The results of the present study, which was conducted among Iranian adolescents, showed a significant association between fast food consumption and higher BMI and waist circumference. The relationship between some nutrients intake such as protein, carbohydrate, SFA, and vitamin C and fast food consumption was also significant. According to the results, NARs for vitamin B1, phosphorus, selenium, and vitamin B2 were lower among those in the highest quartile of fast food consumption compared to lowest quartile. Adolescents in the highest quartile of fast food intake had the highest dietary energy density.
The general concept of fast food may be different in Iran in comparison to western countries. Some Iranian fast foods may be similar to US fast foods like hamburgers, but others may be different. Sausage, calbas, hamburger, and pizza are the most popular consumed fast food among Iranians [10
]. These foods are not offered as a specific commercial brand; each outlet sells its production. So, we have different brands of the mentioned fast foods in Iran. Different producers provide sausages, calbas, hamburgers, and pizza. We have these products in the form of ready to eat in supermarkets and fast food restaurants in Iran. Rusk fish and rusk poultry are also available and usually consumed by fast food consumers. To prepare these foods, a slice of fish/poultry was imbued with rusk powder then it was inundated with oil for deep frying. So they are a rich source of fat, especially trans fatty acids as they are fried with partially hydrogenated vegetable oil [10
]. Because of religious limitations, the content of Iranian sausage and calbas is free of pork meat and lard. Poultry, beef, and veal are used instead of pork meat.
In the present study, fast food consumers had higher BMI and higher waist circumference. Although Schröder et al. drew the same conclusion for the association between fast food consumption and BMI [20
], the current study was conducted in adolescent girls but their study population in Schröder et al. was Spanish subjects aged 25–74 years old. Another study indicated a positive association between fast food intake and BMI in both cross-sectional and longitudinal analysis among young adults [21
]. Probable hypothesis is that more fast food consumption is associated with more energy intake from non-fast-food and fast food sources [20
]. One study exhibited that visiting fast food outlets increased the amount of sugar sweetened beverages intake [22
]. For instance, a significant direct association between fast food intake and sugar sweetened beverages (SSB) has been reported [23
]. Beyond the high sugar content of SSBs, these kinds of beverages may decrease satiety and increased subsequent food intake [21
]. Such findings may explain the mechanism of the relationship between increased energy intake from non-fast-food sources and fast food consumption. The results of the present study revealed that association between fast food intake and obesity was not totally mediated by energy intake. Fat content of fast foods might also have a role in this association. Iranian fast foods are rich source of trans fatty acid and SFAs [10
]. Studies reported the link between trans fatty acids intake and obesity [24
The relevance between dietary quality and fast food consumption was evaluated in current study by some indices. The energy-adjusted distribution of nutrients across quartiles of fast food consumption showed that adolescents in the highest quartile consumed more amount of protein, carbohydrate, fat, saturated fatty acid, and vitamin C. Previous studies have illustrated that saturated fatty acid content of Iranian fast food is high [10
]. Recent data showed that more fast food consumption was related to higher fats and processed meats intake [25
]. High amount of processed meat in different kinds of fast foods can explain the reported positive relationship between fast food consumption and protein intake. Consumption of fast foods is almost accompanied with vegetable salads (e.g., lettuce and cruciferous), pepper, and tomato sauces. On the other hand, fruit and vegetables are the best sources of vitamin C [26
]. So higher vitamin C intake may result from this accompaniment. In the present study dietary quality was inversely associated with fast food intake because NARs for vitamin B1, phosphorus, selenium, and vitamin B2 decreased across quartiles of fast food consumption. White bread is another food consumed along with fast foods in Iranian dietary habits. Thiamine content of white bread is less than whole wheat bread [27
]. Moreover, the conversion of phytic acid to its soluble form in unleavened bread commonly consumed among Iranians is less than leavened ones [28
]. So the content of the dietary phosphorus in these kind of breads have been decreased. As we know, dairy products are considered as vitamin-B2
-rich foods [29
]. On the other hand, Dough, an Iranian traditional beverage derived from yoghurt, is usually replaced with SSBs when subjects consume fast food. So, lower vitamin B2
intake can also be predicted.
Dietary energy density, another index for dietary quality, showed a statistical significant association with fast food intake. Fast foods are considered as energy dense foods and similar relationship between dietary energy density and fast food intake was reported in previous studies [20
In the current study, we have disclosed a significant energy-adjusted positive correlation coefficients between obesity-related anthropometric variables, that is, BMI and waist circumference and fast food consumption. A positive association between the frequency of fast food intake and body weigh after 15-year followup has also been reported from a prospective study [21
]. Cross-sectional studies have reported a same association for fast food consumption and total energy intake [31
]. Frying oils and partial hydrogenated vegetable oils (PHVO) rich in trans fatty acids are used for preparing fast foods in Iran. Previous study showed that consumption of PHVO is a risk factor for systemic inflammation, cardiovascular disease [32
], and obesity [33
The energy density of the fast foods may explain this correlation [34
]. The energy density of fast foods was higher than the recommended energy density in a healthy diet (1100
g versus 525
]. One study suggested that we should choose healthier fast foods with lower energy, fat content, and energy density rather than traditional fast foods [35
]. Low-energy dense diets like high fiber diets could reduce weight and waist circumference [36
]. However, high-energy dense diets could increase the prevalence of obesity and abdominal adiposity [37
These results were affected by some limitations. Due to cross-sectional nature of the present study, causal relationship could not be concluded [38
]. So, prospective studies should be run in this population for more reliable data. Moreover, we used a FFQ consist of 53 food items for dietary assessment. Although the validity and reliability of this FFQ had been assessed, the variation of food choices might be more than 53 food items. Furthermore, our FFQ was not designed to assess the SSBs. Simultaneous evaluation of the SSBs intake and fast food consumption may disclose more aspects of fast food dietary pattern. Small sample size is another limitation. However the sample was drawn through well-conducted random sampling and the study finding showed statistically significant associations. However, data from large sample size studies are more credible.
Several strength points should be addressed regarding the present study. Dietary intake was assessed by a validated FFQ. There are few epidemiological studies about the association between fast food intake, obesity, and dietary quality among this population. Evaluating the diet quality by several indices, that is, NARs, energy density and nutrient intakes, is another strength point.
In conclusion there is a relationship between fast food consumption, obesity, and diet quality indices among Iranian adolescents.