Vitamins (nutrition)

Vitamins: Classification, Functions, and Nutritional Importance

Introduction to Vitamins

Vitamins are essential nutrients required by the body in small amounts to support growth, reproduction, and overall health. Unlike macronutrients (carbohydrates, proteins, and fats), vitamins do not provide energy (calories) but are crucial for maintaining physiological functions and preventing deficiency diseases.

• Definition: Organic compounds needed in small amounts for normal body functioning.

• Discovery: Many vitamins were identified less than 100 years ago.

• Deficiency: Lack of specific vitamins leads to characteristic deficiency symptoms (e.g., vitamin C deficiency causes scurvy; vitamin A deficiency causes night blindness).

• Toxicity: Excessive intake of certain vitamins can cause adverse effects (hypervitaminosis).

Classification of Vitamins

Vitamins are classified based on their solubility:

• Fat-soluble vitamins: Vitamins A, D, E, and K

• Water-soluble vitamins: B-complex vitamins and vitamin C

Fat-Soluble Vitamins

Fat-soluble vitamins are absorbed with dietary fat and stored in the body's fatty tissues and liver. They are released as needed, especially when dietary intake is insufficient.

• Absorption: Requires the presence of dietary fat.

• Storage: Liver is the main depot for vitamin A (and some E and K); muscle stores vitamin D.

• Toxicity risk: Because they are stored, excessive intake (especially of vitamins A and D) can lead to toxicity.

Water-Soluble Vitamins

Water-soluble vitamins are absorbed directly into the bloodstream, primarily in the upper small intestine (except vitamin B12, which is absorbed in the lower portion). They are not stored in significant amounts, so regular intake is necessary.

• Absorption: With water, directly into the bloodstream.

• Excretion: Excess amounts are excreted in urine, reducing the risk of toxicity but increasing the risk of deficiency if intake is inadequate.

• Examples: B-complex vitamins (B1, B2, B3, B5, B6, B7, B9, B12) and vitamin C.

Comparison Table: Fat-Soluble vs. Water-Soluble Vitamins

Property	Fat-Soluble Vitamins (A, D, E, K)	Water-Soluble Vitamins (B-complex, C)
Absorption	With dietary fat, into lymph, then blood	Directly into blood with water
Transport	Require protein carriers	Travel freely in water-filled body fluids
Storage	Stored in liver and fatty tissues	Not stored in significant amounts
Excretion	Less readily excreted	Excess excreted in urine
Toxicity Risk	Higher (especially with supplements)	Lower, but possible with high-dose supplements
Frequency Needed	Periodic (weeks or months)	Frequent (every 1–3 days)

Key Points and Examples

• Fat-soluble vitamins: Can accumulate in the body, so toxicity is a concern with excessive supplementation.

• Water-soluble vitamins: Deficiency can develop quickly if intake is inadequate, as they are not stored.

• Example: Vitamin A is stored in the liver and can cause toxicity if consumed in excess, while vitamin C is excreted in urine and must be consumed regularly.

Summary Table: Main Roles, Deficiency Symptoms, and Food Sources

Vitamin	Main Roles	Deficiency Symptoms	Food Sources
Vitamin A	Vision, immune function, cell growth	Night blindness, xerophthalmia	Liver, eggs, fortified milk, orange/yellow vegetables
Vitamin D	Calcium absorption, bone health	Rickets (children), osteomalacia (adults)	Fortified milk, fatty fish, sunlight exposure
Vitamin E	Antioxidant, protects cell membranes	Nerve and muscle damage (rare)	Vegetable oils, nuts, seeds, green leafy vegetables
Vitamin K	Blood clotting, bone health	Bleeding disorders (rare)	Green leafy vegetables, vegetable oils
B Vitamins	Energy metabolism, red blood cell formation	Varies (e.g., anemia, neurological symptoms)	Whole grains, meats, dairy, legumes
Vitamin C	Collagen synthesis, antioxidant, immune support	Scurvy (bleeding gums, poor wound healing)	Citrus fruits, berries, peppers, green vegetables

Additional info:

• Vitamins are sensitive to heat, light, and oxygen, so food preparation methods can affect their content.

• Bioavailability refers to the proportion of a nutrient that is absorbed and utilized by the body.

• Some vitamins (e.g., vitamin K) can be synthesized by intestinal bacteria, but dietary sources are still important.