This retrospective cohort study was conducted to investigate the association between dietary indices and dietary acid load with CVD mortality in cardiovascular patients of Fasa Persian cohort. The results of this study showed that the CVD mortality rate increased significantly with age and simultaneous onset of hypertension; there was also a significant positive relationship between tobacco use and mortality from cardiovascular diseases. However, it was found that with increasing serum HDL levels, the mortality rate of CVD decreased significantly. In addition, there was a direct and significant association between DII score and dietary acid load with CVD mortality; thus, with an increase in DII score and dietary acid load, the rate of CVD mortality increased by 11% and 2%, respectively, while there was no significant correlation between AHEI and MDS scores with CVD mortality. The current study also revealed that adherence to the DASH diet in cardiovascular patients can reduce the risk of CVD mortality by 20.4%, but this decrease was not statistically significant although it is clinically significant.
In line with our study, Jibin et Al. (17) linked tobacco use and hypertension levels to mortality due to heart disease, stroke, and IHD. Nicotine and carbon monoxide available in tobacco promote the development of atherosclerosis by affecting myocardial oxygen capacity and increasing endothelial damage. Previous studies have also shown that tobacco use is closely linked to high hypertension and stroke. In addition, one of the main risk factors for heart disease is hypertension, and the severity of this disease can be controlled by reducing it.(17, 18)
This study also showed an inverse relationship between serum HDL levels and CVD mortality rate in patients. In this regard, Chantal et al. (19) reported high levels of serum HDL as a protective factor against mortality and cardiovascular diseases in people with diabetes. HDL plays a protective role against cardiovascular diseases and their mortality by clearing cholesterol from the macrophages and increasing endothelial function and antioxidant activities.(19)
Similar to our study, Hoge et al. (8) and Shivappa et al. (9) observed a direct association between CVD mortality and DII score. To explain this association, we can point to the relationship between high DII scores and increased risk of obesity, metabolic syndrome, and insulin resistance. (9, 20–22) These chronic diseases are associated with inflammatory conditions in the body. Inflammatory biomarkers including hs-CRP, IL-6, IL-1B, TNF-α, IL-4, and IL-10 have also been shown to be connected with obesity, diabetes, and CVD. In this regard, previous studies have linked increased DII scores to high levels of cytokines such as IL-1, TNF-α, and CRP. On the other hand, the guidelines of AHA published in 2019 have introduced obesity and diabetes as two risk factors related with CVD mortality.(9, 23, 24)
According to the AHA-2019 guideline, energy, saturated and trans-fatty acids, cholesterol, red meat, and refined grains in the diet not only increase the DII score, but are also linked to increased mortality due to CVDs.(24) In contrast, the anti-inflammatory compounds such as vitamins C, E, beta-carotene, and zinc are associated with a decrease in DII score, and the antioxidant role of these compounds is a factor in the primary and secondary prevention of cardiovascular diseases. (8) In this regard, omega-3 fatty acids and polyphenols (found in vegetables and fruits) are associated with a decrease in DII score and inflammation rate in the body. They are involved in regulating the body's inflammatory processes, improving lipid profile, oxidative stress and endothelial function, and in this way, decreased the risk of chronic disorders such as cardiovascular disease. As a result, if dietary intake leads to inflammatory conditions, it can increase the risk of platelet aggregation and plaque formation at the endothelial cell, predisposing a patient to vascular damage and occlusion and increasing the risk of mortality.(22)
The results of our study demonstrated that there was a significant positive association between dietary acid load and risk of CVD mortality, so that by increasing a score in dietary acid load, the rate of CVD mortality increased by 2%. In of the same line with the present study, Shamima et Al. (6) detected a strong relationship among PRAL, NEAP scores, and CVD mortality rates. Minseon et al. (25) also linked an increased dietary acid load and a higher risk of CVD and all-cause mortality. Previous researches have shown that an increase in consumption of meat and its products, eggs, cheese, refined grains and fish, and a decrease in consumption of fruits and vegetables lead to an increase in the dietary acid load and risk of chronic diseases, including hypertension, cardiovascular diseases and type 2 diabetes and their mortality due to these diseases.