Vitamins can be classified based on their solubility.
Most are water-soluble, meaning they dissolve in water. In contrast, the fat-soluble vitamins are similar to oil and do not dissolve in water.
Fat-soluble vitamins are most abundant in high-fat foods and are much better absorbed into your bloodstream when you eat them with fat.
There are four fat-soluble vitamins in the human diet:
- Vitamin A
- Vitamin D
- Vitamin E
- Vitamin K
Vitamin A plays a key role in maintaining your vision. Without it, you would go blind.
Vitamin A is not a single compound. Rather, it is a group of fat-soluble compounds collectively known as retinoids.
The most common dietary form of vitamin A is retinol. Other forms — retinal and retinoic acid — are found in the body, but absent or rare in foods.
Vitamin A2 (3,4-dehydroretinal) is an alternative, less active form found in freshwater fish (1).
Summary: The main dietary form of vitamin A is known as retinol.
Role and Function of Vitamin A
Vitamin A supports many critical aspects of body function, including:
- Vision maintenance: Vitamin A is essential for maintaining the light-sensing cells in the eyes and for the formation of tear fluid (2).
- Immune function: Vitamin A deficiency impairs immune function, increasing susceptibility to infections (3, 4).
- Body growth: Vitamin A is necessary for cell growth. Deficiency may slow or prevent growth in children (5).
- Hair growth: It is also vital for hair growth. Deficiency leads to alopecia, or hair loss (6).
- Reproductive function: Vitamin A maintains fertility and is vital for fetal development (7).
Summary: Vitamin A is best known for its vital role in maintaining vision. It's also essential for body growth, immune function and reproductive health.
Vitamin A is only found in animal-sourced foods. The main natural food sources are liver, fish liver oil and butter.
The table below shows the amount of vitamin A in 3.5 ounces (100 grams) of some of its richest dietary sources (8):
Vitamin A can also be derived from certain carotenoid antioxidants found in plants. They are collectively known as provitamin A.
Summary: The best dietary sources of vitamin A include liver and fish oil. Sufficient amounts can also be derived from provitamin A carotenoids, like beta-carotene, which are found in vegetables.
The table below shows the recommended daily allowance (RDA) for vitamin A. The RDA is the estimated amount of vitamin A that the vast majority (about 97.5%) of people need to meet their daily requirements.
This table also shows the tolerable upper intake limit (UL), which is the highest level of daily intake considered safe for 97.5% of healthy people (11).
|RDA (IU / mcg)||UL (IU / mcg)|
|Infants||0–6 months||1,333 / 400||2,000 / 600|
|7–12 months||1,667 / 500||2,000 / 600|
|Children||1–3 years||1,000 / 300||2,000 / 600|
|4–8 years||1,333 / 400||3,000 / 900|
|9–13 years||2,000 / 600||5,667 / 1700|
|Women||14–18 years||2,333 / 700||9,333 / 2800|
|19–70 years||2,333 / 700||10,000 / 3000|
|Men||14–18 years||3,000 / 900||9,333 / 2800|
|19–70 years||3,000 / 900||10,000 / 3000|
Summary: The RDA for vitamin A is 3,000 IU (900 mcg) for adult men and 2,333 (700 mcg) for women. For children, it ranges from 1,000 IU (300 mcg) to 2,000 IU (600 mcg).
Vitamin A Deficiency
Vitamin A deficiency is rare in developed countries.
However, vegans may be at risk, since pre-formed vitamin A is only found in animal-sourced foods.
Although provitamin A is abundant in many fruits and vegetables, it is not always efficiently converted into retinol, the active form of vitamin A. The efficiency of this conversion depends on people's genetics (12, 13).
Deficiency is also widespread in some developing countries where food variety is limited. It is common in populations whose diet is dominated by refined rice, white potatoes or cassava and lacking in meat, fat and vegetables.
