The DHD2015-index consists of fifteen components representing the Dutch food-based dietary guidelines of 2015: vegetables, fruit, wholegrain products, legumes, nuts, dairy, fish, tea, fats and oils, coffee, red meat, processed meat, sweetened beverages, alcohol and Na. Additionally, the component 'unhealthy food choices' was added based on the guideline of the Netherlands Nutrition Centre. For every component, the score ranges from 0 (no adherence) to 10 points (complete adherence), resulting in a total score between 0 and 160 points.

Total DHD2015-index score and individual component scores were calculated from the Eetscore FFQ and 3-day food records.

Macronutrient composition of the diet was evaluated by intake of total energy, total carbohydrates and mono- and disaccharides, total protein, plant-based and animal-based protein, total fat, saturated fat, monounsaturated fatty acids and polyunsaturated fatty acids (including alpha-linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid), and dietary fibre. Furthermore, dietary intake of a limited range of micronutrients for which bariatric patients often display low serum levels was assessed: folate, vitamin B12, vitamin D, calcium and iron.