Dietary lipids and the inflammatory response

N-3 polyunsaturated fatty acids and inflammation: from molecular biology to the clinic

The immune system is involved in host defense against infectious agents, tumor cells, and environmental insults. Inflammation is an important component of the early immunologic response. Inappropriate or dysfunctional immune responses underlie acute and chronic inflammatory diseases. The n-6 PUFA arachidonic acid (AA) is the precursor of prostaglandins, leukotrienes, and related compounds that have important roles in inflammation and in the regulation of immunity. Feeding fish oil results in partial replacement of AA in cell membranes by EPA. This leads to decreased production of AA-derived mediators, through several mechanisms, including decreased availability of AA, competition for cyclooxygenase (COX) and lipoxygenase (LOX) enzymes, and decreased expression of COX-2 and 5-LOX. This alone is a potentially beneficial anti-inflammatory effect of n-3 FA. However, n-3 FA have a number of other effects that might occur downstream of altered eicosanoid production or might be independent of this effect. For example, dietary fish oil results in suppressed production of proinflammatory cytokines and can modulate adhesion molecule expression. These effects occur at the level of altered gene expression. Fish oil feeding has been shown to ameliorate the symptoms of some animal models of autoimmune disease and to protect against the effects of endotoxin. Clinical studies have reported that oral fish oil supplementation has beneficial effects in rheumatoid arthritis and among some asthmatics, supporting the idea that the n-3 FA in fish oil are anti-inflammatory. There are indications that the inclusion of fish oil in enteral and parenteral formulae is beneficial to patients.

The ability of fish oil to suppress tumor necrosis factor alpha production by peripheral blood mononuclear cells in healthy men is associated with polymorphisms in genes that influence tumor necrosis factor alpha production

BACKGROUND

Tumor necrosis factor alpha (TNF-alpha) mediates inflammation. High TNF-alpha production has adverse effects during disease. Polymorphisms in the TNF-alpha and lymphotoxin alpha genes influence TNF-alpha production. Fish oil suppresses TNF-alpha production and has variable antiinflammatory effects on disease.

OBJECTIVE

We examined the relation between TNF-alpha and lymphotoxin alpha genotypes and the ability of dietary fish oil to suppress TNF-alpha production by peripheral blood mononuclear cells (PBMCs) in healthy men.

DESIGN

Polymorphisms in the TNF-alpha (TNF*1 and TNF*2) and lymphotoxin alpha (TNFB*1 and TNFB*2) genes were determined in 111 healthy young men. TNF-alpha production by endotoxin-stimulated PBMCs was measured before and 12 wk after dietary supplementation with fish oil (6 g/d).

RESULTS

Homozygosity for TNFB*2 was 2.5 times more frequent in the highest than in the lowest tertile of inherent TNF-alpha production. The percentage of subjects in whom fish oil suppressed TNF-alpha production was lowest (22%) in the lowest tertile and doubled with each ascending tertile. In the highest and lowest tertiles, mean TNF-alpha production decreased by 43% (P < 0.05) and increased by 160% (P < 0.05), respectively. In the lowest tertile of TNF-alpha production, only TNFB*1/TNFB*2 heterozygous subjects were responsive to the suppressive effect of fish oil. In the middle tertile, this genotype was 6 times more frequent than the other lymphotoxin alpha genotypes among responsive individuals. In the highest tertile, responsiveness to fish oil appeared unrelated to lymphotoxin alpha genotype.

CONCLUSION

The ability of fish oil to decrease TNF-alpha production is influenced by inherent TNF-alpha production and by polymorphisms in the TNF-alpha and lymphotoxin alpha genes.

