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Introduction

Humans require 13 essential vitamins, classified as fat-soluble (A, D, E, K) or water-soluble (B complex [B1, B2, B3, B5, B6, B7, B9, B12], C).  Deficiencies can occur with any vitamin (eg, nutritional scarcity, malabsorption).  However, clinically significant toxicities are only seen with fat-soluble vitamins (especially A and D) because they accumulate in tissues and exert dose-dependent effects through nuclear transcription factors.  Water-soluble vitamins rarely cause toxicity because they are readily excreted in the urine.

General principles

All vitamins

In resource-rich regions, dietary deficiency is rare due to fortification of staple foods like grains and cereal.  In the resource-limited regions, several vitamin deficiencies (especially A, D, and B12

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