In the present study which analyzed dietary data collected from four waves over the course of a decade (from 2006 to 2017), we derived three dietary patterns using factor analysis mostly centered on these food groups: 1. A Healthy dietary pattern, characterized by vegetable, fruit, dairy products, liquid oil, and nuts and seeds; 2. A Western dietary pattern featured by soft drinks, snack and dessert, meat, refined grain, and solid fat intake, and 3. A Mixed dietary pattern highlighted by tea and coffee, and simple sugars. Our findings indicated that the structure of these dietary patterns did not seem to be stable over a decade, indicating that the type of food groups that population chose to eat in combination had changed since 2006. Results demonstrating a secular trend in dietary patterns including an emerging adherence of study population to the Mixed dietary pattern, maintenance of the Healthy dietary pattern, and a decline in the Western dietary pattern.
Most previous epidemiological studies which focused on dietary patterns, have analyzed the relation of dietary pattern with risk of chronic diseases [16,17,18,19,20,21], and only a few of them have investigated the secular trend of dietary patterns over time [9, 10, 22, 23]; mostly three common dietary patterns have been identified throughout these investigations. The Healthy or Prudent dietary pattern which is mostly based on fruit, vegetables, dairy products, and liquid oil, Unhealthy or Westernized dietary pattern, mostly characterized by solid fat, snack, soda, and meat; however, the third one, mostly named as modified, new or mixed dietary pattern, differ in each study with different factor loading and food items. Results of the Healthy and Western dietary patterns loaded in the current study are mostly similar to other studies [9, 21]. Adherence of our study population to the three dietary patterns is similar to those of a Korean population, in which the number of participants following the Western dietary pattern declined, and the new dietary pattern increased in population over time .
According to the results of this study, energy intakes of study population remained stable since the first wave. However, the percent of energy from carbohydrate and protein intakes increased whereas percent of energy from fat and all its subtypes (saturated, mono- and polyunsaturated fatty acids) decreased. People have become increasingly aware of the health benefits of vegetable oils and it seems that the sources of fat intake have changed during the last decade, with a significant shift from intakes of solid fat to the liquid oil . Likewise, sources of protein intake have changed from animal to plant based, including legumes.
In term of the trend in food consumption, our findings indicate that fruit and vegetable intakes remained consistent, indicating that policy based-approaches must be considered to increase fruit and vegetable intakes, as consumption of these food groups is barely possible for populations with low income. Whole grain, increased significantly since the first wave, which was in contrast with the trend of other MENA region countries . In the current study, intake of meat as a protein source was stable over the last decade, a finding contrary to the results of other Asian countries, including China and India, where meat intake had increased since westernization . It is important to note that with the growing rate of urbanization, study population consume more snacks and desserts. One of the important points of the current study is that dietary dairy intakes decreased significantly throughout the waves; this finding has been confirmed by the World Health Organization STEP wise approach to Surveillance (STEPS), which indicated that only approximately 18% of Iranian population meet the appropriate amount of dairy intakes .
Based on the results of the current study, the percent of study population who follow the Mixed dietary pattern has increased since the wave 1; this dietary pattern includes of simple sugar, tea and coffee, and whole grain, and is very similar to the traditional dietary pattern of Iranian population. Simple sugar and tea and coffee were loaded in all four waves, showing these food items to be deeply rooted in the traditional dietary pattern of Iranian culture. Interestingly, in the second and mostly the third wave unhealthy food items such as refined grain, and solid fat were added to the Mixed dietary pattern, possibly influence by modified Iranian meals like a western style dinner. The traditional Iranian diet is wheat-based. Tea is the major beverage and dairy products such as yoghurt and cheese was consumed widely ; however, the consumption of dairy products decreased since the previous decade. It is noteworthy that about 38% of our study populations try to maintain a healthy dietary pattern; however, intakes of food groups have changed and they accepted Western-style foods are more consumed, based on changing environmental factors, indicating that Iranians have modified their dietary pattern; in other words it is not totally westernized, although many of unhealthy foods have been added to the traditional dietary patterns of our study population.
The strength of the present study is as follow. Firstly, it was the first study investigating the secular trend of Iranian population dietary patterns and the adherence to these dietary patterns during the last decade. Secondly, the longitudinal design of the study, by which we could track changes in dietary patterns individually. Besides, we believe that our comparisons between years were not affected by the cohort effect, not only because all the subjects were included in at least two waves but also because we adjusted all the results by age in the first wave.
One of the limitations of the present study is that some food groups including dairy products and meat intakes were not categorized into subgroups. For example, dairy products include of low fat dairy, high fat dairy, or meats include of egg, red meat, processed meat, poultry, and fish. Therefore, it is not clear that higher factor loading of these food groups in one of the three dietary patterns is because of which sources of food items; however, we consider all the foods which were categorized in a same groups of the food pyramid. Another important limitation is that the number of foods available in the food supply exceeds by far the number of those available in food composition tables so the present study was unable to capture all the changes in dietary intake, particularly of packaged processed foods. Recall bias was also an inevitable problem when asking participants to remember and report dietary intakes. Finally, information about diet therapies for obesity had not been gathered by demographic questionnaire and was not in the scope of TLGS. Because of that we could not exclude participants with diet therapy for obesity.