n-3 polyunsaturated fatty acids (PUFAs) are a family of healthy dietary fats mainly found in fatty fish, such as salmon and herring, and in walnuts and seeds. The essential fatty acids linolenic acid (precursor for n-6 PUFAs) and α-linolenic acid (ALA, precursor for n-3 PUFAs), that cannot be synthesized endogenously by mammals, derive from the diet (fish and nuts) and can be converted into long-chain (LC) PUFAs via two major enzymes, Δ-5/Δ-6 desaturase and elongase-2/-5.
LC n-3 PUFAs are synthetized through a series of desaturation and elongation steps as shown in Figure 1 [1]. The most common LC n-3 PUFAs, containing 20 or more carbon atoms, with 3–6 double bonds, are eicosapentaenoic acid (EPA), docosapentaenoic acid, and docosahexaenoic acid (DHA) and can be synthetized endogenously or are found in the diet [1]. ALA and stearidonic acid, product of the Δ-6 desaturation of ALA, contain 18 carbon atoms and are termed C18 n-3 PUFAs [1].
Metabolic pathways of the synthesis of n-3 PUFAs from ALA. Essential fatty acid α-linolenic acid (ALA), precursor of endogenous n-3 PUFAs, is obtained from the diet as cannot be synthetized in mammals. A small portion of n-3 PUFAs is endogenously synthetized with most of n-3 PUFAs introduced with diet.
Metabolic pathways of the synthesis of n-3 PUFAs from ALA. Essential fatty acid α-linolenic acid (ALA), precursor of endogenous n-3 PUFAs, is obtained from the diet as cannot be synthetized in mammals. A small portion of n-3 PUFAs is endogenously synthetized with most of n-3 PUFAs introduced with diet.
Δ-6 and Δ-5 desaturases are encoded by the fatty acid desaturase 2 (FADS2) and fatty acid desaturase 1 (FADS1) genes, respectively. The main tissue for the synthesis of LC PUFAs is the liver, but FADS1 and FADS2 are expressed in most tissues [2].
Elongase-2 is highly expressed in many tissues such as liver, brain, lung, and kidney, whereas elongase-5 is more ubiquitously expressed [3]. Elongases extend a fatty acid by 2 carbons as part of a series of sequential reactions. The final step in the synthesis of LC PUFAs (e.g., DHA) occurs in the endoplasmic reticulum, either via peroxisomal oxidation or FADS2-mediated Δ-4 desaturation [4].
Humans can synthetize only a small portion of LC n-3 PUFAs from essential fatty acids; the low conversion rates are around 5%–10% from 18 carbon fatty acids to EPA and 1% from ALA to DHA. The limited endogenous synthesis of LC n-3 PUFAs indicates that their main sources are diets rich in 20 carbon PUFAs (e.g., EPA and DHA), which are contained mainly in fish.
n-3 PUFAs promote modifications in cells’ membrane fatty acid composition which, in turns, alters the cells’ membrane properties, affecting cellular signaling pathways and gene/protein expression [5]. LC PUFAs retain a wide range of beneficial roles: in particular, they confer protection to the cardiovascular and renal system by promoting a reduction in blood pressure, decreased thrombosis, improved vascular reactivity, reduced inflammation, reduced oxidative stress, improved insulin sensitivity, and reduction in triglycerides [5‒7], as shown in Figure 2.
Potential beneficial and less beneficial effects of n-3 PUFAs on the cardiovascular-renal system. Level of n-3 PUFAs supplementation, short- and long-chain n-3 PUFAs ratios, and duration of exposure could drive a beneficial or less beneficial effect on the cardiovascular renal system. More studies are required to better understand n-3 PUFAs metabolisms and their role in physiology and disease settings.
Potential beneficial and less beneficial effects of n-3 PUFAs on the cardiovascular-renal system. Level of n-3 PUFAs supplementation, short- and long-chain n-3 PUFAs ratios, and duration of exposure could drive a beneficial or less beneficial effect on the cardiovascular renal system. More studies are required to better understand n-3 PUFAs metabolisms and their role in physiology and disease settings.
Many associative studies have explored the potential benefit of n-3 PUFAs on cardiovascular morbidity and mortality, but the results have been inconclusive [8]. Similarly, intervention studies, such as the ORIGIN trial in 2012 [9], have demonstrated that daily supplementation with 1 g/day of n-3 PUFAs did not improve cardiovascular morbidity and mortality in patients with altered glucose metabolism (prediabetes and diabetes) at high risk for cardiovascular disease. In another double-blind, placebo-controlled clinical trial, conducted in primary care setting, in patients with multiple cardiovascular risk factors or atherosclerotic vascular disease but not myocardial infarction (of which two-third of patients were diabetic), randomization to 1 g of n-3 PUFAs daily (n = 6,244) did not show any effect on cardiovascular mortality and morbidity when compared to patients (n = 6,269) on placebo (olive oil) [10].
