The Fat-Soluble Vitamins

The Fat-Soluble Vitamins • Vitamin A • Vitamin D • Vitamin E • Vitamin K Dr. Latifah Al-Oboudi2012

Vitamins: Essential Dietary Components • Vitamins are essential, organic compounds needed for important metabolic reactions in the body. • They are not a source of energy. Instead, they promote many energy-yielding and other reactions in the body, thereby aiding in the growth, development, and maintenance of various body tissues. Dr. Latifah Al-Oboudi2012

Vitamins cannot be synthesized in the body at all or are synthesized in insufficient amounts. • Vitamins A, D, E, and K are fat soluble, whereas the B-vitamins and vitamin C are water soluble. Dr. Latifah Al-Oboudi2012

Absorption of vitamins • Fat-soluble vitamins are absorbed along with dietary fat. Thus adequate absorption depends on the efficient use of bile and pancreatic lipase in the small intestine to digest dietary fat and the adequate absorptive capacity of the intestinal mucosa. • Under optimal condition, about 40 to 90% of the fat-soluble vitamins are absorbed when they are consumed in recommended amounts. Dr. Latifah Al-Oboudi2012

Malabsorption of vitamins • Fat malabsorption (resulting from GI tract and pancreatic disease) may cause poor absorption of fat-soluble vitamins. • In disease states that limit fat digestion, fat-soluble vitamin absorption may be compromised, thereby increasing the risk of deficiency in these individuals. Dr. Latifah Al-Oboudi2012

Transport of vitamins • They travel by way of the lymphatic system into general circulation, carried by chylomicrons. Dr. Latifah Al-Oboudi2012

Vitamin Toxicity • Fat –soluble vitamins are excreted less readily from the body than water-soluble vitamins and thus pose a potential threat for toxicity, especially of vitamins A and D. • Toxicities of these fat- soluble vitamins generally occur with high doses of supplements, rather than from foods Dr. Latifah Al-Oboudi2012

Vitamin A • Vitamin A contributes to the maintenance of vision, the normal development of cells (especially mucous-forming cells), and immune function. Dr. Latifah Al-Oboudi2012

Vitamin A • Vitamin A refers to the preformed retinoids and provitamin A carotenoids that can be converted to vitamin A activity. • Retinoids is a collective term for the biologically active forms of vitamin A because, unlike carotenoids, they do not need to be converted to become biologically active. Dr. Latifah Al-Oboudi2012

Vitamin A • Retinoids exist in 3 forms: retinal(an aldehyde), retinol(an alcohol), and retinoic acid. • Carotenoids are yellow-orange pigmented materials in vegetables and fruits, some of which are provitamins---that is, they can be converted into vitamin A. • A plant derivative, known as beta-carotene, along with 2 other carotenoids(alpha-carotene and beta- cryptoxanthin), yields vitamin A after metabolism by the small intestine or liver. Dr. Latifah Al-Oboudi2012

Interconversions of beta-carotene and various retinoids. Notice that the synthesis of retinoic acid is a “dead end” in metabolic terms. Dr. Latifah Al-Oboudi2012

Dr. Latifah Al-Oboudi2012

Vitamin A in Foods • Retinoids (preformed vitamin A) are found in foods of animal origin, such as liver, fish, fish oils, fortified milk and eggs. • A carotenoids are found mainly in dark green and yellow-orange vegetables and fruits, such as carrots, spinach and other greens, winter squash, sweet potatoes, broccoli, mangoes, cantaloupe, peaches, and apricots. Dr. Latifah Al-Oboudi2012

Food sources of vitamin A Dr. Latifah Al-Oboudi2012

At one time the amounts of vitamin A were expressed in International Units (IUs). Today, there are more sensitive means for measuring nutrients. Consequently, milligram (1/1000 of a gram) and microgram (1/1,000,000 of a gram) measurements have generally replaced IUs as the units of measure. However, some food and vitamin supplement labels may still display the older IU value. • Dietary vitamin A activity is currently expressed in Retinol Activity Equivalents(RAE) Dr. Latifah Al-Oboudi2012

Vitamin A Needs • The RDA for vitamin A is 900 µg Retinol Activity Equivalents(RAE) per day for adult men and 700 µg RAE per day for adult women. Dr. Latifah Al-Oboudi2012

Absorption, Transport, Storage, and Excretion of Vitamin A • Up to 90% of retinol is absorbed into the cells of the small intestine. • After absorption, a fatty acid is attached to retinol to form a new retinyl ester. These retinyl esters are packaged into chylomicrons before entering the lymphatic circulation. Dr. Latifah Al-Oboudi2012