A common symptom of early deficiency includes night blindness. As it progresses, it may lead to more serious conditions, such as:
- Dry eyes: Severe deficiency may cause xerophthalmia, a condition characterized by dry eyes caused by reduced tear fluid formation (2).
- Blindness: Serious vitamin A deficiency may lead to total blindness. In fact, it is among the most common preventable causes of blindness in the world (14).
- Hair loss: If you are vitamin A deficient, you may start to lose your hair (15).
- Skin problems: Deficiency leads to a skin condition known as hyperkeratosis or goose flesh (16).
- Poor immune function: Poor vitamin A status or deficiency makes people prone to infections (3).
Summary: Severe vitamin A deficiency may lead to blindness. Other symptoms may include hair loss, skin problems and an increased risk of infections.
Vitamin A Toxicity
Overdosing on vitamin A leads to an adverse condition known as hypervitaminosis A. It's rare, but may have serious health effects.
Its main causes are excessive doses of vitamin A from supplements, liver or fish liver oil. In contrast, high intake of provitamin A does not cause hypervitaminosis.
The main symptoms and consequences of toxicity include fatigue, headache, irritability, stomach pain, joint pain, lack of appetite, vomiting, blurred vision, skin problems and inflammation in the mouth and eyes.
It may also lead to liver damage, bone loss and hair loss. At extremely high doses, vitamin A can be fatal (17).
People are advised to avoid exceeding the upper limit for intake, which is 10,000 IU (900 mcg) per day for adults.
Higher amounts, or 300,000 IU (900 mg), may cause acute hypervitaminosis A in adults. Children can experience harmful effects at much lower amounts (18).
Individual tolerance varies considerably. Children and people with liver diseases like cirrhosis and hepatitis are at an increased risk and need to take extra care.
Pregnant women should also be especially careful, since high doses of vitamin A may harm the fetus. Doses as low as 25,000 IU per day have been linked with birth defects (19).
Summary: High doses of vitamin A may lead to hypervitaminosis A, which is associated with various symptoms. Pregnant women should avoid eating high amounts of vitamin A because of the risk of birth defects.
Benefits of Vitamin A Supplements
While supplements are beneficial for those who suffer from deficiency, most people get enough vitamin A from their diet and do not need to take supplements.
Yet, controlled studies suggest that vitamin A supplements may benefit certain people even if their diet meets the basic requirements.
They protect against measles-associated pneumonia and reduce the risk of death by 50–80%. Studies suggest that vitamin A acts by suppressing the measles virus (22).
Summary: Supplements mainly benefit those who are low or deficient in vitamin A. One exception is children with measles, as studies show that supplements may help treat the disease.
Summary of Vitamin A
Vitamin A, also known as retinol, is a fat-soluble vitamin traditionally associated with vision and eye health.
The most abundant dietary sources of vitamin A are liver, fish liver oil and butter.
It can also be derived from provitamin A carotenoids found in red, yellow and orange vegetables, as well as some leafy, dark-green vegetables.
Deficiency is rare in developed countries, but is most common among people who follow diets lacking in diversity, especially those dominated by rice, white potatoes and cassava.
Early symptoms of vitamin A deficiency include night blindness, and severe deficiency may eventually lead to total blindness.
Nevertheless, while getting enough vitamin A is vital, too much may cause harm.
Nicknamed the sunshine vitamin, vitamin D is produced by your skin when it's exposed to sunlight.
It is best known for its beneficial effects on bone health, and deficiency makes you highly susceptible to bone fractures.
Vitamin D is a collective term used to describe a few related fat-soluble compounds.
Also known as calciferol, vitamin D comes in two main dietary forms:
- Vitamin D2 (ergocalciferol): Found in mushrooms and some plants.
- Vitamin D3 (cholecalciferol): Found in animal-sourced foods, such as eggs and fish oil, and produced by your skin when exposed to sunlight.
Summary: Dietary vitamin D can be classified as vitamin D2, found in mushrooms and plants, and vitamin D3, found in animal-derived foods.
