The pre-operative diet can be either a partial or complete formula based diet or meal replacement and a food based diet [1–4]. A randomized trial of a very low calorie diet showed formula based diets and standard diets are both capable of achieving comparable results on preoperative weight loss before bariatric surgery However patient compliance, tolerance and acceptance were all significantly better after a standard diet [9]. Another study showed that a partial use of a formula diet is more effective in reducing body weight than food-based diets alone perhaps due to a balanced composition of the formula and improved compliance [10]. Whatever be type of diet is used the selection of the amount of carbohydrate intake is most important as it may directly affect the level of the liver fat and liver volume [11]. It has also been shown that a low carbohydrate diet results in reduction of insulin resistance too [12]. Fluid recommendations should be given with all diets from an additional 1 l of fluids to 3.5 l dependent on the type of diet. Micronutrient supplementation is essential in this preoperative phase. Emerging evidence also suggests that immunonutrition formulas may be even better than high protein formulas or regular diet of similar caloric intake [13].

The duration of a preoperative diet varies from 2 to 12 weeks [1–4]. However it has been shown that 80 % of expected liver volume reduction will occur in the first 2 weeks of a very low energy restrictive diet [1]. It has also been demonstrated that compliance to diet restriction will reduce over time relative to the severity of energy restriction [14].

Thus a short term energy restrictive diet or liver shrinkage diet (food based or formula based) of around 2 weeks can be used in the pre surgery period.

The principles of post bariatric surgery nutritional management are diet modifications based on the food texture, consistency and volume. The goal is to provide adequate energy and nutrition while reducing symptoms like dumping syndrome and early satiety [15–17]. Post nutritional requirement was not documented and was not stressed until protein malnutrition and other nutritional deficiencies had appeared [18–22]. In general, post-operative bariatric diet is comprised of four stages each providing a more advanced form of food texture than the previous starting from liquid to solid diet [23].

The recommended dietary allowance for carbohydrate is 130 g/day for adults providing between 45 and 65 % of total energy intake (TEI). The role of carbohydrate in weight loss has been related to glycemic load but the outcomes have been varied. Moize et al. proposed a food pyramid model for bariatric surgery, in which a CHO intake of between 40 and 45 % of daily TEI was recommended [24].

Protein intake should be individualized, assessed, and guided by an experienced dietitian, with reference to gender, age, and weight. A minimal protein intake of 60 g/d and up to 1.5 g/kg ideal body weight per day should be adequate; higher amounts of protein intake – up to 2 g/kg ideal body weight per day – may be required in special situations. The importance of protein intake has been discussed later in chapter 33.

When the stomach size is reduced during bariatric surgery, there is an increase in pH secondary to the reduction of pepsin thereby limiting the early steps in fat digestion. Also malabsorptive procedures like bilio-pancreatic diversion (BPD) have been shown to decrease fat absorption by upto 72 %. This increases the risk for essential fatty acid and fat soluble vitamin deficiencies. Essential fatty acid deficiency symptoms includes dry scaly skin, hair loss, decreased immunity, increased susceptibility to infections, anemia, mood changes, and unexplained cardiac, hepatic, gastrointestinal and neurological dysfunction.

Researchers have shown that high fat diet (50 % of calories) resulted in fat storage and impaired suppression of carbohydrate oxidation. No relationship with oxidation was noted with low (20 % of energy derived from fats) or moderate (30 % of energy derived from fat) fat diets [25]. Thus fats should provide 20–30 % of the total energy. Saturated fat should be decreased and replace it with poly and monounsaturated fats.

Dumping syndrome is commonly reported which mainly occurs after consumption of foods with a high sugar and carbohydrate content, resulting in symptoms of early dumping syndrome such as nausea, dizziness, weakness, rapid pulse, cold sweats, fatigue, cramps, and diarrhea 10–30 min after eating [26]. Some RYGB patients experience late dumping, which occurs 1–3 h after a meal as a result of an exaggerated insulin release and reactive hypoglycemia [26]. To prevent dumping syndrome sugar consumption should be less than 25 g per serving. Hence concentrated sugar containing drinks like soda, juices and frosts should be avoided and natural sugars like dairy and whole fruits can be included.