Vitamins are organic compounds that support metabolism, growth, and overall health. They are classified as either fat-soluble (A, D, E, and K) or water-soluble based on how they dissolve and function in the body. Figure 8.5 shows the two categories of vitamins.

Although our bodies can produce small amounts of certain vitamins, most must be obtained from the foods we eat—reminding us that a varied, nutrient-rich diet is essential for optimal health.

Vitamin A: From Food to Function

Vitamin A is a fat-soluble vitamin obtained from both animal and plant foods, but it comes in different forms. Understanding these forms helps explain how vitamin A supports health and why both food sources and safe intake levels are important.

Retinol (Preformed Vitamin A)

Retinol is the active form of vitamin A found in animal-based foods such as milk, eggs, cheese, and liver. Retinol is easily absorbed and stored in the liver, where it can be released when the body needs it.

This form of vitamin A plays a crucial role in vision, particularly in night vision, supports immune function, and helps maintain normal growth and healthy skin. Because retinol is stored in the body, consuming too much—most often from supplements rather than food—can be toxic and lead to harmful health effects.

Carotenoids (Provitamin A)

Carotenoids are natural pigments that give many fruits and vegetables their red, bright orange, yellow, and dark green colors. They are found in plant foods such as carrots, sweet potatoes, spinach, and mangoes. The most important carotenoid for human nutrition is beta-carotene because the body can convert it into vitamin A. For this reason, beta-carotene is called provitamin A. This conversion is limited, meaning the body changes only a small amount—about 12 micrograms of beta-carotene are needed to produce 1 microgram of retinol.

 Other Helpful Carotenoids

Some carotenoids do not convert into vitamin A but still play important roles in supporting health. These carotenoids act as antioxidants, helping protect cells from damage and supporting eye health.

• Beta-cryptoxanthin is found in orange and red fruits such as oranges, tangerines, papaya, and red peppers.
• Lycopene gives foods a red or pink color and is found in tomatoes, watermelon, and pink grapefruit.
• Lutein is yellow-green and found in dark leafy greens such as spinach, kale, and collard greens.
• Zeaxanthin is found in yellow and orange foods like corn, orange peppers, and egg yolks.

Lutein and zeaxanthin are especially important for eye health, where they help protect the retina from light-related damage.

Dietary Recommendations and Sources of Vitamin A

The Recommended Dietary Allowance (RDA) for vitamin A is expressed in units called retinol activity equivalents (RAE). This system is used because the body converts different carotenoids into vitamin A at different rates. For teens and adults, the vitamin A RDA is 700 micrograms (mcg) RAE for females and 900 mcg RAE for males.

Vitamin A, like many fat-soluble vitamins, is stored in the liver rather than being excreted daily. Because of this storage, liver is the richest food source of vitamin A. Other sources are listed in the image below.

 

Vitamin A’s Role in Vision

The retinoids are aptly named, as their most notable function is in the retina of the eye. Circulating retinol is taken up by the retina, where it’s incorporated into the rhodopsin pigment. Rhodopsin is especially important to our ability to see in low-light conditions.  Low rhodopsin levels impair vision in dim light, a condition known as night blindness.

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Vitamin A is also required for normal cell differentiation, important in every tissue of the body.  If there is a shortage of vitamin A, the eyes are one of the first areas to be affected. Cells in the eye lining produce mucus and tears, which keep the eyes moist and lubricated.  A lack of vitamin A results in dry eyes, a condition known as xerophthalmia. (See Figure 8.8).  Instead of producing mucus, these dysfunctional cells produce a protein called keratin. Keratin  is a hard, structural protein found in nails, hair, and the outer layer of skin, and you can imagine the problems it causes when it accumulates in the eye. Instead of a moist, well-lubricated eye, keratin causes the eye to become hard and dry, resulting in clouded vision. A deficiency in vitamin A can thus impair vision in two ways:

1. Development of night blindness due to a lack of the pigment rhodopsin

2. Development of xerophthalmia, or dry eyes, caused by abnormal cellular differentiation

Night blindness is typically the first symptom of a vitamin A deficiency, followed by xerophthalmia and impaired vision. If the deficiency persists, damage to the keratin in the eye’s lining can lead to permanent blindness.

Vitamin A’s role in cellular differentiation also makes it critical to cells around the body involved in normal growth, development, reproduction, and immune function.

