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Chapter 8 – Vitamins
8.3 The Fat Soluble Four: Vitamins that You Store
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 that the body uses in different ways. 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 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 only changes a small amount—about 12 micrograms of beta-carotene are needed to make 1 microgram of retinol.
In addition to supporting vitamin A needs, carotenoids provide antioxidant protection that helps protect cells from damage. Unlike retinol, carotenoids are not toxic when consumed from foods. Carotenoids are pigments that give many fruits and vegetables their bright orange, yellow, and dark green colors.
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 excellent sources include deep orange vegetables such as sweet potatoes, pumpkins, and carrots, which provide beta-carotene that the body can convert into vitamin A as needed.
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.
Figure 8.7. Normal vision at night compared to night blindness caused by vitamin A deficiency. The image on the left shows what a person with normal vision sees at night, and the image on the right shows the effect that vitamin A deficiency has on poor vision in the dark, known as night blindness. (“Depiction of vision of a person suffering from night blindness “ by Upchar licensed CC BY-SA 4.0.)
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
Figure 8.8. A child with xerophthalmia caused by vitamin A deficiency, demonstrating cloudiness on the surface of the eye. (“Eye with xerophthalmia” by CDC, Public Domain.)
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 cells in the skin 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]
Figure 8.9. The worldwide prevalence of vitamin A deficiency in 1995-2005, based on World Health Organization data. (Image by Our World in Data licensed CC BY 4.0.)
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 swelling of the brain. 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, but this harmless condition goes away once intake is reduced. This condition is known as carotenemia.
Vitamin D: Where Sunlight Meets Skeletal Health
Vitamin D refers to a group of fat-soluble vitamins that the body can make from cholesterol. There are two main forms of vitamin D with biological activity in humans: vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol).
The skin produces vitamin D₃ when it is exposed to sunlight. For most people, more than 90% of vitamin D₃ comes from casual exposure to the sun’s UVB rays, rather than from food. Anything that limits UVB exposure reduces the amount of vitamin D₃ the skin can make. This includes long winters, living at higher latitudes or altitudes, wearing sunscreen or protective clothing, and having darker or tanned skin.
Many people worry about skin cancer risk from sun exposure. While excessive sun exposure is harmful, short periods of sunlight are effective for vitamin D production. Less than 30 minutes of sun exposure to the arms and legs can raise vitamin D₃ levels more effectively than taking very large oral supplements (such as 10,000 IU).
Figure 8.10. The functions of Vitamin D. (Image by Allison Calabrese licensed CC BY 4.0.)
Functions of Vitamin D
Activated vitamin D (calcitriol) works with theparathyroid hormoneto maintain normal blood calcium levels. Without enough vitamin D, the body absorbs less than 15% of the calcium consumed from foods or supplements. This regulation is essential for building and maintaining strong bones.
In children, vitamin D deficiency causes nutritional rickets, a condition marked by soft, weak bones that may become deformed and fracture easily. In adults, deficiency leads to osteomalacia, characterized by low bone mineral density and often accompanied by osteoporosis.
Vitamin D deficiency is common worldwide, particularly among older adults, individuals with darker skin, and people living in northern regions where sunlight exposure is limited during winter months.
Research suggests a clear link between vitamin D and bone health. Observational studies show that people with low vitamin D levels tend to have lower bone mineral density (BMD) and a higher risk of osteoporosis. In contrast, people who regularly eat vitamin D–rich foods—such as salmon—often have better bone health.
Clinical trials provide stronger evidence. A review of eleven clinical trials published in the May 2019 issue of the Journal of the American Medical Association found that older women who took 700–800 IU of vitamin D per day, along with calcium supplements, had a lower risk of hip fractures. This suggests that vitamin D, especially when combined with adequate calcium, plays an important role in protecting bones as we age.[3] A reduction in fracture risk was not observed when people took vitamin D supplements without calcium.