Another study, two-by-two factorial design, randomized, placebo-controlled trial, of vitamin D3 (2,000 IU per day) and marine n-3 PUFAs (1 g daily) conducted as a primary prevention intervention for cardiovascular disease and cancer failed to demonstrate a preventative role of n-3 PUFAs on cardiovascular events [11]. Similarly, n-3 PUFAs (1 g daily) also failed, in another prospective randomized blinded study, to prevent cardiovascular events in patients with diabetes without evidence of atherosclerotic cardiovascular disease [12].
A relatively recent systematic review of randomized clinical trials assessing the effects of dietary n-3 PUFAs supplementation in the diet showed a potential effect on the reduction in cardiovascular morbidity but no effect on all-cause or cardiovascular disease mortality [13]. In parallel to cardiovascular outcome studies, other studies have explored the potential role of n-3 PUFAs on renal disease morbidity and mortality, known to be closely linked with cardiovascular disease as a clinical outcome [14].
As per the heart, there is some evidence, though not strong or definitive, from experimental and clinical studies that n-3 PUFAs could be beneficial for diabetic nephropathy via a reduction in albuminuria, a consolidated marker for cardiorenal disease, an event also observed in nondiabetic population, but any effects on renal function, such as changes in glomerular filtration rate (GFR), have not been observed [15]. Many studies have demonstrated a direct association between assumption of n-3 PUFAs intake and kidney disease protection in both the nondiabetic and diabetic populations [16, 17].
If we consider prospective studies on n-3 PUFAs on renal outcome, a small randomized, placebo-controlled, two-period crossover trial tested the effects of 4 g/day of n-3 PUFAs supplementation on markers of glomerular filtration and kidney injury in patients with type 2 diabetes and proteinuria did not demonstrate, on top of blockade of the renin-angiotensin-aldosterone system, any effect on changes in estimated GFR [18]. Conversely, assumption of n-3 PUFAs showed a significant reduction in 24-h albumin urinary excretion and a parallel reduction of tubular markers of kidney injury such as neutrophil gelatinase-associated lipocalin, liver fatty acid-binding protein, and N-acetyl-beta-D-glucosaminidase [18].
Another study where patients with type 2 diabetes were randomized to receive vitamin D3 (2,000 IU/day) and n-3 PUFAs (ALA, EPA, and DHA; 1 g/day), vitamin D3 and placebo, n-3 PUFAs and placebo, or placebo for both arms of the study did not demonstrate any improvement in estimated GFR, after 5-year treatment [19]. Despite a quite disappointing effect of n-3 PUFAs on chronic kidney disease (CKD) in both the diabetic and general population, Ao et al. [20] have explored, in a recent work, in patients with diabetes, the association between fish oil supplementation, rich in n-3 PUFAs, and risk of CKD.
Ao et al. [20] studied 24,497 patients with diabetes from the UK Biobank of which 7,122 patients reported taking fish oil supplements. They report that, in the fully adjusted model, fish oil use was inversely associated with the incidence of CKD. Participants who took fish oil supplements showed 2.79 years delayed risk for CKD events when compared with nonusers of fish oil. Ao et al. [20] also attempted to dissect the mediators of putative role of fish oil supplementation on CKD incidence by looking at relevant biomarkers. An increased HDL cholesterol, reduced C-reactive protein, and lower HbA1c appear to mediate the association between fish oil use and the risk of CKD in the diabetic population studied [20]. As expected, the protective effect of fish oil supplements seems to be driven by changes in glucose and lipid metabolism and inflammatory response [20].
The authors propose a beneficial role of fish oil use in preventing CKD among patients with diabetes [20]. This work studied a high number of patients with diabetes, but key information on fish oil supplement usage, doses, frequency, and duration is lacking. Further, this study describes an association, and a cause-effect relationship cannot be extrapolated despite an attempt to identify potential mediators or mechanisms.
Despite the study by Ao et al. [20], like many others previously [16, 17], supports an association between the use of fish oil supplementation and a reduced risk of incident CKD in patients with diabetes, it still does not give a clear answer on the putative benefit of LC PUFAs and renal disease risk. The KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease does not include specific recommendations on fish oil supplements for preventing diabetes vascular complications, which include renal disease [21].
