VITAMINS : The Micro-Nutrients in Our Body

Vitamins are essential organic micronutrients required in very small amounts for normal metabolism, growth, and health. They are grouped under biomolecules along with carbohydrates, proteins, and lipids, but unlike macronutrients, they do not provide energy directly.

In biochemistry, the most important vitamin classification is based on solubility into fat‑soluble and water‑soluble vitamins. Understanding this vitamin classification helps students remember sources, functions, RDA, and deficiency diseases, which is very useful for exam‑oriented vitamin notes and short notes on vitamins.

This article provides structured notes on vitamins for MBBS, BSc, B.Pharm, and nursing students, including classification of vitamins, fat‑soluble and water‑soluble vitamin charts and tables, functions, and clinical deficiency features.

Definition of Vitamins

A vitamin is an organic compound required in small quantities in the diet because it cannot be synthesized in adequate amounts by the human body. Most vitamins act as coenzymes or as precursors of coenzymes in numerous biochemical reactions.

From a biochemistry perspective, vitamins are vital micronutrients that participate in energy production, redox reactions, cell growth, tissue repair, and immune function. Vitamin notes in biochemistry, therefore, focus on their chemistry, active coenzyme forms, metabolic roles, recommended dietary allowances, and clinical manifestations of deficiency or toxicity.

General Features of Vitamins

• Vitamins are micronutrients required in microgram or milligram quantities per day.
• They are generally not synthesized in sufficient amounts in humans and must be obtained from the diet or supplements.
• Based on solubility, vitamins are classified into fat‑soluble and water‑soluble groups.
• Fat‑soluble vitamins are stored in the liver and adipose tissue; water‑soluble vitamins are not significantly stored, and excess is excreted in urine.
• Because fat‑soluble vitamins are stored in the body, chronic excessive intake can cause toxicity, while deficiency usually develops more slowly than for water‑soluble vitamins.

These general points are important for vitamin short notes and for answering “How are vitamins classified?” type questions in examinations.

Classification of Vitamins

The most widely used vitamin classification in biochemistry is based on their solubility. Vitamins can be divided into two major categories:

1. Fat-Soluble Vitamins: These are vitamins A, D, E, and K. These vitamins are stored in the liver and fatty tissues until required. As such, they can be harmful if too much is taken in.
2. Water Soluble Vitamins: These are B-complex vitamins (B1, B2, B3, B5, B6, B7, B9, B12) and vitamin C. These vitamins are not stored in the body. As such, they are required more frequently than fat-soluble vitamins.

This classification of vitamins is crucial because absorption, transport, storage, deficiency patterns, and toxicity differ between these two vitamin classes. When you write notes on vitamins or short notes, always start with this basic classification and then expand into individual vitamins.

Fat-Soluble Vitamins

Fat‑soluble vitamins include vitamins A, D, E and K (often remembered by the short form ADEK). These vitamins dissolve in dietary fat, require normal fat digestion and bile for absorption, and are transported in association with lipoproteins.

Because they are stored in the liver and adipose tissue, fat‑soluble vitamins can supply body needs for longer periods, but they also have a higher risk of toxicity if taken in large amounts for a long time. In the biochemistry of vitamins, the key roles of fat‑soluble vitamins are vitamin A in vision and epithelial integrity, vitamin D in calcium and bone metabolism, vitamin E as an antioxidant, and vitamin K in blood coagulation.

Vitamin A

• Vitamin A (retinol) is required for the formation of rhodopsin, a photoreceptor pigment in the retina. Vitamin A helps to maintain epithelial tissues. Usually, the liver stores 90% of the body’s vitamin A.
• The body releases it into the circulation bound to pre-albumin (transthyretin) and retinol-binding protein. β-carotene and other provitamin carotenoids, contained in green, leafy, and yellow vegetables and deep- or bright-colored fruits, are converted to vitamin A.
• Carotenoids are absorbed better from vegetables when they are cooked or homogenized and served with some fats or oils. The Recommended Dietary Allowance (RDA) of vitamin A is 900 micrograms for men and 700 micrograms for women.
• Deficiency impairs immunity and causes skin rashes and common ocular effects such as dry eyes and night blindness.