Many additional health benefits have been associated with higher vitamin D status, including lower rates of cardiovascular disease and certain infections. Laboratory studies in cells and animals suggest that vitamin D may slow the growth of some cancer cells, reduce inflammation, improve insulin secretion, influence immune responses, and affect the development of atherosclerosis. Low vitamin D levels have also been linked to a higher risk of autoimmune conditions such as rheumatoid arthritis, multiple sclerosis, and type 1 diabetes, as well as hypertension.
However, it is important to interpret these findings cautiously. Much of the evidence supporting these broader health effects comes from laboratory and observational studies, which cannot prove cause and effect. Large randomized clinical trials, including the Vitamin D and Omega-3 Trial (VITAL), were designed to better evaluate these proposed benefits. Overall, current evidence does not consistently support high-dose vitamin D supplementation for the prevention of chronic disease in generally healthy adults, highlighting the need to distinguish between association and proven clinical benefit.
Vitamin D Toxicity
Vitamin D toxicity is rare but can occur with excessive supplement use. Too much vitamin D raises blood calcium levels (hypercalcemia), which may increase urinary calcium loss, damage the kidneys, and cause calcium deposits in soft tissues such as the kidneys and blood vessels. Importantly, vitamin D produced from sunlight does not cause toxicity because skin synthesis of vitamin D₃ is tightly regulated.
Vitamin D Requirements and Safety
For most of the population, vitamin D needs are relatively consistent across the life span. Individuals ages 1 through 70 years require 15 micrograms (µg) per day, which is equivalent to 600 international units (IU) of vitamin D. Beginning at age 71, vitamin D needs increase to 20 µg (800 IU) per day to help support bone health and reduce fracture risk associated with aging.
Some professional organizations recommend slightly higher intakes for certain adults. The National Osteoporosis Foundation suggests that adults under age 50 consume 400–800 IU per day, while adults age 50 and older aim for 800–1,000 IU per day, particularly if sun exposure is limited or dietary intake is low.
According to the Institute of Medicine (IOM), the tolerable upper intake level (UL) for vitamin D is 50 µg or 4,000 IU per day for adults. Vitamin D toxicity is rare but can occur with excessive supplement use. Certain medical conditions—including hyperparathyroidism, lymphoma, and tuberculosis—can increase sensitivity to vitamin D and raise the risk of hypercalcemia when intakes are too high. For this reason, high-dose vitamin D supplementation should be monitored by a healthcare provider.
Vitamin E: Antioxidant Protection and Cellular Health
Vitamin E includes eight related compounds, but alpha-tocopherol is the primary form that meets human needs. As a fat-soluble vitamin, it acts as a powerful antioxidant, protecting cell membranes from free-radical damage.
When alpha-tocopherol neutralizes a free radical, it becomes inactive unless it is regenerated. Vitamin C helps restore some of its antioxidant activity, while the rest is excreted. Because vitamin E cannot be recycled indefinitely, regular dietary intake is necessary to maintain healthy levels.
Vitamin E deficiency is rare, and when it does occur, signs and symptoms may be subtle or not immediately apparent. Deficiency is most often linked to nerve degeneration, reflecting vitamin E’s role in protecting nerve cell membranes from oxidative damage. Individuals with fat-malabsorption disorders, such as Crohn’s disease or cystic fibrosis, as well as infants born prematurely, are at greater risk of vitamin E deficiency.
Vitamin E and Chronic Disease: What Does the Evidence Show?
Physiologic Roles of Vitamin E
Vitamin E is a fat-soluble antioxidant that plays several important roles in human health. At recommended intake levels, it:
Supports immune function by helping the body defend against bacteria and viruses
Promotes healthy blood vessel dilation
Helps reduce abnormal blood clot formation
Protects cell membranes from oxidative damage
Despite these important functions, research does not support the routine use of high-dose vitamin E supplements for disease prevention.