To date, the studies conducted to explore potential effect of n-3 PUFAs on cardiovascular and renal protection have not provided a significant convincing evidence, even in prospective randomized trials looking at cardiovascular [8] and renal endpoints [22]. Whether the studies were not adequately powered/designed, or whether no clear n-3 PUFAs renal benefit is present, is a question that still needs answering.
Evolution may provide some understanding on whether PUFAs can have a beneficial role for cardiovascular and renal protection. The genes that encode fatty acid desaturases show signatures of positive selection in multiple populations from African, South Asian, European, and Inuit (originated in eastern Siberia) ancestry [23].
Fatty acid desaturases gene polymorphism has been associated with positive changes in plasma fatty acid composition and possibly plays a protective role in the development of type 2 diabetes [24]. Serum and erythrocyte LC PUFAs levels are not associated with risk of type 2 diabetes, while dietary intake of LC PUFAs is associated with lower risk of type 2 diabetes [25]; these are interesting initial observations that warrant further investigations.
These observations seem promising in supporting a protective role of LC PUFAs synthesis on cardiovascular renal health, but most randomized endpoint studies on cardiorenal outcome have failed, and we should query whether we are asking the right questions and whether the studies are adequately powered. Despite all these negative outcome trials, and the lack of clear-cut evidence on the role of n-3 PUFAs on cardiovascular outcome, the American and European Heart Associations recommend replacing saturated fat by n-3 PUFAs by limiting the intake of meat and consuming more fish at least twice weekly.
The lack of LC-PUFAs’ effects in improving the cardiovascular and renal outcome in the diabetic population could be attributed to an inappropriate (too low or too elevated) dose or treatment duration with n-3 PUFAs. This brings to us an ongoing discussion on potential toxic effect of n-3 PUFAs supplementation [26]. There are few suggestions that the antithrombotic action of n-3 PUFAs could lead to an increased incidence of pro-hemorrhagic effect, such as hemorrhagic stroke (that could occur at very high levels of n-3 PUFAs) [26]. Concerns have also been raised on the n-3 PUFAs-mediated depressive effect of immune functions that could hamper immune response in general, which could be detrimental in some patients [26].
n-3 PUFAs could also lead to an increase in oxidative products. The latter is a quite debated topic, and it contrasts with predicted beneficial n-3 PUFAs-mediated antioxidant effect. n-3 PUFAs have been associated with an increase in free radicals, and there is some evidence of oxidative derivatives of n-3 PUFAs having a pro-neoplastic role via the formation of chemical modifications to DNA (DNA adducts) [26]. Specifically, the presence of double bonds in the acyl chain of PUFAs makes them highly susceptible to lipoperoxidation, a process driven by the formation of free radicals after reaction with oxygen to form lipid hydroperoxides, which are later degraded into small secondary oxidation product such as aldehydes [26].
Whether n-3 PUFAs function as tissue pro or antioxidant is controversial. On one side, some studies support the concept that the high peroxidability of membrane n-3 PUFAs is sufficient to sensitize cells to reactive oxygen species, which will then trigger cellular oxidative stress. In parallel, other studies have demonstrated n-3 PUFAs-mediated protection from oxidative stress, which is believed to be implicated in their putative cardiovascular and renal protection [26].
Along the same lines, conflicting literature exists on the potential role of n-3 PUFAs in stimulating or inhibiting cellular antioxidant response [26]. Overall, it seems that use of recommended doses of n-3 PUFAs (0.5–1 g/day) does not result in an increase in tissues oxidative stress and carcinogenic potential. This has been confirmed by studies where a very low level of oxidative stress has been observed with the use of recommended n-3 PUFAs doses; more studies are clearly needed to consolidate this potential concern [26]. As clinicians, we are called to improve the poor cardiovascular and renal outcome of patients with diabetes and at the same time maintain safety.
Fish oil, as supplement, still needs to demonstrate a value in protecting patients from a renal (and cardiovascular) outcome. Still, potential safety concerns on PUFAs supplementation exist and we need more definitive answers. Diet with fish, rich in n-3 PUFAs, should be supported.
We should “first do no harm” and wait for more conclusive and convincing work to support use of PUFAs supplementation as protective agents against cardiovascular and renal risk in patients with diabetes. More studies are awaited, perhaps studying different n-3 PUFAs dosing and treatment duration in prospective studies.
Conflict of Interest Statement
The author has no conflicts of interest to declare.
Funding Sources
L.G. acknowledges funding from the British Heart Foundation (PG/24/11441) and Diabetes UK (22/0006394).
Author Contributions
L.G. has written and revised the manuscript.