Vitamins are essential constituents of human diet since they are synthesized inadequately or not at all in human body. Vitamin deficiencies affect all ages, the groups most susceptible to deficiencies are pregnant and lactating women, young children because of relatively high need, chronically ill patients and alcoholics. After gastric bypass surgery patients are at high risk of multiple vitamin deficiencies. Conversely, because of widespread use of nutrient supplements, nutrient toxicities are also gaining pathophysiological and clinical importance. Laboratory investigations play an important role in detecting subclinical deficiencies as well as probable toxicities in asymptomatic patients.