Role and Function of Vitamin D
Vitamin D has numerous roles and functions, but only a few are well researched. These include the following:
- Bone maintenance: Vitamin D regulates the circulating levels of calcium and phosphorus, which are the most important minerals for bone growth and maintenance. It promotes the absorption of these minerals from the diet.
- Immune system regulation: It also regulates and strengthens immune system function (23).
Once absorbed into the bloodstream, the liver and kidneys change calciferol into calcitriol, which is the biologically active form of vitamin D. It can also be stored for later use in the form of calcidiol.
Summary: One of the most important functions of vitamin D is the maintenance of calcium and phosphorus levels in blood. It benefits bone health by promoting the absorption of these minerals.
Sources of Vitamin D
Your body can produce all the vitamin D it needs as long as you regularly expose large parts of your skin to sunlight (26).
However, many people spend little time in the sun or do so fully clothed. Justifiably, others cover their skin with sunscreen to prevent sunburns. While sunscreen use is highly recommended, it reduces the amount of vitamin D produced by your skin.
As a result, people generally need to rely on their diets to get enough vitamin D.
Few foods naturally contain vitamin D. The best dietary sources are fatty fish and fish oil, but mushrooms that have been exposed to ultraviolet light may also contain significant amounts.
The chart below shows the amounts of vitamin D in 3.5 ounces (100 grams) of some of its richest dietary sources (8):
In addition, dairy products and margarine often come with added vitamin D.
To find out more ideas of foods you can eat to increase your vitamin D intake, read this article.
Summary: Your body can produce the vitamin D it needs if you regularly expose large parts of your skin to sunlight. However, most people need to get it from their diet or supplements, such as fatty fish or fish oil.
The table below shows the recommended dietary allowance (RDA) and upper limit (UI) for vitamin D (27).
Since no RDA has been established for infants, the values marked with an asterisk are the adequate intake (AI). The AI is similar to the RDA, but based on weaker evidence.
|Age Group||RDA (IU / mcg)||UL (IU / mcg)|
|0–6 months||400 / 10*||1,000 / 25|
|7–12 months||400 / 10*||1,500 / 38|
|1–3 years||600 / 15||2,500 / 63|
|4–8 years||600 / 15||3,000 / 75|
|9–70 years||600 / 15||4,000 / 100|
|70+ years||800 / 20||4,000 / 100|
If you want to find out more about the optimal intake of vitamin D, read this article.
Summary: For children and adults, the RDA for vitamin D is 600 IU (15 mcg). The amount is slightly higher for elderly adults, at 800 IU (20 mcg).
Vitamin D Deficiency
Severe vitamin D deficiency is rare, but mild forms of deficiency or insufficiency are common among hospitalized people as well as the elderly.
Risk factors of deficiency are dark skin color, old age, obesity, low sun exposure and diseases that impair fat absorption.
The most well-known consequences of vitamin D deficiency include soft bones, weak muscles and an increased risk of bone fractures. This condition is called osteomalacia in adults and rickets in children (28).
Other signs of deficiency or insufficiency may include fatigue, depression, hair loss and impaired wound healing.
Summary: The main symptoms of vitamin D deficiency include fatigue, weak muscles, soft bones, an increased risk of fractures and susceptibility to infections.
Vitamin D Toxicity
Vitamin D toxicity is very rare.
While spending a lot of time in the sun doesn't cause vitamin D toxicity, taking high amounts of supplements may harm you.
The main consequence of toxicity is hypercalcemia, a condition characterized by excessive amounts of calcium in the blood.
Symptoms include headache, nausea, lack of appetite, weight loss, fatigue, kidney and heart damage, high blood pressure and fetal abnormalities, to name a few.
People are generally advised to avoid exceeding the upper limit of vitamin D intake, which is 4,000 IU per day for adults.
