Long-Chain Polyunsaturated Fatty Acids Supplementation and Respiratory Infections
Long-chain polyunsaturated fatty acids (LCPUFAs) are essential for the maintenance and optimal functioning of
immune cells. The metabolites of both omega-3 and omega-6 fatty acids play an important role in the synthesis of immune mediators, such as prostaglandins, leukotrienes, thromboxanes, protectins, and resolvins. LCPUFAs are
synthesized by elongation and desaturation of the carbon chain from the essential PUFAs, such as linoleic acid (LA) and alpha linolenic acid (ALA). In the current pandemic, great attention has been paid to immune modulation, with the ultimate goal of infection prevention and treatment of COVID-19 patients.
There are few clinical trials on LCPUFA supplementation for the prevention and management of respiratory tract infections (RTIs). This narrative review aimed to identify whether LCPUFA supplementation was effective at protecting against RTIs. Eight clinical studies were analyzed, 5 of which were conducted in children (aged 0–11 years) and 3 in adults (aged 18–45 years). A total of 4,107 subjects were included. A variety of LCPUFA types and dosages were used for the supplementations, ranging in duration between 4 and 9 months.
The available results point towards a potential beneficial role of LCPUFA supplementation in the prevention of RTIs. In children, the protective effects were reflected by a lower number of infectious episodes, reduced symptoms, and reduced school absences. Positive results were seen with supplemented formula and dietary supplements in the pediatric population. However, some studies have yielded inconclusive findings. The heterogeneity of study design and inclusion criteria are major limitations in our ability to translate these findings into concrete recommendations that benefit patients. Randomized controlled trials designed and powered to address questions on dose, duration,
and outcomes of LCPUFA on RTIs are still needed.
Imhoff-Kunsch B, Stein AD, Martorell R, Parra-Cabrera S, Romieu I, Ramakrishnan U. Prenatal docosahexaenoic acid supplementation and infant morbidity: randomized controlled
trial. Pediatrics. 2011;128(3):e505–12.
• Long-chain polyunsaturated fatty acids (LCPUFAs) and their metabolites are involved in the control of chronic and acute inflammations.
• No data were available on the role of LCPUFAs in COVID-19 disease.
• More focused randomized controlled trials are necessary to evaluate the effect of LCPUFA supplementation.
Background: Long-chain polyunsaturated fatty acids (LCPU- FAs) can actively affect the maintenance and optimal functioning of immune cells. The metabolites of both omega-3 and omega-6 play an important role in the synthesis of different mediators, such as prostaglandins, leukotrienes, thromboxanes, protectins, and resolvins, that can interfere with the virus and modulate inflammation.
Summary: In this narrative review, we aim to identify whether LCPUFA supplementation may be effective in protecting the population against respiratory tract infections. We included only randomized controlled trials performed in both pediatric and adult subjects. Eight papers were selected: five trials were conducted in a pediatric population and three in adults. Different concentrations of fatty acids supplementation were associated with a lower incidence of common respiratory symptoms, except for two studies that did not provide significant results. Most of the studies are of low quality, and respiratory infections were assessed as secondary or even safety outcomes.
Key Messages: No data were available on the role of LCPUFAs in coronavirus disease 2019 (COVID-19). Although these data showed that LCPUFAs may be effective in preventing respiratory tract infections, future studies are still needed to clarify their possible co-adjuvant role in the prevention and treatment of respiratory infections.
It is well known that nutrients can actively affect the maintenance and optimal functioning of immune cells . Long-chain polyunsaturated fatty acids (LCPUFAs) have a very special role in this process because they participate in controlling chronic and acute inflammations. The metabolites of both omega-3 (ω3) and omega-6 (ω6) play an important role in the synthesis of different mediators, such as prostaglandins, leukotrienes, thromboxanes, protectins, and resolvins . Particularly ω3 LCPUFAs could notably help improve the resolution of the inflammatory process and may also interact at different stages of viral infections. In this narrative literature review, we provide information on LCPUFAs and the possible effects of these dietary compounds in promoting better immune function and, therefore, as a co-adjuvant treatment to decrease the severity of disease among those who have been diagnosed with respiratory tract infections.
Among the respiratory viruses, particular attention has been paid to coronavirus disease 2019 (COVID-19), identi\fied for the first time in December 2019 in Wuhan, China. Because of its rapid spread worldwide and the increased infection rate, the WHO declared it a pandemic in March 2020 . In 10-15% of all cases, COVID-19 is complicated by severe pneumonia requiring hospitalization, with a high risk of developing acute respiratory distress syndrome, which can lead to intensive care unit placement of the pa\tient and is often lethal. Other patients may remain asymp\tomatic even if they test positive for the virus . In this context, due to its highly infectious nature and alarming mortality rate, every effort is focused on prevention and treatment to alleviate the suffering of COVID-19 patients, including the immune response.