Absorption, Transport, Storage, and Excretion of Vitamin A • The provitamin A carotenoids can be enzymatically split within the intestinal cells or liver cells to form retinal or, to lesser extent, retinoic acid. • The carotenoid absorption is much lower than that of retinol. • After being absorbed in the small intestine , carotenoids can be cleaved to yield retinal, which is then converted to retinol. Dr. Latifah Al-Oboudi2012

Absorption, Transport, Storage, and Excretion of Vitamin A • The chylomicrons deliver vitamin A to tissue for storage or cellular use. • Storage: Over 90% of the body’s vitamin A stores are found in the liver, with small amounts in adipose tissue, kidneys, bone marrow, testicles, and eyes. Normally, the liver stores enough vitamin A to last for several months to protect against vitamin A deficiency. Dr. Latifah Al-Oboudi2012

Excretion of Vitamin A • Although vitamin A is not readily excreted by the body, some is lost in the urine. • Kidney disease increases the risk of vitamin A toxicity because this urinary route of excretion is compromised. Dr. Latifah Al-Oboudi2012

Functions of Vitamin A ( Retinoids) • Growth and Development. • Cell Differentiation. • Vision. • Immune Function. Dr. Latifah Al-Oboudi2012

Vitamin A deficiency Diseases • Conjunctiva: • Xerophthalmia: Dr. Latifah Al-Oboudi2012

Vitamin A Toxicity • Preformed vitamin A can be quite toxic when taken at doses 2 to 4 times or more the RDA. • Use of vitamin A supplements is especially dangerous during pregnancy because it can lead to fetal malformations. Dr. Latifah Al-Oboudi2012

Vitamin A Toxicity • Consuming the right amount of vitamin A is critical to overall health. Avery low (deficient) or very high (toxic)vitamin A intake (as retinoids) can produce harmful symptoms and can even lead to death. Dr. Latifah Al-Oboudi2012

Golden rice was genetically engineered to synthesize beta-carotene. This rice was developed for use as a fortified food in areas of the world that have limited access to vitamin A-rich foods. Dr. Latifah Al-Oboudi2012

Vitamin D • In presence of sunlight, skin cell synthesize a sufficient supply of vitamin D from a derivative of cholesterol. • Dietary source is not required if synthesis is adequate to meet needs, the vitamin is more correctly classified as a “conditional” vitamin or prohormone (a precursor of an active hormone). Dr. Latifah Al-Oboudi2012

Vitamin D • In the absence of UV light exposure, an adequate dietary intake of vitamin D is essential to prevent the deficiency diseases rickets and osteomalacia and to provide for cellular needs. • After exposure to the sun, humans produce vitamin D₃ (cholecalciferol) from a derivative of cholesterol. • The liver and kidneys each add a hydroxyl group(-OH) to this to yield the active form of vitamin D ( 1,25 dihydroxy D₃, or calcitriol). Dr. Latifah Al-Oboudi2012

Vitamin D₂ in foods • The best food sources of vitamin D are: • Fatty fish (e.g. sardines, mackerel, and salmon). • Cod liver oil. • Fortified milk. • Some fortified breakfast cereals. • Although eggs, butter, liver, and a few brands of margarine contain some vitamin D, large servings must be eaten to obtain an appreciable amount of the vitamin. Thus, these foods are not considered a significant source. Dr. Latifah Al-Oboudi2012

Cod liver oil was a common supplements for children in the US until 1933, when milk was first fortified with vitamin D Dr. Latifah Al-Oboudi2012

Vitamin D₂ in foods • Most fortified foods and supplements containing vitamin D are in the form of ergocalciferol, or vitamin D₂, the same form found naturally in foods. • Ergocalciferol has vitamin D activity in humans, but in lesser amounts than provided by cholecalciferol (vitamin D₃). Dr. Latifah Al-Oboudi2012

Food sources of vitamin D Dr. Latifah Al-Oboudi2012

Vitamin D₃ Formation in the Skin • The synthesis of vitamin D₃ begins with a compound called 7-dehydrocholesterol, a precursor of cholesterol synthesis located in the skin. • 1 ring on the molecule undergoes a chemical transformation, forming the more stable vitamin D₃ (cholecalciferol). • This change allows vitamin D₃ to enter the bloodstream for transport to the liver and kidneys, where it undergoes hydroxylation (the addition of -OH) and subsequent conversion to its bioactive form 1,25 dihydroxy D₃ (calcitriol). Dr. Latifah Al-Oboudi2012

Vitamin D₃ Formation in the Skin • For many individuals, sun exposure provides 80 to 100% of the vitamin D₃ required by the body. • The amount of sun exposure needed , however, depend on the time of day, the geographic location, the season of the year, one’s age, one’s skin color, and the use of sunscreen. Dr. Latifah Al-Oboudi2012

