To our current knowledge, this is the first systematic review and meta-analysis to quantitatively summarize the published evidence on the association between greater adherence to the DASH diet and the risk of developing DM. Data from fifteen studies comprising 57,064 diabetes cases and 557,475 participants, were included in our analyse. The results indicated that high adherence to the DASH diet was associated with a 18% decreased in the risk of DM. In addition, the dose-response analysis showed a linear trend between DASH diet and DM. Sensitivity analysis showed that the pooled effect of adherence to the DASH diet on DM was not significantly altered by excluding a certain study. Overall, our findings add to the evidence of an inverse association between adherence to the DASH diet and DM risk, and support the adoption of adherence to the DASH diet as a primary prevention of DM.
Over the past four decades, the global epidemic of DM has continued to increase(38). According to the latest reports, as of 2021, an estimated 537 million adults worldwide are living with diabetes, 80% of whom live in low- and middle-income countries(3).This continued upward trend reflects the need for preventive measures. Among the multiple risk factors for DM, dietary factors, especially overall dietary patterns have received considerable attention(5). The whole dietary patterns have been determined using either a priori or a posteriori methods in nutritional epidemiology studies(12). For example, the DASH diet, as a priori dietary pattern, emphasizes high consumption of fruits, vegetables, whole grains, nuts and legumes, moderate consumption of low-fat dairy products, as well as low consumption of sodium, sweetened beverages and red and processed meats(14). To date, a substantial amount of epidemiological studies have been performed to examine the relationship between adherence to the DASH diet and DM risk(24–26, 34–37), but these findings remain inconsistent. In the Iran cohort study, Esfandiar and colleagues reported a 13% increased between adherence to DASH diet and the risk of DM (RR = 1.13, 95%CI: 0.88–1.46)(34). By contrast, in the Brazilian Longitudinal Study of Adult Health(ELSA-Brasil) study, Drehmer et al. observed that greater adherence to the DASH diet was not significantly related to the risk of newly diagnosed DM(OR:1.07; 95%CI: 0.84–1.36)(36). The inconsistencies observed in published studies might be explained by sociodemographic disparities between study populations and differences in the adjustment for potential confounders. Furthermore, to date, the relationship between adherence to the DASH diet and DM risk has not yet been studied in a systematic review and dose-response meta-analysis. In this context, identifying the association between adherence to the DASH diet and DM risk through a systematic review and meta-analysis would appear to have value.
In our analyses, we observed a significant negative association between adherence to the DASH diet and DM risk. In line with our findings, several prior studies have reported that “healthy” dietary patterns with some similar components to the DASH diet, were negatively associated with the risk of DM(39). Consistently, a previous meta-analysis of 10 prospective cohort studies including 404,528 participants, revealed that high adherence to “healthy” dietary patterns significantly decreased the risk of type 2 DM(RR:0.86; 95%CI:0.82–0.90)(40). However, in contrast to our findings, found no significant association between adherence to the DASH diet and DM(35–36). Although evidence associating DASH diet to DM remains inconsistent, some possible mechanisms have been reported to explain the observed negative association. First, the DASH diet emphazizing high consumption of fruits, vegetables, whole grains and legumes, is rich in dietary fiber. Increasing evidence has suggested that higher intake of dietary fiber was associated with a lower risk of type 2 DM(41). Moreover, Lattimer and Haub also reported that high intake of dietary fiber, especially soluble fiber, could delay gastric emptying and reduce the absorption of carbohydrates, thereby reducing postprandibular blood glucose and insulin levels(42). Second, vegetables and whole grains that are the most important foods in the DASH diet, have low glycaemic index and load. A recent systematic review and updated meta-analyses of prospective cohort studies showed that diets higher in glycemic index and glycemic load were robustly associated with the risk of type 2 DM(43). Third, high consumption of whole grains has been found to decrease the risk of overweight/ obesity(44), which are recognized risk factors for DM(45). Fourth, numerous studies have suggested that antioxidants, including vitamin C and carotenoids abundant in vegetables and fruits are associated with lower risk of obesity and hypertension, all of which are important risk factors for type 2 DM (46–47). Also, Evans and his colleagues found that dietary antioxidants could protect against oxidative stress accretion, thereby reducing insulin resistance and subsequently improving in insulin secretion (48). Fifth, the beneficial effect of adherence to the DASH diet on DM risk may be related to consumption of low-fat dairy products. Prori studies have demonstrated that higher intake of dairy, especially low-fat dairy, may decrease the risk of type 2 DM in men(49). Finally, the composition of the DASH diet with its emphasis on low consumption of red and processed meats has been repoted to be associated with DM risk. Shu and his colleguages reported that high consumption of red meat was associated with an increased risk of type 2 DM(50). As far as we know, red and processed meats are also rich sources of iron. Epidemiological studies have shown that excessive iron stores in the body may promote insulin resistance, which in turn increases the risk of type 2 DM(51). Likewise, processed meats often contain high levels of nitrates, or nitrites, as well as nitrosamines, which are thought to increase the risk of type 2 DM(50). In general, the aforementioned these mechanisms may explain the beneficial link between high adherence to the DASH diet and DM risk.
