Chemistry of dietary fatty acids

There are 3 major classes of fatty acids: saturated fatty acids (no double bonds); monounsaturated fatty acids (single double bond); and PUFAs (≥2 double bonds). The PUFAs are subclassified into n-3 or n-6 PUFAs according to the site of the double bond (at the third or the sixth position) proximal to the methyl terminus. Enzymes required to introduce the double bonds into the n-3 and n-6 positions are not present in mammals, and therefore, these fatty acids must be obtained from the diet and are accordingly termed as essential fatty acids. The n-3 fatty acids can be derived from fish and plant sources, but with certain fundamental differences. The fish-derived n-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have longer carbon chains and are more unsaturated than the plant-derived n-3 fatty acids, alpha-linolenic acid (ALA), and stearidonic acid.

In a typical Western diet, far more n-6 fat is consumed than n-3 fat mainly because of the abundance of linoleic acid (LA) in soybean, safflower, sunflower, and corn oils, which are readily available for food preparation and are present in many processed foods. The chemical similarity between n-3 and n-6 PUFAs leads to competitive inhibition of metabolism of n-3 by n-6 fatty acids and vice versa. Because LA is the n-6 homolog of ALA, it can decrease metabolism of vegetable oil–derived ALA to EPA and also decrease the level of tissue EPA plus DHA that has arisen from dietary EPA plus DHA. Thus, the relatively high LA intake in the American diet presents an important practical barrier to dietary strategies aimed at elevating tissue levels of EPA plus DHA and exerting their beneficial effects.

Mechanism of action of n-3 fatty acids

A number of antiinflammatory mechanisms have been postulated to account for the beneficial effects of fish oil. These include alteration of arachidonic acid (AA) pathway products favoring antiinflammatory mediators and disruption of adhesion molecule activity.

Both EPA and DHA are homologous to AA. They competitively inhibit the oxidation of AA by cyclooxygenase (COX) to n-6 prostaglandins (PGs). They also inhibit the conversion of AA to leukotrienes (LTs) via 5-lipoxygenase (LOX) enzymes. These PGs and LTs are collectively referred to as eicosanoids (C20 oxylipids). The n-6 eicosanoids PGE2, thromboxane A2 (TXA2), and leukotriene B4 (LTB4) are all proinflammatory. PGE2 causes vasodilatation, increased vascular permeability, and hyperalgesia. TXA2 promotes synthesis of inflammatory cytokines, interleukin (IL) 1 beta, and tumor necrosis factor α (TNF-α) by mononuclear phagocytes. LTB4 is a neutrophil chemoattractant and activator. In addition to reducing the production of n-6 eicosanoids, dietary fish oil has also been shown to inhibit TNF-α and IL-1 synthesis in healthy human beings and patients with RA. With the inhibition of formation of these proinflammatory cytokines by EPA and DHA, the net result is reduction in all the cardinal signs of inflammation, such as pain, warmth, redness, swelling, and loss of function.

Interestingly, it is not simply the downregulation of proinflammatory cytokines that contributes to this antiinflammatory effect. In addition, antiinflammatory cytokines are also increased. EPA and DHA are substrates for antiinflammatory eicosanoid formation. DHA can be converted via 15-LOX to 17(S)- and 17(R)-hydroxyl derivatives to a compound called resolvin D1. Resolvin production via the COX-2 pathway also happens when aspirin is given. Resolvins have antiinflammatory effects in the murine peritonitis model by downregulating leukocyte recruitment and stimulating macrophage removal of apoptotic cells.

Beyond effects on cytokine levels, the n-3 fatty acids, in vitro and ex vivo, have been shown to decrease the expression of adhesion molecules, intercellular adhesion molecule (ICAM) 1, and leukocyte function–associated antigen (LFA) 1. ICAM-1 and LFA-1 have been implicated in the migration of leukocytes in the inflamed synovium in laboratory rodents, and ICAM-1 blockade has been shown to reduce disease activity in RA.

In RA and other inflammatory joint diseases, chronic joint inflammation causes cartilage destruction by the various matrix metalloproteinases (MMPs), which serve as the ultimate effector molecules. N-3 fatty acid supplementation abolishes the expression of messenger RNA (mRNA) for MMP-13 and MMP-3 in human cartilage and reduction of proteoglycan degradation as seen in IL-1 stimulated bovine chondrocytes. Thus, the n-3 fatty acids exert a vast array of antiinflammatory effects via a number of different mechanisms.

Mechanism of action of n-3 fatty acids

A number of antiinflammatory mechanisms have been postulated to account for the beneficial effects of fish oil. These include alteration of arachidonic acid (AA) pathway products favoring antiinflammatory mediators and disruption of adhesion molecule activity.

Both EPA and DHA are homologous to AA. They competitively inhibit the oxidation of AA by cyclooxygenase (COX) to n-6 prostaglandins (PGs). They also inhibit the conversion of AA to leukotrienes (LTs) via 5-lipoxygenase (LOX) enzymes. These PGs and LTs are collectively referred to as eicosanoids (C20 oxylipids). The n-6 eicosanoids PGE2, thromboxane A2 (TXA2), and leukotriene B4 (LTB4) are all proinflammatory. PGE2 causes vasodilatation, increased vascular permeability, and hyperalgesia. TXA2 promotes synthesis of inflammatory cytokines, interleukin (IL) 1 beta, and tumor necrosis factor α (TNF-α) by mononuclear phagocytes. LTB4 is a neutrophil chemoattractant and activator. In addition to reducing the production of n-6 eicosanoids, dietary fish oil has also been shown to inhibit TNF-α and IL-1 synthesis in healthy human beings and patients with RA. With the inhibition of formation of these proinflammatory cytokines by EPA and DHA, the net result is reduction in all the cardinal signs of inflammation, such as pain, warmth, redness, swelling, and loss of function.

