Efficacy of DHA and EPA on Serum Triglyceride Levels of Healthy Participants : Systematic Review

Background Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are categorized as omega-3 poly unsaturated fatty acids (PUFAs) that are present in fish oil, etc. DHA and EPA omega-3 PUFAs have a well-established fasting serum triglycerides (TG) lowering effect that may result in normal lipidemia in hyperlipidemic patients. In general, omega-3 PUFAs, such as DHA and EPA, can be ingested easily, and because they are highly safe, they are assumed to be suitable for controlling fasting serum TG in the serum of those who do not require drug treatment. To the best of our knowledge, however, almost all systematic reviews on the effects of omega-3 PUFAs on lowering fasting serum TG are directed at patients fulfilling the diagnostic criteria of dyslipidemia. Objectives To review and confirm the preventive effect of omega-3 PUFAs against hypertriglyceridemia or the effect on nondrug treatment in patients with a mild disease, a systematic review was conducted to determine whether there was a fasting serum TG-lowering effect in subjects without disease and those with a slightly higher triglyceride level who consumed DHA and/or EPA orally compared to those with placebo or no intake of DHA and/or EPA. Search Methods We evaluated articles from searches of PubMed (1946-February 2016), Ichushi-Web (1977-February 2016), and J Dream III (JST Plus, 1981-February 2016; JMED Plus, 1981-February 2016). The keywords were set as follows: “DHA” or “docosahexaenoic acid” or “EPA” or “eicosapentaenoic acid” and “TG” or “triglyceride” or “triglycerol” or “triacylglycerol” or “neutral lipid.”. In addition to the literature group obtained by the database search, we included participants not suffering from any disease (i.e., excluding mild hypertriglyceridemia). Eligibility Criteria Before the test selection process, the following inclusion criteria were defined. Participants were healthy men and women including those with mild hypertriglyceridemia (fasting serum TG level, 150-199 mg/dL [1.69-2.25 mmol/L)). Intervention was defined as orally ingested DHA and/or EPA. Comparison was made to placebo intake or no intake of DHA and/or EPA. Results were measured for the fasting serum TG level. The test design was RCT, and quasi-RCT. DOI: 10.14302/issn.2379-7835.ijn-18-2469 Freely Available Online www.openaccesspub.org | IJN CC-license DOI : 10.14302/issn.2379-7835.ijn-18-2469 Vol-3 Issue 2 Pg. no.23 Introduction Cardiovascular disease (CVD) is the leading cause of death worldwide and acts as a major barrier to sustainable human development. To address this major global health concern, in 2011, the United Nations officially recognized several noncommunicable diseases, including CVD, and set up an ambitious plan to dramatically reduce the impact of these diseases in all areas [1]. Hypertriglyceridemia is a type of dyslipidemia characterized by an elevated serum triglycerides (TG] level and has been reported by several prospective studies and randomized controlled trials (RCTs) to be a risk factor for CVD. An increased level of circulating TG is an independent risk factor for the onset of CVD. Hokanson and Austin reported that a fasting serum TG level of 88 mg/dL or more increases the risk of CVD development by 14% and 37% in men and women, respectively [2]. Therefore, lowering or maintaining a low level of fasting serum TG level reduces the risk of CVD. Fatty acids are comprised of lipids, which are present in almost all parts of the human body. Fatty acids are divided broadly into two categories, saturated and unsaturated fatty acids. Unsaturated fatty acids are further classified into two categories: monounsaturated and poly unsaturated fatty acids (PUFAs). The PUFAs are further divided into two categories: the omega-3 series (metabolic cascade starts with α-linoleic acid (ALA)) and omega-6 series (metabolic cascade starts with linoleic acid (LA)). Docosahexaenoic acid (DHA) and Eicosapentaenoic acid (EPA) are categorized as omega-3 fatty acids [3]. Certain fatty acids, such as ALA and LA, cannot be synthesized in humans, and thus must be obtained in the diet. ALA, a type of omega-3 fatty acid, is converted into DHA and EPA in the body. DHA and EPA also exist naturally in some foods. LA, which is a type of omega-6 fatty acid, is converted to arachidonic acid (AA). DHA and EPA are derived from ALA by a similar biochemical pathway as AA. Omega-3 fatty acids generally lower fasting serum TG levels and very low-density lipoprotein (VLDL) levels in serum among hyperlipidemic patients. Correspondig author: Yohei Kawasaki, Biostatistics Section, Clinical Research Center, Chiba University Hospital, 1-8-1 Inohana, Chuo-ku, Chiba-shi, Chiba, 260-8677, Japan, Email: ykawasaki@chiba-u.jp Running title: Efficacy of DHA and EPA on serum triglyceride levels of healthy participants


