D and Subsequent Occurrence of Type 2 Diabetes.” Epidemiology 19 (5): 666–71. https://pubmed.ncbi.nlm.nih.gov/18496468/
  19. Matsumoto, T., C. Igarashi, Y. Takeuchi, S. Harada, T. Kikuchi, H. Yamato, and E. Ogata. 1991. “Stimulation by 1,25-Dihydroxyvitamin D3 of in Vitro Mineralization Induced by Osteoblast-like MC3T3-E1 Cells.” Bone 12 (1): 27–32. https://pubmed.ncbi.nlm.nih.gov/2054233/
  20. Atkins, Gerald J., Paul H. Anderson, David M. Findlay, Katie J. Welldon, Cristina Vincent, Andrew C. W. Zannettino, Peter D. O’Loughlin, and Howard A. Morris. 2007. “Metabolism of Vitamin D3 in Human Osteoblasts: Evidence for Autocrine and Paracrine Activities of 1 alpha,25-Dihydroxyvitamin D3.” Bone 40 (6): 1517–28. https://pubmed.ncbi.nlm.nih.gov/17395559/
  21. Ruohola, Juha-Petri, Ilkka Laaksi, Timo Ylikomi, Riina Haataja, Ville M. Mattila, Timo Sahi, Pentti Tuohimaa, and Harri Pihlajamäki. 2006. “Association between Serum 25(OH)D Concentrations and Bone Stress Fractures in Finnish Young Men.” Journal of Bone and Mineral Research: The Official Journal of the American Society for Bone and Mineral Research 21 (9): 1483–88. https://pubmed.ncbi.nlm.nih.gov/16939407/
  22. Burgi, Alina A., Edward D. Gorham, Cedric F. Garland, Sharif B. Mohr, Frank C. Garland, Kenneth Zeng, Kerry Thompson, and Joan M. Lappe. 2011. “High Serum 25-Hydroxyvitamin D Is Associated with a Low Incidence of Stress Fractures.” Journal of Bone and Mineral Research: The Official Journal of the American Society for Bone and Mineral Research 26 (10): 2371–77. https://pubmed.ncbi.nlm.nih.gov/21698667/
  23. Gao, Qi, Tingyan Kou, Bin Zhuang, Yangyang Ren, Xue Dong, and Qiuzhen Wang. 2018. “The Association between Vitamin D Deficiency and Sleep Disorders: A Systematic Review and Meta-Analysis.” Nutrients 10 (10). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6213953/
  24. Huang, Wei, Shivani Shah, Qi Long, Alicia K. Crankshaw, and Vin Tangpricha. 2013. “Improvement of Pain, Sleep, and Quality of Life in Chronic Pain Patients with Vitamin D Supplementation.” The Clinical Journal of Pain 29 (4): 341–47. https://pubmed.ncbi.nlm.nih.gov/22699141/
  25. Mohammad Shahi, Majid, Seyed Ahmad Hosseini, Bizhan Helli, Mohammad Hosein Haghighyzade, and Mohammad Abolfathi. 2017. “The Effect of Vitamin D Supplement on Quality of Sleep in Adult People with Sleep Disorders.” Tehran University Medical Journal TUMS Publications 75 (6): 443–48. https://tumj.tums.ac.ir/article-1-8272-en.html
  26. Aranow, Cynthia. 2011. “Vitamin D and the Immune System.” Journal of Investigative Medicine: The Official Publication of the American Federation for Clinical Research 59 (6): 881–86. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3166406/
  27. Urashima, Mitsuyoshi, Takaaki Segawa, Minoru Okazaki, Mana Kurihara, Yasuyuki Wada, and Hiroyuki Ida. 2010. “Randomized Trial of Vitamin D Supplementation to Prevent Seasonal Influenza A in Schoolchildren.” The American Journal of Clinical Nutrition 91 (5): 1255–60. https://pubmed.ncbi.nlm.nih.gov/20219962/
  28. Schwalfenberg, Gerry Kurt. 2017. “Vitamins K1 and K2: The Emerging Group of Vitamins Required for Human Health.” Journal of Nutrition and Metabolism 2017 (June): 6254836. https://www.hindawi.com/journals/jnme/2017/6254836/
