Biochemistry Course: Fat-Soluble Vitamins

Biochemistry for Year 1 Students, Helwan National University

• Course: Biochemistry for Year 1 students
• Module: Human Body Function (HBF 102)
• Academic Year: 2024-2025
• Semester: 1
• Faculty: Faculty of Medicine, Helwan National University
• Lecturers: Dr. Seham Adel and Assoc. Prof. Dr. Ahmed M.A. Akabawy

Fat-Soluble Vitamins

• Vitamins are chemically unrelated organic compounds.
• Humans cannot synthesize fat-soluble vitamins in adequate quantities.
• Fat-soluble vitamins are obtained from the diet.
• Fat-soluble vitamins are absorbed, absorbed, and transported with the fat of the diet.
• Significant quantities of fat-soluble vitamins are stored in the liver and adipose tissue.
• Excessive consumption of vitamins A and D can lead to toxic quantities in the body.

Classification of Vitamins

• Vitamins are grouped into water-soluble and fat-soluble categories.
  • Water-soluble vitamins: Folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid.
  • Fat-soluble vitamins: Vitamins A, D, K, and E.

Vitamin A

• Vitamin A is a collective term for biologically active molecules called retinoids.
• Retinol is found in animal tissues and is involved in vision, reproduction, growth, and maintenance of epithelial tissues.
• Retinoic acid is derived from the oxidation of retinal.
• β-carotene (found in plants) is converted into retinol in the intestine, but the conversion is inefficient.
• Vitamin A is essential for normal differentiation of epithelial tissues and mucus secretion.

Vitamin A - Functions

• Visual Cycle: Vitamin A is a component of the visual pigments in rod and cone cells. Rhodopsin, a visual pigment in the rod cells, consists of 11-cis retinal bound to opsin. Light exposure causes isomerization, bleaching, and release of all-trans retinal. This process triggers a nerve impulse to the brain.
• Growth: Vitamin A deficiency leads to decreased growth rate, especially in children. Bone development may also be delayed.
• Reproduction: Vitamin A is essential for normal reproduction, spermatogenesis (in males), and preventing fetal resorption (in females). Retinoic acid acts in maintaining the reproductive system but in the visual cycle it is inactive.
• Epithelial Tissues: Vitamin A is crucial for maintenance of epithelial tissues and mucus secretion.

Vitamin A - Distribution

• Liver, kidney, cream, butter, and egg yolk are good sources of preformed vitamin A.
• Yellow and dark green vegetables, and fruits contain carotenoids, serving as vitamin A precursors.

Vitamin A - Clinical Indications

• Dietary Deficiency: Night blindness is an early sign. Severe deficiency can lead to xerophthalmia (dryness of the conjunctiva and cornea) and potentially blindness. This condition is often seen in children in developing tropical countries.
• Acne: Retinoic acid and its derivatives are effective treatments for acne and psoriasis. Topical tretinoin is often used for mild acne cases.

Vitamin A - Toxicity

• Excessive intake of vitamin A can lead to a toxic syndrome called hypervitaminosis A.
• Pregnant women should avoid excessive doses due to the potential for congenital malformations in the developing fetus.
• Isotretinoin is specifically contraindicated in women of childbearing potential, unless used for very severe, disfiguring acne cases.

Vitamin D

• The D Vitamins are a group of sterols that have hormone-like function.
• The bioactive molecule is 1,25-dihydroxycholecalciferol (1,25-diOH-D3).
• Vitamin D regulates calcium and phosphorus plasma levels.
• It increases calcium and phosphate uptake in the intestines.
• It minimizes calcium loss through the kidneys.
• It stimulates bone resorption.

Vitamin D - Distribution

• Sources of preformed vitamin D are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3).
• 7-dehydrocholesterol, an intermediate in cholesterol synthesis, is converted into cholecalciferol in the dermis and epidermis of humans exposed to sunlight.

Vitamin D - Clinical Indications

• Bone Diseases: Vitamin D deficiency in children leads to rickets, and in adults, to osteomalacia. Insufficient sunlight exposure and/or low dietary intake, especially in infants and the elderly, are risk factors.
• Other Conditions: Sufficient vitamin D levels have been correlated with fall prevention and increased muscle strength in older individuals.

Vitamin D - Toxicity

• High doses of vitamin D can lead to hypervitaminosis D.
• Symptoms include loss of appetite, nausea, thirst, and stupor.
• Hypercalcemia (high blood calcium levels), results from enhanced calcium absorption and bone resorption.
• Calcium deposits occur in organs, particularly arteries and kidneys.

Vitamin K

• Vitamin K is an important coenzyme for the maturation and activation of various blood clotting factors.
• In plants, vitamin K exists as phylloquinone (vitamin K1).
• In the intestinal bacterial flora, vitamin K exists as menaquinone (vitamin K2).
• Synthetic vitamin K is menadione.
• The activated form of clotting factors requires vitamin K.
• The prothrombin-calcium complex binds to phospholipids on platelets increasing the rate of prothrombin to thrombin conversion.

Vitamin K - Distribution

• Vitamin K is found in cabbage, kale, spinach, egg yolk, and liver.
• Extensive synthesis of vitamin K occurs in the gut bacteria.

Vitamin K - Clinical Indications

• Deficiency: Deficiency is rare due to gut bacteria production. However, antibiotics can reduce gut bacterial production, which can lead to hypoprothrombinemia, potentially causing a bleeding tendency, especially in the young and malnourished.
• Newborns: Newborns are often given an intramuscular vitamin K injection because their gut bacteria is not yet fully developed, and human breast milk provides only a small amount relative to the body's needs.

Vitamin E

• The E vitamins are collectively known as tocopherols, with α-tocopherol being the most active.
• Vitamin E acts as an antioxidant, preventing non-enzymatic oxidation of cell components such as polyunsaturated fatty acids.
• The primary function is to protect against free radical damage.

Vitamin E - Distribution

• Vegetable oils are rich sources of vitamin E.
• Liver and eggs contain moderate amounts.

Vitamin E - Deficiency/Toxicity

• Vitamin E deficiency is largely restricted to premature infants; adults are rarely deficient.
• Signs of deficiency include erythrocyte sensitivity to peroxide and abnormal cellular membranes.
• Vitamin E has very low toxicity and is safe at doses of 300 mg/day.

Test your knowledge on fat-soluble vitamins and their functions in the human body. This quiz covers their absorption, roles during growth, potential toxicity, and effects of deficiency. Ideal for students in the biochemistry course HBF-102.