While there was a significant inverse relationship between adherence to the DASH diet and DM, high heterogeneity of the present study was also observed(I2 = 89.1%; P < 0.001). To this end, we performed subgroup analyses based on study design(cohort vs case-control studies), country (Western vs Asian countries), age(> 50y vs < 50y), and comparison(Q5 vs.Q1/Q4 vs.Q1/T3 vs.T1). The results suggested that significant heterogeneity might be mainly due to the differences in age and country. When the results were stratified by age and country, heterogeneity decreased from 89.1–64.6%, 55.0%, respectively. Although significant heterogeneity cannot be fully explored, there are several possible explanations for the high heterogeneity. First, given the differences in DASH diet in Eastern and Western populations, although RRs/HRs/ORs were from the highest category(with the lowest category as a reference), the definition of DASH diet may vary slightly in different studies, leading to significant heterogeneity. Second, the results were pooled from different populations with different dietry habits, which might result in significant heterogeneity. Additionally, fourteen of included studies used FFQs to collect dietary data. Thus, recall bias about dietary intakes is inevitable. Third, despite adjustments for potential confounders have been included in all the included studies, a certain degree of residual or unmeasured confounding factors may exsit. Finally, significant heterogeneity remained in subgroup analyses, indicating the presence of some unmeasured confounding factors.
Strengths and limitations
This systematic review and meta-analysis has several strengths and limitations. First, this is the first systematic review and dose-response meta-analysis so far evaluating the association between adherence to the DASH diet and DM risk. Our findings add the available evidence that high adherence to the DASH diet can reduce the risk of DM. Second, the rigorous selection of articles was carried out according to the pre-determined inclusion and exclusion criteria. Third, there were no obvious signs of publication bias in the funnel plot, and the Egger’s and Begg’s tests for publication bias were non- significant. Fourth, the quality assessment showed that the majority of studies included in the present meta-analysis were of high quality. Fifth, subgroup and sensitivity analyses were used to further explore the potential sources of heterogeneity, thereby improving the accuracy of the study results. Finally, the reported ORs/RRs /HRs were multivariate and all included studies had adjusted for some potential confounders, including age, physical activity and total energy intake, which can affect the relationship between adherence to the DASH diet and DM risk. Despite these strengths, several limitations should be taken into account when interpreting our findings. First, in our analyses, two of included studies used the case-control design, which was more susceptible to recall and selection bias, than cohort design. In addition, potential confounding by pre-existing and undiagnosed diseases should be acknowledged. Thus, prospective cohort studies or randomized controlled trials are needed to further confirm the exact association between DASH diet and DM risk. Second, fourteen of the included studies used an FFQ to collect dietary intake data, which may have caused misclassification and resulted in the under-or overestimation of DASH diet consumption. In addition, dietary intake data were self-reported and this might also lead to recall and selection biases. Third, the levels of the highest and the lowest categories of DASH diet scores were inconsistent in the included studies, which might have attenuated the true association between adherence to the DASH diet and DM risk. Also, adherence to DASH diet may have altered over the follow-up. Studies reported that adherence to DASH diet can change over time because of change in socioeconomic factors such as poverty, globalization, and more access to energy-dense foods. Fourth, significant heterogeneity was found in this study. Although we performed subgroup and sensitivity analyses to explore the potential sources of heterogeneity, we could not ascertain and explain the sources of inter-study heterogeneity sufficiently. Fifth, although high adherence to the DASH diet was associated with decreased risk of DM, the results should be interpreted with caution. Meanwhile, there was also an inconsistent adjustment for potential confounders in the included studies. As a result, the data included in our analyses may suffer from differing degrees of completeness and accuracy. Finally, this study had a geographical restriction, as the majority of included studies came from the United States, where the dietary intakes were markedly different from the Asian countries. This leaded to a reduction in the heterogeneity of this meta-analysis and hence, further large prospective studies and randomized controlled trials are needed to confirm our findings in different regions and populations.