The production of vitamin D₃ in the skin decreases by about 70% when one reaches the age of 70. older people are advised to get small amounts of sun exposure, or to take vitamin D supplements to prevent deficiency. • The large amount of melanin (skin pigment) in dark-skinned individuals may block UV light and prevent adequate vitamin D₃ synthesis. Dr. Latifah Al-Oboudi2012

Scientists recommend that people expose their hands, face, and arms to UV light at least 2 or 3 times a week for 10 to 15 minutes. • Individuals with dark skin may need sun exposure of 3o minutes or more (or vitamin D supplementation). • People who do not receive enough UV light exposure to synthesize adequate amounts of vitamin D₃ should make certain that they have adequate sources of vitamin D in their diets. Dr. Latifah Al-Oboudi2012

Vitamin D Needs • The adequate intake for vitamin D is 5µg/day (200 IU/day) for people under age 51, 10µg/day (400 IU/ day) for people between 51 and 70, and 15µg/day (600 IU/ day) for older adults. • Older adults may need 20 to 25µg/day (800 to 1000 IU/ day) from a combination of vitamin D-fortified foods and a supplement to decrease the risk of bone loss and other chronic diseases. • The daily value used on food and supplements labels is 10 µg. • Breastfed infants recommended to be given a vitamin D supplement of 5 µg/day (200 IU) until they are weaned to infant foods fortified with, or rich in, vitamin D . Dr. Latifah Al-Oboudi2012

Absorption, Transport, Storage, and Excretion of Vitamin D • About 80% of vitamin D₂ is incorporated (along with other dietary fats) into micelles in the small intestine, absorbed, and transported to the liver by chylomicrons through the lymphatic system. Dr. Latifah Al-Oboudi2012

Whether synthesized in the skin or obtained from dietary sources, vitamin D ultimately function as a hormone: 1,25(OH)₂ vitamin D₃ (calcitriol) Dr. Latifah Al-Oboudi2012

Function of Vitamin D • Vitamin D has hormone like functions, which help regulate the body’s concentration of calcium and phosphorus Dr. Latifah Al-Oboudi2012

Figure 12-13 The active vitamin D Hormone– 1,25 (OH)₂ vitamin D₃ --and parathyroid hormone interact to control blood calcium concentration. Low blood calcium is a trigger for the following actions, all of which raise blood calcium levels. Parathyroid hormone (PTH) and 1,25(OH)₂ vitamin D₃ mobilize calcium from the bone. PTH also a. Reduces calcium excretion by the kidneys. b. stimulates kindly synthesis of 1,25(OH)₂ vitamin D₃. 1,25(OH)₂ vitamin D₃ stimulates intestinal calcium absorption. Conversely, when calcium levels in the blood become too high, the hormone calcitonin responds by promoting calcium disposition in the bone (see chapter 14)/. 1 2 3 Dr. Latifah Al-Oboudi2012

Function of Vitamin D • Vitamin D promotes increased intestinal absorption of calcium and phosphorus from foods to maintain blood levels of these minerals. This makes calcium and phosphorus available for body cells and for incorporation into bones when there is more than needed for basic functions. Dr. Latifah Al-Oboudi2012

Function of Vitamin D • When blood levels of calcium and phosphorus start to fall, vitamin D (with PTH from the parathyroid gland) can release calcium and phosphorus from bone into the blood to restore blood levels of these minerals. • This action can eventually weaken the bones if it continues for a prolonged period of time, it helps provide the calcium and phosphorus needed for many basic life functions. If the bones did not supply calcium and phosphorus for these functions, a person could quickly have serious, even fatal, health consequences. Thus, vitamin D preserves these important functions even if dietary intakes of these minerals are inadequate. Dr. Latifah Al-Oboudi2012

Function of Vitamin D • Vitamin D has important functions beyond its role in maintaining calcium and phosphorus homeostasis and bone health. • Vitamin D also is involved in immune function and cellular metabolism. Dr. Latifah Al-Oboudi2012

Function of Vitamin D • Vitamin D also may be involved in cell cycle regulation. • Additionally, vitamin D may decrease the risk of certain types of infections and autoimmune diseases, such as multiple sclerosis, through its action in the immune system and offer protection against diabetes, hypertension, and certain cancers. Dr. Latifah Al-Oboudi2012

Vitamin D Deficiency Diseases • Vitamin D deficiency results in harmful changes in bone, a condition known as rickets in children and osteomalacia in adults. Dr. Latifah Al-Oboudi2012

The bone deformities and bowed legs of rickets, a vitamin D deficiency disease in children. Dr. Latifah Al-Oboudi2012

Vitamin D Toxicity • Vitamin D toxicity can occur from excessive vitamin D supplementation, causing the deposition of calcium in the kidneys, heart, and lungs. Dr. Latifah Al-Oboudi2012

The Fat-Soluble Vitamins