On Thursday, people across the United States will be filling their plates with a variety of traditional Thanksgiving dishes: roast turkey, stuffing, potatoes, and cranberries. In doing so, they are embracing the philosophy of “eating everything in moderation,” also known as dietary diversity. Most individuals also practice dietary diversity in their daily eating habits; however, a study recently published in PLOS ONE found that “everything in moderation” may not be optimal for metabolic health.
Maintaining proper nutrition is key to remaining healthy and avoiding diseases like obesity and type 2 diabetes. Confused by the wealth of seemingly contradictory nutrition research, many people adhere to the philosophy of dietary diversity. Similarly, the United States Dietary Guidelines recommend eating a variety of foods within each of the food groups, which include fruits, vegetables, dairy, protein-containing foods, and grains. No single food or food group can supply all the nutrients one needs, so some level of dietary diversity is necessary to maintain one’s health. In theory, dietary diversity also makes meals more interesting and less repetitive, which may make it easier to stick to a healthy eating plan.
Although nutrition researchers do not fully agree on an optimal diet, there is much common ground. One recommended diet is the DASH (Dietary Approaches to Stop Hypertension) diet, originally developed for individuals with high blood pressure. This diet is rich in fruits, vegetables, and low-fat dairy, and it can help individuals reduce their sodium intake. Nutrition researchers have created the Alternative Healthy Eating Index (AHEI), which ranks foods based on their association with chronic disease. High AHEI scores indicate a diet rich in fruits, vegetables, whole grains, and unsaturated fatty acids, which is associated with low chronic disease risk. In both of these systems, unprocessed, plant-based foods are preferred, and processed snacks and desserts should be limited.
How do diet diversity and diet quality relate to each other? In a 2015 PLOS ONE study, researchers found that “everything in moderation” may not be the best dietary rule of thumb. In fact, researchers discovered that dietary diversity was associated with increased weight gain in some groups. People who consumed a wide variety of foods were also more likely to consume unhealthy foods, which may explain their increased weight gain. Based on these results, the researchers suggest that eating a small number of highly nutritious foods may be more effective at preventing chronic disease than eating “everything in moderation.”
Obesity, type 2 diabetes, and the importance of preventive nutrition
The effects of urbanization, poor nutrition, and a sedentary lifestyle have compounded to create a global pandemic of obesity and metabolic disease, with the Americas, the Western Pacific and parts of Asia especially hard-hit. In the U.S., two-thirds of all adults are considered overweight, with one in three meeting the criteria for obesity, as determined by body mass index (BMI). The health burden of obesity continues to rise. According to the World Health Organization, the global prevalence of obesity has more than doubled from 1980 to 2014. Obesity is more than just too much weight: it increases the risk of a number of severe metabolic diseases, including type 2 diabetes and heart disease. The economic costs of obesity are also high, with medical spending due to obesity estimated at 147-210 billion dollars per year in the United States. Obesity is associated with lower productivity and work attendance; absenteeism alone costs 4.3 billion dollars annually.
Individuals who follow good nutritional practices are less likely to develop severe metabolic disease, such as obesity and type 2 diabetes, but research shows a high-quality diet combined with exercise can also help reduce the severity of existing obesity and metabolic disease, or in some cases reverse it altogether. For these reasons, public health efforts to improve eating patterns are of the utmost importance. Since 1980, the U.S. Departments of Agriculture (USDA) and Health and Human Services (HHS) have advised U.S. citizens to eat a variety of foods in moderation. This message is best illustrated by the iconic food pyramid, used in health communications in the 1990s and early 2000s.
For de Oliveira Otto and colleagues, the trouble with “eating everything in moderation” is that the health effects of this dietary practice had not yet been evaluated in nutritional studies. If diet diversity has positive health impacts, the practice should achieve to things: 1. Reduce an individual’s consumption of unhealthy food items and 2. Reduce his/her risk of obesity and type 2 diabetes. If, on the other hand, a diverse diet promotes unhealthy food choices, nutrition guidelines should be retooled.