Higher amounts, ranging from 40,000–100,000 IU (1,000–2,500 mcg) per day, may cause symptoms of toxicity in adults when taken daily for one or two months. Keep in mind that much lower doses may harm young children.
To learn more about how much vitamin D you can safely take, read this article.
Summary: Vitamin D is toxic in high doses. The most serious symptoms are caused by dangerously high levels of calcium in blood, which may harm the heart and kidneys.
Benefits of Vitamin D Supplements
For people who spend little time in the sun and seldom eat fatty fish or liver, supplements can be very beneficial.
They may also have many other benefits in people with vitamin D deficiency, but more studies need to examine their effects in people with sufficient vitamin D levels.
Summary: Health professionals advise most people to take vitamin D supplements to prevent deficiency. Supplements may improve general health and reduce the risk of infections.
Summary of Vitamin D
Vitamin D is sometimes called the sunshine vitamin. This is because your skin can produce all the vitamin D you need, given enough sunlight.
Nevertheless, most people don't get enough vitamin D from sunlight alone. Also, few foods naturally contain high amounts of vitamin D, making supplements necessary.
The richest natural sources of vitamin D include fatty fish, fish oil and mushrooms that have been exposed to sunlight or ultraviolet light.
As a powerful antioxidant, vitamin E protects your cells against premature aging and damage by free radicals.
Vitamin E is a family of eight structurally similar antioxidants that are divided into two groups:
- Tocopherols: Alpha-tocopherol, beta-tocopherol, gamma-tocopherol and delta-tocopherol.
- Tocotrienols: Alpha-tocotrienol, beta-tocotrienol, gamma-tocotrienol and delta-tocotrienol.
Alpha-tocopherol is the most common form of vitamin E. It makes up around 90% of the vitamin E in the blood.
Summary: Vitamin E is a group of related compounds divided into tocopherols and tocotrienols. Alpha-tocopherol is the most common type.
Role and Function of Vitamin E
Vitamin E's main role is to act as an antioxidant, preventing oxidative stress and protecting fatty acids in your cell membranes from free radicals (37).
These antioxidant properties are enhanced by other nutrients, such as vitamin C, vitamin B3 and selenium.
In high amounts, vitamin E also acts as a blood thinner, reducing the blood's ability to clot (38).
Summary: Vitamin E's key role is to serve as an antioxidant, protecting cells against free radicals and oxidative damage.
The richest dietary sources of vitamin E include certain vegetable oils, seeds and nuts. The chart below shows some of the best sources of vitamin E and the amount found in 3.5 ounces (100 grams) of these foods (8):
Other rich sources include avocados, peanut butter, margarine, fatty fish and fish liver oil.
Summary: The best sources of vitamin E are certain vegetable oils, nuts and seeds.
The table below shows the RDA and tolerable upper limit for vitamin E intake. The values marked with an asterisk are the adequate intake, since no RDA values are available for infants.
|RDA (IU / mg)||UL (IU / mg)|
|Infants||0–6 months||6 / 4*||Not known|
|7–12 months||8 / 5*||Not known|
|Children||1–3 years||9 / 6||300 / 200|
|4–8 years||11 / 7||450 / 300|
|9–13 years||17 / 11||900 / 600|
|Adolescents||14–18 years||23 / 15||1,200 / 800|
|Adults||19–50 years||23 / 15||1,500 / 1,000|
|51+||18 / 12||1,500 / 1,000|
Summary: Among adults, the RDA for vitamin E is 23 IU (15 mg). For children and adolescents, the RDA ranges from 9 IU (6 mg) to 23 IU (15 mg), depending on the age group.
Vitamin E Deficiency
Vitamin E deficiency is uncommon and is never detected in people who are otherwise healthy.
It happens most often in diseases that impair the absorption of fat or vitamin E from food, such as cystic fibrosis and liver disease.
Symptoms of vitamin E deficiency include muscle weakness, walking difficulties, tremors, vision problems, poor immune function and numbness.
