Our finding from the current large-scale observational study indicated that the top category of the DASH diet might significantly associate with a lower level of serum TC, TG, LDL, and LDL/HDL ratio. Besides, high adherence to the MED diet might directly relate to increasing the HDL level after controlling potential confounders. Also, adjustment of confounding variables revealed a lower odds of LDL/HDL ratio in the third tertile of the MED diet. Moreover, in the high adherence to the DASH diet (third tertile), lower odds of high serum TC level and high LDL/HDL ratio were observed respectively.
Numerous studies have reported relations between the MED diet and the risk of CVDs (7, 16, 39). However, few have focused specifically on lipid profile as a primary outcome. Also, an update of meta-analysis which included 12 studies and 1,574,299 participants indicated that adherence to a MED diet was associated with a significantly lowered risk of mortality from cardiovascular diseases (40). Previous studies suggested that adherence to the MED diet might be associated with lower odds of high TC (21). Besides, in line with our findings, a high level of HDL with adherence to the MED diet was determined by another study (20).
A publication on the Nurses' Health Study cohort (NHS) over 24 y of follow-up suggests that adherence to a DASH-style diet might contribute to lowered risks of coronary heart disease and stroke in middle-aged women (35). Likewise, in the line with our results, the DASH diet has been effective in lowering plasma TC level and LDL/HDL ratio (41). It is believed that the DASH diet because of its beneficial components has a favorable effect on lipid profiles. Higher amounts of fruit and vegetables in the DASH diet that increase the content of dietary fiber and phytoestrogens might respond to its beneficial effects on serum TG, total cholesterol, and LDL levels (41–43). It is proposed that a high intake of legumes in the DASH diet might also be associated with its beneficial effects on lipid profiles (44). Also, the DASH diet contains higher amounts of non-hydrogenated vegetable oils that might contribute to favorable effects on lipid profiles. Several studies suggest that consuming edible vegetable oils have a modulation effect on blood pressure and serum lipid profiles (45, 46). Similar beneficial effects on serum levels of TC, TG, LDL, and LDL/HDL ratio were founded in our study as well.
It should be noted that we observed some differences between the effects of the MED diet and the DASH diet on blood lipids. In the present study, the MED diet was tended to be directly associated with a high level of HDL, simultaneously lower odds of LDL/HDL ratio, while there is not any significant association between MED and other lipid profile; Nevertheless, the DASH diet performed somewhat better than the MED, having significant inverse associations with high TC, TG, LDL levels, and LDL/HDL ratio. The difference in the study region between the current study and previous studies may be one of the main reasons why such results were reported. The MED dietary pattern in Mediterranean countries differs from other countries like Iran (47). Although the most consumed cereals belong to white rice and refined grains in Iran (48), the majority of people consume brown rice and whole grains in the Mediterranean region (47). Besides, the amount of fish intake and its cooking method is different between these regions which results in differences in the omega-3 intake. For instance, in Mediterranean countries, olive oil is consumed as the main component in their diet and also to prepare fishes. The nutritional value of fish may increase through elevated absorption of antioxidants, phenols, and vitamins by olive oil consumption (47). Nevertheless, Iranian people used corn and sunflower oil to prepare fish and other foods, which contain less proportion of unsaturated fatty acid especially oleic acid than olive oil (49).
An important question that arises concerning our findings is whether the non-protective effect of the MED diet is reliable, due to Iranian adults consume a low amount of whole-grain foods (< 10 g/day) and consume more refined grains (such as rice and white bread) that might affect our findings (50). Moreover, we tried to take lifestyle into account by adjusting all analyses for physical activity and smoking. However, the odds produced by the adjusted and unadjusted models were quite similar. This finding suggests we identified an effect of diet, rather than other factors, on odds of lipid profile, although we cannot rule out residual confounding because of the suboptimal measurement of these factors.
This study has several limitations as follows: First, our study was a cross-sectional study; thus, determining causal relationships between observed findings and lipid profile is not possible. We tried to minimize the potential confounding effects by excluding individuals with CVDs, type 2 diabetes, hypertension, cancer, and other chronic diseases since their serum blood lipids and also their dietary pattern might be affected. Likewise, participants that might have changed their dietary patterns because of illness were excluded. Second, we used semi-quantitative FFQs to collect dietary assessment data with trained interviewers. Although using a validated questionnaire, the nature of FFQ is likely to have misclassification. Third, the study was conducted on selected participants from Yazd Greater Area. Therefore, the generalization of our findings to the whole Iranian population should be done with caution. Moreover, although we tried to control the maximum number of potential confounding variables, residual confounding from unknown or unmeasured confounders cannot be excluded. On the other hand, the present study benefited from a large sample size, which might be its key strength. The study also exploited the information available on several non-dietary variables, allowing us to control for their supposed confounding effect in the analyses.