The Immune System and LCPUFAs
The immune system is a network of biological processes with the main aim to protect the organism from pathogenic organ\isms and, therefore, from disease. Overactivity of the immune system can display many forms, including an excessive reaction to allergens or autoimmunity, while underactivity of the immune system (immunodeficiency) can be due to primary immunodeficiency diseases or can be the result of other conditions, such as cancer, immunosuppressants, and HIV/AIDS. These conditions increase the susceptibility to other infections.
LCPUFAs are an essential component of immune cells, and their relationships are an ongoing matter of research. It has been reported that in lymphocytes, monocytes, and neutrophils, arachidonic acid (ARA) constitutes about 20% of total fatty acids, while ω3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) constitute 1 and 2.5%, respectively .
LCPUFAs are synthesized by elongation and desaturation of the carbon chain from the essential PUFAs: linoleic acid (LA) for the ω6 series and alpha-linolenic acid (ALA) for the ω3 series . The metabolic pathway consists of successive carbon chain elongation and desaturation steps as illustrated in Figure 1. Due to the different roles and effects of the ω6 and ω3 series, it is important to understand the effect of the ω6 to ω3 ratio on the immune system, in particular the ratio of LA/ALA.
LA, defined as an essential fatty acid in mammals because of their inability to synthesize it, is common in the human diet, being widely distributed in foods . In many vegetable oils, it represents more than 50% of the lipid content; high amounts of LA are also present in nuts, while lower levels are found in cereals (more in whole grains), legumes, some meats, eggs, and dairy products . LA has been reported to be the substrate for CYP450 enzymes, leading to the formation of linoleic epoxides 9,10-epoxyoctadecenoic acid (9,10-EpOME), and 12,13-epoxyoctadecenoic acid (12,13-EpOME) known as leukotoxin and isoleukotoxin . The epoxides are then metabolized by the soluble epoxide hydrolases into the dihydroxy derivatives 9,10-DiHOME and 12,13-DiHOME. DiHOMEs may play a dual role in inflammation, stimulating neutrophil chemotaxis at low concentrations, while inhibiting neutrophil respiratory burst at higher concentrations . Like LA, ALA is defined as an essential fatty acid in mammals , and its principal diet sources are nuts, leafy vegetables, and seed oils. After absorption, it can be catabolized into longer-chain and more unsaturated fatty acids, such as EPA and DHA, but, like LA conversion into ARA within the ω6 series, the endogenous production of ALA derivatives is low in humans.
Arachidonic Acid and Its Metabolites and the Immune System
ARA is the main ω6 product and is present esterified to the 2-position in membrane phospholipids . Upon release from membrane phospholipids by the activity of the cytosolic phospholipase A2 (PLA2), ARA is enzymatically metabolized by several oxygenases into eicosanoids, a large family of mostly proinflammatory molecules. The main ARA metabolites involved in the immune system are prostaglandin E2 (PGE2), leukotriene B4 (LTB4), and thromboxane A2 (TXA2). PGE2 inhibits T-cell and NK-cell proliferation, as well as interferon-y (IFN-y) and interleukin-12 (IL-12) production after binding cell surface receptors [11, 12]. PGE2 also inhibits B-cell activation secondary to IL-4 stimulation in a specific manner and enhances IgE and IgG1 production .
LTB4 enhances T-cell recruitment, inhibits de novo induced Tregs generation, and increases IL-17 cytokine production during T-cell differentiation. Also, it regulates the migration of various lymphoid-derived cell types . TXA2 inhibits naive T-cell proliferation. Moreover, it inhibits T-cell interaction with dendritic cells and increases mature T-cell proliferation and activation .
Regarding the role of ARA and SARS-CoV-2, two recent reviews resume some pivotal points. Ripon et al.  describe the role of the ARA metabolic cascade during the cytokines storm due to virus infection, concluding with the recommendation to use bioactive lipids, nonsteroidal anti-inflammatory drugs, steroids, cell phospholipase A2 (cPLA2) inhibitors, and specialized pro-resolving mediators to treat COVID-19 disease, while Das  explains and supports the use of ARA to counteract SARS-CoV-2 due to its active role against viruses and to reduce inflammation.
ω3EPA, DHA, and Their Metabolites and the Immune System
ω3 LCPUFAs act to inhibit the inflammatory response both in a direct and indirect pathway: DHA reduces endoplasmic reticulum stress and reactive oxygen species production in mitochondria, inhibits Toll-like receptor activation, and upregulates cytoprotective proteins, intracellular antioxidants, and anti-inflammatory and detoxifying enzymes via the activation of nuclear factor erythroid 2-related factor 2 (NRF2). Both DHA and EPA regulate the inflammation response acting on the expression of oxidized low-density lipoprotein receptor 1, plasminogen activator inhibitor 1, TXA2 receptor, vascular cell adhesion molecule-1, monocyte chemoattractant protein-1, and intercellular adhesion molecule 1 . EPA, docosapentaenoic acid, and DHA are the precursors of several oxygenated lipids. The resulting compounds have been collectively described as a novel genus of specialized pro-resolving lipid mediators and are endowed with potent anti-inflammatory, pro-resolution activities within the immune system. This molecular family includes D- and E-series resolvins, protectins, maresins [18, 19], and maresin conjugates in tissue regeneration, which have been proved effective in limiting inflammation and accelerating the physiological resolution of the inflammatory response [18, 20].
