Paradoxically, the obese are more susceptible to malnutrition. The more obese they are, the greater the risk for micronutrient deficiencies particularly in fat-soluble vitamins and anti-oxidants. Several micronutrient deficiencies could be worsened because of poor intake and absorption difficulty after bariatric surgery. Micronutrient deficiencies could not only decrease the effect of bariatric surgery but also cause various diseases such as anemia, neurologic impairment, or osteoporosis. Micronutrient deficiencies should be monitored, recognized, and corrected properly in bariatric patients in order to maximize the effect of surgery and improve the health-related quality of life.