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

Vitamins are often categorized by how they dissolve. Fat-soluble vitamins are concentrated in high-fat foods and are absorbed far more effectively into the bloodstream when consumed alongside dietary fat.

Most vitamins dissolve in water, but fat-soluble vitamins behave like oils and do not mix with water.

The human diet contains four fat-soluble vitamins:

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

This article offers an in-depth look at fat-soluble vitamins, covering:

• health advantages
• physiological roles
• primary food sources

Vitamin A

Vitamin A is crucial for preserving eyesight. Without adequate amounts, vision fails.

Types

Vitamin A is not a single molecule but a collection of fat-soluble compounds called retinoids.

The most prevalent dietary form is retinol. Other biologically active forms — retinal and retinoic acid — exist in the body but are rare or absent in foods.

Vitamin A2 (3,4-dehydroretinal) is a less potent variant found in some freshwater fish (1).

Summary: Retinol is the principal dietary form of vitamin A.

Role and function of vitamin A

Vitamin A supports many essential bodily processes, including:

• Vision support. It is vital for the light-sensitive cells in the eye and for producing tear fluid (2).
• Immune health. Lack of vitamin A weakens immune responses and raises infection risk (3, 4).
• Growth. It’s required for cellular growth—insufficiency can stunt growth in children (5).
• Hair maintenance. Adequate vitamin A supports hair growth; deficiency can cause alopecia (6).
• Reproduction. It helps sustain fertility and is important for fetal development (7).

Summary: Vitamin A is widely known for its role in vision; it also contributes to growth, immunity, hair health, and reproductive function.

Dietary sources

Preformed vitamin A is present only in animal-based foods. Top natural sources include:

• liver
• fish liver oil
• butter

The chart below displays vitamin A amounts in 100 grams of some of the richest foods (8):

Vitamin A can also be produced from plant carotenoids, referred to as provitamin A.

Beta-carotene is the most efficient provitamin A and is abundant in vegetables like carrots, kale, and spinach (9, 10).

Summary: Top dietary vitamin A sources are liver and fish oil, but provitamin A carotenoids such as beta-carotene from vegetables can also supply adequate amounts.

Recommended intake

The following table lists the recommended daily allowance (RDA) for vitamin A, which should cover the needs of about 97.5% of people, and the tolerable upper intake limit (UL), the maximum daily amount likely to be safe (11).

Summary: The RDA for vitamin A is 900 mcg RAE for adult men and 700 mcg RAE for adult women; for children it ranges from 300–600 mcg RAE.

Vitamin A deficiency

In wealthy countries, deficiency is uncommon.

Vegans may face higher risk because preformed vitamin A occurs only in animal foods.

Although many plants contain provitamin A, conversion to active retinol can be inefficient and varies by genetics (12, 13).

Deficiency is still widespread in some low-income regions where diets lack variety and are dominated by refined staples like white rice, potatoes, or cassava.

Early sign of deficiency is night blindness. If it worsens, it may cause:

• Dry eyes. Severe deficiency can produce xerophthalmia, or very dry eyes, due to reduced tear production (2).
• Blindness. Extreme deficiency can cause permanent blindness and is a leading preventable cause worldwide (14).
• Hair loss. Insufficient vitamin A can result in hair loss (15).
• Skin issues. Deficiency may cause hyperkeratosis, sometimes called gooseflesh (16).
• Weakened immunity. Low vitamin A increases infection risk (3).

Summary: Severe lack of vitamin A can cause blindness; other effects include hair and skin problems and greater vulnerability to infections.

Vitamin A toxicity

Excessive vitamin A intake causes hypervitaminosis A, a potentially serious condition.

Common causes include very high supplement doses, consuming large amounts of liver, or using fish liver oil. High intakes of provitamin A do not cause this condition.

Symptoms and consequences include:

• fatigue
• headache
• irritability
• abdominal pain
• joint pain
• loss of appetite
• vomiting
• blurred vision
• skin issues
• inflammation of the mouth and eyes

It can also cause:

• liver injury
• bone demineralization
• hair loss

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

Healthcare experts warn against surpassing the adult upper limit of 3,000 mcg preformed vitamin A per day.

Children and people with liver conditions have lower tolerance and face greater risk at smaller doses (18).

Pregnant women should be particularly cautious because high vitamin A intake can harm the developing fetus (19).