Influence of dietary conjugated linoleic acid isomers on early inflammatory responses in male broiler chickens

1. The influence of dietary conjugated linoleic acid isomer (CLA, 0 and 10 g/kg) on the metabolic and physiological responses to immune stimulation induced by a single injection of Salmonella enteritidis lipopolysaccharide (LPS) or repeated injections of LPS and Sephadex G-50 was determined in male broiler chicks. 2. In experiment 1, 10-d-old chicks were fed on experimental diets for 14 d and half of the birds fed on each diet were injected intraperitoneally with LPS (1.5 mg/kg body weight). In experiment 2,7-d-old chicks were fed on experimental diets for 18 d. Immune stimulation was started at 19 d old and continued for 5 d. Half of the birds fed on each diet were injected intraperitoneally with 0.25 mg/kg body weight of LPS at 19, 21 and 23 d of age, and with 250 mg/kg body weight of Sephadex at 20 and 22 d of age to stimulate the immune system. 3. In experiment 1, giving CLA prevented an increase in blood heterophil to lymphocyte ratio 7 h after a single injection of LPS, and increases in plasma ceruloplasmin and alpha 1 acid glycoprotein (AGP) 24 h after the injection, but not 7 h after the injection. CLA also prevented a decrease in food intake for 24 h after LPS injection. 4. In experiment 2, the CLA diet partially prevented reductions in body weight gain and weight gain to feed intake ratio caused by repeated injections of LPS and Sephadex. Feeding CLA prevented increases in plasma ceruloplasmin and AGP at 24 d of age caused by repeated injections of LPS and Sephadex, but not at 20 d of age. 5. These results suggest that feeding CLA alleviates some undesirable metabolic and physiological changes induced by immunological stimulation in male broiler chicks.

Fatty acids and lymphocyte functions

The immune system acts to protect the host against pathogenic invaders. However, components of the immune system can become dysregulated such that their activities are directed against host tissues, so causing damage. Lymphocytes are involved in both the beneficial and detrimental effects of the immune system. Both the level of fat and the types of fatty acid present in the diet can affect lymphocyte functions. The fatty acid composition of lymphocytes, and other immune cells, is altered according to the fatty acid composition of the diet and this alters the capacity of those cells to produce eicosanoids, such as prostaglandin E2, which are involved in immunoregulation. A high fat diet can impair lymphocyte function. Cell culture and animal feeding studies indicate that oleic, linoleic, conjugated linoleic, gamma-linolenic, dihomo-gamma-linolenic, arachidonic, alpha-linolenic, eicosapentaenoic and docosahexaenoic acids can all influence lymphocyte proliferation, the production of cytokines by lymphocytes, and natural killer cell activity. High intakes of some of these fatty acids are necessary to induce these effects. Among these fatty acids the long chain n-3 fatty acids, especially eicosapentaenoic acid, appear to be the most potent when included in the human diet. Although not all studies agree, it appears that fish oil, which contains eicosapentaenoic acid, down regulates the T-helper 1-type response which is associated with chronic inflammatory disease. There is evidence for beneficial effects of fish oil in such diseases; this evidence is strongest for rheumatoid arthritis. Since n-3 fatty acids also antagonise the production of inflammatory eicosanoid mediators from arachidonic acid, there is potential for benefit in asthma and related diseases. Recent evidence indicates that fish oil may be of benefit in some asthmatics but not others.

Polyunsaturated fatty acids, inflammation, and immunity

The fatty acid composition of inflammatory and immune cells is sensitive to change according to the fatty acid composition of the diet. In particular, the proportion of different types of polyunsaturated fatty acids (PUFA) in these cells is readily changed, and this provides a link between dietary PUFA intake, inflammation, and immunity. The n-6 PUFA arachidonic acid (AA) is the precursor of prostaglandins, leukotrienes, and related compounds, which have important roles in inflammation and in the regulation of immunity. Fish oil contains the n-3 PUFA eicosapentaenoic acid (EPA). Feeding fish oil results in partial replacement of AA in cell membranes by EPA. This leads to decreased production of AA-derived mediators. In addition, EPA is a substrate for cyclooxygenase and lipoxygenase and gives rise to mediators that often have different biological actions or potencies than those formed from AA. Animal studies have shown that dietary fish oil results in altered lymphocyte function and in suppressed production of proinflammatory cytokines by macrophages. Supplementation of the diet of healthy human volunteers with fish oil-derived n-3 PUFA results in decreased monocyte and neutrophil chemotaxis and decreased production of proinflammatory cytokines. Fish oil feeding has been shown to ameliorate the symptoms of some animal models of autoimmune disease. Clinical studies have reported that fish oil supplementation has beneficial effects in rheumatoid arthritis, inflammatory bowel disease, and among some asthmatics, supporting the idea that the n-3 PUFA in fish oil are anti-inflammatory and immunomodulatory.

