Fat-Soluble Vitamins

Fat-soluble vitamin includes vitamin A, D, E and K that are released, absorbed and transported with fat in our diet. All the fat-soluble vitamins are isoprenoids as they are produced by the condensation of four isoprene units. The deficiency and over intake of these vitamins can cause serious health problems and toxicity in our body.

Vitamin A, D, E and K refer as fat-soluble vitamins because fatty tissues absorb these vitamins. It also shows the property of water insolubility. Fat-soluble vitamins must be added to our diet for healthy living.

Content: Fat-Soluble Vitamins

  1. Definition
  2. Types

Definition of Fat-Soluble Vitamins

Fat-soluble vitamins can be defined as the vitamins (A, D, E and K) absorbed by our lymphatic system, as these are lipophilic. These vitamins are generally stored in the liver, adipose tissues and skeletal muscles and digested by the bile secretions in the liver. Fat-soluble vitamins are essential micronutrients that remain in our bodies for a more extended period.

Each form of these vitamins play a vital role in our body and performs distinct tasks. A person with an impaired fat absorption system may suffer severe health issues due to a deficiency of fat-soluble vitamins. Fat-soluble vitamins can derive from the plant source, animal source and in the form of dietary supplements.

Types of Fat-Soluble Vitamins

Fat-soluble vitamins comprise four major types, namely vitamin A, D, E and K.


Vitamin-A includes two subclasses, namely retinoids and carotenes. The absorption of vitamin-A from the gastrointestinal tract depends on fat metabolism, which may be affected by iron and zinc deficiency in a person.


Vitamin-A comprises a group of active forms of chemical compounds that are collectively called “Retinoids”.  The class of retinoids includes retinol, retinal and retinoic acid.

  1. Retinol or Vitamin-A1 is the most active form that exists as “retinyl ester” in animal tissue, having a long chain of fatty acids. It is a form of primary alcohol. Retinol comprises a cyclohexenyl ring with an unsaturated side chain, having four double bonds.
  2. Retinal (a derivative of vitamin-A) also refers to retinaldehyde that derives by the oxidation of retinol at the C-15 group by retinol dehydrogenase. It plays a functional role in promoting rod and cone cells of the retina. 3, 4-dehydroretinal is less active comparative to the retinol and is also called Vitamin-A2.
  3. Retinoic acid (an active form of vitamin-A) obtains by the oxidation of retinaldehyde through the enzymatic activity of retinal dehydrogenase. Unlike retinal, it cannot be interconverted into either retinal or retinol and participates in gene expression like a steroid hormone.


It is another class of vitamin-A, which includes α, β and γ carotenes and is also called Provitamin-A carotenes. Among these, β-carotene is the most common form that serves as a precursor of vitamin-A. The oxidation of β-carotene forms two molecules of retinol.

Key functions of vitamin-AMajor sources of vitamin-AToxicity of vitamin-A causesDeficiency of vitamin-A
Vision maintenanceFishHypervitaminosisNight blindness
Immune healthEgg yolkLiver damageXeropthalmia or eye dryness
Cell growth and developmentMilkReduced bone densityHyperkeratosis
Hair growthCod liver cellsHair loss problems
Fetal developmentMango

Sources: Vitamin-A can obtain from different sources like:
Vegetable sources: Carrots, pumpkin, kale, spinach, asparagus, pepper etc.
Fruits: Apricots, peaches, mango, squash etc.
Animal source: Fish, meat, milk, egg yolk, butter, cheese etc.

Functions: Retinol functions to create visual pigments in the eye and maintains overall eye health. Along with zinc, vitamin-A can reduce the chances of macular degeneration in the old population. β-carotene is also a form of vitamin-A that functions in skin repairing, as it has an antioxidant property that keeps our skin glow and healthy.

Deficiency: A person with a vitamin-A deficiency may concur with refractive illnesses like night-blindness (in children) and xeropthalmia (in an old age group).