Vitamin D

• Vitamin D has two primary forms: D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D3 is synthesized in the skin by exposure to sunlight (ultraviolet radiation) and obtained in the diet, chiefly in fish liver oils and egg yolks.
• In some developed countries, milk and other foods are fortified with vitamin D. Human breast milk is low in vitamin D, containing an average of only 10% of the amount in fortified cow’s milk.
• Vitamin D is a prohormone with several active metabolites that act as hormones. The liver metabolizes vitamin D3 to 25(OH)D, which is then converted by the kidneys to 1,25(OH)2D (1,25-dihydroxycholecalciferol, calcitriol, or active vitamin D hormone).
• 25(OH)D, the primary circulating form, has some metabolic activity, but 1,25(OH)₂D is the most metabolically active. Inadequate exposure to sunlight may cause vitamin D deficiency.
• Deficiency impairs bone mineralization, causing rickets in children and osteomalacia in adults, and may contribute to osteoporosis.
• The current recommendations from the Institute of Medicine are 200 IU/day from birth through age 50, 400 IU for those aged 51 to 70, and 600 IU for those over 70 years.
• These recommendations were established by determining the level of vitamin D that was enough to prevent bone demineralization or rickets.
• The safe, tolerable upper intake level of vitamin D is 10,000 IU/day.
• Randomized trials using the currently recommended intake of 400 IU of vitamin D per day have shown no appreciable reduction in fracture risk. In contrast, experiments using 700-800 IU of vitamin D per day found lower fracture incidence. Adults should be consuming at least 1000 IU per day of vitamin D to maintain blood serum levels that are effective for strengthening the bones.

Vitamin E

• Vitamin E is a group of compounds (including tocopherols and tocotrienols) that have similar biologic activities. The most biologically active is α-tocopherol, but β-, γ-, and δ-tocopherols also have essential biologic activity.
• These compounds act as antioxidants, which prevent lipid peroxidation of polyunsaturated fatty acids in cellular membranes. Plasma tocopherol levels vary with the total plasma lipid levels. Normally, the plasma α-tocopherol level is 5 to 20 μg/mL. Dietary vitamin E deficiency is common in developing countries.
• Vitamin E deficiency causes degeneration of the axons of neurons (nerve cells), resulting in neurologic deficits and fragility of red blood cells, which are diagnosed with hemolytic anemia.
• Taking vitamin E supplements is not recommended because studies have found an increased risk of heart failure and general mortality. Vitamin E is found in spinach, watercress, mustard greens, and many other green leafy vegetables.
• Good sources of vitamin E are oily plant seeds such as peanuts and sunflower kernels.

Vitamin K

• Vitamin K1 (phylloquinone) is dietary vitamin K. Dietary fat enhances its absorption. Infant formulas contain supplemental vitamin K.
• Vitamin K2 refers to a group of compounds (menaquinones) synthesized by bacteria in the intestinal tract; the amount synthesized does not satisfy the vitamin K requirement.
• The Recommended Dietary Allowance (RDA) is 120 micrograms for men and 90 micrograms for women. Vitamin K controls the formation of coagulation factors II (prothrombin), VII, IX, and X in the liver.
• In healthy adults, dietary vitamin K deficiency is uncommon because vitamin K is widely distributed in green vegetables such as kale, spinach, and mustard greens. The bacteria of the healthy gut also synthesize menaquinones.

Deficiency and Toxicity of Fat‑Soluble Vitamins

Deficiency of fat‑soluble vitamins occurs in conditions such as prolonged low‑fat diet, fat malabsorption, chronic liver disease, pancreatic disorders, and obstructive jaundice. The most important deficiency manifestations include:

• Vitamin A: night blindness, xerophthalmia, Bitot’s spots, and keratomalacia.
• Vitamin D: rickets in children, osteomalacia, and osteoporosis in adults.
• Vitamin E: hemolytic anemia, peripheral neuropathy in severe deficiency.
• Vitamin K: hemorrhagic disease of the newborn, easy bruising, and prolonged bleeding time.

Because fat‑soluble vitamins are stored in the body, chronic intake above recommended levels may lead to toxicity, especially with vitamins A and D. Students should remember both deficiency and toxicity aspects when writing fat‑soluble vitamin notes or short notes.

Water-Soluble Vitamins

Water‑soluble vitamins include all the B‑complex vitamins and vitamin C. These vitamins are soluble in water and are not stored in large amounts in the body, with the exception of vitamin B12, which is stored in the liver.

