Design and baseline data in the BAriatic surgery SUbstitution and Nutrition study (BASUN): a 10-year prospective cohort study

Criteria for bariatric surgery in public healthcare in Sweden are in essence similar to those advocated in international guidelines, including BMI > 35 kg/ m² with obesity-related comorbidities such as diabetes and sleep apnoea, or BMI > 40 kg/m² without comorbidities [7]. Contraindications include drug or alcohol abuse, unstable psychiatric disorders, age under 18 years, cancer during the last 5 years, or poor general health condition [8]. A multidisciplinary board assesses the eligibility of all persons to undergo surgical treatment. Only Roux-en-Y gastric bypass (RYGB) or sleeve gastrectomy (SG) are used as the primary surgical procedure in the public health care in Sweden. Laparoscopic antecolic antegastric RYGB is carried out as previously described [9, 10]. Laparoscopic SG is carried out in line with state-of-the art principles described at the Fifth International Consensus Conference on SG [11]. The final choice of surgical method to be used in each individual case was made in consensus between the operating surgeon and the patient, taking into consideration the patients’ preference as well medical conditions. Postoperative follow-up is conducted in accordance with Nordic guidelines for follow-up and dietary supplementation after bariatric surgery [12]. The operating unit conducts follow-up at 6 weeks, 6 and 12 months after surgery, then the patients are referred to their primary health care units that continue to do yearly follow-ups. All patients are prescribed dietary supplementation with iron 100 mg/day, vitamin B12 1 mg/day, calcium 500 mg and vitamin D 800 U-combinations twice daily, and multivitamin and mineral preparations containing at least 1,4 mg thiamine, 400 μg folate and 14 mg zinc, twice daily. During the first 2 months after surgery all operated patients were prescribed proton pump inhibitors, e. g. omeprazole 20 mg once daily.

Medical treatment is offered to all persons 18 years of age or older with BMI > 35 kg/ m², who do not qualify, or are not willing to undergo surgical treatment. If there are no contraindications, such as binge eating, severe psychological disturbance or medical condition, the intervention starts with a very low energy diet (VLED) period for 12, 16 or 20 weeks depending on the starting weight (BMI 35–39.9, 40–49.9 or ≥ 50 kg/m², respectively). During this period the daily intake is 450–800 kcal as well as a recommended 1.5–2 L of fluids per day. The patients have appointments with a nurse at the start and then after 2, 5, 8 and 12 weeks.

The VLED period is followed by a period of food reintroduction lasting 12 weeks guided and monitored by a dietitian or clinical nutritionist. Each meal has an energy content of 300–475 kcal and for every meal that was introduced monthly (breakfast, lunch, dinner) a VLED meal was removed. To estimate the energy need for each individual for weight reduction, the Harris Benedict sex-specific equations are used multiplied with a physical activity level of 1.3 to 1.4 and minus 30% representing the estimated energy deficit to achieve weight reduction [13]. After the food reintroduction period the patients continues with such energy-restricted diet. The dietary advice is based on the Nordic Nutrition Recommendation and each patient receives written instructions for a diet of 1400–1600 kcal per day (15–20% energy percent (E%) protein, 30 E% fat and 50–55 E% carbohydrates) that was divided into three to five meals [13]. During the food reintroduction period, the patients had monthly appointments with a dietician for the remainder of the 12-month treatment period. The patients have appointments with a physician after 6 months and 12 months for addition of GLP-1 receptor agonists, SGLT-2 inhibitors, orlistat, or a combination of bupropion and naltrexone when appropriate as well as assessment of blood tests. Advice on physical activity was given by all professions at all visits.

Demographic data at baseline was collected. This includes age, gender, civil status, working condition, education, community, country of birth, medication, comorbidities, smoking habits, and a 4-days dietary recall. Physical measurements were conducted regarding height and weight. Certain blood samples were analysed immediately, while additional blood samples were collected and stored in a biobank until future analysis. Results from blood work that was collected before the start of treatment was gathered from electronic medical records. Information on blood tests and measurements are given in Table 1.

Specific questionnaires are completed by the study participants at baseline and will be completed at 2, 5 and 10 years of follow-up as shown in Table 2. The questionnaires cover three areas: gastrointestinal symptoms and eating problems, physical activity and quality of life, and psychological health.

In future analyses of the data, Cox regressions and machine learning techniques will we used to evaluate importance of baseline conditions and characteristics for the long-term results in the three study groups.