Most dietary guidelines recommend dietary diversity.
In other words, they say that people should eat lots of different kinds of food in moderation. “Everything in moderation” is a popular phrase.
However, it is unclear whether following this recommendation has any effects on people’s health.
A group of scientists tried to answer this question by examining the association of dietary diversity and abdominal obesity and type 2 diabetes.
Dietary diversity has never been clearly defined.
Most previous studies simply define it as the number of different foods you eat in a certain time frame.
However, this leaves out some important factors, such as how the foods are spaced out or how different the foods actually are.
A few previous studies have investigated the effects of a diverse diet, but most focused only on diversity within selected food groups.
For example, one observational study links eating many different types of fruits and vegetables to a reduced risk of type 2 diabetes (1).
Another observational study showed that eating a variety of vegetables may help prevent weight gain. However, eating many varieties of unhealthy foods may increase the risk of weight gain (2).
These studies suggest that dietary diversity can affect health in different ways, depending on what foods are eaten.
A team of researchers from the US examined how a varied diet affects the risk of obesity and type 2 diabetes.
Basic Study Design
This study was a secondary analysis of the Multi-Ethnic Study of Atherosclerosis, a prospective observational study in people of Caucasian, Hispanic, African and Chinese descent.
The purpose of this secondary analysis was to examine the association of dietary quality with abdominal obesity and type 2 diabetes.
A total of 2505 men and women, 45 to 84 years old, participated. None of them had type 2 diabetes. At the beginning of the study, their diet was estimated using a food frequency questionnaire.
To estimate dietary diversity, the researchers assessed:
- Count: How many different foods were eaten more than once per week.
- Evenness: How the diversity was spread out over the week.
- Dissimilarity: How different the foods were.
After 5–7 years, the study staff measured body weight and waist circumference. Fasting blood sugar was measured 10–11 years later.
Bottom Line: This was a prospective observational study examining how the risk of weight gain and diabetes is linked with dietary diversity or quality.
Finding 1: Dietary Diversity Is Not Associated With Dietary Quality
The major finding of the study is that a diverse diet is not necessarily a healthy diet.
The count and evenness measures of dietary diversity were only weakly linked with dietary quality.
Conversely, greater food dissimilarity was linked with lower dietary quality — a lower intake of healthy foods and a higher intake of unhealthy foods.
Bottom Line: A diverse diet is not necessarily a healthy diet. Eating many different types of food doesn’t guarantee that those foods are high in quality.
Finding 2: Dietary Diversity May Promote Weight Gain
Greater food dissimilarity was linked with higher gain in waist circumference, which is a measure of belly fat.
This is probably because a higher food dissimilarity was associated with a higher intake of unhealthy food.
Supporting this, one observational study showed that greater dietary variation was associated with a higher intake of unhealthy foods (3).
Bottom Line: Eating many different kinds of food does not necessarily prevent belly fat. In fact, increasing diversity may even promote higher calorie intake and weight gain.
Finding 3: A Varied Diet Was Not Linked With Type 2 Diabetes
During the 10-year follow-up period, 24% of the participants were diagnosed with type 2 diabetes.
However, this was not linked to dietary diversity. Likewise, no significant associations were found when foods were categorized as either healthy or unhealthy.
In fact, a greater variety in fruit and vegetable intake did not seem to be associated with a reduced risk of type 2 diabetes.
This is inconsistent with one observational study suggesting that eating a greater variety of fruits and vegetables may help prevent type 2 diabetes (1).
On the other hand, the present study found high dietary quality to be significantly associated with less risk of type 2 diabetes.
This means that diets should focus on high-quality food, rather than on diversity.
Bottom Line: A diverse diet did not protect against type 2 diabetes. However, a high-quality diet was linked with less risk of type 2 diabetes.
The use of food frequency questionnaires to assess dietary variation is one of the main limitations of the study.
Food frequency questionnaires can only include a few types of food. For example, the fruit and vegetable category included only 23 different foods.
For this reason, dietary diversity may have been biased or underestimated.
Additionally, this was an observational study, not a controlled trial. Although the study was designed well, it can not prove causation.
For example, it can show that people who eat a diverse diet have a higher risk of weight gain, but it can not prove that the diverse diet itself caused the weight gain.
Bottom Line: The study was designed well. However, the use of food frequency questionnaires to assess dietary intake limited the researchers’ ability to fully evaluate dietary diversity.
Summary and Real-Life Application
In short, this study does not support the idea that eating whatever you want, even in moderation, promotes a healthier diet.
However, this doesn’t mean dietary diversity is a bad thing.
For example, in developing countries with limited food supplies, greater dietary diversity would increase nutrient intake (5).
It just means that, for many people, diet should focus on eating quality, whole foods — but not trying to eat “everything in moderation.”