[Nutritional implications of bariatric surgery on the gastrointestinal tract]

Anatomical change in the anatomy of the gastrointestinal tract after bariatric surgery leads to modification of dietary patterns that have to be adapted to new physiological conditions, either related with the volume of intakes or the characteristics of the macro- and micronutrients to be administered. Restrictive diet after bariatric surgery (basically gastric bypass and restrictive procedures) is done at several steps. The first phase after surgery consists in the administration of clear liquids for 2-3 days, followed by completely low-fat and high-protein content (> 50-60 g/day) liquid diet for 2-4 weeks, normally by means of formula-diets. Soft or grinded diet including very soft protein-rich foods, such as egg, low-calories cheese, and lean meats such as chicken, cow, pork, or fish (red meats are not so well tolerated) is recommended 2-4 weeks after hospital discharge. Normal diet may be started within 8 weeks from surgery or even later. It is important to incorporate hyperproteic foods with each meal, such egg whites, lean meats, cheese or milk. All these indications should be done under the supervision of an expert nutrition professional to always advise the patients and adapting the diet to some special situations (nausea/vomiting, constipation, diarrhea, dumping syndrome, dehydration, food intolerances, overfeeding, etc.). The most frequent vitamin and mineral deficiencies in the different types of surgeries are reviewed, with a special focus on iron, vitamin B12, calcium, and vitamin D metabolism. It should not be forgotten that the aim of obesity surgery is making the patient loose weight and thus post-surgery diet is designed to achieve that goal although without forgetting the essential role that nutritional education has on the learning of new dietary habits contributing to maintain that weight loss over time.