Towards the end of the 19th century, scientists discovered something very important about food. Before that, people believed that a healthy diet only needed carbohydrates, fats, proteins, minerals, and water. But later, they found that our body also needs some extra nutrients in very small amounts. These nutrients were called vitamins. This discovery was important because it showed that even a small lack of these nutrients can cause serious diseases.

In 1897, Christiaan Eijkman made an important discovery. He found that a disease called beriberi could be controlled by adding rice polishings (the outer layer of rice) to people’s diet in areas where the disease was common.

Later, scientists studied these rice polishings and found that they contained a substance with nitrogen, which they thought was an amine. Because of this, they first used the word “vitamine” meaning “vital amine.” However, later research showed that not all these substances were amines, so the name was shortened to vitamins.

In simple terms, vitamins are small but very important nutrients that our body needs to stay healthy.

Definition of Vitamins

A vitamin is defined as a naturally occurring essential organic constituent of the diet which, in minute amounts, is required for the maintenance of normal metabolic activities of tissues and overall physiological function. Although required in very small quantities, vitamins play indispensable roles in regulating biochemical and physiological processes.

It is important to note that the presence of an amine group is not necessary for a substance to be classified as a vitamin.

Vitamins are generally obtained from the diet. However, certain vitamins or their precursors can be synthesized within the body to a limited extent. For example, the amino acid tryptophan can be converted into nicotinic acid (niacin). Similarly, intestinal microflora synthesize small amounts of vitamin K and some members of the B-complex vitamins, although this contribution is usually not sufficient to meet total body requirements.

Vitamin D (cholecalciferol) is unique in that it is synthesized in the skin from 7-dehydrocholesterol upon exposure to ultraviolet (UV) radiation.

Provitamins and Precursors

Some substances present in food or synthesized in the body act as provitamins, which are biologically inactive precursors converted into active vitamins after metabolism.

Examples include:

• β-carotene → Vitamin A (retinol)
• 7-dehydrocholesterol → Vitamin D3 (cholecalciferol)

These conversions are essential for maintaining adequate vitamin status in the body.

Biological Role of Vitamins

Vitamins do not serve as direct sources of energy. Instead, they function primarily as:

• Coenzymes or coenzyme precursors
• Regulators of metabolic pathways
• Essential components for enzymatic reactions

Many vitamins, especially those of the B-complex group, act after conversion into biologically active coenzyme forms. Examples include:

• Thiamine pyrophosphate (TPP) – derived from vitamin B1
• Flavin adenine dinucleotide (FAD) – derived from vitamin B2
• Nicotinamide adenine dinucleotide (NAD) – derived from niacin (B3)
• Coenzyme A – derived from pantothenic acid (B5)

Deficiency of vitamins leads to specific clinical deficiency diseases, whereas excessive intake of certain vitamins—particularly fat-soluble vitamins—may result in toxicity, a condition known as hypervitaminosis.

Classification of Vitamins

Vitamins are classified primarily on the basis of their solubility into two major groups:

1. Fat-soluble vitamins

These include:

• Vitamins A, D, E, and K

Fat-soluble vitamins are generally stored in body tissues, particularly in the liver and adipose tissue. Because of this storage capacity, they are not required in large daily amounts; however, excessive intake may lead to toxicity due to accumulation in the body.

2. Water-soluble vitamins

These include:

• Vitamin C (ascorbic acid)
• Vitamin B-complex group

Water-soluble vitamins are generally not stored in significant amounts in the body (with the exception of vitamin B12 and folate to some extent). Excess amounts are usually excreted in urine, reducing the risk of toxicity but increasing the need for regular dietary intake.

Vitamers

Several chemically related compounds may exhibit the biological activity of a given vitamin. Such compounds are known as vitamers, also referred to as vitamin isomers or vitamin-active analogues. These compounds may differ slightly in structure but can perform similar biological functions.

Recommended Daily Dietary Allowance of Vitamins

The National Research Council (NRC) of the United States periodically establishes recommended intake levels for essential nutrients, including vitamins. These values are intended to ensure optimal health and prevent deficiency states.

These recommendations are generally higher than the minimum amounts required to prevent overt deficiency diseases, as they account for normal physiological variations, growth, and metabolic demands.
In modern nutritional science, these values are referred to as:

• Recommended Dietary Allowances (RDA)
• Dietary Reference Intakes (DRI)

These values may vary slightly between countries depending on dietary habits and updated nutritional research.

Recommended Daily Dietary Allowance of Vitamins

Variation in Requirements

It is also important to note that vitamin requirements are not fixed and may vary depending on physiological and pathological conditions. Requirements may increase significantly during:

• Physical and psychological stress
• Infectious diseases
• Surgical recovery and trauma
• Pregnancy and lactation
• Malabsorption syndromes and gastrointestinal disorders

In such conditions, increased metabolic demand or impaired absorption necessitates higher vitamin intake to maintain normal physiological function.