Previous observational studies reported that consumption of fish once or twice per week was associated with lower risks of fatal coronary heart disease (CHD), but cohort studies are subject to confounding factors and biases and are unable to establish causality [13,14,15]. Hence, there has been an increasing interest among the scientific community to assess whether n-3 PUFA has a protective effect on CVD risk and its related outcomes such as myocardial infarction (MI), stroke, CHD and CVD mortality. Several trials studying the cause-and-effect relationship between n-3 PUFA intake and its effects on CVD and its outcomes have subsequently reported “discordant trial results” [8]. Recent evidence from SRMAs demonstrated a significant reduction in incidence of MI, CHD, total CVD, and CVD mortality after supplementation with n-3 PUFA [9,11,16,17]. Additionally, a dose–response meta-analysis showed that a dose of 10 g/day × years increments of n-3 PUFA for a longer duration was associated with a 13% reduction in major cardiovascular events [9].

Large-scale trials, with doses of n-3 PUFA (EPA, DHA, or EPA + DHA) ranging from 0.85 to 4 g daily and with a follow-up ranging from 3.5 to 4.9 years reported significant reductions in CVD, CHD, MI, stroke, and their related fatalities [18,19,20,21] (Table 1). The Gruppo Italiano perlo Studio della Sopravvivenza nell’Infarto Miocardico (GISSI) Prevenzione trial, an open-label RCT involving 11,324 participants with recent MI reported a 20% reduction in the primary composite of death, non-fatal MI and non-fatal stroke in the group receiving 1 g of EPA + DHA daily for 3.5 years [21]. Another open-label RCT, the GISSI-Heart Failure (HF) trial with 7046 patients with evidence of heart failure significantly reduced CVD mortality by 8% after a supplementation of 0.85–0.88 g of EPA + DHA every day for almost 4 years [19]. The Japan EPA Lipid Intervention Study (JELIS), an open-label trial conducted among 18,645 hypercholesterolemic patients further reduced major coronary events by 19% after a supplementation of 1.8 g of EPA and 5–10 mg of statin daily for 4.6 years [18]. A recent randomized, double-blind, placebo-controlled trial, the Reduction of Cardiovascular Events with Icosapent Ethyl-Intervention Trial (REDUCE-IT), reported a 25% reduction in the primary composite endpoint of cardiovascular death, nonfatal myocardial infarction (MI), nonfatal stroke, coronary revascularization, or unstable angina after a daily intake of 4 g of EPA daily for a median follow-up of 4.9 years [20].

In contrast, eleven large-scale trials with doses of n-3 PUFA ranging from 0.84 to 4 g daily and with a follow-up of 1 to 7.4 years reported no significant association with CVD and its sub types [22,23,24,25,26,27,28,29,30,31,32] (Table 1). Two of these large-scale trials, The Outcome Reduction with an Initial Glargine Intervention (ORIGIN) [25] and a Study of Cardiovascular Events in Diabetes (ASCEND) [29] studied the effects of n-3 PUFA on cardiovascular outcomes in patients with impaired fasting glucose or with diabetes, but without evidence of atherosclerotic CVD. Both had more than 12,000 participants enrolled, taking 0.84 g of DHA + EPA for more than 6 years, and still failed to show any significant reduction in CVD and its related outcomes [25,29]. Moreover, the Vitamin D and Omega-3 Trial (VITAL), a study of 25,871 subjects with no previous CVD risk factors taking 0.84 g EPA + DHA with 2000 IU of vitamin D daily for more than 5 years also reported no significant reduction in major cardiovascular events or mortality [30]. Likewise, two other large-scale trials, namely the Long-Term Outcomes Study to Assess Statin Residual Risk with Epanova in High Cardiovascular Risk Patients with Hypertriglyceridemia (STRENGTH) and the Omega-3 fatty acids in Elderly with Myocardial Infarction (OMEMI) demonstrated null findings after combined EPA + DHA therapy [31,32]. The STRENGTH trial was even stopped early because of a low possibility of demonstrating any clinical benefits [31].

In 2020 alone, cancer was the leading cause of morbidity and death globally with approximately 19.3 million new cancer cases and 10 million cancer-related deaths [33]. Since cancer incidence and mortality are on the rise, it has been suggested that the intake of n-3 PUFA may reduce cancer risk by regulating metabolic pathways and inflammatory responses, oxidative stress, and changes in the composition of membrane affecting cell signaling pathways [34,35]. It was demonstrated in vitro and in animal studies that n-3 PUFA may inhibit breast cancer growth [36]. Asian countries with the highest intake of n-3 PUFA had a significantly lower rate of breast cancer compared to Europe and United States [37]. A greater incidence of breast cancer was further observed among Asian–American women migrating to the West due to a shift from their traditional fish-based diet to a high-fat Western diet [38]. Earlier evidence focusing on Asian populations showed a protective benefit of n-3 PUFA, with a 20% and 26% reduction in breast cancer risk in both Japanese and Singaporean Chinese populations, respectively [39,40]. A more recent dose–response meta-analysis demonstrated a 6% decrease in breast cancer risk for every 1% increase in circulating n-3 PUFA [41].