Summary: Vitamin E deficiency is rare, but can cause muscle weakness, susceptibility to infections, neurological problems and poor vision.
Vitamin E Toxicity
Overdosing on vitamin E is difficult when it is obtained from natural dietary sources. Cases of toxicity have only been reported after people have taken very high doses of supplements.
Yet, compared to vitamin A and D, overdosing on vitamin E appears to be relatively harmless.
It may have blood-thinning effects, counteracting the effects of vitamin K and causing excessive bleeding. Thus, people who take blood-thinning medications should avoid taking large doses of vitamin E (38, 41, 42).
Additionally, at high doses of more than 1,000 mg per day, vitamin E may have pro-oxidant effects. That is, it can become the opposite of an antioxidant, potentially leading to oxidative stress (43).
Summary: Vitamin E appears to be less toxic at high doses than vitamin A and D. However, high doses may cause excessive bleeding and oxidative stress.
Benefits and Risks of High Vitamin E Intake or Supplements
High vitamin E intake from food or supplements has been linked with a number of benefits.
One form of vitamin E, gamma-tocopherol, was found to increase blood flow by promoting the dilation of blood vessels, potentially reducing blood pressure and the risk of heart disease (44).
Gamma-tocopherol supplements may also have a blood-thinning effect as well as reducing levels of "bad" LDL cholesterol (45).
In contrast, other studies suggest that high-dose vitamin E supplements may be harmful, even when they don't cause any obvious symptoms of toxicity.
Given the potentially adverse effects of vitamin E supplements, they cannot be recommended at this point. High-quality studies are needed before solid conclusions can be reached about the long-term safety of these supplements.
Summary: Vitamin E supplements may reduce the risk of heart disease, but the evidence is conflicting. Some studies suggest that high-dose supplements are harmful. More studies are needed.
Summary of Vitamin E
Vitamin E is a group of powerful antioxidants, the most common of which is alpha-tocopherol.
Its main function is to serve as an antioxidant and protect the body's cells against damage by free radicals.
The most abundant dietary sources of vitamin E include vegetable oils, nuts and seeds. Deficiency is very rare in healthy people.
Vitamin K plays a key role in blood clotting. Without it, you would run the risk of bleeding to death.
Vitamin K is actually a group of fat-soluble compounds divided into two main groups:
- Vitamin K1 (phylloquinone): Found in plant-sourced foods, phylloquinone is the main form of vitamin K in the diet (49).
- Vitamin K2 (menaquinone): This variety of vitamin K is found in animal-sourced foods and fermented soy products, like natto. Vitamin K2 is also produced by gut bacteria in the colon (50, 51).
Additionally, there are at least three synthetic forms of vitamin K. These are known as vitamin K3 (menadione), vitamin K4 (menadiol diacetate) and vitamin K5.
Summary: Vitamin K is a family of compounds. The main dietary forms are vitamin K1, found in plant foods, and vitamin K2, found in animal-derived foods and fermented soy products.
Role and Function of Vitamin K
Vitamin K plays an essential role in blood clotting. In fact, the "K" stands for "koagulation," the Danish word for coagulation, which means clotting.
But vitamin K has other functions as well, including supporting bone health and helping prevent the calcification of blood vessels, potentially reducing the risk of heart disease (52).
Summary: Vitamin K is vital for blood clotting and supports bone health.
The best dietary sources of vitamin K1 (phylloquinone) are leafy green vegetables, whereas vitamin K2 (menaquinone) is mainly found in animal-sourced foods and fermented soy products.
The table below shows some of the main sources of vitamin K1 and the amounts found in 3.5 ounces (100 grams) of these foods (8):
In contrast to phylloquinone, menaquinone is only found in small amounts in certain high-fat, animal-sourced foods, such as egg yolks, butter and liver.
It is also found in certain soy foods, such as natto.