(6, 25) In contrast to the consumption of potassium bicarbonate, magnesium, fiber, vitamin C, calcium and phytochemicals, which exist abundantly in fruits and vegetables (as part of a healthy diet); are probably associated with reduced dietary acid load and a lower risk of cardiovascular diseases.(6, 7, 16) Partial examination shows that a rise in acid load of diet, which is associated with an increase in blood acid levels, leads to the excretion of sodium in the body and an increase in cortisol secretion and insulin resistance. In this regard, the serum level of potassium reduces due to increased potassium excretion, which leads to an increase in hypertension by affecting vasodilation.(26) On the other hand, increased cortisol levels are related with metabolic syndrome and increased cardiovascular diseases and mortality risk.(27, 28) Insulin resistance is also caused by a reduced tendency for insulin to bind to its receptor in an acidic environment, which is directly related to the risk of CVD and all-cause mortality. (29, 30)
Phillips et al. (31) and Fung et al. (32) showed when the adherence to the DASH diet increases, the risk of cardiovascular diseases and mortality decreases. This decrease is due to the consumption of more vegetables, fruits, nuts and seeds, whole grains and low-fat dairy products and restrictions on the consumption of red meat, sugars, sweets, beverages, and total and saturated fat. The mechanism of impact of this diet can be attributed to its compounds such as potassium, sodium, magnesium, calcium, fiber, and antioxidants. In this diet, high fiber intake leads to reduction in the levels of LDL, cholesterol, triglycerides, hypertension, CRP, and the risk of obesity and overweight, and improves insulin sensitivity and endothelial function.(4) DASH diet with emphasis on magnesium and potassium sources such as dark green leafy vegetables, seeds, nuts, and whole grains can play a role in controlling hypertension and stroke. Magnesium and potassium reduce inflammatory cytokines and increase nitric oxide levels, thereby affecting the severity of heart diseases and mortality due to them.(33, 34) According to our results, it was observed that adherence to the DASH diet in cardiovascular patients could reduce the risk of CVD mortality by 20.4%; however, this decrease was not statistically significant. In the same line with our study, Ant Aigren et al.\ did not found a strong relationship among DASH score and mortality from stroke.(3)
In the present study, we did not find a significant correlation between AHEI score and CVD related deaths. Contrary to our study, Emily et al. (35) reported that higher AHEI score led to lower risk of cardiovascular diseases and mortality by 16% and 34%, respectively. The AHEI score can assess the quality of the diet and predict the risk of death due to chronic diseases. The index measures consumption of whole grains, fruits, vegetables, fish, dairy, processed red meat, and alcohol. Also, Akbaraly TN et al. (5) in their study found that higher AHEI score had a significant inverse relationship with CVD mortality, while similar to the current study, no significant association was observed between this score and cancer related mortality. The most important reason for the inconsistency of the results of our study with other studies can be the short follow-up time.
In their study during 4 years, Emily et al. (4) recorded 1385 mortality cases from HF. In their study, similar to ours, there was no statistically significant relationship between MDS score and mortality. While Hodge et al. found that the more the adherence to the Mediterranean diet, the lower the risk of mortality at a young age. The Mediterranean diet improves systolic hypertension and lowers CRP, fibrinogen, oxidative stress and serum cholesterol, thereby reducing the incidence of chronic disorders and their mortality.(8, 34)
One of the main weaknesses of this study was the short follow-up time. Also, in this study, recall diet was not examined for nutritional assessment and only FFQ was used. Another weakness of this study was the recording of nutritional data in the baseline state and no other dietary record was recorded during the follow-up period; as a result, we were unable to assess the patients' dietary changes. Using self-reporting for food evaluation can also increase overestimation and underestimation. In this study, we failed to assess important factors such as disease severity, type of treatment during illness, and evaluation of health care that are associated with mortality. One of the strengths of this study is the study of dietary indices instead of evaluating specific micronutrients in the controls and inhibit of cardiovascular disease and its mortality. Also, in this study, four important indices were examined simultaneously, and this is of great importance. To summarize and comment definitively in this regard, it is suggested that studies with more follow-up years and the use of cohort studies should be carried out in other regions simultaneously.