The Four Fat-Soluble Vitamins – Vitamin A, D, E and K

Vitamins and minerals play a wide range of roles in your body to keep your cells, tissues, and organs functioning optimally. While these groups of nutrients tend to be lumped together, vitamins play vastly different roles than minerals. And furthermore, each vitamin has its own unique list of duties depending on its structure, function, and chemical breakdown.

And then comes the water-soluble versus fat-soluble aspect. You’ve probably heard of fat-soluble and water-soluble vitamins, but what exactly does that mean?

This article will highlight the four fat-soluble vitamins, describe their primary functions, and explain why each of these nutrients is vital for bone health.

What Are The 4 Fat-Soluble Vitamins?

The four fat-soluble vitamins include vitamins A, D, E, and K.

The solubility of a vitamin (water versus fat) comes down to the structure of the vitamin, which indicates how each nutrient is absorbed and where it’s stored. 

The water-soluble vitamins (B and C) are dissolved in water and, therefore, easily excreted. 

Fat-soluble vitamins, on the other hand, are dissolved in fat. This means that in order to absorb them, you need fat in the matrix of your food, and once they’re absorbed into your tissues, they can build up over time as they get stored with your fat.

For this reason, it’s much more common for people to experience fat-soluble vitamin toxicity than water-soluble vitamin toxicity. 

The Main Functions of Fat-Soluble Vitamins

Fat-soluble vitamins perform a range of vital activities in your body. Below are some of the main functions that each of the vitamins is responsible for. 

Vitamin A

Vitamin A plays a crucial role in cell growth and differentiation, contributing to the normal formation and maintenance of the heart, lungs, eyes, and other organs. Vitamin A is also involved in[1][2]:

• Immune function
• Cellular communication
• Growth and development
• Male and female reproduction

Vitamin D

Along with its vital role in bone health (to be discussed more below), vitamin D is involved in[3]:

• Reduction of inflammation
• Cellular growth
• Neuromuscular function
• Immunity 
• Glucose metabolism 

Vitamin E

Vitamin E is most well-known as an antioxidant. In fact, “vitamin E” is actually a term used for a collective of fat-soluble compounds with distinctive antioxidant activities. In addition to its role as an antioxidant, vitamin E is also involved in[4]:

• Gene expression
• Cell signaling 
• Immunity
• Inhibition of platelet aggregation
• Reducing inflammation

Vitamin K

Vitamin K plays an important role in blood clotting but also assists with:

• Regulating blood calcium levels
• Facilitating energy production
• Maintaining heart health

In addition to each of their individual contributions, the fat-soluble vitamins work in concert with each other to balance one another and improve overall health and vitality. 

For example, studies suggest that vitamin A can reduce the risk of vitamin D toxicity and vice versa. Similarly, adequate levels of vitamin K may decrease symptoms of vitamin A and E toxicity[5][6].

Therefore, each of the four fat-soluble vitamins is not only necessary to human health, but their ratios and proper intakes are also vitally important as they all work synergistically with one another.

The Fat-Soluble Vitamins and Bone Health

Vitamin D

Vitamin D is unique in that, traditionally, the best way to get vitamin D is through sunlight. Unfortunately, today most people spend the majority of their time indoors. Furthermore, studies show that even if you do spend a fair amount of time outside, depending on the latitude of where you live you may not be getting any vitamin D absorption. For instance, one study found that in certain areas of the Northern US, sun exposure during the months of November to February results in zero vitamin D synthesis[7]. 

For both of these reasons, vitamin D supplementation is becoming commonplace.

Vitamin D is vitally important for bone health. Specifically, this vitamin plays a role in regulating circulating calcium and phosphorus levels by enhancing their absorption from food. Calcium and phosphorus are two of the most important minerals for bone health and integrity, which means that when vitamin D is low, it puts your bones at risk for bone mineral density loss and the potential development of osteoporosis or fractures[8]. 

Vitamin D comes in two forms, D2 and D3. While D2 is found in mushrooms and in small amounts in other plants, D3 comes from animal sources like eggs and fish. Of the two, D3 is much more readily absorbed and utilized[9]. 

Vitamin E

Vitamin E is one of the most potent fat-soluble antioxidants and plays a crucial role in protecting the membrane of your cells from free radicals and oxidative damage. 

Studies show that vitamin E may help protect against osteoporosis due to several different mechanisms[10]:  

• First, there is evidence that vitamin E may enhance osteoblast activity (the bone cells that are responsible for creating new bone in place of old bone). 
• Second, cell studies show that vitamin E may enhance the proliferation of mesenchymal stem cells (MSCs), which are found in bone marrow and are responsible for making and repairing skeletal tissues.
• Third, studies show that vitamin E may suppress the formation of osteoclast cells (the cells responsible for breaking down bone). 

Furthermore, a clinical study found that women with low vitamin E levels during postmenopause were at greater risk for developing osteoporosis[11]. 

