1. Introduction
Malaysia is among the world’s biggest consumers of fish, eating at least 56.5 kg of fish per person each year [
1]. More than three quarters of Malaysians consume fish at least twice per week, eating 168 g of fish per day [
2]. Eating fish on a regular basis can be beneficial for our health in many ways. For example, fish could help to reduce the risk of cardiovascular disease (CVD) and improve the chances of survival following a heart attack, as displayed in several observational studies [
3,
4,
5], although not all agree [
6]. Researchers believe that the heart health benefit of fish is more promising in those fish rich in omega-3 fatty acids, which is attributed to the ability of fish to ameliorate hypertriglyceridemia [
7,
8] and resolve inflammatory processes [
9]. A recent meta-analysis demonstrated that consuming oily fish was associated with significant reductions in plasma triglycerides and an increase in HDL-cholesterol [
10]. Omega-3 fatty acids reduce plasma lipid levels by inhibiting triacylglycerol and VLDL synthesis in liver, increasing fatty acid oxidation, as well as promoting the synthesis of membrane phospholipids [
11,
12,
13]. Increased dietary EPA intake appears to compete with arachidonic acid (AA, omega-6 fatty acids) for the same desaturation enzymes, and in turn produces eicosanoids that are less potent than those produced from AA [
14]. Pro-resolving mediators such as resolvins and protectins derived from EPA and DHA also helps in regulating the inflammatory response [
9]. Omega-3 fatty acids are essential polyunsaturated fatty acids that must obtain from diet due to the lack of delta-12 and delta-15 desaturases in humans [
15]. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are the two main long-chain omega-3 fatty acids commonly found in marine sources [
16]. Marine fish, especially salmon, are the principal source of EPA and DHA [
17]. In Malaysia, people consume mostly farmed Atlantic salmon, particularly Norwegian salmon [
18]. However, salmon can be costly and require importation for tropical countries such as Malaysia. Yellowstripe scad (YSS,
ikan selar kuning) is one of the most frequently consumed local fish in Malaysia [
2]. Many Asian populations like to consume it as snack in the form of dried fish while Malaysians usually deep-fry and serve it with nasi lemak. It is not only commonly available in Malaysia but also affordable in price. Recent studies showed that YSS could provide a comparable eicosapentaenoic acid and docosahexaenoic acid (EPA+DHA) content to farmed Atlantic salmon (879 mg/100 g vs. 947 mg/100 g) [
19,
20]. However, studies exploring the benefits of YSS are scarce. The current study previously demonstrated an alteration of leptin and prothrombotic parameter upon the consumption of YSS and salmon [
21]. In this paper, we aimed to investigate the effects of consuming YSS as compared with salmon on lipid profile and inflammatory markers among healthy overweight subjects.
4. Discussion
The present study investigated the effect of YSS as compared with salmon on the lipid profile and inflammatory markers among healthy overweight subjects. A crossover design was selected for several reasons. Firstly, the intervention is evaluated within the same participants, thus eliminating the between-subject variability [
44]. Secondly, this study design is more sensitive than parallel-group design to detect small differences between equivalent treatments such as YSS and salmon that have similar EPA+DHA content [
45]. Thirdly, since each participant serves as his or her own control, fewer subjects are required to achieve a similar statistical power [
46]. The 8-week duration of the diet period was chosen as this time frame was sufficient to incorporate EPA+DHA into tissues [
47] and to induce notable effects in the lipid profile [
8] and inflammation [
48]. A washout period of 8 weeks allowed to reduce the possible carry-over effect from the crossover study design, as indicated by the insignificant changes between the baseline values of YSS and salmon.
The consumption of YSS did not induce significant benefits on the lipid profile. Instead, salmon improved triglycerides and VLDL-cholesterol levels. Published interventional data displaying the lipid-lowering effects of dietary fish are conflicting. A crossover trial by Lindqvist et al. [
49] documented that the triglyceride-lowering effect by 6-week consumption of herring fish (equivalent to 1200 mg EPA+DHA/day) was no difference from pork and chicken fillets (equivalent to 400 mg EPA+DHA/day). The author concluded that total dietary composition (protein, carbohydrates, and fat) rather than increased fish intake plays a crucial role in improving the triglyceride level. Unfortunately, a later study did not support this hypothesis [
8]. The 8-week, parallel-arm, randomized intervention study demonstrated a dose-dependent relationship between the triglyceride-lowering effect of fatty fish diets and their EPA+DHA intake, suggesting that dietary EPA+DHA intake played a higher role than total dietary composition [
8]. In contrast, the current results reported a significant triglyceride-lowering effect by salmon compared with YSS, although both dietary fish had similar EPA+DHA content. With that, whether total dietary composition or high omega-3 fish diet contribute to the lowering effect on triglycerides is far less certain.
