Cardiovascular diseases are the primary cause of death in many countries, including Iran. There is a known relationship between serum lipid concentration and atherosclerosis lesions (1). A 25% reduction in cholesterol concentration reduces the incidence of coronary events by about 50% (2). Blood lipids control, especially low-density lipoprotein, can lead to ischemic events reduction. High blood lipids including high total cholesterol (TC), low-density lipoprotein cholesterol (LDL), and triglycerides are known risk factors for heart disease. The risk of death due to cardiovascular disease decreases by increased high-density lipoprotein which is a protective factor (3).
Today, the incidence of hyperlipidemia is increasing due to changes in lifestyle along with the industrialization of societies and changes in dietary habits, and a reduction in physical activity. Abnormal and chronic increase of cholesterol and triglyceride leads to a lot of problems, including stenosis and occlusion of the vessels in different parts of the body, especially in the heart (4). Currently, various drug groups are used to reduce hyperlipidemia. These drugs can cause gastrointestinal complications such as indigestion, flatulence, constipation, diarrhea and abdominal cramps (5). To our knowledge, there has been no study investigating the complications of dill extract.
 In recent years, there has been a trend towards the use of herbal medicine in the treatment and prevention of diseases, with a dramatic increase worldwide, especially in Iran. Dill is one of the plants used in herbal medicine (6). Medicinal plants have played an important role in treating diseases throughout the world for a long time. Currently, there is a lot of interest in restoring the use of herbal medicines in the form of standard extracts, partly due to numerous side effects and high costs of chemical drugs (7).
Dill is a perennial herb of the Apiaceae family, which is found in most parts of the world (8). Several pharmacological effects have been identified for dill, such as anti-hypertensive (9-11), antiseptic, anticonvulsant, anti-flatulence, anti-vomiting, antispasmodic, pain relief and blood lipid level reduction (12). The extract of this plant has also been shown to be antibacterial (13), antifungal, anticonvulsant, antioxidant, and anti-cancer as well as having, stomach acid secretion and glucose, lipid and cholesterol reduction effects (14). Several studies have examined the effect of dill extract on serum lipid levels (8, 10, 11, 15).
Despite the significant effect of the plant in some clinical trials and contradictions in some other studies, no meta-analysis has been carried out so far. This study aimed to determine the general effect of dill on blood lipid levels, which is of great importance for clinical interventions.

 

 
Figure1. Flowchart of reviewed and searched articles

Of the 30 articles in the database, 12 clinical trials met the meta-analysis criteria. The characteristics of these studies are shown in Table 1.
Of the total studies, the overall reduction in cholesterol after administration of dill extract was estimated in 3 studies: statistically insignificant in Kazemi and Mansuri’s studies but the reduction was significant in Mobasseri’s (p <0.05) (16,18,19). By combining the results of the three studies and using the random effects model, it was shown that the extract had a significant effect on cholesterol reduction. The standard mean difference (SMD) in cholesterol reduction was SMD = -0.56 (95% CI: -0.90, -0.22) (Figure 2(a)). In three studies, using a randomized model with a sample size of 69 people, the dill extract showed an average of 19.22 mg / dl cholesterol reduction (Figure 2(b)).

Table 1. Characteristic of studies included in the review

A

B
Figure2. Standard mean difference (SMD) (a) Weighted mean difference (WMD) (b) of total cholesterol (TC) reduction before and after intervention in the case group. Squares show point estimate of TC and lines show 95% CI TC in each study.  The diamond shows pooled estimate of studies.