Vitamin E and Cardiovascular Disease
Vitamin E reduces LDL oxidation, and it was therefore hypothesized that vitamin E supplementation would protect against atherosclerosis. However, large clinical trials have not consistently found evidence to support this hypothesis. In fact, in the “Women’s Angiographic Vitamin and Estrogen Study,” postmenopausal women who took 400 international units (264 milligrams) of vitamin E and 500 milligrams of vitamin C twice per day had higher death rates from all causes.[4]
Other studies have not confirmed the association between increased vitamin E intake from supplements and increased mortality. There is more consistent evidence from observational studies that a higher intake of vitamin E from foods is linked to a decreased risk of dying from a heart attack.[5][6]
Vitamin E’s Role in Cancer
The large clinical trials that evaluated whether vitamin E was linked to cardiovascular disease risk also examined cancer risk. These trials, called the HOPE-TOO Trial and Women’s Health Study, did not find that vitamin E at doses of 400 international units (264 milligrams) per day or 600 international units (396 milligrams) every other day reduced the risk of developing any form of cancer.[7][8]
Vitamin E and Eye Health
Oxidative stress plays a role in age-related vision loss, known as age-related macular degeneration (AMD). The condition primarily occurs in people over the age of 50 and is characterized by the progressive loss of central vision resulting from damage to the macula, the center of the retina.
Vitamin E and Dementia
The brain’s high glucose consumption makes it more susceptible to oxidative stress than other organs. Oxidative stress has been implicated as a major contributing factor to dementia and Alzheimer’s disease. Some studies suggest vitamin E supplements delay the progression of Alzheimer’s disease and cognitive decline, but again, not all of the studies confirm the relationship. A recent study, with over 5,000 participants, published in the July 2010 issue of the Archives of Neurology, demonstrated that individuals with the highest intakes of dietary vitamin E were 25 percent less likely to develop dementia than those with the lowest intakes. [9]
More studies are needed to better assess the dose and dietary requirements of vitamin E and, for that matter, whether other antioxidants lower the risk of dementia, a disease that not only devastates the mind but also puts a substantial burden on loved ones, caretakers, and society in general.
Vitamin E Toxicity
Currently, researchers have not found any adverse effects from consuming vitamin E in food. Although that may be the case, supplementation of alpha-tocopherol in animals has been shown to cause hemorrhage and disrupt blood coagulation. Extremely high levels of vitamin E can interact with vitamin K-dependent clotting factors, inhibiting blood clotting.[10]
Dietary Reference Intakes for Vitamin E
The RDA for vitamin E is 15 mg of alpha-tocopherol per day for adults, including those who are pregnant or lactating. This amount is sufficient to support vitamin E’s primary role as an antioxidant, protecting cell membranes from oxidative damage. Most people can meet the RDA through foods such as vegetable oils, nuts, seeds, and fortified cereals.
The Tolerable Upper Intake Level (UL) for vitamin E is 1,000 mg per day, a level set to reduce the risk of adverse effects. High-dose vitamin E supplements are generally not recommended for the general population, as intakes well above the RDA have not consistently shown added health benefits and may increase the risk of bleeding, particularly in people taking blood thinners. For most individuals, obtaining vitamin E from foods rather than supplements is the safest and most effective approach.
Dietary Sources of Vitamin E
Add some nuts to your salad and make your own dressing to get a healthy dietary dose of vitamin E.
Figure 8.13. Image by rawpixel, courtesy of Pixabay.
Vitamin E is found in many foods, especially those higher in fat, such as nuts and oils. Some spices, such as paprika and red chili pepper, and herbs, such as oregano, basil, cumin, and thyme, also contain vitamin E. (Keep in mind spices and herbs are commonly used in small amounts in cooking and therefore are a lesser source of dietary vitamin E.) See Figure 8.14 for foods high in vitamin E.