The atherogenic index of plasma (AIP) is the most sensitive marker for predicting cardiovascular risk compared with other atherogenic indices, including atherogenic coefficient [
50]. AIP values increase with the cardiovascular risk [
39]. In this study, AIP reduced significantly after the consumption of salmon as compared with YSS, indicating that salmon is superior in improving the lipid profile and the occurrence of cardiovascular events. Derosa et al. [
51] found an increase in HDL-cholesterol and decreased triglycerides after 18 months of omega-3 supplementation. Although consuming fresh fish seems to provide more promising benefits in AIP than omega-3 supplement [
52], research exploring the effect dietary fish on AIP is still scarce. Devadawson et al. [
53] showed a statistical difference between inland fish and sea fish eaters on the AIP level; however, which group of fish eaters had a better profile was not stated clearly.
Moreover, HDL-cholesterol was significantly increased by 3.9% or 0.06 mmol/1 from baseline with the consumption of salmon. According to Gordon et al. [
54], each increase in baseline HDL-cholesterol of 0.03 mmol/1 is associated with a 6% decrease in the risk of death from CVD. In this case, the improvement of salmon on HDL-cholesterol presented in the current study would be unequivocally of great benefit to the population. Previously, LDL-cholesterol was significantly reduced following 8-week consumption of Namibia hake, a type of white fish rich in omega-3 fatty acids (equivalent to 642 mg EPA+DHA/day) [
55]. The current study findings, however, are not in line with the previous evidence. Being also a type of white fish rich in omega-3 fatty acid, the consumption of YSS (equivalent to 1000 mg EPA+DHA/day) did not exert significant effect LDL-cholesterol. The study by Vazquez et al. [
55] explained that the observed reduction in serum LDL-cholesterol may not be a direct effect of omega-3 fatty acid content in fish; instead, a reduced intake of saturated fats from other protein sources may lead to lower LDL-cholesterol among subjects during the period of fish intake. These results are in keeping with the hypothesis from the previous study that reduced saturated fat intake rather than omega-3 content of fish contributed to a lower level of LDL-cholesterol. Future trials are warranted to confirm this hypothesis.
Despite the beneficial difference in lipid profile observed in the salmon group, it should be noted that the mean baseline lipid levels in both diet groups were within the normal range or at borderline. According to the Clinical Practice Guideline of Malaysia, a person is diagnosed with dyslipidaemia when he/she has a total cholesterol of >5.2 mmol/l, triglycerides of >1.7 mmol/l, HDL-cholesterol of <1.0 mmol/l (males) or <1.2 mmol/l (females), and/or LDL-cholesterol of <3.0 mmol/l or <3.8 mmol/l (if triglycerides >4.5 mmol/l) [
56]. Although the observed improvement in the salmon group may not be clinically meaningful to the study population, it may at least be useful as primary prevention. Moreover, it is possible that other dietary nutrients and physical activity levels could have influenced the results, although both diet groups did not show significant differences in terms of diet quality [
57,
58]. The increasing trend of carbohydrate intake and reducing trend of physical activity level observed in the YSS group after 8 weeks were much greater than the salmon group, though not statistically significant.
In terms of inflammatory markers, current results demonstrated no significant changes with the consumption of YSS. On the contrary, salmon was found to exert a significant suppression effect on the pro-inflammatory marker IL-6 compared with YSS. Serum TNF-α was also decreased significantly in the salmon group after 8 weeks but not significantly in between-group difference. Results of the present study corroborate those reported by Zhang et al. [
8] regarding the beneficial effect of 8-week salmon consumption on lowering and IL-6 and TNF-α levels. Conversely, a later randomized trial using mixed fish, equivalent to approximately 800 mg/day of EPA+DHA, did not support the beneficial effect of consuming omega-3 rich fish for 8 weeks on serum cytokines levels [
44]. It is believed that the dietary EPA+DHA intake in Grieger et al. [
59] (800 mg/day) was lower than that in Zhang et al. [
8] (1600 mg/day), and the present study (1000 mg/day) to improve the inflammatory markers.