Summary: Very high vitamin A intake can cause hypervitaminosis A with various symptoms. Pregnant women should avoid excessive intake due to birth defect risks.

Benefits of vitamin A supplements

Supplements help those who are deficient, but most people obtain sufficient vitamin A from food and don’t need extra.

Still, some controlled trials show supplements can benefit specific conditions even when basic dietary needs are met.

For example, vitamin A supplementation has been shown to assist in treating measles in children (20, 21), lowering measles-related pneumonia and cutting mortality by 50–80%, possibly by inhibiting the measles virus (22).

Summary: Supplements are mainly useful for those with low vitamin A; a notable exception is children with measles, where supplementation has shown clear benefit.

Summary of vitamin A

Vitamin A (retinol) is a fat-soluble nutrient closely linked to eye health.

Rich food sources include liver, fish liver oil, and butter.

Provitamin A carotenoids found in colorful vegetables and dark leafy greens can also supply the vitamin.

Deficiency is uncommon in developed nations but occurs in populations with limited dietary diversity.

Early deficiency presents as night blindness; severe forms may lead to permanent blindness.

While it’s important to obtain enough vitamin A, excessive intake can be hazardous. Pregnant women should avoid very high intakes to prevent birth defects.

Vitamin D

Often called the sunshine vitamin, vitamin D is synthesized in the skin when exposed to sunlight.

It’s best known for supporting bone health; insufficient vitamin D significantly raises the risk of fractures.

Types

Vitamin D refers to several related fat-soluble compounds, collectively known as calciferols.

The two primary dietary forms are:

• Vitamin D2 (ergocalciferol). Found in some plants and mushrooms.
• Vitamin D3 (cholecalciferol). Present in animal-based foods like eggs and fish oil and produced in skin exposed to sunlight.

Summary: Dietary vitamin D exists mainly as D2 (plants, mushrooms) and D3 (animal foods and produced in skin).

Role and function of vitamin D

Vitamin D performs several functions, though only a few are well established:

• Bone health. It regulates blood calcium and phosphorus—minerals crucial for bone formation and maintenance—by enhancing their absorption.
• Immune regulation. It helps modulate immune responses and bolster immunity (23).

After absorption, the liver and kidneys convert calciferol into calcitriol, the active hormonal form. Vitamin D can also be stored as calcidiol for later use.

Vitamin D3 is converted into the active form more effectively than D2 (24, 25).

Summary: A key role of vitamin D is maintaining calcium and phosphorus levels to support bone health.

Sources of vitamin D

Your body can synthesize adequate vitamin D if substantial areas of skin are regularly exposed to sunlight (26).

However, many people get little sun exposure or protect their skin with clothing or sunscreen, which reduces vitamin D production.

Consequently, dietary intake becomes important. Few foods naturally provide vitamin D—fatty fish and fish oils are the best sources, and ultraviolet-exposed mushrooms can also contain meaningful amounts.

The chart below illustrates vitamin D content in 100 grams of some top sources (8):

Many dairy products and margarines are fortified with vitamin D.

To learn more food options for raising vitamin D intake, consult this article.

Summary: If you get enough sun exposure, your body can make vitamin D; otherwise, you generally need dietary sources or supplements like fatty fish or fish oil.

Recommended intake

The table below lists the recommended dietary allowance (RDA) and upper limit (UL) for vitamin D (27).

For infants, values marked with an asterisk indicate adequate intake (AI) rather than an RDA, reflecting weaker evidence.

For more detail on optimal vitamin D intake, read this article.

Summary: The RDA for most children and adults is 600 IU (15 mcg) of vitamin D; older adults are recommended to have 800 IU (20 mcg).

Vitamin D deficiency

Severe deficiency is infrequent, but mild insufficiency is common in hospitalized patients and the elderly.

Risk factors include:

• darker skin
• advanced age
• obesity
• limited sun exposure
• conditions that impair fat absorption

Classic outcomes of deficiency are soft bones, weak muscles, and higher fracture risk—known as osteomalacia in adults and rickets in children (28).

Low vitamin D status is also linked to compromised immunity, greater infection risk, and autoimmune disorders (29, 30).

Other signs may include fatigue, depression, hair loss, and slower wound healing. Observational studies also associate low vitamin D with higher cancer mortality and cardiovascular risk (31, 32).

Summary: Symptoms of vitamin D deficiency include tiredness, weak muscles, softening of bones, increased fracture risk, and vulnerability to infections.