Introduction
Cardiovascular disease (CVD) is the leading cause of death worldwide and acts as a major barrier to sustainable human development.To address this major global health concern, in 2011, the United Nations officially recognized several noncommunicable diseases, including CVD, and set up an ambitious plan to dramatically reduce the impact of these diseases in all areas [1].
Hypertriglyceridemia is a type of dyslipidemia characterized by an elevated serum triglycerides (TG] level and has been reported by several prospective studies and randomized controlled trials (RCTs) to be a risk factor for CVD.An increased level of circulating TG is an independent risk factor for the onset of CVD.
Hokanson and Austin reported that a fasting serum TG level of 88 mg/dL or more increases the risk of CVD development by 14% and 37% in men and women, respectively [2].Therefore, lowering or maintaining a low level of fasting serum TG level reduces the risk of CVD.
Fatty acids are comprised of lipids, which are present in almost all parts of the human body.Fatty acids are divided broadly into two categories, saturated and unsaturated fatty acids.Unsaturated fatty acids are further classified into two categories: monounsaturated and poly unsaturated fatty acids (PUFAs).The PUFAs are further divided into two categories: the omega-3 series (metabolic cascade starts with α-linoleic acid (ALA)) and omega-6 series (metabolic cascade starts with linoleic acid (LA)).Docosahexaenoic acid (DHA) and Eicosapentaenoic acid (EPA) are categorized as omega-3 fatty acids [3].
Certain fatty acids, such as ALA and LA, cannot be synthesized in humans, and thus must be obtained in the diet.ALA, a type of omega-3 fatty acid, is converted into DHA and EPA in the body.DHA and EPA also exist naturally in some foods.LA, which is a type of omega-6 fatty acid, is converted to arachidonic acid (AA).DHA and EPA are derived from ALA by a similar biochemical pathway as AA.Omega-3 fatty acids generally lower fasting serum TG levels and very low-density lipoprotein In regard to low-density lipoprotein (LDL) level, omega-3 fatty acids increase it or had no influence among the subjects.
Since it has five cis-type double bonds, the molecule is not a linear structure; hence, its melting point is low and it is easily oxidized.It is almost odorless just after purification, but it undergoes auto-oxidation quickly in air and begins to smell.Peroxide is also unstable, and the volatile component is comprised mainly of the carbonyl compound of the secondary product due to the polymerization and decomposition that causes a fishy odor.It is widely distributed as a major constituent of the fatty acids in marine organisms, such as fish, mollusks, crustaceans, seaweed, and microorganisms.
In particular, various sardines, mackerels, saury, and so forth which are blue-backed fish.
DHA is also a PUFA and has 22 carbon atoms and six double bonds, and is abbreviated as 22:6 omega-3.It is the final metabolite of omega-3 PUFA, with the first double bond on the third carbon counted from the methyl group end and starting from ALA (18:3 omega-3).Since it has six cis double bonds, it has a large curved molecular structure; hence, the melting point of a DHA-containing lipid is low, such as is the case for EPA.Moreover, it is extremely easy to oxidize, and readily generates a fishy odor that is mainly composed of a carbonyl compound.DHA is present in various marine animals and microorganisms, including fish, crustaceans, mollusks, microorganisms, etc. Fish At present, there is well-known scientific agreement that omega-3 fatty acids intake should be increased and omega-6 fatty acid intake should be decreased to promote health; however, it is unknown whether the desired ratio of omega-6 and omega-3 fatty acids exists in meals, and how much omega-6 fatty acid ingestion is necessary to inhibit omega-3 production when large amounts of omega-6 are ingested.