Excess water‑soluble vitamins are usually excreted in urine, so deficiency can develop more rapidly when dietary intake is poor.

The major roles of water‑soluble vitamins include energy production, amino acid metabolism, DNA synthesis, and antioxidant protection.

This section is useful for “water‑soluble vitamins notes,” “water‑soluble vitamins and their functions,” and “water‑soluble vitamin deficiency diseases.”

Thiamin (Vitamin B1)

• Thiamine, or thiamine (vitamin B1), is widely available in the diet. Thiamine is involved in carbohydrate, fat, amino acid, glucose, and alcohol metabolism.
• It is essentially nontoxic. Thiamine deficiency (causing beriberi) is most common among people subsisting on highly refined rice or other carbohydrates in developing countries.
• Bean sprouts, brewer’s yeast, and fortified cereals are excellent sources of thiamine.

Riboflavin (Vitamin B2)

• Riboflavin (Vitamin B2) is involved in carbohydrate metabolism as an essential coenzyme in many oxidation-reduction reactions. Riboflavin is essentially nontoxic.
• Riboflavin deficiency usually occurs with other B-vitamin deficiencies. Symptoms and signs include a sore throat, lesions of the lips and mucosa of the mouth, glossitis, conjunctivitis, seborrheic dermatitis, and normochromic-normocytic anemia.
• Riboflavin is found in mushrooms, yeast, and meats such as beef, pork, and lamb.

Niacin (Vitamin B3)

• Niacin (Vitamin B3 or nicotinic acid) derivatives include nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), which are coenzymes in oxidation-reduction reactions vital in cell metabolism.
• Dietary niacin deficiency causes pellagra, a disease characterized by dermatitis, gastrointestinal disorders, and mental disturbances.
• Primary deficiency results from extremely inadequate intake of both niacin and the amino acid tryptophan, which usually occurs in areas where maize (Indian corn) constitutes a substantial part of the diet.
• Mushrooms and fish are good sources of niacin.

Pantothenic Acid (Vitamin B5)

• Pantothenic acid (Vitamin B5) is widely distributed in foods, and high amounts are found in whole grain cereals, legumes, eggs, and meat.
• Pantothenic acid is needed to form coenzyme-A (CoA) and is critical in the metabolism and synthesis of carbohydrates, proteins, and fats.
• Adults require about 5 mg/day. Pantothenic acid is found in mushrooms, yeast, and the liver.

Pyridoxin (Vitamin B6)

• Vitamin B6 includes a group of closely related compounds: pyridoxine, pyridoxal, and pyridoxamine.
• They are metabolized in the body into pyridoxal phosphate, which acts as a coenzyme in many essential reactions in the blood, central nervous system, and skin metabolism.
• Vitamin B6 is essential in the biosynthesis of heme and nucleic acid, as well as in lipid, carbohydrate, and amino acid metabolism.
• Vitamin B6 is found in a variety of vegetables and meats. Many breakfast portions of cereal are fortified with vitamin B6.
• Some natural sources of Vitamin B6 are brewer’s yeast, Chinese cabbage (Pak-Choi), and red and green peppers.

Biotin (Vitamin B7)

Biotin (Vitamin B7) acts as a coenzyme for carboxylation reactions essential to fat and carbohydrate metabolism. An adequate intake for adults is 30 μg/day. Liver, egg yolks, green vegetables, and whole grains are rich sources of biotin.

Folic Acid (Vitamin B9)

• Folate, also called Vitamin B9, is involved in the maturation of red blood cells and the synthesis of purines and pyrimidines, which, in folic acid, are required for the development of the fetal nervous system.
• Adequate folic acid intake before conception and throughout the first trimester of pregnancy helps prevent specific brain and spinal cord defects such as spina bifida.
• Folate is absorbed in the duodenum and upper jejunum. The US recommended daily dose of folate is 400 μg and the upper limit is 1000 μg. Folate is essentially nontoxic.
• Deficiency produces megaloblastic anemia indistinguishable from that due to vitamin B12 deficiency. A lack of folates in old age significantly increases the risk of developing dementia.
• Folic acid is found in dried peas, dried beans, yeast, and leafy green vegetables such as spinach, endive, lettuce, and mustard greens.