Vitamin K

There are two forms of vitamin K, K1 and K2. While vitamin K1 plays an essential role in blood clotting, vitamin K2 is the form that is vital for bone health.

Specifically, Vitamin K2 directs calcium into your bones and away from your arteries, assisting in both bone and heart health. When vitamin K2 is low, it can lead to arterial calcification, which sets the stage for cardiovascular disease. When you have balanced levels of vitamin K2, on the other hand, it keeps your arteries clear of excess calcium while also helping your bones maintain their mineral density. 

A common misunderstanding is that excess calcium supplementation increases arterial calcification when, in fact, studies show that it’s a lack of vitamin K2 that is likely the culprit[12]. 

Vitamin K2 is also involved with the production of osteocalcin in your bones, which helps to keep your bones strong. This may be why research shows that low vitamin K2 levels are linked to low bone mineral density[13]. 

Vitamin A

The proper balance of vitamin A is essential to bone health, as both too much and too little may increase your risk of fractures. As mentioned previously, all four fat-soluble vitamins are needed to balance one another, so making sure that you’re getting enough vitamins D, E, and K will also help you maintain a proper ratio of vitamin A. 

Studies indicate that adequate vitamin A through food or supplements helps to maintain healthy bones through this nutrient’s impact on bone cell formation.

Furthermore, research shows that poor bone health outcomes due to high levels of vitamin A are more pronounced in individuals with vitamin D deficiency, indicating that it may not be vitamin A itself causing an issue but rather an imbalance in fat-soluble vitamins[14]. 

The Recommended Intakes for Fat-Soluble Vitamins

As mentioned, these four vitamins work synergistically with one another to optimize their balance and enhance each other’s function. As such, it’s important to get the right amount of each nutrient in a balanced ratio depending on your age and gender. 

Below are the recommended daily intakes for each fat-soluble vitamin.

Vitamin A: 700 mcg/day for adult females and 900 mcg//day for adult males[15]. 

Vitamin D: The National Institute of Health recommends that adults under 71 need 600 IU daily, with that number increasing to 800 IU for adults over the age of 71. However, this dosage may be a bit conservative as The International Osteoporosis Foundation recommends a higher dose of 800-1000 IU daily for adults over 60[16]. 

Getting at least 800 IU per day is a good idea for anyone concerned about bone health.

Vitamin E: 15 mg/day for females and males over the age of 14. Taking more in excess does not result in any added benefits[17]. 

Vitamin K: 90 mcg/day for females 19 and older and 120 mcg/day for males 19 and older[18]. 

Takeaway 

While getting a balance of nutrients in your diet is always important, when it comes to the fat-soluble vitamins, it’s even more essential. Due to their ability to impact one another, maintaining a healthy balance of these nutrients is crucial not only for bone health but for the health of all of your organ systems. 

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FAQs

References

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2. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/#en1
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4. https://ods.od.nih.gov/factsheets/VitaminE-HealthProfessional/
5. Frigg, M., and J. Broz. “Relationships between vitamin A and vitamin E in the chick.” International Journal for Vitamin and Nutrition research. Internationale Zeitschrift fur Vitamin-und Ernahrungsforsch
6. https://www.westonaprice.org/health-topics/abcs-of-nutrition/vitamin-a-on-trial-does-vitamin-a-cause-osteoporosis/#dprotects
7. Webb, A.R., et al. “Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin.” J Clin Endocrinol Metab (1988): 373-378.
8. DeLuca, Hector F. “The metabolism and functions of vitamin D.” Steroid Hormone Resistance (1986): 361-375.
9. Shieh, Albert, et al. “Effects of high-dose vitamin D2 versus D3 on total and free 25-hydroxyvitamin D and markers of calcium balance.” The Journal of Clinical Endocrinology & Metabolism 101.8 (2016): 3070-3078.
10. Wong, Sok Kuan, et al. “The molecular mechanism of vitamin E as a bone-protecting agent: a review on current evidence.” International journal of molecular sciences 20.6 (2019): 1453.
11. Mata-Granados, José M., et al. “Lower vitamin E serum levels are associated with osteoporosis in early postmenopausal women: a cross-sectional study.” Journal of bone and mineral metabolism 31.4 (2013): 455-460.
12. Beulens, Joline WJ, et al. “High dietary menaquinone intake is associated with reduced coronary calcification.” Atherosclerosis 203.2 (2009): 489-493.
13. Weber, Peter. “Vitamin K and bone health.” Nutrition 17.10 (2001): 880-887.
14. Yee, Michelle Min Fang, et al. “Vitamin A and bone health: a review on current evidence.” Molecules 26.6 (2021): 1757.
15. https://www.mayoclinic.org/drugs-supplements-vitamin-a/art-20365945
16. https://www.osteoporosis.foundation/patients/prevention/vitamin-d
17. https://www.hsph.harvard.edu/nutritionsource/vitamin-e/
18. https://ods.od.nih.gov/factsheets/VitaminK-HealthProfessional/#h6