Figure 8.14. Vitamin E content of various foods. (Image by Allison Calabrese licensed CC BY 4.0, with images by Remi Yuan, Annie Spratt, and Engin Akyurt on Unsplash.)
Food Recommendation
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 refers to a group of fat-soluble vitamins with similar chemical structures. Its primary and best-known role is in blood clotting. Vitamin K functions as a coenzyme for proteins involved in coagulation, which are constantly circulating in the bloodstream. When a blood vessel is injured, platelets adhere to the damaged area and form a temporary plug. Vitamin K-dependent clotting proteins then help stabilize this plug and stop bleeding. Without adequate vitamin K, normal blood clotting cannot occur.
Vitamin K deficiency can lead to bleeding disorders. Although deficiency is relatively rare, individuals with conditions that impair fat absorption—such as liver disease, pancreatic disease, celiac disease, or other malabsorption disorders—are at increased risk. Signs and symptoms of deficiency include frequent nosebleeds, easy bruising, bleeding gums, broken blood vessels, and heavy menstrual bleeding. Because vitamin K is required for clotting, high supplemental intakes can interfere with the action of the anticoagulant medication warfarin. Calcium also plays an important supporting role in activating vitamin K-dependent clotting proteins.
In addition to its role in blood coagulation, vitamin K is important for bone health. It is required for the modification of osteocalcin, a protein involved in bone formation and remodeling. While the full functions of osteocalcin and other vitamin K-dependent proteins in bone are still being studied, research suggests that low dietary intake of vitamin K is associated with an increased risk of bone fractures.
Dietary Reference Intake and Food Sources for Vitamin K
The AI of vitamin K for adult females is 90 micrograms per day, and for males it is 120 micrograms per day. A UL for vitamin K has not been set. The Food and Nutrition Board (FNB) has not established a UL for vitamin K because it has a low potential for toxicity.
Vitamin K is present in many foods. It is found in highest concentrations in green vegetables such as broccoli, cabbage, kale, parsley, spinach, and lettuce. Additionally, vitamin K can be synthesized by bacteria in the large intestine. The exact amount of vitamin K synthesized by bacteria that is actually absorbed in the lower intestine is not known, but it is likely to contribute less than 10 percent of the recommended intake. Newborns have low vitamin K stores, and it takes time for the sterile newborn gut to acquire the good bacteria it needs to produce vitamin K. So, it has become a routine practice to inject newborns with a single intramuscular dose of vitamin K. This practice has basically eliminated vitamin K-dependent bleeding disorders in babies.[11]
The table below shows the food sources, adult RDA, main functions, deficiency conditions, groups at risk, the toxicity, and upper limit (UL) for the four fat-soluble vitamins.
Table 8.2: Fat-Soluble Vitamins
Vitamin
Sources
Recommended Intake for Adults
Major functions
Deficiency diseases and symptoms
Groups at risk of deficiency
Toxicity
UL
Vitamin A (retinol, retinal, retinoic acid, carotene, beta-carotene)
Antioxidant, vision, cell differentiation, reproduction, and immune function
Xerophthalmia, night blindness, eye infections; poor growth, dry skin, impaired immune function
People living in poverty (especially infants and children), premature infants, pregnant and lactating women, and people who consume low-fat or low-protein diets
Hypervitaminosis A: Dry, itchy skin, hair loss, liver damage, joint pain, fractures, birth defects, swelling of the brain
Carotenia: orange skin
3000 mcg/day
Vitamin D