Triglycerides were decreased in Kazemi, Yazdanpanah, Mansouri and Mobasseri’s studies, but they were not statistically significant in all studies (16-19). By combining the results of the four studies and using the random effect model, it was shown that dill extract had a significant effect on the triglyceride reduction. The standard mean difference of triglyceride reduction was estimated to be (0.95% CI: -0.62, -0.04) SMD = -0.33 (Figure 3(a)). In the four studies examined, using a randomized model with a sample size of 91 subjects in the case group, it was shown that dill extract reduced blood glucose levels by an average of 25.47 mg / dl (Figure 3(b)).
In the study of Kazemi, Mansouri and Mobasseri, low density lipoprotein (LDL) was decreased using dill extract. LDL reduction was statistically significant in Kazemi and Mobasseri’s studies (p <0.05) (16,18,19). However, in Mansuri's study, LDL reduction was not statistically significant. By combining the results of the three studies using the random effects model, dill had a significant effect on LDL reduction. The standard mean difference in LDL reduction, was estimated to be SMD = -0.60 (0.95% CI: -0.94, -0.25) (Figure 4(a)) (19). In three studies, using a randomized model with a sample size of 69 subjects in the case group, it was shown that the dill extract reduced LDL by an average of 14.01 mg / dl, (Figure 4(b)). The Weighted mean difference in LDL reduction before and after the intervention was WMD = -14.01 mg / dl (95% CI: -22.14, -5.89), which was statistically significant (p <0.05).

A

B
Figure 3. Standardized mean difference (SMD) (a) and Weighted mean difference (WMD) (b) of triglyceride reduction before and after intervention in the case group. Squares show point estimate of TG and lines show 95% CI TG in each study.The diamond shows pooled estimate of studies.
 

A

B
Figure 4. Standard mean difference (SMD) (a) and Weighted mean difference (WMD) (b) of LDL reduction before and after intervention in the case group .Squares show point estimate of LDL and lines show 95% CI LDL in each study.The diamond shows pooled estimate of studies.
 

Discussion

In a systematic review, a number of articles were identified during the search. After reviewing the titles of the articles, article abstracts and various aspects examined in the study were entered into a checklist. After a final evaluation, the researcher obtained 12 articles and their full text. Systematic review and meta-analysis of the findings obtained from clinical trials indicated effectiveness of dill in reducing lipid levels in the body. Our results showed that dill reduced cholesterol, triglycerides, and low-density lipoprotein (LDL), which was statistically significant; however, there was no significant effect on high-density lipoprotein (HDL).
Given that the accuracy of the conclusions in meta-analysis studies based on the mean and standard deviation of the studies before and after the intervention was higher compared with the time when the p-values were combined, it seems that dill has no significant effect on HDL, but its effect on cholesterol, triglycerides and LDL is statistically significant. Based on the mean and standard deviation of the studies, none of the variables was significant in the placebo group. Similarly, by combining p-value results of the studies in placebo group, it was shown that only cholesterol and triglycerides were statistically significant.
The results of the Hajj Hashemi and the Abbasi studies showed that the extract of aerial part of dill decreased triglycerides and low density lipoprotein in male rats (8). Madani et al., reported that the aerial part of dill extract had a significant effect on decreasing the concentration of low density lipoproteins and triglycerides in diabetic animals (24).
The exact mechanism for reduction of blood lipids by dill extract is unknown, but some researchers have reported that the reduction effect of the aerial part of dill extract may be related to the flavonoid composition of quercetin (26). Based on the results of this study, quercetin in dill significantly reduces glucose, cholesterol and low density lipoprotein (26-28). Blood lipid
 reduction after dill use was reported differently in various studies. Given the effect of the dose and duration of the use, this may probably be the main reason for the difference in blood lipid levels in different studies.

 

Limitation

There were some limitations in this article including the access to full text of some articles. Incomplete reported information in some studies, and variations in quality of articles. The main limitation of this meta-analysis, like any other overview, is that the participants in the study and the results in the various studies are not completely similar.

 

Conclusion

Dill reduces cholesterol, triglyceride, and low-density lipoprotein, but does not have a significant effect on high-density lipoprotein levels. With regard to the results of this study, it is recommended that dill be used in the treatment of hyperlipidemia because of lower side effects and low price, however, because complications are not investigated, the recommendation needs more evaluations and studies.

 

Acknowledgements

None.

 

Conflicts of Interest

There is no conflict of interest.