Researchers at the Tufts Educational Policy
Committee reviewed the database of the Third National

Health and Nutrition Examination Survey (NHANES III;
1988-1994) and investigated the intake of omega-3 fatty acids in the United States.ALA intake was significantly lower in males than in females, and greater in adults than in children.It became clear that there were fewer subjects with CVD than without a history of CVD.Only 25% of the population ingested DHA and EPA in a given day.The average daily intake was 14 g for LA, 1.33 g for ALA, 0.04 g for EPA, and 0.07 g for DHA.
ALA is present in green leafy yellow vegetables, nuts, vegetable oils (such as canola and soybean oils), and especially linseed or linseed oil.Good sources of DHA and EPA include seafoods (fish, crustaceans, mollusks, seaweeds and their oils and fish eggs).LA is present in several foods consumed by Americans, such as meat and vegetable oils (safflowers, sunflowers, corns, soybeans, and so forth), as well as processed foods using these oils.Daily consumption of ALA recommended by the Institute of Medicine was set at 1.1-1.6 g and LA at 11-17 g for adults, but the daily adequate intake of DHA and EPA were not set [4].

Eligibility Criteria
The following inclusion criteria were defined prior to the test selection process: Participants were healthy adult men and women including those with mild hypertriglyceridemia (fasting serum TG level, 150-199 mg/dL (1.69-2.25 mmol/L)).[2] Intervention was defined as orally ingested DHA and/or EPA.[3] A comparison was made for placebo intake or no intake of DHA and/or EPA.[4] Results were measured according to the fasting serum TG level.[5] The test design was RCT, and quasi-RCT.
Based on these requirements, two reviewers (Y.T and H. M) independently selected studies and extracted data regarding the study characteristics and outcomes from the selected studies.

Result
We found 812 reports from the database retrieval, collections, and other cited references.A total of 53 duplicated studies were excluded.We selected 193 of 759 reports that were at the primary (title and summary) screening stage.Finally, 37 reports meeting the eligibility criteria were extracted at second (full text) screening stage.Figure 1 summarizes the selection process steps.Characteristics of the 37 documents selected are listed in Table 1 together with bibliographic information.Fasting serum TG levels of control and intervention groups of the 37 reports are listed in Furthermore, there was no evidence of harmful effects due to the intake of DHA and/or EPA.

