Fat Soluble Vitamins Overview

Fat Soluble Vitamins

• Vitamins are organic nutrients needed in small amounts for metabolism, growth, reproduction, and tissue maintenance.
• Fat-soluble vitamins are absorbed and transported like lipids (bound to proteins).
• Fat-soluble vitamins are stored mostly in the liver.
• Manifestation of deficiencies is slower.
• Excesses are not excreted, and toxic levels are more likely to develop.
• These vitamins are required periodically.
• Examples of fat soluble vitamins are A, D, E, and K.

Vitamin A

• Vitamin A exists in 3 active forms: Retinol, Retinal, and Retinoic acid.
• These are collectively called retinoids.
• Retinoids are found in animal tissues.
• Carotenoids, found in plants, function as provitamin A.
• Carotenoids can be broken down to produce retinol.
• Beta carotene in dietary sources converts to retinol.
• Bile and dietary fat are needed for the absorption of retinol.
• Vitamin A is stored in the liver and adipose tissue.

Role of Vitamin A in Metabolism

• Vision is regulated by cis-retinal regeneration via isomerization.
• Retinoic acid and retinol behave like steroid hormones, regulating epithelial cell growth and differentiation.
• Retinol is also essential for maintaining reproduction.
• Carotenoids function as antioxidants and may decrease cancer risk.

Vitamin A Sources

• Preformed vitamin A is in animal products like liver, fish, eggs, and dairy.
• Provitamin A carotenoids are found in plants like carrots, spinach, broccoli, and other dark green/yellow vegetables and fruits.

Vitamin A Deficiency

• Defective night vision (nyctalopia) is a sign of Vitamin A deficiency.
• Severe deficiency causes xerophthalmia (drying of the conjunctiva and cornea) and keratomalacia (resulting in blindness).

Vitamin A Toxicity

• Taking large amounts of preformed vitamin A or supplements for extended periods can be toxic.
• Pregnant women should avoid excessive Vitamin A intake. Excessive intake during pregnancy can cause congenital malformations in the developing fetus.

Vitamin D

• Vitamin D exists in 2 forms: D2 (Ergocalciferol) found in plant-based foods, and D3 (Cholecalciferol) found in animal-based tissues.

Vitamin D Synthesis

• With the help of UV light, 7-dehydrocholesterol in the skin can be converted to cholecalciferol.

Vitamin D Activation

• Cholecalciferol undergoes activation in two stages:
  • 25-hydroxylation in the liver.
  • 1α-hydroxylation in the kidney.
• The final active form of vitamin D is 1,25-dihydroxycholecalciferol (also called calcitriol).

Vitamin D Role

• Vitamin D promotes the formation of the protein calbindin in intestinal mucosal cells, aiding calcium and phosphorus absorption.
• Increases calcium and phosphorus reabsorption in kidney tubules.
• Supports healthy calcium levels and bone growth.
• Growing children require higher vitamin D levels than adults.

Vitamin D Food Sources

• Egg yolks, fatty fish and fish liver oils, butter, fortified milk.

Vitamin D Deficiency

• Children's deficiency results in rickets. Symptoms include weak, soft, and bent bones (bow legs, knock-knees, pigeon chest, rickety rosary, bossing of frontal bones) due to insufficient bone mineralization. It may also cause growth retardation.
• Adult deficiency results in osteomalacia, increasing susceptibility to fractures.

Vitamin D Toxicity

• Long-term use of vitamin D supplements can lead to calcium deposits in soft tissues (e.g., hardening of arteries) and kidney stones.

Vitamin E

• Vitamin E, or tocopherols, act as antioxidants.
• Inactivates reactive oxygen species like superoxide, peroxide, and hydroxyl radicals, protecting lipids, membranes, and DNA from oxidative damage.

Vitamin E Food Sources

• Nuts and peanuts, plant oils (sunflower, safflower, corn, soybean).
• Wheat germ oils.
• Cooking and freezing can destroy vitamin E.
• Stored in adipose tissue.

Vitamin E Deficiency

• Rare; associated with fat malabsorption diseases.

Vitamin K

• Vitamin K exists in 3 forms:
  • K1 (Phylloquinone), found in green vegetables.
  • K2 (Multiprenylmenaquinone), synthesized by intestinal bacteria.
  • K3 (Menadione), a synthetic water-soluble form.

Role of Vitamin K

• Plays a key role in blood clotting (coagulation) as a coenzyme.
• Activates clotting factors II, VII, IX, and X through carboxylation.
• The anticoagulant drugs Warfarin and Dicumarol inhibit the maturation of vitamin K-dependent clotting proteins.

Vitamin K Food Sources

• Green leafy vegetables (spinach, asparagus), cabbage family, carrots, liver, and eggs.
• Bacteria in the small intestine synthesize vitamin K.

Vitamin K Deficiency

• Rare in adults but can be caused by long-term antibiotic treatment or fat malabsorption.
• In newborns, deficiency can cause bleeding disorders. Newborn infants often get a vitamin K shot at birth to prevent these problems.

This quiz explores the essential aspects of fat-soluble vitamins, focusing on their types, absorption, storage, and roles in metabolism. Special emphasis is placed on Vitamin A, including its active forms and dietary sources. Test your knowledge on the critical functions and characteristics of these important nutrients.