It acts as hormone precursors. The dermis and epidermis layer of the skin gives vitamin-D3, cholecalciferol, or calciol derives non-enzymatically from 7-dehydrocholestrol on exposure to sunlight. Further, it is transported to the liver via transcalciferin and converted to 1, 25- dihydroxycholecalciferol through different enzyme systems present in the liver. 1, 25- dihydroxycholecalciferol regulates calcium levels in the intestine, kidney and bones and participates in gene expression. UV-breakdown of ergosterol gives vitamin-D2, ergocalciferol or ercalciol.

Key functions of vitamin-DMajor sources of vitamin-DToxicity of vitamin-D causesDeficiency of vitamin-D
Bone strengtheningMilkHypercalcimiaRickets
Maintenance of immune healthFortified grainsOsteomalacia or bone softening

Sources: Vitamin-D can obtain from different sources like:
Vegetable sources: Mushrooms, spinach etc.
Fruits: Orange, strawberries, banana etc.
Animal source: Fish, meat, milk, butter, cheese, cod liver oil etc.
Other sources: Whole grains, dry fruits, sunlight etc.

Functions: Vitamin-D performs a vital role in the homeostasis of calcium and phosphorus in the blood. Besides this, it also promotes bone health and strengthens the immune system. It reduces the risk of multiple sclerosis, heart disease, flu etc.

Deficiency: A person with a vitamin-D deficiency may concur with bone weakness like rickets, osteomalacia or osteoporosis in children and adults, respectively.


It occurs in eight different naturally occurring tocopherols, among which α-tocopherol is the most common and active form. The primary function of vitamin E acts as an antioxidant to prevent the cell components from oxidative damage by the free radicles and against premature ageing.

Key functions of vitamin-EMajor sources of vitamin-EToxicity of vitamin-E causesDeficiency of vitamin-E
Antioxidant propertyMilkBlood thinning effectsMuscle weakness
Fortified grainsOxidative stress due to its pro-oxidant propertyPoor reflexes
Green leafy vegetablesFertility disorders
Dry-fruitsPoor immune system

Sources: Vitamin-E can obtain from different sources like:
Vegetable sources: Cabbage, tomatoes, pepper, olives, kale, spinach etc.
Fruits: Papaya, squash, apricots, avocado, kiwi, pomegranate etc.
Animal source: Fish, meat, cod-liver oil etc.
Other sources: Nuts, cold-pressed plant oils, coconut oil etc.

Functions: Vitamin E mainly shows an antioxidant property that protects the cell against free radicals and strengthens immune health to fight off infection. Apart from this, vitamin E promotes muscle or cell repair and also reduces digestive problems. It also plays a key role in producing the prostaglandin hormone that maintains blood pressure levels and muscle contraction.

Deficiency: A person with a vitamin-E deficiency may concur with muscle weakness, anaemia, neurodegenerative disorders, poor vision etc.


It exists in different forms, such as vitamin-K1 and vitamin-K2. Vitamin-K1 is naturally present in plants as phylloquinone, whereas vitamin-K2 is present in intestinal bacterial flora as menaquinone. It participates in the post-translational modifications of various blood clotting factors and helps in blood coagulation.

Key functions of vitamin-KMajor sources of vitamin-KToxicity of vitamin-K causesDeficiency of vitamin-K
Promotes blood clottingGreen leafy vegetablesNot identifiedReduced bone density
Supports bone healthMilkBlood hemorrhage
Prevents calcification of blood vesselsGrapes

Sources: Vitamin-K can obtain from different sources like:
Vegetable sources: Cabbage, turnip, broccoli, lettuce, kale, beans, asparagus, cauliflower, corn etc.
Fruits: Avocado, kiwi, green apple, grapes, strawberries etc.
Animal source: Fish, meat, chicken, cod-liver oil etc.
Other sources: Olive oil, wheat, soybeans, oats etc.

Functions: Vitamin-K performs a key role in blood clotting, prevent excessive bleeding, promotes bone health, wound healing property etc.

Deficiency: A person with a vitamin-K deficiency may concur with the reduced bone density, blood haemorrhage etc. Warfarin is an anticoagulant that is a structural analogue of vitamin-K that hinders the carboxylation of prothrombin.

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