There are several possible explanations for the varied effects of YSS and salmon on health outcomes. First, dietary inclusion of YSS and salmon did not effectively increase serum EPA+DHA despite their similar omega-3 content. When the omega-3 fatty acids were studied individually, higher EPA intake reflected from salmon, which had higher EPA content, increased the serum EPA level of subjects after 8 weeks. There is growing evidence that EPA and DHA exert differential health benefits [
60,
61]. The current findings acknowledge that the observed changes may mainly be associated with dietary intake of EPA. However, research remains inconclusive as to whether EPA or DHA is better to improve outcomes [
60,
61]. Considering the potentially independent effects of EPA or DHA on outcomes would be meaningful in the future. Second, YSS and salmon have different omega-6 to omega-3 fatty acids (n6:n3) ratio. A recent study found that even with similar EPA+DHA content, the beneficial effects on lipid profile and inflammatory markers tend to be greater when consumed fish with a lower ratio of omega-6 to omega-3 fatty acids (n6:n3 ratio) [
62]. If this effect is confirmed, it could explain the significant changes in lipid profile and inflammatory markers observed in the salmon group instead of the YSS group since salmon had a lower n6:n3 ratio than YSS (0.3 vs. 0.2) [
19,
20]. Taken together, we can speculate that higher dietary EPA intake and lower n6:n3 ratio may be associated with greater beneficial effects on the lipid profile and inflammatory markers. Further studies are necessary to confirm this hypothesis.
As discussed in our previous study [
63], the amount of fish given was nearly double the recommended serving size (150 g) [
64], although the intention was to mimic the omega-3 intake recommendation (1000 mg/day or 7000 mg/week) [
23]. Consuming such an amount of fish is unlikely to be practical in the long term. Harris et al. [
65] observed that incorporating EPA+DHA into blood lipids was equally effective when provided an identical amount of EPA+DHA from oily fish on a weekly basis or from fish oil supplementation on a daily basis. However, an in vitro study carried out by Browning et al. [
66] reported to have better EPA+DHA status when consuming a moderate amount regularly (daily) instead of a higher amount intermittently (twice per week), despite both administrations providing an identical amount of EPA+DHA per week. A recent in vivo study suggested that having a large dose of omega-3 fatty acids once per week is more effective than a smaller dose delivered daily [
67]. Additional research to compare the effectiveness of taking EPA+DHA derived from oily fish consumption on a daily and intermittently basis is warranted.
To our best knowledge, this is the first randomised crossover study that compared the health benefits of YSS and salmon matched for EPA+DHA content. This study design is one of the most powerful designs for examining the efficacy of dietary treatments [
68]. However, there are several limitations that should be addressed in this study. First, the dietary data and the serum EPA+DHA compliance biomarker showed no increase in the YSS period and very little increase in the salmon period, despite a goal of reaching an intake of approximately 1000 mg/day EPA+DHA. The results raise doubts about whether the subjects truly adhered to the study protocol, although subjects reported a good compliance index based on meal photos taken. Poor compliance is not uncommon in randomized trials with free-living subjects. One of the possible reasons for the non-compliance in this study is that the fish was given at nearly twice the recommended serving size, which was not usual. Therefore, subjects may fail to finish the diet treatment. Moreover, YSS is rather small and bony and is commonly prepared by deep-frying. Instead, the fish was steamed to retain most of its nutrients in this study. This little bony fish may cause hassle to the eating process. Moreover, salmon is a favourite fish of people but is also expensive; not all could afford it. It cannot rule out the possibility that subjects may share the diet treatment with family or friends. All these factors may relate to subject’s non-compliance in this study [
69,
70]. The primary purpose of self-administered two-day 24HR was to monitor the compliance with dietary intake. Although the intake of other nutrients was generally not different between groups, we did not specify which two days were to be recorded. This presents a further limitation to the study.
Second, in the earlier work of this study, which reported preliminary findings for the first intervention period [
42], the YSS group had higher total cholesterol and HDL-cholesterol after 8 weeks [
42]. Nevertheless, the modulation of YSS on these outcomes did not last on the second intervention period. A possible explanation is that subjects’ compliance was compromised due to the duration of intervention and complexity of the diet treatment [
69]. At the second intervention phase, subjects in the second group may have less motivation since it was more hassle to consume the bony fish YSS than salmon.
Third, even though there were statistical differences of decreasing IL-6 and TNF-α after 8 weeks of salmon consumption and between-group difference on IL-6 concentration, the changes were very small. Therefore, it is difficult to conclude the beneficial effects of salmon on these biomarkers.
Forth, the sample size was calculated based on the effect size of the primary outcome while the variables discussed in this paper were the secondary outcomes. This study had relatively wide confidence intervals that crossed zero for most variables. This may indicate an inadequate sample size and the study may not have adequate power to detect existing differences. We considered that this study would produce possible differences in the study outcomes due to results demonstrated by previous clinical trials of omega-3 rich fish [
8,
23,
54,
55,
59]. Therefore, more extensive human clinical trials with larger sample sizes are necessary.