Nutritional modulation of immune function

The inflammatory response to injury and infection, although an essential part of immune function, carries the risk of severe tissue depletion and immunosuppression. These outcomes increase morbidity and delay recovery. Evidence is accumulating that single-nucleotide polymorphisms in the genes controlling pro-inflammatory cytokine production adversely influence the response. Immunonutrition provides a means of modulating the inflammatory response to injury and infection, and thereby improves clinical outcome. n-3 Polyunsaturated fatty acids (n-3 PUFA), glutamine, arginine, S amino acids and nucleotides are important components of immunonutrient mixes. While animal model studies suggest that all these components may exert a beneficial effect in patients, the number of large randomized placebo-controlled trials utilizing immunonutrition is fairly limited and the observed effects are relatively small. Meta-analyses suggest that while immunonutrition may not reduce mortality rates, a reduction in hospital length of stay, decreased requirements for ventilation and lower infection rates are achieved by this mode of nutrition. The present paper discusses some underlying reasons for the difficulty in demonstrating the clinical efficacy of immunonutrition. Paramount among these reasons is the antioxidant status and genetic background of the patient. A number of studies suggest that there is an inverse relationship between inflammation and T-cell function. Immuno-enhancive effects have been shown in a number of studies in which n-3 PUFA, glutamine and N-acetyl cysteine have been employed. All these nutrients may exert their effects by suppressing inflammation; n-3 PUFA by direct suppression of the process and glutamine and N-acetyl cysteine by acting indirectly on antioxidant status. Glutamine and nucleotides exert a direct effect on lymphocyte proliferation. Preliminary data suggests that not all genotypes are equally sensitive to the effects of immunonutrition. When further studies have been conducted to discern the precise interaction between each individual's genotype of relevance to the response to injury and infection, and immunonutrients, the level of precision in the application of immunonutrition will undoubtedly improve.

The scientific basis of immunonutrition

Substrates with immune-modulating actions have been identified among both macro- and micronutrients. Currently, the modes of action of individual immune-modulating substrates, and their effects on clinical outcomes, are being examined. At present, some enteral formulas are available for the clinical setting which are enriched with selected immune-modulating nutrients. The purpose of the present paper is to review the scientific rationale of enteral immunonutrition. The major aspects considered are mucosal barrier structure and function, cellular defence function and local or systemic inflammatory response. It is notable that in critical illness the mucosal barrier and cellular defence are impaired and a reinforcement with enteral immunonutrition is desirable, while local or systemic inflammatory response should be down regulated by nutritional interventions. The results available from clinical trials are conflicting. Meta-analyses of recent trials show improvements such as reduced risk of infection, fewer days on a ventilator, and reduced length of intensive care unit and hospital stay. Thus, a grade A recommendation was proclaimed for the clinical use of enteral immune-modulating diets. Improvement in outcome was only seen when critical amounts of the immune-modulating formula were tolerated in patients classified as being malnourished. However, in other patients with severe sepsis, shock and organ failure, no benefit or even disadvantages from immunonutrition were reported. In such severe conditions we hypothesize that systemic inflammation might be undesirably intensified by arginine and unsaturated fatty acids, directly affecting cellular defence and inflammatory response. We therefore recommend that in patients suffering from systemic inflammatory response syndrome great caution should be exercised when immune-enhancing substrates are involved which may aggravate systemic inflammation.

Kill and cure: dietary augmentation of immune defences against colon cancer

At its most fundamental, cancer is a genetic disease resulting from inherited or acquired mutations in tumour suppressor genes and proto-oncogenes. Environmental factors, including ingested food components, interact with genetic inheritance to determine individual cancer risk. There is growing evidence that the immune system exerts selective pressure during neoplastic development. Tumour cells that evade this immunosurveillance because they are non-antigenic or because they defend themselves successfully against immune attack have a survival advantage. Effective chemopreventative agents will include dietary components that enhance the immune system's ability to identify transformed cells and to target them for apoptosis.