The Fat-Soluble Vitamins: A, D, E, and K

On Dec 08,2024 In fitness diet

Vitamins can be classified based on their solubility. Fat-soluble vitamins are most abundant in high fat foods and are much better absorbed into your bloodstream when you eat them with fat.

Most vitamins are water-soluble, meaning they dissolve in water. In contrast, fat-soluble vitamins are similar to oil and do not dissolve in water.

There are four fat-soluble vitamins in the human diet:

  • vitamin A
  • vitamin D
  • vitamin E
  • vitamin K

This article provides a comprehensive overview of the fat-soluble vitamins that includes:

  • health benefits
  • functions
  • main dietary sources

Vitamin A

Vitamin A plays a key role in maintaining your vision. Without it, you would go blind.

Types

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 your body function, including:

  • Vision maintenance. Vitamin A is essential for maintaining the light-sensing cells in your eyes and for the formation of tear fluid (2).
  • Immune function. Vitamin A deficiency impairs your immune function, increasing susceptibility to infections (3, 4).
  • Body growth. Vitamin A is necessary for cell growth in your body. 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.

Dietary sources

Vitamin A is only found in animal-sourced foods. The main natural food sources are:

  • liver
  • fish liver oil
  • butter

The table below shows the amount of vitamin A in 3.5 ounces (100 grams) of some of its richest dietary sources (8):

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Vitamin A can also be derived from certain carotenoid antioxidants found in plants. They are collectively known as provitamin A.

The most efficient of these is beta-carotene, which is abundant in many vegetables, such as carrots, kale, and spinach (9, 10).

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.

Recommended intake

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 maximum daily intake considered unlikely to cause adverse health effects (11).

RDA (mcg RAE)UL (mcg RAE)Infants0–6 months4006007–12 months500600Children1–3 years3006004–8 years4009009–13 years6001,700Women14–18 years7002,80019–70 years7003,000Men14–18 years9002,80019–70 years9003,000Summary

The RDA for vitamin A is 900 mcg RAE for adult men and 700 mcg RAE for women. For children, it ranges from 300 mcg RAE to 600 mcg RAE.

Vitamin A deficiency

Vitamin A deficiency is rare in developed countries.

However, vegans may be at risk since preformed 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 your 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 you 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, a 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
  • inflammation in the mouth and eyes

It may also lead to:

  • liver damage
  • bone loss
  • hair loss

At extremely high doses, vitamin A can be fatal (17).

Healthcare professional advise against exceeding the upper limit for intake, which is 3,000 mcg of preformed vitamin A per day for adults.

Higher amounts 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 (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 have a 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.

For instance, vitamin A supplements may help treat measles in children (20, 21).

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 food variety, 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.

Pregnant women should be extra careful not to eat excessive amounts of vitamin A because of the risk of birth defects.

Vitamin D

Nicknamed the sunshine vitamin, vitamin D is produced when your skin when is exposed to sunlight.

It is best known for its beneficial effects on bone health. Deficiency makes you highly susceptible to bone fractures.

Types

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). This form is found in mushrooms and some plants.
  • Vitamin D3 (cholecalciferol). This form is found in animal-sourced foods, such as eggs and fish oil, and produced when your skin is exposed to sunlight.
Summary

Dietary vitamin D can be classified as vitamin D2, which is found in mushrooms and plants, and vitamin D3, which is 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 your diet.
  • Immune system regulation. It also regulates and strengthens your immune system function (23).

Once absorbed into the bloodstream, your 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.

Vitamin D3 is more efficiently converted into calcitriol than vitamin D2 (24, 25).

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 your skin produces.

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):

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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.

Recommended intake

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 it’s based on weaker evidence.

Age GroupRDA (IU / mcg)UL (IU / mcg)0–6 months400 / 101,000 / 257–12 months400 / 101,500 / 381–3 years600 / 152,500 / 634–8 years600 / 153,000 / 759–70 years600 / 154,000 / 1,00070+ years800 / 204,000 / 1,000

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 older 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 people who are hospitalized as well as older adults.

Risk factors of deficiency are:

  • darker skin tone
  • older age
  • obesity
  • low sun exposure
  • 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).

Vitamin D deficiency is also associated with poor immune function, increased susceptibility to infections, and autoimmune diseases (29, 30).

Other signs of deficiency or insufficiency may include fatigue, depression, hair loss, and impaired wound healing.

