The Dutch Healthy Diet index 2015 ^{1) 2)} is a diet score derived from one of the FFQ processing steps; the FFQ item results. This DHD index is available for ~144.000 adult participants for the FFQ heart and ~68.500 for the FFQ heart and petals (section: Nutrition (Diet scores) and Secondary & linked variables).
The DHD index scores for Lifelines were developed by researchers from Wageningen University and Research and is a tool to assess a participant's own diet quality and adherence to the 2015 Dutch dietary guidelines. On the Lifelines data, the Lifelines Diet Score has been developed as well, which is a tool to assess relative diet quality to the overall Lifelines population.

The DHD index can be requested in the Lifelines catalogue. When this data has been used in your research, you will have to include a reference to the the following papers:

• Baart, A. Mireille, et al. "Dietary intake in the lifelines cohort study: Baseline results from the flower food frequency questionnaire among 59,982 participants." Nutrients 14.1 (2021): 48.
• Baart, A. Mireille, et al. "Assessment of the Dutch Healthy Diet index 2015 in the Lifelines cohort study at baseline." European Journal         of Clinical Nutrition (2023): 1-11.

Dietary intake data is very difficult to work with in scientific research, as it covers a great variety of aspects. The DHD2015-index is a simple measure of adherence to the 2015 Dutch dietary guidelines. The index consists of sixteen components, where the sixteenth component is a recent addition to include unhealthy foods, based on a guideline of the Netherlands Nutrition Centre. For all components a minimum of 0 points and a maximum of 10 points can be allocated, with a higher score indicating a better adherence ot the guidelines.

The calculation of the DHD index consisted of two steps which have been executed for the FFQ heart only, and for the FFQ heart and petals. The first being the calculation of the intake of food products that are included in the different components of the DHD index, and the second calculating the DHD index itself.

In this first step, the FFQ items are assigned to their specific components. For example: for the FFQ heart only calculation, the grams per day of item number 82 (peanuts and nuts with hot meal) and 96 (peanuts and nuts inbetween meals) are added to the category “Nuts”. Some special cases are discussed below:

• Some items fall into two categories. For example: for the FFQ heart only calculation, item number 25 (chocolate milk) is added to the category “Dairy” and “Sugarsweetened beveraged and fruit juices”.
• Some items are divided into two categories. For example: for the FFQ heart only calculation, 77.96% of the intake of item number 65 (other types of fish) is added to the category “Fish_lean” and 22.04% of the intake of this item is added to the category “Fish_fat”. These percentages are assumptions that are based on the Dutch national Food consumpumption Survey ³⁾.
• Some items are not included. FFQ items that are not related to one of the 2015 Dutch Dietary Guidelines, or for which too many assumptions have to be made, are not added to a specific component and thus are not used to calculate the DHD index.

Once the food items have been added to their specific category, an aggregate variable per category is calculated to obtain a total intake of the product group per participant per day.

In the second step of the calculations, the scores per DHD component are calculated. With the final step being the calculation of the overall DHD index. Please note: Components 3b (grain ratio), 10 (coffee), and 15 (sodium) cannot be calculated for the FFQ heart and/or heart+petals version due to the items being unavailable or lack of distinction.

* All DHD indices are included as assessment 'sec' as the indices include several assessments.