*Geek Box: Diet Quality Index
Diet quality indices provide a means of quantitatively assessing diet, to better characterise healthfulness of an overall dietary pattern, rather than isolated nutrient variables alone. There are a range of indices commonly used in research, including the Diet Quality Index, Healthy Eating Index and Alternate Healthy Eating Index, and the Mediterranean Diet Score – these are the most internationally recognised and validated for assessment of diet quality in various populations.
Diet quality indices provide a means to quantitatively assess the healthfulness of dietary pattern, given “quality” is an ambiguous term for research purposes. There are a number of ways an index may assess diet quality: basing the index on food groups and nutrient intakes is the most common, for example and index may have “red meat” as a food group, and then thresholds in grams per day upon which higher or lower scores are assigned. Conversely, polyunsaturated fats may have a threshold in percentages, which higher or lower scores assigned based on PUFA intake as percentage of total energy intake.
As an example of a common form of diet quality index, let’s consider the Alternate Healthy Eating Index, which consists of 11 dietary components with a maximum of 10-points for each, contributing to a total score of 110. The dietary components are based on an adequacy principle for foods whose consumption is associated with lower risk of disease, including vegetables, wholegrains, whole fruit, nuts and legumes, long-chain omega-3 fatty acids, and polyunsaturated fats: the points score rises with increasing consumption of the dietary component up to a maximum score of 10, which is individualised to the food group: for EPA/DHA a max score is 250mg/d on average or 2 x 114g oily fish servings per week, for wholegrains a max score is 90g/d in men and 75g/d in women.
In contrast, dietary components associated with negative health outcomes, including sugar-sweetened beverages and fruit juices, red/processed meats, trans fats, and sodium have a points score inverse to consumption. For example, the max score for sodium is <1,612mg/d, while a score of 0 for sodium >5,271mg/d. Due to positive associations with health outcomes associated with moderate alcohol consumption, a maximum score is available for moderate alcohol intake, while high intake scores a standard value of zero.
Diet quality index’s may change over time: in the AHEI, for example, a max score is provided if trans fats are <0.5%, but due to changes in the food supply this score is often a default 10 points, not necessarily reflective of diet quality. Thus, diet quality indices can be flexible and evolve to reflect different dietary patterns and factors. However, an index chosen to asses diet quality in a given population should be appropriate to that population.