Measuring dietary diversity and its effects
de Oliveira Otto et al. analyzed dietary diversity as part of the Multi-Ethnic Study of Atherosclerosis (MESA). Individuals recruited to this study range from 45-84 years old, without diagnosed heart disease. Four major ethnic subgroups are represented, as obesity and metabolic disease susceptibility vary by ethnicity. Study participants filled out a standard nutrition questionnaire covering 120 food items. They indicated the number of times per day or week a particular food was consumed, as well as the portion size (small, medium, large.) For this subanalysis, researchers excluded participants with diagnosed diabetes, as well as those with very low (<600) or very high (>6000) calories consumed per day.
Dietary diversity is difficult to calculate because it can mean many different things. Is a diet diverse when someone eats many foods, or must those foods also be different from each other? To obtain a holistic view of diversity, researchers measured three parameters: food count, dissimilarity and evenness. The simplest measure, food count, is the number of food items consumed at least once per week. Food dissimilarity represents how different the consumed foods are based on 12 properties, including plant/animal source and various nutrients (see Fig. 2 for an example). Food evenness looks at the percentage of calories obtained from each food; someone who eats about the same number of calories from a large number of foods has high diet evenness.
To look at the long-term effects of diet diversity on health, researchers tracked participants for five years. Researchers measured changes in weight and waist circumference, and checked if participants had developed diabetes over the monitoring period. Waist circumference is a measure of abdominal fat, a factor known to increase one’s risk of type 2 diabetes. Results were adjusted for potential confounding factors, including physical activity and smoking.
After five years of monitoring, all ethnic groups showed increased waist circumference, positively correlated with food dissimilarity but not food count or evenness. These differences were more pronounced in Chinese and Black populations. When comparing participants in the lowest and highest quintiles of diet dissimilarity, Chinese populations showed a 2.5-fold increase and Black populations showed a five-fold increase in waste circumference. Type 2 diabetes risk was not associated with any measure of diet diversity.
Researchers also examined diet quality to see how it related to diet diversity and metabolic health, using both the DASH and AHEI metrics. For both DASH and AHEI, food count and/or evenness did not correlate with diet quality, but greater diet dissimilarity was associated with lower diet quality scores, which likely explains the increase in waist circumference. In contrast, higher AHEI diet quality scores were associated with reduced type 2 diabetes risk. These results shows that diet diversity may lead to consumption of both healthy and unhealthy foods.
Conclusions and open questions
de Oliveira Otto et al. conclude that diet diversity doesn’t improve diet quality or provide additional health benefits. Instead, they believe nutritional guidelines should focus on bringing diet quality to the forefront. Reading the findings of this study opens up a number of additional questions that merit further exploration in follow-up studies. First, scientists ought to examine whether diet diversity is associated with higher caloric intake, since an individual is consuming more types of food. Dubbed the “buffet effect” by psychologists, this result might indicate that diet diversity has helped fuel the obesity epidemic.
Second, examining other metabolic parameters will generate a more robust assessment of dietary effects on metabolic health. Type 2 diabetes affects about eight percent of the U.S. population, but metabolic syndrome, which is known to increase the risk of developing type 2 diabetes and heart disease, affects nearly 35 percent of U.S. adults in total, and 50 percent of individuals over 60 years of age. Individuals with metabolic syndrome fit three or more of the following criteria: high triglycerides, low HDL cholesterol (also called “good” cholesterol), high blood pressure, high blood sugar, and high abdominal obesity. This study examined the last two parameters, but measuring all of these values would help researchers better understand how dietary diversity influences chronic disease rates and the health of the total population. Longer-term studies, for example, over 10-20 years, would also be very informative, as chronic disease risk increases dramatically as people age.
“Americans with the healthiest diets actually eat a relatively small range of healthy foods,” said senior author Dariush Mozaffarian. So, if “eating everything in moderation” isn’t the way to go, public health experts will need to find a new way to communicate healthy nutritional patterns. The 2015 US Dietary Guidelines will be released later this year, and given the rising rates of obesity and metabolic disease, there’s never been a more important time for good dietary advice.
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