Fatty Acid Biochemistry

Fatty acids are divided into three groups on the basis of the number of double bonds they contain (saturated fatty acids (no double bond), monounsaturated fatty acids (one double bond), and polyunsaturated fatty acids (PUFAs) (≥2 double bonds)). C18 fatty acids are prominent in the diet and provide the index fatty acids for the above classification. PUFAs are further grouped according to the site of the double bond proximal to the methyl (omega) terminus as n-6 or n-3. Vertebrates do not have the enzymes required to introduce double bonds in the n-3 and n-6 positions; therefore these fatty acids must be obtained from the diet and are thus known as essential fatty acids.

In general, the Western diet contains more n-6 fats than n-3 fats due to the dominance in processed foods and visible fats of soybean, safflower, sunflower, and corn oils, which contain the n-6 fat linoleic acid (LA; 18:2n-6). The n-3 homologue of LA, α-linolenic acid (ALA; 18:3n-3), is present in flaxseed oil, which is generally a minor dietary component. LA and ALA may be used in energy metabolism or be converted to the C20 fatty acids arachidonic acid (AA) and eicosapentaenoic acid (EPA), respectively. AA and EPA are then incorporated into cell membranes and tissues. EPA can be further metabolized to the n-3 fatty acid docosahexaenoic (DHA).

The critical process linking fatty acids and inflammation is the metabolism of AA and EPA to eicosanoids, which act as inflammatory mediators. AA is metabolized via cyclooxygenase (COX) to n-6 eicosanoids (prostaglandin [PG] E₂, thromboxane [TX] A₂ or via 5-lipoxygenase [5-LOX] to n-6 leukotrienes [LTs]). In comparison, EPA is metabolized via COX and 5-LOX to n-3 PGs and LTs, respectively. In comparison to the n-6 eicosanoids produced from AA, EPA is a poor COX substrate such that n-3 PGs are not as readily produced (Figure 68-1). EPA and DHA competitively inhibit production of most n-6 eicosanoids, with prostacyclin (PGI₂) being an exception. Increased dietary consumption of n-3 fatty acids such as EPA increases the proportion of EPA incorporated in cellular membranes and tissues partly at the expense of AA incorporation. The net result is an alteration in the balance of n-3/n-6 eicosanoid production (see Figure 68-1).

Figure 68-1 Metabolism of linoleic acid (LA) and α-linolenic acid (ALA) to n-6 and n-3 prostaglandins and leukotrienes. AA, arachidonic acid; COX, cyclooxygenase; EPA, eicosapentaenoic acid; IL-1β, interleukin-1β; LOX, lipoxygenase; LTB, leukotriene B; PGE, prostaglandin E; TNF, tumor necrosis factor; TXA, thromboxane A.

Proinflammatory Actions of Eicosanoids

In general the n-6 eicosanoids (PGE₂ and TXA₂) are proinflammatory, whereas the n-3 eicosanoids are either less potent in their effects (TXA₃) or less abundant (PGE₂). TXA₂ promotes interleukin-1β (IL-1β) and tumor necrosis factor (TNF) production by mononuclear cells,² whereas PGE₂ results in vasodilatation, increased vascular permeability, and hyperalgesia (see Figure 68-1). PGE₃ is edemogenic, although little is produced. LTB₅ is 10 to 30 times less potent than LTB₄ as a neutrophil chemotaxin.

Effect of n-3 Fatty Acids on Proinflammatory Cytokine Production

IL-1β and TNF production may be reduced as a consequence of dietary n-3 fatty acid supplementation. Although some of this cytokine inhibition is mediated through effects on eicosanoids, there also appears to be eicosanoid-independent cytokine inhibition. For example, fatty acids may have direct effects on intracellular signaling mechanisms including nuclear factor κB (NFκB) and PPAR-γ, thereby affecting cytokine production.³

The resolvins (resolution phase interaction products) are derived from n-3 fatty acids via COX-2 with increased production in the presence of aspirin. Resolvins derived from EPA are known as E-resolvins, whereas those derived from DHA are known as D-resolvins. The resolvins have a variety of anti-inflammatory actions including inhibition of TNF-induced transcription of IL-1β and inhibition of human polymorphonuclear leukocyte transendothelial migration (for review, see Kohil and Levy’s study⁴). The identification of resolvins provides another mechanism through which n-3 fatty acids contribute to inhibition of inflammatory cytokine production.