Summary: Vitamin K1 is abundant in many leafy green vegetables, while vitamin K2 is found in low amounts in animal-sourced foods and fermented soy foods.
The table below shows the adequate intake (AI) values for vitamin K.
AI is similar to the RDA, which is a daily intake level thought to meet the requirements of 97.5% of people, but AI is based on weaker evidence than RDA.
Summary: The adequate intake (AI) of vitamin K is 90 mcg for women and 120 mcg for men. For children and adolescents, the AI ranges from 30–75 mcg, depending on the age group.
Vitamin K Deficiency
Unlike vitamins A and D, vitamin K isn't stored in the body in significant amounts. For this reason, consuming a diet lacking in vitamin K may lead you to become deficient in as little as a week (53).
People who do not efficiently digest and absorb fat are at the greatest risk of developing vitamin K deficiency. This includes those who suffer from celiac disease, inflammatory bowel disease and cystic fibrosis.
Use of broad-spectrum antibiotics may also raise the risk of deficiency, as well as very high doses of vitamin A, which seem to reduce vitamin K absorption.
Without vitamin K, your blood wouldn't clot and even a small wound could cause unstoppable bleeding. Fortunately, vitamin K deficiency is rare, since the body only needs small amounts to maintain blood clotting.
Low levels of vitamin K have also been linked with reduced bone density and increased risk of fractures in women (55).
Summary: Deficiency in vitamin K may lead to excessive bleeding. Diseases that interfere with fat absorption increase the risk of deficiency.
Vitamin K Toxicity
Unlike the other fat-soluble vitamins, natural forms of vitamin K have no known symptoms of toxicity.
As a result, scientists have not been able to establish a tolerable upper intake level for vitamin K. Further studies are needed.
Summary: The maximum safe dosage of vitamin K is unknown and no symptoms of toxicity have been identified.
Benefits of Vitamin K Supplements
Several controlled studies have examined the effects of vitamin K supplements in humans. These studies show that vitamin K supplements — vitamin K1 and vitamin K2 — may decrease bone loss and reduce the risk of bone fractures (58, 59).
Additionally, taking vitamin K2 supplements at 45–90 mg per day slightly increased the survival of patients with liver cancer (60).
Finally, vitamin K1 supplements taken at 0.5 mg every day for three years slowed the development of insulin resistance in older men, compared to a placebo. No significant differences were detected in women (63).
Summary: Limited evidence suggests that vitamin K supplements may improve bone health, reduce the risk of heart disease and increase survival among liver cancer patients.
Summary of Vitamin K
Vitamin K is a group of fat-soluble compounds divided into vitamin K1 (phylloquinone) and vitamin K2 (menaquinone).
Vitamin K1 is mainly found in leafy green vegetables, whereas vitamin K2 comes from animal-sourced foods, such as liver, butter and egg yolks.
Small amounts are also produced by gut bacteria in the colon.
Deficiency impairs the blood's ability to clot, causing a risk of excessive bleeding.
There is limited evidence on the health benefits of supplements among people who aren't deficient. However, a few controlled studies suggest that vitamin K supplements benefit bone and heart health.
There are four fat-soluble vitamins in the human diet: A, D, E and K. They are essential for health and play many important roles in the body.
With the exception of vitamin D, most of them are easy to get from a diverse diet, especially if you eat plenty of nuts, seeds, vegetables, fish and eggs.
These vitamins tend to be abundant in fatty foods and you can enhance their absorption by adding fat or oil to an otherwise low-fat meal.
Few foods are naturally rich in vitamin D. It is abundant in fatty fish and fish oil, but also formed by your skin when you're exposed to sunlight.
For this reason, vitamin D deficiency is a problem for people who follow an inadequate diet and spend most of their time indoors.
While you generally do not need to supplement with vitamin A, E and K, taking vitamin D supplements is widely recommended.
For optimal health, make sure you get all of the fat-soluble vitamins in adequate amounts.