Vitamin D toxicity

Toxicity from vitamin D is rare.

Excessive sun exposure does not cause toxicity, but very high supplemental doses can.

The primary issue is hypercalcemia—excessive blood calcium—which may present with headache, nausea, poor appetite, weight loss, fatigue, kidney and heart damage, high blood pressure, and fetal harm.

Adults are generally advised not to exceed 4,000 IU per day. Chronic intakes of 40,000–100,000 IU daily for 1–2 months can produce toxicity in adults; much lower doses can be dangerous for children.

To learn safe supplement dosing, read this article.

Summary: High doses of vitamin D can be toxic, chiefly because they raise blood calcium to dangerous levels that can damage the heart and kidneys.

Benefits of vitamin D supplements

For people with minimal sun exposure and little intake of fatty fish or liver, supplements can be very valuable.

Regular supplementation appears to reduce mortality in hospitalized patients and elderly residents of care homes (33, 34), and may lower the risk of respiratory infections (35, 36).

Supplements likely benefit people who are deficient, but more research is needed for those with adequate levels.

Summary: Health professionals commonly recommend vitamin D supplements to prevent deficiency; they may improve overall health and reduce infection risk.

Summary of vitamin D

Vitamin D gets its nickname from being produced in skin exposed to sunlight.

Most people don’t obtain enough from sunlight alone, and few foods naturally contain it, making supplements necessary for many.

Top natural sources are fatty fish, fish oil, and UV-exposed mushrooms.

Deficiency is linked to osteomalacia in adults and rickets in children, both causing soft or fragile bones.

Vitamin E

Vitamin E is a potent antioxidant that defends cells from premature aging and free-radical damage.

Types

Vitamin E comprises eight related compounds divided into two classes:

• Tocopherols: alpha-, beta-, gamma-, and delta-tocopherol
• Tocotrienols: alpha-, beta-, gamma-, and delta-tocotrienol

Alpha-tocopherol is the predominant form in human blood, comprising roughly 90% of circulating vitamin E.

Summary: Vitamin E includes tocopherols and tocotrienols, with alpha-tocopherol being the most common form.

Role and function of vitamin E

Its principal role is acting as an antioxidant, protecting cell membrane fats from oxidative damage by free radicals (37).

These antioxidant effects are supported by other nutrients such as vitamin C, niacin (vitamin B3), and selenium.

In high doses, vitamin E can also thin the blood by reducing clotting capacity (38).

Summary: Vitamin E primarily works as an antioxidant to shield cells from free-radical injury.

Dietary sources

Rich vitamin E foods include certain vegetable oils, seeds, and nuts. The chart below lists top sources and amounts per 100 grams (8):

Other good sources include avocados, peanut butter, margarine, fatty fish, and fish liver oil.

Summary: The best vitamin E sources are certain vegetable oils, nuts, and seeds.

Recommended intake

The table below provides RDA and tolerable upper limit values for vitamin E. Values marked with an asterisk are AI for infants, where no RDA exists (39).

Summary: For adults, the RDA for vitamin E is 15 mg; for children and teens it ranges from 6–15 mg depending on age.

Vitamin E deficiency

Deficiency is rare in otherwise healthy people and usually occurs only with disorders that impair fat absorption, like cystic fibrosis or severe liver disease.

Deficiency symptoms can include:

• muscle weakness
• difficulty walking
• tremors
• vision disturbances
• immune weakness
• numbness

Long-standing severe deficiency may lead to anemia, cardiovascular problems, neurological decline, blindness, dementia, and impaired motor control (40, 41).

Summary: Vitamin E deficiency is uncommon but can cause muscle weakness, infection risk, neurological issues, and visual problems.

Vitamin E toxicity

Toxicity from foods is unlikely; reported cases are usually due to very large supplemental doses.

Compared with vitamins A and D, excessive vitamin E tends to be less dangerous but still requires attention.

High doses can thin the blood and oppose vitamin K’s clotting actions, raising bleeding risk, especially in people on anticoagulants (38, 42, 43).

At doses above 1,000 mg/day, vitamin E may act as a pro-oxidant, potentially causing oxidative stress (44).

Summary: Vitamin E is less toxic than vitamins A and D, but high doses can increase bleeding risk and may induce oxidative stress.

