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Avitaminosis is any disease caused by chronic or long-term vitamin deficiency or caused by a defect in metabolic conversion, such as tryptophan to niacin. They are designated by the same letter as the vitamin.
Conversely hypervitaminosis is the syndrome of symptoms caused by over-retention of fat-soluble vitamins in the body.
Avitaminoses (and their lacking factors) include:
- Vitamin A deficiency (retinal and carotenoid precursors) causes xerophthalmia or night blindness.
- Thiamine deficiency (vitamin B1) causes beriberi.
- Niacin deficiency (vitamin B3) causes pellagra.
- Vitamin B12 deficiency (cyanocobalamin) leads to megaloblastic anemia and subacute combined degeneration of spinal cord.
- Vitamin C deficiency (ascorbic acid) leads to scurvy.
- Vitamin D deficiency (cholecalciferol, ergocalciferol or lack of sunlight) causes rickets.
- Vitamin K deficiency (phylloquinone or menaquinone) causes impaired coagulation.
See also 
- Essential nutrient
- Illnesses related to poor nutrition
- Vitamin#Human vitamins for more details.
- Orthomolecular medicine