In recent years, the recourse to obesity surgery to treat morbid obesities has grown. The number of "malabsorptive" interventions, such as the gastric bypass (RYGB: Roux-en-Y gastric bypass) increases each year. The RYGB, which combines two mechanisms promoting weight loss, restriction and malabsorption, has proven its effectiveness in term of weight loss and improvement of obesity-associated co-morbidities. However this intervention involves a profound change in digestive physiology and is the source of nutritional and metabolic complications. The deficits observed most frequently concern proteins, iron, calcium, vitamin B12 and vitamin D. The deficiencies in vitamin B1 are rare but potentially serious. Multidisciplinary follow-up is essential to ensure prevention, diagnosis and treatment of these complications. Based on an analysis of the literature, this article summarizes the various nutritional complications observed after RYGB and the means to diagnose it. It proposes practical recommendations for follow-up, preventive supplementation and treatment of these deficiencies, both generally and in the more specific case of a pregnancy after RYGB.