**Abstract**

Bariatric surgery (BS) results in significant weight loss and improvement of obesity associated comorbidities. Despite the benefits achieved with these operations, deficiencies of vitamins and other micronutrients are common. Such deficiencies may become clinically significant if not discovered and treated early. Therefore, it is imperative to undertake thorough screening, and have sound preventive strategies in place in order to make BS a safer procedure. This chapter will provide the multidisciplinary bariatric team with a comprehensive review of micronutrient deficiencies before and after bariatric surgery. The focus will be on the most common micronutrient deficiencies that are encountered in various types of BS procedures, including water soluble vitamins, fat-soluble vitamins, minerals and trace elements deficiencies, as well as protein malnutrition. The chapter starts with an overview of the causes of micronutrient deficiencies in patients with obesity and before undergoing BS. It reviews the screening of patients for preexisting micronutrient deficiencies prior to their BS. Then the chapter addresses the potential causes and mechanisms leading to such deficiencies after BS. It then conducts an in depth discourse of the prevalence of deficiencies by the type of BS, the presenting symptoms, and the investigations required for the diagnoses. The chapter will also discuss the management of each deficiency according to the severity of the symptoms. The chapter also reviews the recent updated guidelines for standard nutritional care post BS. We will finally conclude with a framework of the preventive strategies for optimal care to ensure long term success post-surgery.

**Keywords:** obesity, bariatric surgery, micronutrients, deficiencies, water soluble vitamins, fat soluble vitamins, minerals, trace elements, protein malnutrition