  29. Gajic-Veljanoski, O., A. M. Bayoumi, G. Tomlinson, K. Khan, and A. M. Cheung. 2012. “Vitamin K Supplementation for the Primary Prevention of Osteoporotic Fractures: Is It Cost-Effective and Is Future Research Warranted?” Osteoporosis International: A Journal Established as Result of Cooperation between the European Foundation for Osteoporosis and the National Osteoporosis Foundation of the USA 23 (11): 2681–92. https://link.springer.com/article/10.1007%2Fs00198-012-1939-4
  30. DiNicolantonio, James J., Jaikrit Bhutani, and James H. O’Keefe. 2015. “The Health Benefits of Vitamin K.” Open Heart 2 (1): e000300. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4600246/
  31. Pan, Min-Hsiung, Katarzyna Maresz, Pei-Sheng Lee, Jia-Ching Wu, Chi-Tang Ho, Janusz Popko, Dilip S. Mehta, Sidney J. Stohs, and Vladimir Badmaev. 2016. “Inhibition of TNF-α, IL-1α, and IL-1β by Pretreatment of Human Monocyte-Derived Macrophages with Menaquinone-7 and Cell Activation with TLR Agonists In Vitro.” Journal of Medicinal Food 19 (7): 663–69. https://www.liebertpub.com/doi/abs/10.1089/jmf.2016.0030?journalCode=jmf
  32. Tanner, S. Bobo, and Susan A. Harwell. 2015. “More than Healthy Bones: A Review of Vitamin D in Muscle Health.” Therapeutic Advances in Musculoskeletal Disease 7 (4): 152–59. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4530385/
  33. Shuler, Franklin D., Matthew K. Wingate, G. Hunter Moore, and Charles Giangarra. 2012. “Sports Health Benefits of Vitamin D.” Sports Health 4 (6): 496–501. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3497950/
  34. Close, G. L., J. Russell, J. N. Cobley, D. J. Owens, G. Wilson, W. Gregson, W. D. Fraser, and J. P. Morton. 2013. “Assessment of Vitamin D Concentration in Non-Supplemented Professional Athletes and Healthy Adults during the Winter Months in the UK: Implications for Skeletal Muscle Function.” Journal of Sports Sciences 31 (4): 344–53. https://pubmed.ncbi.nlm.nih.gov/23083379/
  35. Close, Graeme L., Jill Leckey, Marcelle Patterson, Warren Bradley, Daniel J. Owens, William D. Fraser, and James P. Morton. 2013. “The Effects of Vitamin D(3) Supplementation on Serum Total 25[OH]D Concentration and Physical Performance: A Randomised Dose-Response Study.” British Journal of Sports Medicine 47 (11): 692–96. https://pubmed.ncbi.nlm.nih.gov/23410885/
  36. Wyon, Matthew A., Yiannis Koutedakis, Roger Wolman, Alan M. Nevill, and Nick Allen. 2014. “The Influence of Winter Vitamin D Supplementation on Muscle Function and Injury Occurrence in Elite Ballet Dancers: A Controlled Study.” Journal of Science and Medicine in Sport / Sports Medicine Australia 17 (1): 8–12. https://pubmed.ncbi.nlm.nih.gov/23619160/
  37. “Endurance.” n.d. .https://www.sciencedirect.com/topics/medicine-and-dentistry/endurance
  38. McFarlin, Brian K., Andrea L. Henning, and Adam S. Venable. 2017. “Oral Consumption of Vitamin K2 for 8 Weeks Associated With Increased Maximal Cardiac Output During Exercise.” Alternative Therapies in Health and Medicine 23 (4): 26–32. https://pubmed.ncbi.nlm.nih.gov/28646812/
  39. Sumida, S., Iwamoto, J., Kamide, N., & Otani, T. (2012). Evaluation of bone, nutrition, and physical function in Shorinji Kempo athletes. Open access journal of sports medicine, 3, 107–114. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3781905/
  40. Iwamoto J, Takeda T, Uenishi K, Ishida H, Sato Y, Matsumoto H. Urinary levels of cross-linked N-terminal telopeptide of type I collagen and nutritional status in Japanese professional baseball players. J Bone Miner Metab. 2010;28(5):540-546. https://pubmed.ncbi.nlm.nih.gov/20162436/