Effects of n-3 Fatty Acids on MHC Expression

The number of MHC molecules expressed on antigen-presenting cells (APCs) is an important determinant of T cell response to antigen. Patients with RA have high levels of MHC class II expression on T cells and synovial lining cells.⁵ In vitro studies show that EPA and/or DHA reduces monocyte expression of HLA-DR and HLA-DP molecules and reduces the ability of monocytes to present antigen to autologous lymphocytes.⁶ Thus n-3 fatty acids may have an anti-inflammatory effect via suppression of pathogenic T cell activation through inhibition of APC function.

Effect of n-3 Fatty Acids on Adhesion Molecule Expression

Adhesion molecules expressed on endothelial cells and leukocytes mediate the transit of cells from the circulation into tissues. Intercellular adhesion molecule-1 (ICAM-1) and its cognate receptor, leukocyte function-associated antigen (LFA)-1, have been shown to be important in migration of leukocytes into inflamed synovium in animal models.⁷ ICAM-1 blockade has also been reported to reduce disease activity in RA.⁸ In vitro n-3 fatty acids decrease human monocyte ICAM-1 and LFA-1 expression.⁶ In addition, dietary n-3 fatty acid supplementation reduces soluble ICAM-1 and vascular cell adhesion molecule-1 (VCAM-1) plasma concentrations,⁹ although whether cell surface expression of these adhesion molecules is also reduced has not been reported.

Effect of n-3 Fatty Acids on Degradative Enzymes

Proteinases have a pivotal role in cartilage degradation and bone erosion. n-3 Fatty acids added in vitro can suppress proteinases ADAMTS-4, ADAMTS-5, and MMP-3 in IL-1α stimulated bovine chondrocytes.¹⁰ This inhibition of chondrocyte proteases is a mechanism through which n-3 fatty acids may inhibit cartilage degradation and bone erosion.

The RANK/RANKL/OPG pathway is also important in bone pathophysiology in RA. Increased RANK/RANKL and decreased OPG contribute to bone erosion. Three months of dietary fish oil supplementation has been reported to decrease the RANK/OPG ratio, which may help prevent bone resorption that leads to erosions.¹¹

Importance of the Balance of n-3 and n-6 Fatty Acids in the Inflammatory Process

The balance of AA and EPA can be altered through dietary fatty acid intake. In humans, the conversion of dietary ALA to tissue EPA is inefficient and fish/fish oils are a more effective way to increase EPA and DHA in tissues. Changes in AA/EPA ratios in tissues have downstream effects on eicosanoid production and the resulting proinflammatory/anti-inflammatory environment. Dietary supplementation with fish oil in humans results in decreased production of PGE₂,¹² TXA₂,¹² and LTB₄¹³ with increased production of TXA₃¹⁴ and LTB₅.¹⁵ These data provide a mechanistic basis for beneficial effects of dietary n-3 fatty acid supplementation in the control of inflammatory diseases. Dietary fish oil supplements have been shown to increase vascular production of prostacyclin (PGI₂).¹⁶ Although the role of PGI₂ in inflammation is not well defined, it is a potent vasodilator and inhibits platelet aggregation, as well as disaggregating platelets. These effects likely contribute to the protective effects of dietary fish and fish oils against thrombotic vascular events. Importantly, patients with several of the major rheumatic diseases (e.g., RA, systemic lupus erythematosus [SLE] and gout) are at high risks for serious cardiovascular events and mortality, to which nonsteroidal anti-inflammatory drug (NSAID)–associated COX-2 inhibition may also contribute.

Omega-3 Fatty Acid Consumption

The long chain n-3 fats, EPA and DHA, are most abundant in fish and fish oils. A protective effect against RA of fish intake has been reported. For example, the Seattle Women’s Health Study showed reduced risk for developing RA in subjects consuming two or more fish meals per week with an adjusted odds ratio (OR) of 0.57 (95% confidence interval [CI], 0.35 to 0.93) compared with subjects consuming less than one fish meal per week.²⁷ A more recent population-based, case-controlled study reported a modest decrease in the risk of RA in subjects who consume oily fish one to seven times per week compared with those who seldom or never consumed fish (OR, 0.8; 95% CI, 0.6 to 1.0), which did not change even allowing for RF and anticyclic citrullinated protein (CCP) antibody status.²⁸

Red Meat and Protein Consumption

High consumption of red meat has been associated with an increased risk of inflammatory polyarthritis (OR, 1.9; 95% CI, 0.9 to 4.0).²⁹ Although one study reported that meat and offal were associated with an increased risk of developing RA,³⁰ this was not confirmed in other studies.^31,32 Whether the association between red meat consumption and inflammatory arthritis is causative remains unclear, although the presence of significant amounts of AA in red meat may provide some explanation for the association.