Deficiency and Toxicity of Vitamin A and Carotenoids

Another symptom of vitamin A deficiency is hyperkeratosis, which occurs when skin cells overproduce the protein keratin (as in the eye, with xerophthalmia), making the skin rough and irritated.

Vitamin A deficiency is rare in developed countries, but globally, it is the leading cause of preventable blindness in children. According to a 2023 report, an estimated 250,000 to 560,000 children lose sight each year due to vitamin A deficiency, and half die within a year of developing blindness, likely due to infection. ^[1]  According to UNICEF, 30% of children under 5 have vitamin A deficiency worldwide.^[2]

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Vitamin A toxicity occurs when excessive amounts of preformed vitamin A are consumed, typically from supplements rather than food. Symptoms can include dry, itchy skin, loss of appetite, dizziness, nausea, joint pain, and brain swelling. In severe cases, it can lead to liver damage, coma, or death. Toxicity typically develops when supplements are taken in excess of the Upper Limit (UL) of 3,000 mcg per day for prolonged periods. These high intakes are not typically seen in a normal diet, although rare cases have been reported among Arctic explorers who consumed large amounts of bear or seal liver, which is extremely high in vitamin A.

Too much vitamin A during pregnancy is especially dangerous because it can cause birth defects. For this reason, pregnant individuals should carefully check supplement labels and avoid high-dose vitamin A supplements. Some prescription acne treatments made from synthetic vitamin A, such as Acutane,  should also never be used during pregnancy due to this risk.

In contrast, beta-carotene and other carotenoids do not cause vitamin A toxicity or birth defects. The body converts beta-carotene to vitamin A only when needed, helping prevent excess buildup. As a result, beta-carotene is commonly used as a source of vitamin A in prenatal supplements.

Very high intakes of beta-carotene from foods can cause the skin to appear yellow or orange. The discoloration is often most noticeable on the nose, palms of the hands, and soles of the feet, and disappears when beta-carotene intake is reduced. Notice the orange tint on the child’s nose. This harmless yellow-orange discoloration is a sign of carotenemia, caused by consuming large amounts of beta-carotene-rich food.

Vitamin D: Where Sunlight Meets Skeletal Health

Vitamin D is a fat-soluble vitamin that can be produced in the skin when it is exposed to ultraviolet B (UVB) rays from sunlight. The two major forms are vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol). Factors such as season, latitude, sunscreen use, clothing, and skin pigmentation can reduce vitamin D production.

Functions of Vitamin D

Activated vitamin D works with parathyroid hormone to regulate blood calcium levels and support bone health. Without enough vitamin D, the body absorbs much less calcium from food, increasing the risk of weak bones.

Vitamin D deficiency causes rickets in children, resulting in soft, poorly mineralized bones. In adults, deficiency can lead to osteomalacia and contribute to osteoporosis. Deficiency is more common among older adults, individuals with darker skin, and people with limited sun exposure.

Health Benefits

The strongest evidence for vitamin D relates to bone health. Research shows that adequate vitamin D, especially when combined with calcium, helps maintain bone density and reduce fracture risk in older adults. Although low vitamin D status has been associated with other health conditions, current evidence does not support high-dose vitamin D supplements for preventing chronic disease in healthy adults.

Vitamin D Toxicity

Vitamin D toxicity is rare and usually results from excessive supplement use. Too much vitamin D can raise blood calcium levels (hypercalcemia), potentially damaging the kidneys and other tissues. Vitamin D produced from sunlight does not cause toxicity.

Vitamin D Requirements

Most people ages 1–70 need 15 µg (600 IU) of vitamin D per day. Adults over age 70 need 20 µg (800 IU) per day. The Upper Intake Level (UL) for adults is 100 µg (4,000 IU) per day.

Vitamin E: Antioxidant Protection and Cellular Health

Vitamin E is a fat-soluble vitamin that acts as an antioxidant, helping protect cell membranes from damage caused by free radicals. Alpha-tocopherol is the primary form that meets human needs.

Vitamin E also supports immune function and helps maintain healthy blood vessels. Because it works closely with other antioxidants, such as vitamin C, regular dietary intake is important.

Vitamin E deficiency is rare but can occur in people with fat-malabsorption disorders, such as Crohn’s disease or cystic fibrosis, and in premature infants. Deficiency may lead to nerve and muscle damage.

