Vitamins: Overview, types, functions, clinical points

Vitamins are organic compounds needed in very small amounts to keep metabolism, growth, and tissue repair running smoothly. They are grouped mainly by how they dissolve and behave in the body: water‑soluble vitamins and fat‑soluble (lipid‑soluble) vitamins.

Water‑soluble vitamins:

Water‑soluble vitamins dissolve in water, enter the bloodstream directly from the intestine, are not stored in large amounts, and excess is usually excreted in urine. Because body stores are small, they must be supplied regularly in the diet, and toxicity is uncommon compared with fat‑soluble vitamins

Members of the group:

Water‑soluble vitamins include the entire B‑complex and vitamin C:​

• B1 (thiamine)
• B2 (riboflavin)
• B3 (niacin)
• B5 (pantothenic acid)
• B6 (pyridoxine)
• B7 (biotin)
• B9 (folate/folic acid)
• B12 (cobalamin)
• Vitamin C (ascorbic acid)

General biochemical roles:

Most B‑complex vitamins act as coenzymes or parts of coenzymes in energy‑yielding pathways of carbohydrate, fat, and protein metabolism. Vitamin C functions mainly as a reducing agent and antioxidant and is essential for collagen synthesis, immune defense, and iron absorption.

​Absorption, storage, and excretion:

Water‑soluble vitamins are absorbed from the small intestine through specific transporters or diffusion directly into portal blood. With the important exception of vitamin B12, they are stored only in small quantities; daily or frequent intake is therefore necessary to prevent deficiency. Surplus amounts that exceed tissue needs are filtered by the kidneys and eliminated in urine, which is why high intakes usually produce little toxicity but can cause characteristic changes such as bright‑yellow urine with excess riboflavin

Dietary sources (overview):

Water‑soluble vitamins are widely distributed in foods.​

• B‑complex vitamins are abundant in whole grains, pulses, meat, fish, eggs, dairy, green leafy vegetables, and fortified cereals.​
• Vitamin C is rich in citrus fruits, guava, berries, tomatoes, peppers, and many fresh vegetables.​
  Vitamin B12 is an exception because meaningful amounts occur only in animal‑derived foods and fortified products, making vegans particularly vulnerable to deficiency.

Clinical points:

Deficiency of water‑soluble vitamins typically produces symptoms within weeks to months because stores are limited. Classic examples include beriberi (B1), pellagra (B3), megaloblastic anemia (folate, B12), peripheral neuropathy (B1, B6, B12), and scurvy (vitamin C). Treatment usually involves correcting the diet and using oral supplements; parenteral therapy is reserved for severe deficiency or malabsorption, as in vitamin B12 deficiency with intrinsic‑factor loss.

Fat‑soluble (lipid‑soluble) vitamins:

Fat‑soluble vitamins dissolve in lipids, are absorbed together with dietary fat, and can be stored in the liver and adipose tissue. Because they accumulate, deficiency develops slowly but excessive intake, especially from supplements, can cause toxicity.

Members of the group:

There are four fat‑soluble vitamins:​

• Vitamin A (retinoids and carotenoids)
• Vitamin D (D2 and D3)
• Vitamin E (tocopherols and tocotrienols)
• Vitamin K (K1, K2)

Absorption, transport, and storage:

Fat‑soluble vitamins are absorbed in the small intestine along with dietary lipids, incorporated into chylomicrons, and transported via the lymphatic system to the bloodstream. Bile salts and normal fat digestion are essential; disorders such as cholestasis, pancreatic insufficiency, or intestinal fat malabsorption (e.g., celiac disease) often produce combined deficiencies of A, D, E, and K. After delivery to the liver, these vitamins are stored or bound to specific transport proteins and distributed to tissues; for example, vitamin A is stored as retinyl esters and vitamin D as calcidiol.

Overview of functions:

Each fat‑soluble vitamin has distinct but overlapping roles:​

• Vitamin A: vision (especially night vision), epithelial integrity, immune function, and reproduction.
• Vitamin D: regulation of calcium and phosphate balance, bone mineralization, and modulation of immune and endocrine pathways.
• Vitamin E: antioxidant defense, protecting cell membranes and lipoproteins from oxidative damage.
• Vitamin K: activation of clotting factors and several proteins involved in bone and vascular health.

Because of their storage, these vitamins help buffer short‑term fluctuations in dietary intake but can reach toxic levels if chronically ingested in large doses, particularly vitamins A and D.

Dietary sources (overview):

Fat‑soluble vitamins are mainly associated with fat‑containing foods.​

• Vitamin A: liver, dairy fat, egg yolk, and orange‑yellow or dark‑green vegetables rich in provitamin A carotenoids​.
• Vitamin D: fatty fish, fish‑liver oils, fortified milk and cereals, and endogenous synthesis in skin under sunlight.​
• Vitamin E: vegetable oils, nuts, seeds, and whole grains.​
• Vitamin K: leafy green vegetables, certain plant oils, and bacterial synthesis in the intestine.

Clinical points:

Fat‑soluble vitamin deficiencies often indicate chronic fat malabsorption or severe dietary restriction. Typical features include night blindness and xerophthalmia (vitamin A), osteomalacia/rickets (vitamin D), neuropathy and hemolysis (vitamin E), and bleeding tendency due to impaired coagulation (vitamin K). Management focuses on treating the underlying malabsorptive disorder, providing appropriate dietary fat, and supplementing the deficient vitamin in controlled doses to avoid toxicity.