Benefits and risks of high vitamin E intake or supplements

Elevated vitamin E intake from foods or supplements has been linked with potential benefits.

Gamma-tocopherol has been shown to improve blood flow by dilating blood vessels, possibly lowering blood pressure and heart disease risk (45).

Gamma-tocopherol supplements may also thin the blood and lower LDL cholesterol (46).

However, some research suggests that high-dose vitamin E supplements can be harmful. Observational studies have linked supplementation with increased prostate cancer risk and higher all-cause mortality (47, 48, 49).

Because of potential risks, routine high-dose vitamin E supplementation cannot be widely recommended until more rigorous research clarifies long-term safety.

Summary: Vitamin E supplements may offer cardiovascular benefits, but evidence is mixed and some studies suggest harm with high doses. More research is required.

Summary of vitamin E

Vitamin E consists of strong antioxidants, chiefly alpha-tocopherol.

Its principal task is to guard cells from free-radical damage.

Major dietary sources include vegetable oils, nuts, and seeds, and deficiency is rare in healthy individuals.

Supplements may help in certain situations, but long-term safety and overall benefits remain debated.

Vitamin K

Vitamin K is essential for blood clotting. Without it, uncontrolled bleeding can occur, which may be life-threatening.

Types

Vitamin K refers to a family of fat-soluble compounds grouped mainly into:

• Vitamin K1 (phylloquinone). Predominantly found in plant foods and is the primary dietary form (50).
• Vitamin K2 (menaquinone). Present in animal products and fermented soy foods like natto, and also synthesized by intestinal bacteria (51, 52).

There are also synthetic variants such as vitamin K3 (menadione), K4 (menadiol diacetate), and K5.

Summary: Vitamin K includes K1 from plant foods and K2 from animal and fermented foods; both are important dietary forms.

Role and function of vitamin K

Vitamin K is indispensable for proper blood clotting—its name derives from “koagulation,” the Danish word for coagulation.

It also contributes to bone strength and may help prevent arterial calcification, potentially lowering cardiovascular risk (53).

Summary: Vitamin K is needed for clotting and supports bone health.

Dietary sources

Vitamin K1 is abundant in leafy green vegetables, while K2 is found in certain animal foods and fermented soy products.

The table below shows K1 content in 100 grams of some common foods (8):

Unlike phylloquinone, menaquinone appears in small amounts in high-fat animal foods such as egg yolks, butter, and liver, and in fermented products like natto.

Summary: Leafy greens are rich in vitamin K1, while vitamin K2 is present in smaller amounts in animal-derived and fermented foods.

Recommended intake

The table below presents adequate intake (AI) recommendations for vitamin K. AI is similar to RDA but based on weaker evidence.

Summary: The AI for vitamin K is 90 mcg for women and 120 mcg for men; for children and teens it ranges from 30–75 mcg.

Vitamin K deficiency

Unlike vitamins A and D, vitamin K is not stored substantially in the body, so deficiency can develop quickly—potentially within a week—without adequate intake (54).

Those with poor fat digestion (e.g., celiac disease, inflammatory bowel disease, cystic fibrosis) are at higher risk. Broad-spectrum antibiotics and very high vitamin A intake may also promote deficiency by reducing K absorption.

Large doses of vitamin E can antagonize vitamin K’s clotting effects (42, 55).

Without enough vitamin K, blood clotting is impaired and minor injuries can result in significant bleeding. Fortunately, deficiency is uncommon because the body needs only small amounts.

Low vitamin K is associated with decreased bone density and higher fracture risk in women (56).

Summary: Vitamin K deficiency increases bleeding risk; conditions that disturb fat absorption raise the likelihood of deficiency.

Vitamin K toxicity

Natural forms of vitamin K have no established toxicity symptoms, so no upper limit has been set.

However, synthetic vitamin K3 (menadione) may cause adverse effects at high levels (57, 58).

Summary: No clear toxicity has been identified for natural vitamin K forms; the safety ceiling remains undefined.

Benefits of vitamin K supplements

Controlled trials indicate vitamin K1 and K2 supplements may slow bone loss and reduce fracture risk (59, 60).

Vitamin K2 at 45–90 mg/day has modestly improved survival in people with liver cancer in some studies (61).

Observational data also suggest higher K2 intake may be linked to reduced heart disease risk, but randomized trial evidence is limited (62, 63).

Vitamin K1 supplementation (0.5 mg daily for