Tea and Coffee Consumption

Tea and coffee have been identified as potential risk factors for the development of RA. In the Finnish National Health Study, consumption of four or more cups of coffee per day was associated with an increased risk of RF-positive, but not RF-negative, RA after adjustment for potential confounders such as age, smoking, and gender (relative risk [RR], 2.2; 95% CI 1.13 to 4.27).³³ By contrast, the Iowa Women’s Health Study reported no association between daily caffeine intake and risk for RA. However, women who consumed four or more cups of decaffeinated coffee per day were at increased risk for RA compared to non–coffee drinkers (RR, 2.58; 95% CI, 1.63 to 4.06). Furthermore, women who consumed three or more cups of tea per day had a reduced risk of RA (RR, 0.39; 95% CI, 0.16 to 0.97).³⁴ More recent studies have not shown an association between tea/coffee and RA.^31,35

Although the epidemiologic evidence for an association between coffee/tea and RA is equivocal, interest has been sustained by the presence of metabolically active agents in these beverages. For example, the major catechin in tea inhibits induction of inducible nitric oxide synthase (iNOS) by stimulated macrophages.³⁶ iNOS generates highly reactive free radical products while di-imidating substrate arginine moieties to citrulline. Citrullinated peptides/proteins are recognized immunogens that provide a focus for autoimmunity in RA.

Alcohol Consumption

Alcohol consumption may reduce the risk of developing RA. In a case-control study of 515 patients with RA, alcohol consumption was associated with a reduced risk of anti-CCP positive RA.³⁷ A dose-dependent inverse relationship between alcohol consumption and risk of RA has also been demonstrated from two independent case-control studies (Swedish EIRA and Danish CACORA).³⁸ The reduction in risk of RA was more pronounced in patients with the shared epitope compared those without the shared epitope and most pronounced in smokers with the shared epitope.³⁸

With regard to candidate mechanisms for this putative reduction in risk for RA, alcohol has been shown to downregulate the production of proinflammatory cytokines and upregulate production of the anti-inflammatory cytokine IL-10.^39,40 Furthermore, in a murine model of arthritis, ethanol almost totally prevented the development of collagen-induced arthritis and in those mice that did develop arthritis, disease was less severe. These anti-inflammatory effects of ethanol were associated with reduced leukocyte migration, downregulation of NFκB, and reduced production of the proinflammatory cytokines IL-6 and TNF but not the anti-inflammatory cytokine IL-10.⁴¹

Vitamin D

As noted earlier, vitamin D has anti-inflammatory actions. Although vitamin D has been implicated in reducing the risk of the autoimmune diseases, diabetes,⁴² and multiple sclerosis, the association with risk of RA is less clear.⁴³

The Iowa Women’s Health Study reported that a higher intake of vitamin D was associated with a reduced risk of RA (RR, 0.67; 95% CI, 0.44 to -1.00, P = 0.05) in women aged 55 to 69 years.⁴⁴ However, in a more recent large study of 186,389 women followed for 22 years, there was no association between dietary intake of vitamin D and the risk of developing RA.⁴⁵ However, apart from supplements, the main source of vitamin D is de novo synthesis in the skin and estimated dietary intake may be a poor predictor of serum vitamin D concentrations. In a study of 79 patients with RA, no association was found between prior serum vitamin D concentrations and subsequent development of RA.⁴⁶ However, it is notable that the geometric means for both cases and controls in this study were only half the lower reference level of 60 nmol/L, which has been set subsequently to reflect a level that suppresses secondary hyperparathyroidism due to vitamin D insufficiency.

Antioxidants and Risk of Rheumatoid Arthritis

Oxygen free radicals (e.g., nitric oxide, superoxide, hydroxyl radical) are implicated in the tissue damage observed in RA.⁴⁷ Antioxidants including vitamin E (α-tocopherol), vitamin C (ascorbic acid), β-carotene, and selenium may have a protective role against tissue damage caused by these oxygen free radicals. This combined with evidence that markers of antioxidant nutritional status are lower in patients with established RA compared with normal controls⁴⁸ led to the hypothesis that antioxidants may protect against the development of RA. Despite this biologic plausibility, the available data do not provide clear evidence for a protective effect of antioxidants as dietary supplements in relation to the development of RA.

