C₆H₈O₆ — Vitamin C

Vitamin C, also known as ascorbic acid, is an essential water-soluble vitamin required for numerous physiological functions in the human body. Its chemical formula is \(C_6H_8O_6\). This compound is renowned for its role in boosting the immune system, aiding collagen synthesis, healing wounds, and maintaining healthy skin and connective tissues.

Discovered in the early 20th century as the anti-scurvy vitamin, Vitamin C remains one of the most important dietary nutrients. It acts as a powerful antioxidant, protecting the body from oxidative stress caused by free radicals. Since humans cannot synthesize Vitamin C endogenously, it must be obtained from food sources such as citrus fruits, strawberries, guavas, and green vegetables.

Vitamin C is a six-carbon compound related structurally to glucose. It exists primarily in two forms: the L-ascorbic acid (biologically active form) and D-ascorbic acid (inactive form). The structure consists of a five-membered lactone ring with multiple hydroxyl (–OH) groups, making it highly soluble in water.

Its ability to donate electrons from its hydroxyl groups gives it strong reducing properties, which is the foundation of its antioxidant behavior. The molecule can be easily oxidized to dehydroascorbic acid (C6H6O6), which can also revert back to ascorbic acid under physiological conditions, maintaining redox balance in the body.

Vitamin C is abundant in many fruits and vegetables. The richest natural sources include:

• Citrus fruits like oranges, lemons, and grapefruits
• Kiwi, guava, and strawberries
• Vegetables such as bell peppers, broccoli, Brussels sprouts, and spinach
• Indian gooseberry (Amla) — one of the richest sources in the world

In plants, Vitamin C functions as a reducing agent and cofactor in enzymatic reactions involved in photosynthesis and metabolism. In animals, it serves critical biochemical roles in enzyme activation and tissue maintenance.

Vitamin C is a crystalline solid that dissolves readily in water and ethanol but is insoluble in nonpolar solvents. It decomposes at elevated temperatures and in the presence of light or air, as it is easily oxidized. Therefore, storage and cooking methods significantly affect Vitamin C retention in foods.

• Optical Activity: Only the L-isomer of ascorbic acid exhibits vitamin activity.
• Reducing Property: Acts as a strong reducing agent, converting ferric ions (Fe³⁺) to ferrous ions (Fe²⁺).
• Stability: Decomposes under heat and alkaline conditions.
• Acidity: A weak acid with a pKa around 4.2 due to the enediol structure that releases protons easily.

These properties explain its effectiveness in redox reactions and its sensitivity to oxidation in storage.

Most mammals can synthesize Vitamin C in their liver or kidneys from glucose through the uronic acid pathway. Humans, some primates, and guinea pigs lack the enzyme L-gulonolactone oxidase, preventing endogenous production and necessitating dietary intake.

Industrially, Vitamin C is synthesized through the Reichstein process, which involves fermentation and chemical oxidation steps:

Modern biotechnological methods use microbial fermentation to improve yields and reduce environmental waste.

• Collagen Synthesis: Vitamin C acts as a cofactor for the enzymes prolyl and lysyl hydroxylase, essential in the hydroxylation of proline and lysine during collagen formation. This supports skin, cartilage, bone, and connective tissue integrity.
• Antioxidant Protection: Neutralizes free radicals and regenerates other antioxidants like Vitamin E.
• Immune Support: Enhances the production of white blood cells and increases the efficiency of the immune response.
• Iron Absorption: Converts dietary non-heme iron (Fe³⁺) into its absorbable form (Fe²⁺) in the intestine.
• Neurotransmitter Synthesis: Participates in converting dopamine to norepinephrine.

Regular intake of Vitamin C is essential for wound healing, resistance to infection, and maintenance of vascular health.

Deficiency of Vitamin C leads to a disease known as Scurvy, characterized by fatigue, swollen gums, joint pain, and poor wound healing due to impaired collagen synthesis. In severe cases, scurvy can cause anemia, bleeding gums, and even death if left untreated.

Other deficiency-related issues include weakened immunity, dry skin, hair loss, and increased oxidative damage. Fortunately, consuming a diet rich in fresh fruits and vegetables can easily prevent these conditions.

The recommended daily intake (RDI) for adults is around 65–90 mg/day, with an upper safe limit of 2000 mg/day.

• Immune System Booster: Strengthens the body’s natural defenses against infections.
• Skin Health: Promotes healthy skin by aiding collagen formation and protecting against UV damage.
• Wound Healing: Accelerates tissue repair and reduces inflammation.
• Heart Health: Reduces risk of cardiovascular disease by lowering oxidative stress and improving endothelial function.
• Anti-Aging Effects: Prevents premature skin aging caused by free radicals.
• Food Industry: Used as a preservative and antioxidant (E300) to prevent oxidation in processed foods.
• Pharmaceutical Use: Used in tablets, syrups, and supplements to prevent or treat Vitamin C deficiency.

Vitamin C is considered very safe, as excess amounts are excreted through urine. However, extremely high doses (above 2000 mg/day) may cause mild side effects like diarrhea, nausea, and stomach cramps. In people with kidney disorders, excessive Vitamin C intake can contribute to kidney stone formation due to increased oxalate excretion.

As it is sensitive to air, light, and heat, Vitamin C should be stored in airtight containers and kept away from sunlight to prevent degradation.