Swordfish, salmon, tuna, orange juice (fortified), milk (fortified), sardines, egg, synthesized from sunlight
600-800 IU/day (15-20 mcg/day)
Absorption and regulation of calcium and phosphorus, maintenance of bone
Rickets in children: abnormal growth, misshapen bones, bowed legs, soft bones; osteomalacia in adults
Breastfed infants, older adults, people with limited sun exposure, people with dark skin
Calcium deposits in soft tissues, damage to the heart, blood vessels, and kidneys
4000 IU/day (100 mcg/day)
Vitamin E
Sunflower seeds, almonds, hazelnuts, peanuts
15 mg/day
Antioxidants protect cell membranes
Broken red blood cells, nerve damage
People with poor fat absorption, such as premature infants
Inhibition of vitamin K clotting factors
1000 mcg/day from supplemental sources
Vitamin K
Vegetable oils, leafy greens, are synthesized by intestinal bacteria
90-120 mcg/day
Synthesis of blood-clotting proteins and proteins needed for bone health and cell growth
Song P, Adeloye D, Li S, et al. The prevalence of vitamin A deficiency and its public health significance in children in low- and middle-income countries: A systematic review and modeling analysis. J Glob Health. 2023;13:04084. Published 2023 Aug 11. doi:10.7189/jogh.13.04084 ↵
UNICEF. Vitamin A deficiency. UNICEF Data: Monitoring the Situation of Children and Women. Published August 25, 2025. Accessed January 8, 2026. https://data.unicef.org/topic/nutrition/vitamin-a-deficiency/ ↵
Yao P, Bennett D, Mafham M, et al. Vitamin D and Calcium for the Prevention of Fracture: A Systematic Review and Meta-analysis. JAMA Netw Open. 2019;2(12):e1917789. Published 2019 Dec 2. doi:10.1001/jamanetworkopen.2019.17789 ↵
Waters DD, et al. Effects of Hormone Replacement Therapy and Antioxidant Vitamin Supplements on Coronary Atherosclerosis in Postmenopausal Women: A Randomized Controlled Trial. JAMA. 2002; 288(19), 2432–40. ↵
HOPE and HOPE-TOO Trial Investigators. Effects of Long-Term Vitamin E Supplementation on Cardiovascular Events and Cancer. JAMA. 2005; 293, 1338–47. http://jama.ama-assn.org/content/293/11/1338.long., Accessed February 27. 2026. ↵ ↵
9. Lee IM, et al. Vitamin E in the Primary Prevention of Cardiovascular Disease and Cancer: The Women’s Health Study. JAMA.2005; 294, 56–65. http://jama.ama-assn.org/content/294/1/56.long. Accessed October 5, 2017. ↵ ↵
Lonn E, Bosch J, Yusuf S, Sheridan P, Pogue J, Arnold JM, Ross C, Arnold A, Sleight P, Probstfield J, Dagenais GR; HOPE and HOPE-TOO Trial Investigators. Effects of long-term vitamin E supplementation on cardiovascular events and cancer: a randomized controlled trial. JAMA. 2005 Mar 16;293(11):1338-47. doi: 10.1001/jama.293.11.1338. PMID: 15769967 ↵
Goodman M, Bostlick RM, Kucuk O, Jones DP. Clinical trials of antioxidants as cancer prevention agents: past, present, and future. Free Radic Biol Med. 2011; 51(5), 1068–84. https://www.ncbi.nlm.nih.gov/pubmed/21683786. Accessed February 27, 2026 ↵
Devore EE, et al. Dietary Antioxidants and Long-Term Risk of Dementia. Arch Neurol. 2010; 67(7), 819–25. ↵
Dietary Supplement Fact Sheet: Vitamin E.National Institutes of Health, Office of Dietary Supplements. http://ods.od.nih.gov/factsheets/VitaminE-QuickFacts/. Updated March 26, 2021. Accessed February 27, 2026. ↵ ↵
The process in which a cell changes from one cell type to another. Usually, the cell changes to a more specialized type.
A hormone made by the parathyroid glands, located next to the thyroid in the neck, that helps regulate calcium levels in the blood by increasing calcium when levels are too low.
A disease that is characterized by softening of the bones due to poor calcium deposition within them because of a lack of vitamin D in the body.
A disease that is characterized by defective bone formation that may be due to a vitamin D deficiency or a lack of sunlight exposure.