Observational studies have also linked low vitamin D levels or deficiency with an increased risk of dying from cancer and an elevated risk of heart attacks (31, 32).

Summary

The main symptoms of vitamin D deficiency include fatigue, weak muscles, soft bones, 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 does not 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 1 or 2 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.

Regularly taking supplements seems to prolong people’s lives, especially those who are hospitalized or older adults living in care facilities (33, 34).

Supplements may also reduce the risk of respiratory tract infections (35, 36).

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

Healthcare 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.

Vitamin D deficiency is traditionally associated with osteomalacia in adults or rickets in children. Both diseases are characterized by brittle or soft bones.

Vitamin E

As a powerful antioxidant, vitamin E protects your cells against premature aging and damage by free radicals.

Types

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.

Dietary sources

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):

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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.

Recommended intake

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 (39).

RDA (mg)UL (mg)Infants0–6 months4Not known7–12 months5Not knownChildren1–3 years62004–8 years73009–13 years11600Adolescents14–18 years15800Adults19–50 years151,00051+ years121,000Summary

Among adults, the RDA for vitamin E is 15 mg. For children and adolescents, the RDA ranges from 6 mg to 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 when diseases that impair the absorption of fat or vitamin E from food, such as cystic fibrosis and liver disease, are present.

Symptoms of vitamin E deficiency include:

  • muscle weakness
  • walking difficulties
  • tremors
  • vision problems
  • weakened immune function
  • numbness

Severe, long-term deficiency may lead to anemia, heart disease, serious neurological problems, blindness, dementia, poor reflexes, and the inability to fully control body movements (40, 41).

Summary

Vitamin E deficiency is rare, but it 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 vitamins A and D, overdosing on vitamin E appears to have a less harmful effect, but it still requires medical intervention.

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, 42, 43).

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 (44).

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 (45).

Gamma-tocopherol supplements may also have a blood-thinning effect as well as a reduction effect on LDL (bad) cholesterol levels (46).

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.

For instance, observational studies show that taking vitamin E supplements is linked with an increased risk of prostate cancer and death by all causes (47, 48, 49).

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 serving as an antioxidant and protecting your body’s cells against free-radical damage.

The most abundant dietary sources of vitamin E include vegetable oils, nuts, and seeds. Deficiency is very rare in healthy people.

While supplements may provide certain health benefits, not all scientists agree. The long-term safety of vitamin E supplements is a matter of debate.

Vitamin K

Vitamin K plays a key role in blood clotting. Without this essential vitamin, there’s a higher risk of excessive bleeding, which could lead to death.

Types

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 (50).
  • 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 (51, 52).

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 (53).

Summary

Vitamin K is vital for blood clotting and supports bone health.

Dietary sources

The best dietary sources of vitamin K1 (phylloquinone) are leafy green vegetables, while 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):

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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’s 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.

Recommended intake

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.

Al (mcg)Infants0–6 months27–12 months2.5Children1–3 years304–8 years559–13 years60Adolescents14–18 years75Women18+ years90Men18+ years120Summary

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 is not 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 (54).

People who do not efficiently digest and absorb fat are at the greatest risk of developing vitamin K deficiency. This includes those who have 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.

Mega-doses — very large doses — of vitamin E may also counteract the effects of vitamin K on blood clotting (42, 55).

Without vitamin K, your blood will not clot, which increases the risk that 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 (56).

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.

In contrast, a synthetic form of vitamin K, known as menadione or vitamin K3, may have some adverse effects when consumed in high amounts (57, 58).

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 (59, 60).

Additionally, taking vitamin K2 supplements at 45–90 mg per day has shown to slightly increase the survival of people with liver cancer (61).

Observational studies also suggest that a high intake of vitamin K2 may lower the risk of heart disease. However, the evidence from controlled studies is limited and inconclusive (62, 63).

Finally, vitamin K1 supplements taken at 0.5 mg every day for 3 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, while 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 are not deficient. However, a few controlled studies suggest that vitamin K supplements benefit bone and heart health.

The bottom line

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 diet that includes a variety of foods, 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’s 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 can occur in people who do not eat a diet filled with a variety of nutrient-rich foods and regularly spend time indoors, which may be due to a variety of factors, including medical reasons or personal choice.

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 the fat-soluble vitamins in adequate amounts. If you have questions about your vitamin intake or you’re considering supplementation, talk with a healthcare professional.