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Vitamins are essential organic substances that facilitate the utilization of other nutrients rather than serving as energy sources or building materials. They predominantly act as coenzymes, assisting enzymes in specific chemical actions like glucose oxidation for energy. Most vitamins must be consumed through food or supplements, though some are synthesized internally or by intestinal bacteria.

Fat-soluble vitamins like A, D, E, and K are absorbed with dietary lipids and stored in body tissues, making overconsumption potentially toxic. Water-soluble vitamins like B-complex and vitamin C are absorbed with water, scarcely stored in lean tissue, and excess amounts are eliminated through urine, making toxicity from dietary sources rare.

Vitamin A deficiency causes night blindness, difficulty seeing in low-light conditions. Prolonged deficiency leads to xerophthalmia, characterized by corneal dryness and damage that can result in permanent blindness. These conditions highlight the critical importance of adequate vitamin A intake for maintaining vision.

Vitamin D deficiency weakens bones, causing rickets in children and osteomalacia in adults, characterized by bone pain, muscle weakness, and increased fracture risk. Overconsumption leads to hypervitaminosis D, elevating blood calcium levels and causing nausea, vomiting, constipation, confusion, and kidney damage.

B vitamin deficiencies cause distinct conditions: thiamine deficiency causes beriberi with fatigue and nerve damage; riboflavin deficiency causes mouth sores and skin inflammation; niacin deficiency causes pellagra with dermatitis and diarrhea; folic acid deficiency in pregnant women may cause neural tube defects in the developing fetus.

Vitamins C, E, and A function as antioxidants, counteracting harmful free radicals generated during metabolic processes that use oxygen. These antioxidants pass destructive free electrons between molecules until a compound like glutathione absorbs it and is excreted in urine, protecting cells from oxidative damage.

Vitamin K is essential for proper blood clotting; deficiency impairs this process, resulting in profuse bleeding and bruising. Newborns are particularly susceptible to vitamin K deficiency due to limited stores at birth. Excessive synthetic vitamin K supplements may interfere with certain medications and cause adverse effects.