Vitamin B12 (Cyanocobalamin)

• Cobalamin is a general term for compounds with biologic vitamin B12 activity. These compounds are involved in nucleic acid metabolism, methylation, and myelin synthesis and repair.
• They are necessary for the formation of healthy red blood cells. Vitamin B12 is released in the stomach’s acid environment and is bound to R protein. Pancreatic enzymes cleave the B12-R protein complex in the small intestine. After cleavage, intrinsic factor, secreted by parietal cells in the gastric mucosa, binds to vitamin B12.
• Intrinsic factors are required for the absorption of vitamin B12, which takes place in the terminal ileum. The Recommended Dietary Allowance (RDA) is 2.4 micrograms, which is the amount found in 3 ounces (85 grams) of meat. Vitamin B12 is found in clams, oysters, turkey, chicken, beef, and pork.
• Dietary vitamin B12 deficiency usually results from inadequate absorption, but deficiency can develop in vegans who do not take vitamin supplements. Deficiency causes megaloblastic anemia, damage to the white matter of the spinal cord and brain, and peripheral neuropathy, which is characterized by tingling or numbness in the hands or feet.

Vitamin C

• Vitamin C (ascorbic acid) plays a role in collagen, carnitine, hormone, and amino acid formation. It is essential for wound healing and facilitates recovery from burns.
• Vitamin C is also an antioxidant, supports immune function, and facilitates the absorption of iron.
• In developed countries, deficiency can occur with general undernutrition, but severe deficiency (causing scurvy) is uncommon. Symptoms of deficiency include fatigue, depression, and connective tissue defects such as gingivitis, rash, internal bleeding, or impaired wound healing.
• The Recommended Dietary Allowance (RDA) is 75 milligrams for women, 90 milligrams for men. The tolerable upper intake level of vitamin C is approximately 2 grams (2000 mg) per day.
• Higher amounts can cause stomach upset and diarrhea. Vitamin C is found in fresh fruits and vegetables. Citrus fruits like oranges and lemons are good sources of vitamin C.

Vitamins as Biomolecules in Metabolism

Vitamins are important biomolecules because they act as coenzymes or coenzyme precursors in many metabolic pathways. Without their coenzyme forms, many enzymes of carbohydrate, lipid and protein metabolism cannot function efficiently.

For example, thiamine (vitamin B1) is required for oxidative decarboxylation of pyruvate and α‑ketoglutarate, while niacin (vitamin B3) and riboflavin (vitamin B2) provide coenzymes for redox reactions in the electron transport chain. This is why your biomolecules vitamins notes should always link each vitamin to its corresponding enzyme reactions and metabolic pathways.

What is the basic difference between fat-soluble and water-soluble vitamins?

• Vitamins can be grouped into two categories, fat-soluble and water-soluble. Fat-soluble vitamins are dependent upon fats for absorption into the bloodstream.
• They are stored in fat cells found in the liver, kidneys, and wherever else fat tissue is found. Because they can be stored, fat-soluble vitamins don’t have to be ingested daily.
• In fact, taking more fat-soluble vitamins than the body needs can be harmful. This usually occurs when someone is taking vitamin supplements.
• To avoid taking toxic levels of fat-soluble vitamins, do not exceed the Recommended Dietary Allowances (RDA) which are listed on vitamin bottles.
• The RDA for nutrients is published by the Food and Nutrition Board of the National Research Council.
• Fat-soluble vitamins include vitamins A, D, E, and K. Water-soluble vitamins include Vitamins C, B12, and B complex vitamins.
• Except for B12, the body stores water-soluble vitamins for a very short period of time.
• The body rids itself of excess amounts through urination, so water-soluble vitamins must be ingested daily, according to the suggested RDA.

Summary of the Vitamins

• Vitamins are essential organic micronutrients that must be supplied in the diet because they are not synthesized sufficiently in the human body.
• Vitamins classification is mainly based on solubility: fat‑soluble vitamins (A, D, E, K) and water‑soluble vitamins (B‑complex and C).
• Fat‑soluble vitamins are stored in the liver and adipose tissue, have a higher risk of toxicity and are important in vision, bone health, antioxidant defense and blood clotting.
• Water‑soluble vitamins are poorly stored (except B12), excess is excreted in urine, and they function mainly as coenzymes in energy metabolism, amino acid metabolism and collagen synthesis.
• For exam answers and notes on vitamins, always mention classification, sources, functions, RDA, deficiency and any important toxicity features.