Fat Soluble Vitamins and their Functions in the Human Body

carrot juice - Vitamin A sourceVitamins are essential nutrients and micronutrients to the human body. They usually come in the forms of organic chemical compounds that human body can’t produce entirely on its own or can produce, but in sufficient quantities, and thus, must be supplemented in the forms of supplemental diets or pills. Vitamins are classified according to the chemical and biological actions of their vitamers, not by their chemical structures that are designated alphabetically in accordance to their biological activities. A vitamer is a chemical compound that displays a specific biological action in association with a specific vitamin. There are, at present, thirteen generally recognized vitamins worldwide. These 13 vitamins are further subdivided into two groups: the fat soluble vitamins and the water-soluble vitamins.


Understanding Fat Soluble Vitamins

Fat soluble vitamins do not dissolve in water unlike those water soluble vitamins and are rather similar to oil. These group of vitamins are commonly found in fatty foods. Of the 13 vitamins, four vitamins belong to fat soluble vitamins group. These are vitamins A, D, E and K.


Vitamin A


Vitamin A is a group of unsaturated organic compounds and one of the fat soluble vitamins that includes retinol (Vitamin A1), retinal, provitamin A carotenoids (notably -carotene), retinoic acid, and 3,4-dehydroretinal (Vitamin A2). Retinol is the most common vitamer of this vitamin. Vitamin A, in the form of retinol, was first discovered in 1913 in cod fish liver oil and has been known for maintaining the eyesight ever since. The main source of this vitamin is the food taken in and absorbed by stomach in the presence of bile salt and intestinal juice.

Retinol is prevalent in animal livers (like chicken, turkey, pork, and beef), fish’s liver, dairy products like milk, cheese, and butter, eggs, and cod liver oil. Provitamin A compounds like carotene, on the other hand, is abundant in ripe yellow fruits, oranges, leafy vegetables like lettuce and spinach, squash, pumpkins and carrots.

Retinal or retinaldehyde can be ingested directly into the body by means of diet. The retinoic acid, for example, can be produced inside the human body by biosynthesis. The body can produce retinal from one or two carotenoids (α-carotene and/or -carotene) or from -cryptoxanthin. On the other hand, retinoic acid is produced in the body by means of two oxidation sequence. The first synthesis is the oxidation of retinol into retinaldehyde; then, from retinaldehyde to retinoic acid.

Vitamin A2, 3, or 4-dehydroretinal can be found from fresh water fish. This less active vitamer is an alternative source of vitamin A.


Other Benefits of Vitamin A


As mentioned above, Vitamin A is vital in the maintenance of eyesight, especially, the vision during darkness. It helps to conserve the delicate light-sensitive cells of the retina and is instrumental in the formation of tears. Aside from these benefits, the action of this vitamin to the body includes:

  • Support to the immune system. Vitamin A deficiency can impair the body’s inherent immunity, hinder the normal regeneration of damaged mucosal barriers, and limits the abilities of macrophages, neutrophils and NK cells.
  • Promotes body growth, especially, with children. A test conducted in Sudan shows that children regularly given dietary intake of Vitamin A are taller and heavier than their ordinary counterparts.
  • Promotes hair growth. According to studies, there are inconclusive evidences that Vitamin A may be essential in the reversal of hair regression.
  • Supports the reproductive system. Retinoic Acid is instrumental to embryo development and supports both male and female reproductive system.


Severe Vitamin A deficiency can lead to hyperkeratosis, keratomalacia, night blindness and/or total blindness. Symptoms may also include dry eyes, hair loss, weak immune system, and skin problems. On the other hand, overdose can lead to Hypervitaminosis A.


Vitamin D


Vitamin D is a group of secosteriods or fat soluble vitamins that cause intestinal absorption of calcium, phosphate, and magnesium which are essential to bone’s maintenance. Dubbed as the “sunshine vitamin,” Vitamin D is produced by exposing a large area of our skin to sunlight, UV light in particular. However, people don’t readily expose their skins to sunlight for considerable time and use sunblock if they do. Sunblock reduces the amount of sunlight that goes to the skin and gets synthesized. Because of the reduced amount of vitamin that the body absorbs, dietary intake of this vitamin is usually needed


Vitamin D comes in two forms:


  • Cholecalciferol or Vitamin D3 is obtained exposing the skin to sunlight and by consuming fish oils, salmon and mackerel, canned tuna and sardines and eggs.
  • Ergocalciferol or Vitamin D2 is obtainable from eating mushroom, thalli, and lichen.


When absorbed into the blood, the kidney and liver synthesize these inactive secosteriod into its more active form—calcitriol. Cholecalciferol is more efficiently synthesized into calcitriol.


There are numerous potential functions for this vitamin; however, most are insufficiently studied or have inconclusive results. Vitamin D is well known for its contribution to the maintenance of the skeletal system. This vitamin regulates phosphate and calcium level and allows these minerals to be readily absorbed by the body. Calcium and phosphate are vital nutrients that promote bone growth and homeostasis.


Aside from bone maintenance, Vitamin D is likewise, known to regulate the body’s immune system, particularly the autoimmune function, and promote immunological tolerance.


The deficiency in Vitamin D can lead to bone ailment such as osteomalacia and rickets which are both associated with the weakening of bones. On the other hand, an overdose can lead to a condition called hypervitaminosis D or vitamin D toxicity.


Vitamin E


Vitamin E consists of two groups of fat soluble vitamins or vitamers, namely: tocopherols and tocotrienols. Each group is further subdivided into four and carry the first four letters of the Greek alphabet—alpha, beta, gamma, and delta. Alpha (α-) tocopherol is the most common vitamer and makes up to 90% of Vitamin E in the blood. Tocopherol is commonly found in olive, corn, and sunflower oils. Tocotrienol on the other hand, abounds in vegetable and palm oils, almonds and hazelnuts.


This vitamin acts mainly as an antioxidant and inhibits oxidative stress. This antioxidant feature of this vitamin is further enhanced in the presence of Vitamin B3, C and selenium. Moreover, this vitamin acts like a blood “thinner” that reduces blood clotting.


Deficiency in Vitamin E is a rare condition and is only possible to occur among people with liver disease. Some symptoms related to the vitamin deficiency include difficulty in walking, problematic eyesight, numbness, and muscle weakness. Overdose of Vitamin E can lead to vitamin toxicity that can further result into congestive heart failure.


Vitamin K


Vitamin K is one of the fat soluble vitamins that is vital to blood coagulation. The “K” comes from Koagulation—a Danish term for coagulation. Aside from coagulating blood, this vitamin binds the calcium to the bones and other tissues and inhibits calcification in the blood vessels. Vitamin K is also used to counter the anticoagulant effects of the drug warfarin.


Vitamin K comes in the form of Phylloquinone (Vitamin K1) or Menaquinones (Vitamin K2). Phylloquinone is abundant in plant-sourced foods like leafy and green veggies that could be eaten raw or cooked. On the other hand, menaquinones are animal-sourced foods like liver, egg yolk, and butter.


The lack of Vitamin K can have dire effect on a person’s health that could lead to bleeding diathesis. Unlike other vitamins, Vitamin K is stored in our body in only limited quantities that can be exhausted in a week. Moreover, people who are suffering from inflammatory bowel disease, cystic fibrosis, and celiac disease could not digest food properly; hence, they are at risk of having vitamin K deficiency. High doses of vitamins A and E respectively may impair the absorption and coagulation abilities of vitamin K. On the positive side, natural forms of this vitamin (K1 and K2) do not have any ill effect on a person and can be used to counteract the effects of anticoagulant warfarin.