Vitamin E and Health

Researchers have studied vitamin E for its potential role in preventing heart disease, cancer, dementia, and other chronic diseases. However, current evidence does not support the routine use of high-dose vitamin E supplements for disease prevention. The greatest benefits appear to come from consuming vitamin E-rich foods rather than supplements.

Vitamin E Toxicity

Vitamin E toxicity from foods has not been reported. However, very high doses from supplements may interfere with vitamin K-dependent blood clotting and increase the risk of bleeding.

Dietary Reference Intakes  and Sources for Vitamin E

The RDA for vitamin E is 15 mg of alpha-tocopherol per day for adults. The UL is 1,000 mg per day, although most people can meet their needs by eating a variety of vitamin E-rich foods rather than taking supplements.

Vitamin E is found primarily in vegetable oils, nuts, seeds, and fortified cereals. Smaller amounts are present in leafy green vegetables and some fruits. Figure 8.14 highlights foods that are naturally rich in vitamin E.

FOOD TIP

To increase your dietary intake of vitamin E from plant-based foods, focus on meals that combine leafy greens, seeds, nuts, and healthy plant oils, which are some of the richest natural sources of this fat-soluble vitamin. For example, a spinach salad provides a strong nutritional base, as spinach contains vitamin E, vitamin C, and carotenoids that support overall antioxidant defense. Adding fresh tomatoes provides additional antioxidants, while sunflower seeds significantly boost vitamin E intake, as they are among the most concentrated plant sources of this nutrient.

Vitamin K: Holding Blood and Bones Together

Vitamin K is a group of fat-soluble vitamins best known for its essential role in blood clotting. It acts as a coenzyme that activates several clotting proteins circulating in the bloodstream.

When a blood vessel is injured, platelets quickly form a temporary plug. Vitamin K-dependent clotting proteins then stabilize this plug, helping stop the bleeding. Without adequate vitamin K, normal blood clotting cannot occur.

Vitamin K also plays an important role in bone health. It activates osteocalcin, a protein involved in building and remodeling bone. Although researchers are still studying its full role in bone health, low vitamin K intake has been associated with an increased risk of bone fractures.

Vitamin K Deficiency and Symptoms

Vitamin K deficiency is uncommon because it is found in many foods and is produced in small amounts by bacteria in the large intestine. However, people with conditions that reduce fat absorption—such as liver disease, pancreatic disease, celiac disease, or other malabsorption disorders—are at greater risk.

Signs of vitamin K deficiency include easy bruising, frequent nosebleeds, bleeding gums, heavy menstrual bleeding, and prolonged bleeding after an injury. Because vitamin K is essential for clotting, high-dose vitamin K supplements can interfere with the blood-thinning medication warfarin. Calcium also works with vitamin K to activate several clotting proteins.

Dietary reference intake and food sources

The AI for vitamin K is 90 mcg/day for women and 120 mcg/day for men. No Upper Intake Level (UL) has been established because vitamin K has a low potential for toxicity.

Vitamin K is abundant in green leafy vegetables, including kale, spinach, broccoli, cabbage, parsley, and lettuce. Smaller amounts are found in vegetable oils and other foods. Although bacteria in the large intestine produce some vitamin K, this contributes only a small portion of daily needs.

Newborns are born with very low vitamin K stores, and it takes time for beneficial gut bacteria to become established. For this reason, newborns routinely receive a vitamin K injection shortly after birth to prevent potentially serious bleeding disorders. This simple practice has dramatically reduced vitamin K deficiency bleeding in infants.^[3]

Although each fat-soluble vitamin has a unique role, together they support vision, healthy bones, antioxidant protection, blood clotting, and normal growth. Table 8.X provides a quick review of their major functions, common food sources, and deficiency conditions

Attributions

This section is an adaptation of:

• “Vitamins Important for Vision” and “Vitamin D: Important to Bone Health and Beyond” in Nutrition: Science and Everyday Application, v. 1.0 by Alice Callahan, PhD; Heather Leonard, MEd, RDN; and Tamberly Powell, MS, RDN licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
• “Fat-Soluble Vitamins” in Human Nutrition: 2020 Edition by University of Hawai‘i at Mānoa Food Science and Human Nutrition Program licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.