However, the evidence of n-3 PUFA on cancer development and cancer mortality risk remains contradictory when considering all cancer types. Epidemiological studies demonstrating significant reductions in cancer risk may have overestimated the effects of n-3 PUFA supplementation, leading us to believe that estimates of cancer risk need to be assessed by cancer type and it is not possible to generalize across all cancers. An earlier SRMA of observational studies showed significant associations between n-3 PUFA and cancer incidence, especially colorectal, lung, prostate, and breast cancer [42]. More than three decades ago, the Lyon Diet Heart Study, a trial consisting of 605 MI survivors, reported that supplementing the Mediterranean diet with a margarine rich in n-3 PUFA reduced cancer risk by 61% after a 4 year follow up [43]. In contrast, a current SRMA of RCTs extensively examining the high-quality evidence of the effects of n-3 PUFA on cancer demonstrated little or no effect on cancer diagnosis and cancer mortality [12]. Consequently, the same study reported a slight increase in prostate cancer risk after an intake of ALA, although the certainty of evidence was of low-quality [12]. Most recently, large-scale RCTs such as GISSI-P, JELIS, ORIGIN, ASCEND, and VITAL showed null effects on cancer incidence and cancer mortality after more than 3.5 years of follow-up and with doses of EPA + DHA combined ranging from 0.84–1.8 g [18,29,30,44,45,46]. Some small scale RCTs also examining the effects of n-3 PUFA on cancer incidence and mortality for a mean follow-up of 1.5 years found no association with cancer outcomes [47,48,49].

With more than 700 million people projected to be diagnosed with type 2 diabetes by 2045, there is a need to prevent and manage this endocrine disease because of its association with increased risks of CVD and cancer [50]. Type 2 diabetes is also associated with a variety of modifiable risk factors that can be prevented by following a healthy diet and lifestyle [51]. Thus, people are now turning to n-3 PUFA supplementation to manage the symptoms associated with type 2 diabetes such as elevated blood glucose and insulin resistance [52]. Over the years, animal and cell culture studies have shown that n-3 PUFA may prevent type 2 diabetes through anti-inflammatory properties, insulin signaling, changing cell membrane function and controlling expression of glucose metabolism genes [53]. For the past decade, there has been an influx of studies investigating the effects of n-3 PUFA on the prevention and management of type 2 diabetes [52]. But as observed in the case of CVD and cancer prevention and treatment, the evidence for a protective effect of n-3 PUFA on type 2 diabetes and its related parameters also remains inconsistent [51,52].

A SRMA of 12 RCTs studying the effects of n-3 PUFA on glucose control reported no reduction in fasting insulin (FINS), glycosylated hemoglobin (HbA1c), and HOMA of insulin resistance (HOMA-IR) levels [51]. Another SRMA of 25 RCTs investigating the effects of n-3 PUFA supplementation on type 2 diabetes treatment or prevention showed a significant reduction in fasting blood glucose and insulin resistance, but no significant effect on HbA1c was observed [52]. Trials examining the effects of n-3 PUFA on type 2 diabetes among those with impaired glucose tolerance or with metabolic syndrome also found no significant changes in HbA1c, fasting glucose, and insulin sensitivity with the doses of n-3 PUFA ranging from 0.6 to 6 g per day for a duration ranging from 8 weeks to 1 year [54,55,56,57,58]. Conversely, other trials reported a significant reduction in fasting blood glucose, insulin resistance, and HbA1c after supplementing the diet with dosage of n-3 PUFA ranging from 0.41 to 3 g/day for a follow-up of 8 weeks to 1 year as well [59,60,61,62,63]. One particular RCT conducted among patients with vitamin D deficiency reported a significant increase in HbA1c in the n-3 PUFA group after 8 weeks of supplementation with 0.3 g of n-3 PUFA daily and 50,000 IU of vitamin D weekly [64].

Age-related macular degeneration (AMD) accounts for 8.7% of blindness globally in people 60 years and older, while the number of people with AMD is expected to increase to 288 million by 2040 [71]. Consumption of n-3 PUFA has been postulated as a potentially effective strategy to protect against AMD [72]. Evidence from prospective cohort, cross-sectional, and case-control studies has previously shown an association with intake of fatty fish consumption or n-3 PUFA and reduced risk of AMD [73,74,75]. A newly published dose–response meta-analysis of 11 prospective cohorts reported that an increment of 1 g of EPA + DHA daily reduced early AMD risk by 50–60% [76]. Conversely, a review examining high quality evidence from only two RCTs demonstrated that supplementation of n-3 PUFA for up to 5 years did not reduce the incidence or risk of progression to advanced AMD [77].

The Age-Related Eye Disease Study 2 (AREDS2), a large-scale study of 4203 participants with a mean age of 73.1 years and at risk for progression to advanced AMD, reported that 1 g of EPA + DHA combined with lutein and zeaxanthin did not reduce the progression to advanced AMD after a median follow-up of 5 years [78]. Another large-scale trial, the VITAL study found no overall effect of n-3 PUFA on AMD incidence or progression among 25,871 participants with a mean age of 67.1 years after a median follow-up of 5.3 years [79].