A nonsystematic review of the literature was initially performed in May 2021 and then updated in November 2021. The search was carried out via PubMed (www.ncbi.nlm.nih.gov/pubmed), Embase (www. emabase.com), Web of Science (www.isiknowledge.com), and the Cochrane library (www.cochranelibrary.com). In this narrative review, we aim to identify whether LCPUFA supplementation may be effective in protecting the population against respiratory tract infections. Only human studies reported in English were considered. Letters to the editor, abstracts, and proceedings from scientific meetings were excluded from the analysis. Two authors (V.C. and A.M.) independently selected the articles, retrieved and assessed the potentially relevant ones. Eligible were clinical trials on LCPUFA supplementation (intervention) and respiratory tract infection (outcome), including participants of all ages and from any region of the world. Secondary references of identified papers were also screened.
A total of 8 RCTs were identified [21-28], which are detailed in Table 1. They included 4,107 subjects. Five trials were performed in a pediatric population (age 0-11 years) and 3 studies in adults (age 18-45 years). One study was conducted in Italy, 1 in Mexico, 1 in Denmark, 1 in France, 1 in Africa, 2 in the USA, and 1 multicenter study was conducted in Malaysia, the Netherlands, Poland, Portugal, and Thailand. A wide range of LCPUFA dosages and types was used for supplementation. The duration of the intervention was 4-9 months. Three studies showed a reduced incidence of upper respiratory tract infections or infective episodes in children when fed with DHA, ARA or ALA/LA, or EPA/DHA, respectively, in the first year of life [21-23].
First, Birch et al.  reanalyzed data from two cohorts to investigate the incidence of allergic and respiratory diseases in children with a DHA/ARA-supplemented formula (DHA/ARA as 0.32-0.36% and 0.64-0.72% of total fatty acids, respectively) during infancy. They found lower odds of developing upper respiratory infection in the DHA/ARA- supplemented group for 1 year. Second, Venuta et al.  designed an interventional study in children between 36 and 49 months of age to evaluate the impact of supplementation of LA (596 mg/day) and ALA (855 mg/day) on respiratory tract infections. The authors observed a reduced number of infectious episodes, days of fever, and absence from school at 120-180 days from intervention in children receiving supplementation. Malan et al.  supplemented 321 children aged 6-11 years with 50 mg Fe++, 420 mg DHA, and 80 mg EPA given on 4 days/week. The supplementation was associated with a 3-fold lower odds of school absence (OR: 0.30, 95% CI: 0.11, 0.82), but no associations with other outcomes were found.
Two more studies analyzed LCPUFA supplementation in formula milk. Specifically, Lapillonne et al.  observed that infants fed with formula containing 17 mg DHA and 34 mg ARA/100 kcal had a lower incidence and delayed onset of respiratory illnesses (bronchitis/bronchiolitis) and of their symptoms (croup, nasal congestion, cough), as well as less diarrhea requiring medical attention, when compared to infants who received formula without DHA and ARA. On the contrary, Chatchatee et al.  in an international multicenter interventional study failed to demonstrate a reduced risk of infection in 767 healthy children fed with growing-up milk with the addition of 1.2 g/100 mL of short-chain galacto- oligosaccharides/long-chain fructo-oligosaccharides (sc- G0S/ICFOS) (9:1) and 19.2 mg/100 mL of ω3 LCPUFAs (EPA + DHA, 4:6).
Two studies involved pregnant women. Imhoff-Kunsch et al.  studied the effect on infant morbidity of 400 mg of DHA taken daily by mothers in the prenatal period. This double-blind randomized placebo-controlled trial demonstrated a lower occurrence of cold symptoms in the DHA group with respect to placebo at 1 and 3 months of age (37.6 vs. 44.6% and 37.8 vs. 44.1%, respectively). Moreover, a shorter duration of nasal secretion, fever, and difficulty breathing at 6 months (RR: 0.87, 95% CI: 0.77-0.98; RR: 0.80, 95% CI: 0.66-0.98; and RR: 0.46, 95% CI: 0.24-0.87, respectively) was reported. Another trial with the same design by Bisgaard et al.  studied 736 pregnant women supplemented with 2.4 g/day of ω3 LCPUFA (55% EPA and 37% DHA) in the third trimester and concluded that the infants of supplemented mothers had a reduced risk of both wheeze or asthma and of infections of the lower respiratory tract. Lastly, only 1 RCT investigated the effect of 2 g/day of conjugated LA in a population aged 18-45 years and showed no consistent effects on subjects affected by human rhinovirus colds.