Vitamins

General Characteristics of Vitamins

• Definition: Organic substances required in small amounts; 13 vitamins are essential for humans.

• Water-Soluble Vitamins: Includes Thiamin ($B_1$), Riboflavin ($B_2$), Niacin ($B_3$), Biotin ($B_7$), Vitamin C, $B_6$, $B_{12}$, Folate, and Pantothenic acid. These are fragile, heat-sensitive, not stored (except $B_{12}$), and excreted by the kidneys.

• Fat-Soluble Vitamins: Vitamins A, D, E, and K.

Thiamin (Vitamin $B_1$)

• Structure: Contains a Sulfur ring and a Nitrogen ring; bonds are heat and alkaline sensitive.

• Biochemical Function: Key coenzyme in the form of Thiamine Pyrophosphate ($TPP$) for energy-producing reactions, including Pyruvate DEH, $\alpha$-Ketoglutarate DEH, Branched chain oxo-acid DEH, and Tansketolase.

• Nervous System: Essential for glucose metabolism, synthesis of myelin lipids, and Acetylcholine production. Thiamin Triphosphate regulates chloride channels.

• Deficiency Disorders:

  • Beriberi: Characterized by weakness, poor coordination, and paralysis.

  • Wernicke-Korsakoff Syndrome: Common in malnourished alcoholics due to impaired absorption and phosphorylation. Wernicke’s encephalopathy is reversible with injections; Korsakoff’s psychosis involving confabulation is irreversible.

• Recommended Intake: $1.1-1.2\,mg/day$.

Riboflavin (Vitamin $B_2$)

• Functions: Precursor for active coenzymes Flavin mononucleotide ($FMN$) and Flavin adenine dinucleotide ($FAD$), which act as electron carriers in the Electron Transport Chain and for flavoenzymes like Succinate DEH and Glutathione reductase.

• Properties: Light sensitive but heat stable; primary source is milk.

• Deficiency: Ariboflavinosis (skin lesions, confusion, and nuclear cataracts).

• Recommended Intake: $1.1-1.3\,mg/day$.

Niacin (Vitamin $B_3$)

• Active Forms: Nicotinic acid and nicotinamide, which form active coenzymes $NAD$ (glycolysis and TCA cycle) and $NADP$ (biosynthetic pathways and Fatty Acid synthesis).

• Synthesis: Can be synthesized de novo from the precursor Tryptophan.

• Deficiency: Pellagra, characterized by the "4 D’s": Dermatitis, Diarrhea, Dementia, and Death.

• Recommended Intake: $35\,mg/day$.

Biotin (Vitamin $B_7$ or Vitamin H)

• Function: Enables ATP-driven carboxylation (transfer of $CO_2$) for substrates. Key enzymes include Pyruvate carboxylase, Acetyl CoA carboxylase, and Propionyl carboxylase.

• Deficiency: Rare; often caused by Biotinidase deficiency (genetic disorder). Symptoms include hair loss, rash, and brittle nails.

• Adequate Intake (AI): $30\,\mu g/day$.

Vitamin C (Ascorbic Acid)

• Functions: Antioxidant, collagen cross-linking, neurotransmitter synthesis, and enhancement of iron absorption by keeping it in the ferrous state ($Fe^{2+}$).

• Properties: Most unstable vitamin; destruction is accelerated by copper and iron utensils.

• Deficiency: Scurvy (occurs at < 10\,mg/day), involving poor wound healing, bleeding gums (ruptured capillaries), and loose teeth.

• Recommended Dietary Allowance (RDA): $60\,mg/day$ (increased for smokers and during pregnancy).

Vitamin E (Tocopherol)

• Function: Primary antioxidant that protects Polyunsaturated Fatty Acids ($PUFA$) in cell membranes from Reactive Oxygen Species ($ROS$). Its function is restored by Vitamin C.

• Interactions: Requirement increases with higher $PUFA$ consumption.

• Deficiency: RBC Hemolysis and nerve damage; occurs in premature infants or those with long-term low-fat diets.

• Toxicity: High doses inhibit blood clotting by interfering with Vitamin K.

• Recommended Intake: $8-10\,mg\,\alpha\text{-TE}$.

Vitamin A

• Forms:

  • Retinoids: Preformed Vitamin A (retinol, retinal, retinoic acid) from animal sources.

  • Carotenoids: Precursors like $\beta$-carotene from orange fruits and vegetables.

• Storage and Transport: Stored in the liver as Retinyl palmitate and circulated via Retinol Binding Protein ($RBP$).

• Biological Functions: Vision cycle (retinol and retinal), gene expression and cell differentiation (retinoic acid), and antioxidant activity (carotenoids).

• Retinoic Acid and Skin: Regulates gene expression by binding to intra-nuclear receptors to improve acne and wrinkles.

• Deficiency: Most common cause of non-accidental blindness; also leads to night blindness and increased infection risk.

• Toxicity: Acute ($100 \times RDA$) causes nausea and dizziness; Chronic ($10 \times RDA$) causes liver damage and bone abnormalities.

• Recommended Intake: $800-100\,\mu g\,RE$ (as per transcript).