Dear Colleagues,

There is a growing interest in the health profession community in diet quality. The interest likely derives from several decades of primarily reductionist nutrition research that has focused on individual dietary nutrients and often failed to find substantial benefits from increasing or avoiding just one nutrient at time. Possible additive or synergistic effects of manipulating multiple dietary components simultaneously could lead to more substantial benefits than manipulating single nutrients. However, moving from single to multiple dietary nutrients or components increases the complexity of the interventions and the interpretations of the findings. For example, rather than manipulating the intake of two specific fatty acids in the diet and contrasting their effect on a particular health outcome like blood lipid profile, a different type of contrast would be a Mediterranean Diet vs. a Vegan-Low-Fat diet. A particular challenge here is that it is possible to create many possible combinations of foods that might meet the general guidelines of a particular diet, such as the Mediterranean diet, and some of those would likely be considered by nutrition scientists to be more, and some less, healthy. One approach to addressing this would be to try to agree on some general guidelines for a set of Mediterranean diet examples that would all be considered to be of high diet quality, and contrast these to another diet, such as a Vegan-Low-Fat diet, which is similarly aligned with a high diet quality. While there is general agreement on the concept, there is no consensus on a single definition for this term.

Diet quality has been defined by some as being nutrient-dense. This would suggest that the food or menu has a high content of nutrients relative to the energy content. However, this begs the question of which nutrients and in what quantities. How important is the amount of specific nutrients relative to the number of different nutrients found in a single food or menu item? Additionally, are all nutrients considered equally, or are those nutrients that are harder to obtain in a typical eating plan more important than those nutrients that are easily obtained by most people in a typical eating plan?

For the nutrition research community, the concept of diet quality takes on an additional level of complexity. Beyond defining it, researchers want to be able to measure this in their research studies. Optimally the metrics for diet quality would be accurate, reproducible, and sensitive to change when measured. Nutrition researchers would like to be able to assess where on the spectrum of diet quality an individual might be at one point in time, and then be able to demonstrate that an intervention has led to detectable improvement in diet quality. Different nutrition researchers conduct different types of studies. Some researchers conduct tightly controlled studies with small numbers of subjects of short durations where food is purchased prepared and distributed to study subjects, allowing for maximal control over food intake. Other researchers work with larger sets of study subjects over longer periods of time and try to teach their study participants to make these changes on their own—shopping, preparing, and consuming in their home environment rather than a research environment. Diet quality can be important in both of these scenarios, but there are different measurement or assessment issues to consider when food is being provided to study participants vs. when it is not.

In some ways, as it has been said of other topics, diet quality may be like pornography—difficult to clearly define, but you know it when you see it. The objective of this issue of the journal is to assemble a range of metrics and perspectives on diet quality, and make as many of them as possible available in a single location. The purpose will be to offer those interested in the topic of diet quality the opportunity to review and select from them for specific uses. In each example there will likely be pros and cons vs. other examples, and these may be specific to certain populations or situations. Some will have been created with greater attention to validation than others, and this will be considered and presented. Some will have been created with greater attention to ease of use and generalizability than others, and this will be considered and presented. Once a collection of these definitions and metrics is assembled and presented, it is likely that several common and essential components will be evident that will help to better define overall what the term diet quality is, and how best it can be utilized. Hopefully this will help in the ongoing global quest for food and beverage production, preparation, distribution and consumption aligned with better diet quality, and leading to better health for all.

Prof. Christopher Gardner and Dr. Jennifer Hartle
Guest Editor

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• nutrient-dense
• energy-dense
• vitamins
• minerals
• phytochemicals
• whole grains
• fiber
• glycemic index
• glycemic load
• added sugars
• artificial sweeteners
• packaged food
• processed food
• hyperpalatibility
• bliss point
• fresh
• antioxidants
• taste, etc.