Discussion
The aim of this study was to confirm the preventive effect of DHA and/or EPA on hypertriglyceridemia or the effect on nondrug treatment for people with a slightly higher fasting serum TG level.
A systematic review examined whether oral DHA and/or EPA compared to placebo or no DHA and/or EPA would lower serum TG levels in participants without disease and for those with a slightly higher fasting serum TG level.Among the 37 RCTs, there were 16 healthy subjects and the remaining 21 subjects had slightly higher fasting serum TG levels.Among the former 16 RCTs, significant differences were found in the five double-blind RCTs with a high evidence level, and four studies suggested a lowering effect, although there were no significant differences.Considering that a ceiling effect exists for healthy subjects, this result might suggest the magnitude of the preventive effect of DHA and/or EPA.Among the 21 RCTs targeting people with for the synthesis of TG [46].Furthermore, omega-3 fatty acids are assumed to inhibit TG synthesis in the liver by inhibiting important enzymes involved in hepatic TG synthesis, such as phosphatidic acid phosphatase and diacylglycerol acyltransferase [47].Moreover, it has been reported to increase the removal of fasting serum TG from circulating VLDL and chylomicron particles [48,49].
DHA and EPA, the major omega-3 fatty acids, have been reported to lower fasting serum TG levels; however, they are known to have different effects on LDL and high density lipoprotein (HDL) [50][51][52].In a direct comparative study, in a meta-analysis comparing the effects of DHA and EPA, DHA was associated with a greater decrease in fasting serum TG and a greater increase in LDL than EPA.DHA also increased HDL compared to placebo, but EPA did not [51].Further studies are needed to clarify the mechanisms and significance of these differences [50][51][52].infarction and coronary heart disease [53,54].When the DPA level in the serum decreases, the risk of peripheral arterial disease such as vascular plaque formation increases [55,56].DPA has a stronger inhibitory action on platelet aggregation than DHA and EPA [57].Like DHA and EPA, DPA has been reported to decrease the expression of inflammatory genes [58].As the fasting serum TG-lowering mechanism of action of long-chain omega-3 fatty acids differs from that of other lipid-lowering drugs, such as statins, they can potentially provide complementary benefits on the lipid profile when administered in combination [35].This is supported by a study examining the synergistic effect of the lowering action of fasting serum TG by omega-3 fatty acids in addition to statin therapy [59][60][61][62].
This research had certain limitations.There was a possibility that sampling bias existed in the studies used and there was language bias due to the database search using only English and Japanese keywords; however, all reports adopted in this study were peer-reviewed RCTs, the quality of each research was thought to be high, the bias risk was roughly not a problem, and the quality of scientific evidence could be sufficiently judged.In this systematic review, meta-analysis could not be performed due to several reasons, mainly clinical heterogeneity; however, the evidence level of an individual RCT is considered to be sufficiently high, that is, it can be said that DHA and/or EPA intake can reduce and maintain a suitable level of fasting serum TG.
In modern society, the importance of functional foods is increasing in terms of medical economics; however, it will be necessary to accumulate evidence from interventional studies targeting healthy people and perform meta-analysis.
with high DHA content include sardines, sauries, skipjack tunas, amberjacks, tunas, and mackerel, and in particular, DHA is present in squid liver oil and fat near the eyeballs of tuna.In recent years, it has become clear that DHA and EPA have various physiological activities.DHA is the major PUFA present in the brain and is important for brain development and function.The synapses contain abundant DHA, suggesting that DHA is involved in neuron signaling.DHA also is required for the production of a group of compounds called resolvin, which are involved in the body's reaction to inflammation in the brain.Resolvin synthesized specifically from DHA and EPA helps to relieve inflammation caused by ischemic stroke (reduction of blood flow).EPA also suppresses the production of inflammatory compounds, such as cytokines and alleviates inflammatory reactions.Omega-6 fatty acids account for more than 10 times the omega-3 fatty acids in most American meals.
somewhat higher fasting serum TG levels, several reported reduced fasting serum TG levels after oral ingestion of DHA and/or EPA, suggesting that oral intake of DHA and/or EPA suppresses the progression to hypertriglyceridemia. Thus, DHA and/or EPA dietary intake could contribute to decreasing the number of persons who require medicine to control their fasting serum TG level.Although several previous studies have reported the fasting serum TG lowering effect of DHA and/or EPA in subjects with hyperlipidemia, our study strongly suggests that the effect is maintained among the subjects with borderline hyperlipidemia and normal lipidemia.Overall, the studies involving dietary interventions assessed in our review revealed that consuming 133-10,440 mg of DHA and EPA produces fasting serum TG lowering effects in healthy or slightly higher fasting serum TG level individuals.EPA is already used as an ethical drug, and thus, its effect can be considered to be well established; however, the mechanism of omega-3 fatty acids, such as DHA and EPA, to lower the fasting serum TG level, remains unclear.There are some hypothetical mechanisms, including inhibition of diacylglycerol acyltransferase, increase in plasma lipoprotein lipase activity, decrease in liver lipid production, and increase in liver beta oxidation [43].Based on the results of the preclinical and clinical trials, omega-3 fatty acids have been proposed as exerting a decreasing action on fasting serum TG via numerous mechanisms.For example, it is believed to reduce lipid production in the liver by suppressing the expression of sterol regulatory element binding protein-1c.This is due to the downregulation of expression of cholesterol, fatty acids, and TG synthase [44, 45].It also is presumed to increase beta-oxidation of fatty acids, and consequently, the TG are suppressed by decreasing the substrate necessary

Freely
Available Online www.openaccesspub.org| IJN CC-license DOI : 10.14302/issn.2379-7835.ijn-18-2469Vol-3 Issue 2 Pg.no.-36 Research on most omega-3 fatty acids is directed toward DHA and EPA; however, recently omega-3 docosapentaenoic acid (DPA) also has been drawing attention.The level of DPA in serum has is individually associated with a reduction in the risk of myocardial

Table 2 [
. For the total risk of bias, both studies were assessed as having an "overall low risk of bias" (data not show).Among the 37 reports used to qualitatively the results, 25 revealed a decrease in fasting serum TG level due to oral ingestion of DHA and/or EPA.Sixteen studies on subjects without disease and 21 on subjects with slightly higher fasting serum TG levels were separated and subjected to stratified analysis.Ten of the 16 (normal TG participant) and 15 of the 21 studies (slightly higher TG participant), respectively, intake of an at least 133 mg/day of DHA and/or EPA intervention revealed a statistically significant decrease in the fasting serum TG level between the intervention group and Freely Available Online www.openaccesspub.org| IJN CC-license DOI : 10.14302/issn.2379-7835.ijn-18-2469Vol-3 Issue 2 Pg.no.-26

Table 1 .
Characteristics of the selected 37 documents.