Fat-soluble vitamins

There are two types of vitamins including fat soluble and water soluble, which affects their functions in human body. Vitamins A, D, E, and K are called the fat-soluble vitamins, because they are soluble in organic solvents and are absorbed and transported in a manner similar to that of fats.

Fat-soluble vitamins play integral roles in a multitude of physiological processes such as vision, bone health, immune function, and coagulation.

The body absorbs these vitamins as it does dietary fats. They do not dissolve in water. Despite structural differences between fat-soluble vitamins, they are absorbed and transported similarly due to their low solubility in hydrophilic media.

After absorption into enterocytes, fat-soluble vitamins become packaged into chylomicrons, which then get secreted into the lymphatic system before entering the bloodstream. Chylomicrons are needed to absorb fat-soluble vitamins and carry fats and cholesterol from the small intestine into the bloodstream. Chylomicrons are metabolized by lipoprotein lipase, which causes the release of fat-soluble vitamins into tissues for use and storage.

While water-soluble vitamins are quickly absorbed with the excess being released, fat-soluble vitamins are slower to dissolve, and the excess is stored in the liver. This means that excessive amounts of fat-soluble vitamin supplements can cause problems such as vitamin toxicity.

Human body can get deficiencies in fat-soluble vitamins when the fat intake is too low or if body cannot absorb it. Some drugs (weight-loss medications) and certain diseases (cystic fibrosis) can cause these problems.
Fat-soluble vitamins

Fat soluble vitamin: Vitamin A

Vitamin A is best known for its role in vision, but it is also crucial for proper growth, reproduction, immunity and cell differentiation.

The term vitamin A is generally used to refer to a group of compounds that possess the biological activity of all-trans retinol.

Vitamin A is normally transported in the blood linked to a specific protein, retinol binding protein (RBP). 

Specific proteins on cell surfaces and within cells are also involved with intracellular transport of the vitamin. 

Vitamin A is fat soluble and is primarily stored in the liver, where RBP is synthesized. In a well nourished person, vitamin A stores are generally sufficient to last many months on a vitamins A-deficient diet before signs of deficiency appear.

The initial symptoms of vitamin A deficiency are night blindness and keratinization of hair follicles.

Continued deficiency leads to damage to eye tissue and irreversible blindness.

The body uses three active forms of vitamin A, known collectively as the retinoids. These compounds are retinol, the alcohol form of vitamin A; retinal, the aldehyde form of vitamin A; and retinoic acid, the acid of vitamin A.

Retinol is the active form of vitamin A and has a yellow color. Carotene is the precursor of retinal and is a bright orange.

The US recommended Daily Allowance (RDA) of vitamin A for adults is 5000 IU (1000 retinol equivalents).

Some carotenoids (found in deep-yellow and dark green vegetables) can be converted to vitamin A during digestion.

In the US diet, approximately half of the vitamin A activity is derived from B-carotene and other carotenoids.

Vitamin A is found primarily in selected foods if animal origin, especially liver; dairy products (cheese, milk, and butter); eggs, fish; such tuna, sardines and herring, fish oil such as cod liver.
Fat soluble vitamin: Vitamin A