Low serum concentrations of vitamin E, β-carotene, retinol, and selenium have been reported to be weakly associated with an increased risk for developing RA in some but not all studies.^49–51 The strongest association between risk of RA and antioxidants was found for a combined antioxidant index rather than any one specific antioxidant.⁴⁹

A higher dietary intake of β-cryptoxanthin (a carotenoid found in fruit and vegetables) and zinc may protect against the development of RA.^52,53 Low vitamin C intake has also been associated with an increased risk of inflammatory polyarthritis with an adjusted OR of 3.3 (95% CI, 1.4 to 7.9) for the lowest tertile of vitamin C intake (<55.7 mg/day) compared with the highest intake tertile (>94.9 mg/day).⁵⁴ However, another study found no association between intake of vitamin C, vitamin E, zinc, or selenium and the development of RA.³¹

The Women’s Health Study, a randomized, double-blind, placebo-controlled trial designed to evaluate the effects of low-dose aspirin and vitamin E in the primary prevention of cardiovascular disease and cancer, has been used to address the influence of vitamin E supplementation (600 IU on alternate days) on the development of RA. During the 10-year follow-up period, 106 cases of RA occurred with no significant association between vitamin E supplementation and incidence of RA. A statistically insignificant trend toward inverse association between vitamin E and development of seropositive RA was seen on subgroup analysis.⁵⁵

Obesity and Rheumatoid Arthritis

Two studies have reported that obesity increases the risk of RA,^56,57 whereas two other studies report no association.^58–60 Increased plasma concentrations of the adipokines leptin, adiponectin, and visfatin have been observed in patients with RA compared with healthy controls.⁶¹ Furthermore, visfatin and leptin were associated with increased and reduced radiographic joint damage, respectively.⁶¹ These data combined with the proinflammatory state observed in obesity suggest that BMI may affect disease activity and outcomes in RA, whereas, perhaps paradoxically, increased BMI has been associated with less radiographic damage.^58,62 An alternative explanation is that patients with more active inflammatory disease, who tend to develop more radiographic erosions, may have lower BMI and rheumatoid cachexia.

Dietary Factors and Gout

Several large clinical studies have identified an association between high intake of meat and seafood (but not total protein intake) and both SU concentrations⁶³ and gout.⁶⁴ In comparison, the risk of gout is reduced with higher intake of low-fat dairy products and long-term coffee consumption.^64,65 More recently, fructose, which is found in corn syrup, sugar-sweetened soft drinks, and fruit juices, has been associated with hyperuricemia and gout.^66,67 Fructose increases SU through increased purine degradation.⁶⁸ Furthermore, urate and fructose share a common transporter within the kidney (SLC2A9).⁶⁹ Alcohol consumption other than moderate wine intake is associated with an increased risk of gout, with beer conferring a higher risk than liquor.⁷⁰

Dietary vitamin C supplementation has been shown to reduce the risk of gout (relative risk of gout with no supplementation vs. 1000 to 1499 mg vitamin C/day, 0.66 [95% CI, 0.49 to 0.88]).⁷¹ Vitamin C (ascorbic acid) is an important vitamin that can only be obtained through dietary intake. Because ascorbic acid is water soluble, it is not stored within the body and thus must be regularly supplemented through the diet to maintain the ascorbic acid pool. Dietary sources of ascorbic acid include fresh fruits and vegetables, in particular citrus fruits and green leafy vegetables such as broccoli.

Fasting and Gout

Prolonged periods (2 weeks to 8 months) of fasting in obese subjects result in a significant increase in SU and in some cases the development of gout.⁷² In patients with previously documented gout, a 1-day fast led to an increase in SU of 0.5 to 2.1 mg/dL with a mean rise of 1.1 mg/dL, with refeeding associated with a return in SU to baseline levels after 24 hours.⁷³ Explanations for the increase in SU during fasting include increased urate production, decreased excretion due to decreased glomerular filtration rate, altered renal tubular transport of uric acid, and competition with ketones for renal tubular excretion.⁷⁴

Obesity and Gout

High BMI predisposes to gout.⁷⁵ In a study comparing obese men (mean BMI 34 ± 4 kg/m²) with healthy controls (BMI, 21 ± 1 kg/m²), SU concentrations were raised to a similar degree in the obese subjects (≈8.0 ± 1.6 mg/dL vs. controls, 5.2 ± 0.81 mg/dL), irrespective of body fat distribution (predominantly visceral or predominantly subcutaneous). By contrast, 80% of subjects with hyperuricemia and accumulation of fat subcutaneously had low 24-hour urinary urate excretion compared with 10% of their counterparts with fat accumulation predominantly in a visceral distribution. These data suggest that the mechanism of hyperuricemia may vary depending on body fat distribution.⁷⁶ In a separate study using computed tomography to determine transverse area of abdominal fat components, visceral fat was shown to correlate strongly with SU, whereas no relationship was seen between SU and BMI or subcutaneous fat area.⁷⁷