Interestingly, it is not simply the downregulation of proinflammatory cytokines that contributes to this antiinflammatory effect. In addition, antiinflammatory cytokines are also increased. EPA and DHA are substrates for antiinflammatory eicosanoid formation. DHA can be converted via 15-LOX to 17(S)- and 17(R)-hydroxyl derivatives to a compound called resolvin D1. Resolvin production via the COX-2 pathway also happens when aspirin is given. Resolvins have antiinflammatory effects in the murine peritonitis model by downregulating leukocyte recruitment and stimulating macrophage removal of apoptotic cells.

Beyond effects on cytokine levels, the n-3 fatty acids, in vitro and ex vivo, have been shown to decrease the expression of adhesion molecules, intercellular adhesion molecule (ICAM) 1, and leukocyte function–associated antigen (LFA) 1. ICAM-1 and LFA-1 have been implicated in the migration of leukocytes in the inflamed synovium in laboratory rodents, and ICAM-1 blockade has been shown to reduce disease activity in RA.

In RA and other inflammatory joint diseases, chronic joint inflammation causes cartilage destruction by the various matrix metalloproteinases (MMPs), which serve as the ultimate effector molecules. N-3 fatty acid supplementation abolishes the expression of messenger RNA (mRNA) for MMP-13 and MMP-3 in human cartilage and reduction of proteoglycan degradation as seen in IL-1 stimulated bovine chondrocytes. Thus, the n-3 fatty acids exert a vast array of antiinflammatory effects via a number of different mechanisms.

Clinical studies of fish oil in RA

In addition to the pharmacologic and laboratory evidence, there have been 14 double-blind, randomized controlled trials reporting the clinical outcomes and the beneficial effects of fish oil in RA. Almost all the 14 trials have been undertaken in patients with late disease, with mean duration of more than 10 years. The outcomes measured in these trials often included reduction in the tender joints and duration of morning stiffness after starting fish oil treatment. These analyses concluded that fish oil provided a reduction in the number of tender joints and duration of the morning stiffness. These benefits in RA have also been supported by meta-analysis and mega-analysis of outcomes after 12 weeks of fish oil supplementation. Goldberg and Katz conducted a meta-analysis of 17 randomized control trials that included assessment of the analgesic effects of n-3 fatty acid supplementation in patients with RA or joint pain secondary to inflammatory bowel disease. It concluded that fish oil decreased the intensity of joint pain, duration of morning stiffness, number of tender and swollen joints, and the use of nonsteroidal antiinflammatory drugs (NSAIDs). Furthermore, in 2 studies, some subjects who ingested fish oil were able to sustain a lower NSAID dose even after fish oil supplements were discontinued. The beneficial antiinflammatory effects of fish oil in RA are generally delayed by up to 12 weeks after they are started. There is also a dose response for time of onset of symptomatic benefits, with reduction in latency seen with higher doses of fish oil. Clinical benefits have been observed to last for up to 6 weeks after discontinuing therapy. However, most of the studies of fish oil in RA have involved relatively a small number of subjects (N = 16–67). Also, all the clinical trials to date in RA have been in patients with a long-standing disease. The role of fish oil in early disease is not well explored. Trials investigating long-term outcomes of n-3 fatty acid supplementation in a setting where multiple and changing medications are used are also lacking.

Other inflammatory diseases

Consumption of n-3 PUFAs suppresses inflammatory processes that would be extended to management and trials of other inflammatory diseases. Several recent trials have provided evidence that n-3 PUFA supplementation could be useful in treatment of human immunoglobulin A (IgA) nephropathy and lupus nephritis, whereas others suggest such supplementation might be without benefit. Duffy and colleagues in their double-blind, double placebo controlled factorial trial concluded that dietary supplementation with fish oil may be beneficial in modifying symptomatic disease activity in patients with systemic lupus erythematosus (SLE). The reported favorable influence of fish oil on relapse rates in Crohn disease is also worth noting. N-3 fatty acids lower plasma triglyceride levels, particularly in persons with hypertriglyceridemia, by inhibiting the synthesis of very-low-density lipoprotein cholesterol and triglycerides in the liver.

They have been reported to have a dose-response blood pressure–lowering effect in patients with hypertension and with little or no effect in patients with normal blood pressure. N-3 fatty acids have been shown in epidemiologic and clinical trials to reduce the incidence of cerebrovascular disease. Evidence from prospective secondary prevention studies suggest that EPA plus DHA consumption ranging from 0.5 to 1.8 g/d (either as fatty fish or supplementation) significantly reduces subsequent cardiac and all-cause mortality. One study showed increased regression and decreased progression of coronary lesions in patients taking 1.5 g/d of fish oil for 2 years, as assessed by angiography.

Thus, some have suggested that fish oil could have a cardioprotective role, which warrants further investigations.