Is eating a mixed diet better for health and survival?
To answer this question, Hadis Mozaffari, a Vanier Scholar and PhD candidate in Human Nutrition in LFS, Zeinab Hosseini, PhD fellow in Kinesiology at the University of Saskatchewan, Jacynthe Lafrenière, Scientific Project Coordinator at Health Canada, and Annalijn I. Conklin, Assistant Professor in the Faculty of Pharmaceutical Sciences, reviewed the health impacts of dietary diversity. They did a statistical analysis of 20 longitudinal studies, which looked at the link between a diverse diet and chronic disease impacts.
Why is this study important?
Diets consist of different food items with complex components that affect the absorption and metabolism of one another. Many studies to date have assessed the effect of only a single food item or nutrient on one health outcome, and commonly measured diet quality based on micro-/macro-nutrients. Research in the field of nutritional epidemiology is starting to move from nutrient-based approaches towards diet-based approaches. In that sense, one of the most important aspects of this study is that dietary diversity is one of the food-based approaches to assessing the quality of a person’s overall diet.
Historically, dietary diversity has been considered as a key component of healthy eating because it provides the essential nutrients that the body needs for its normal function. However, studies have provided inconsistent findings on health effects of dietary diversity — some show a reduced risk of cardiovascular diseases, diabetes and cancer while others show increased energy consumption that can lead to a greater risk of obesity. In order to reconcile these mixed findings, the researchers designed this study to review all of the existing literature on this topic using the Cochrane Collaboration method, a robust way to provide the best evidence in medical research.
In addition to mixed diets being a longstanding concept in nutritional sciences, dietary diversity also has relevance in health psychology: it can improve appetite and interest in food, especially for older adults or people who live alone. In an aging population, dietary diversity can be an important intervention to ensure an adequate intake of food.
What is ‘dietary diversity’ and how can it be measured?
While there is no single definition of dietary diversity, the researchers found that all of the definitions fall within two broader definitions: between group diversity and within group diversity. For example, a diet that includes fruits, vegetables, dairy products, grains and meat has greater diversity between food groups compared to a diet that only includes fruit and vegetables. A diet that includes different types of meats like poultry, fish and red meat has a greater diversity within the meat food group compared to a diet that only includes red meat.
In terms of measuring dietary diversity, there is currently no standardized methodology, which the researchers shared was one of the limitations of current research in this area.
Existing studies are often based on simply counting the number of different food items or food groups in a diet but this raised questions that the researchers attempted to unpack in their study. For example, a diet might consist of multiple food items but if those food items fall within only a couple food groups, is that a “genuinely diverse” diet? If so, does it have a health impact? These concepts have existed for a long time, the researchers explained, but no one has thoroughly addressed them through a systematic review of the literature.
What are the impacts?
The results of this study might have impacts on clinical practice guidelines and nutrition policy because it can encourage diversity of food groups in diet as part of, for example, a multi-intervention program for the prevention of diseases.
The study also has implications for improving the clarity of dietary guidelines and what it means for Canadians to ‘eat a diverse diet’ to better translate this concept into health-promoting eating practices.
In 2019, Canada’s Food Guide was updated to show a plate of only three food groups, compared to six in previous versions. In their study, the researchers make it clear that three food groups are not enough to constitute a healthy diet and that at least five different food groups are needed on a regular basis to improve overall health and survival.
How can people incorporate dietary diversity in their diet?
People can make their diets more diverse by actively eating from all five main food groups. These include fruits, vegetables, grains, meats (and protein alternatives), and dairy products.
While we all know that greater amounts of vegetables are good for our health, the study showed that it is also important to eat different subtypes of vegetables, for example roots and tubers (starchy vegetables), dark green leafy vegetables like broccoli, and vitamin A-rich vegetables like carrots.
Overall, the most diverse diets can be obtained when people go a step further and not only consume different types of food groups, but also different food sub-types or food items from each of these food groups.
What are the next steps?
The researchers’ view is that nutrition policy and counseling need to shift from general advice on ‘eating a variety of foods’ to a more clear and specific food-based recommendation following evidence that reflects an optimal number of different food items from all of the food groups and their subtypes. However, the science is not fully developed to adequately inform guidelines and public health nutrition policy.
There is still very limited robust and reliable data on the health outcomes of dietary diversity across and within all food groups and much of current research lacks a standardized and operationalized measure to assess different aspects of diet diversity. One of the key findings of this review for future work is to improve researchers’ classification systems and terminology to be able to make comparisons between studies.
The researchers call on nutrition scientists to improve and harmonize measurement approaches and to assess dietary diversity beyond describing simple relations between the diversity of core food groups and health outcomes.
To read the study, which appeared in Critical Reviews in Food Science and Nutrition: Is eating a mixed diet better for health and survival?: A systematic review and meta-analysis of longitudinal observational studies, click here.