Today, bariatric surgery is the most effective treatment of obesity. Restrictive surgeries are aimed at reducing the volume of the stomach. Such operations have the least adverse effects in terms of the physiology of digestion. However, they are less effective in losing weight. Mixed operations (gastric bypass surgery, biliopancreatic diversion) decrease the volume of the stomach and the length of the small intestine. Mixed operations tend to provide more pronounced results in a short time. However, by seriously disrupting the digestion and absorption of the food, they may result in severe micronutrient deficiencies. The assimilation of lipophilic substances is severely impaired as the biliopancreatic system is almost disconnected from the digestive tract. Furthermore, the assimilations of not only fats and essential fatty acids but also all fat-soluble vitamins are interrupted. In most cases, these patients need to take high-dose micronutrient supplements for their whole life. Usually, deficiencies of vitamins A, D, E, C, and vitamins of group B, especially B1, B6, B12, niacin, and folate, as well as iron, calcium, phosphate, magnesium, zinc, copper, and selenium, are found after operations. Restrictive operations, although less traumatic for the digestive system, significantly disrupt the digestion and absorption of nutrients, especially those in which the stomach plays a significant role in its digestion. These are proteins, folate, niacin, vitamin B12, iron, and copper. Bariatric surgical procedures are like a double-edged sword. They can be highly effective in the treatment of obesity and related complications; nevertheless, they are surgeries with serious potential for complications.