  41. Kidd PM. Vitamins D and K as pleiotropic nutrients: clinical importance to the skeletal and cardiovascular systems and preliminary evidence for synergy. Altern Med Rev. 2010;15(3):199-222. https://pubmed.ncbi.nlm.nih.gov/21155624/
  42. Goolsby, M. A., & Boniquit, N. (2017). Bone Health in Athletes. Sports health, 9(2), 108–117. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5349390/
  43. Russo, Cosimo Roberto. 2009. “The Effects of Exercise on Bone. Basic Concepts and Implications for the Prevention of Fractures.” Clinical Cases in Mineral and Bone Metabolism: The Official Journal of the Italian Society of Osteoporosis, Mineral Metabolism, and Skeletal Diseases 6 (3): 223–28. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2811354/
  44. O’Keefe, James H., Nathaniel Bergman, Pedro Carrera-Bastos, Maélan Fontes-Villalba, James J. DiNicolantonio, and Loren Cordain. 2016. “Nutritional Strategies for Skeletal and Cardiovascular Health: Hard Bones, Soft Arteries, rather than Vice Versa.” Open Heart 3 (1): e000325. https://openheart.bmj.com/content/3/1/e000325
  45. “Browse Articles.” 2020. Springer Nature. September 15, 2020. https://www.nature.com/ejcn/articles
  46. Hathcock, John N., Andrew Shao, Reinhold Vieth, and Robert Heaney. 2007. “Risk Assessment for Vitamin D.” The American Journal of Clinical Nutrition 85 (1): 6–18. https://pubmed.ncbi.nlm.nih.gov/17209171/
  47. Ross, A. Catharine, Joann E. Manson, Steven A. Abrams, John F. Aloia, Patsy M. Brannon, Steven K. Clinton, Ramon A. Durazo-Arvizu, et al. 2011. “The 2011 Report on Dietary Reference Intakes for Calcium and Vitamin D from the Institute of Medicine: What Clinicians Need to Know.” The Journal of Clinical Endocrinology and Metabolism 96 (1): 53–58. https://pubmed.ncbi.nlm.nih.gov/21118827/
  48. Knapen, M. H. J., N. E. Drummen, E. Smit, C. Vermeer, and E. Theuwissen. 2013. “Three-Year Low-Dose Menaquinone-7 Supplementation Helps Decrease Bone Loss in Healthy Postmenopausal Women.” Osteoporosis International: A Journal Established as Result of Cooperation between the European Foundation for Osteoporosis and the National Osteoporosis Foundation of the USA 24 (9): 2499–2507. https://pubmed.ncbi.nlm.nih.gov/23525894/
  49. “Google Books.” n.d. https://tinyurl.com/MK-7research
  50. “Should You Supplement with Vitamin K?” 2019. Examine.com. November 12, 2019. https://examine.com/nutrition/supplementing-vitamin-k/
  51. “Vitamin D: Uses, Side Effects, Interactions, Dosage, and Warning.” n.d.https://www.webmd.com/vitamins/ai/ingredientmono-929/vitamin-d
  52. Pucaj, Kresimir, Henrik Rasmussen, Mona Møller, and Tom Preston. 2011. “Safety and Toxicological Evaluation of a Synthetic Vitamin K2, Menaquinone-7.” Toxicology Mechanisms and Methods 21 (7): 520–32. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3172146/
  53. Institute of Medicine (US) Panel on Micronutrients. 2001. Vitamin K. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK222299/
  54. “Vitamin D: Uses, Side Effects, Interactions, Dosage, and Warning.” n.d. https://www.webmd.com/vitamins/ai/ingredientmono-929/vitamin-d
  55. “Drugs & Medications.” n.d. https://www.webmd.com/drugs/2/drug-164466/vitamin-k2-oral/details/list-interaction-medication