n-3 PUFA are widely found in foods such as plant oils and deep-sea fish and are named for the multiple double bonds in their chemical structure(Reference Bhatt, Budoff and Mason1) (Fig. 1). They mainly include three types(Reference Kousparou, Fyrilla and Stephanou2): ALA, EPA and DHA. Among them, ALA(Reference Yuan, Xie and Huang3) is the most common plant-based n-3 PUFA, typically found in certain plant oils (such as flaxseed oil, chia seed oil, hemp oil, etc.) and nuts (such as walnuts, hazelnuts, cashews, etc.). EPA and DHA(Reference Tomczyk, Heileson and Babiarz4) are primarily found in marine organisms, especially in deep-sea fish (such as salmon, mackerel and sardines) and certain algae.
Sources of n-3 PUFA (ALA, EPA and DHA) in foods and their chemical structures.
EPA and DHA are considered the two types of n-3 PUFA with the strongest biological effects and play an important role in human health(Reference Kousparou, Fyrilla and Stephanou2,Reference Tomczyk, Heileson and Babiarz4–Reference Blaauw, Calder and Martindale6) . Since the human body cannot synthesise EPA and DHA on its own, and the conversion efficiency of ALA to EPA and DHA in the body is low(Reference Doughman, Krupanidhi and Sanjeevi7–Reference Richard and Monk9), it is crucial to ensure an adequate intake of EPA and DHA to maintain overall health. The WHO recommends that adults should consume at least 250 mg of EPA and DHA daily, with the specific amount varying according to individual needs(Reference Salem and Eggersdorfer10). For example, pregnant and breastfeeding women have a higher demand for DHA, and it is recommended that they consume at least 200 mg more DHA per day than the general population(Reference von Schacky11). In addition, the American Heart Association recommends that adults consume fatty fish rich in n-3 PUFA at least twice a week to ensure adequate intake of EPA and DHA, thereby promoting cardiovascular health(Reference Kris-Etherton, Harris and Appel12).
However, in reality, most people consume far less n-3 PUFA than the recommended amount, especially those who do not frequently eat fish or seafood(Reference Koutentakis, Surma and Rogula13). In addition, with the widespread consumption of processed foods and fast food, plant oils rich in n-6 PUFA, such as soybean oil, corn oil, sunflower oil, etc., are widely used in the production of foods like French fries, cookies, potato chips, mayonnaise, dressings and various snacks. Excessive consumption of these foods can lead to an imbalance in the n-3:n-6 PUFA intake ratio, which may negatively impact health(Reference Kousparou, Fyrilla and Stephanou2,Reference Yamashima, Ota and Mizukoshi14,Reference Mostafa, Gutierrez-Tordera and Mateu-Fabregat15) . Therefore, supplementing with n-3 PUFA through dietary supplements(Reference Sherratt, Lero and Mason16), such as fish oil, algae oil, etc., has become an important approach in modern nutrition and health management. This helps to address deficiencies in the daily diet, balance the intake of EPA and DHA and promote overall health.
Despite the generally insufficient intake of n-3 PUFA in modern diets, their potential health benefits have garnered widespread attention, especially regarding their role in the prevention and treatment of certain diseases. The purpose of this review is to outline the current clinical applications of n-3 PUFA, with a particular focus on research advancements and potential uses in the field of orthopaedics.
Clinical application progress
In recent years, n-3 PUFA have shown significant effects in the prevention and treatment of various diseases (Table 1), particularly in CVD and neurological disorders.(Reference Rodriguez, Lavie and Elagizi17–Reference von Schacky21). In CVD, n-3 PUFA reduce the risk of heart attacks and strokes through multiple mechanisms, including lowering blood lipid levels, reducing triglycerides, inhibiting platelet aggregation and decreasing atherosclerosis and vascular inflammation(Reference Sherratt, Libby and Budoff22–Reference Sala-Vila, Fleming and Kris-Etherton25). In addition, n-3 PUFA can lower blood pressure in hypertensive patients by regulating ion channels in the blood vessels(Reference Musazadeh, Kavyani and Naghshbandi26). In neurological disorders, n-3 PUFA, especially DHA, are crucial for brain health(Reference DiNicolantonio and O’Keefe27). Multiple studies have shown that n-3 PUFA can improve neural conduction, alleviate symptoms of depression, protect nerve cells, enhance memory and learning abilities and slow down cognitive decline in Alzheimer’s disease(Reference Zuo, Wu and Xiang28–Reference Lu, Qiao and Mi32).
Clinical applications of n-3 PUFA in common diseases
The therapeutic potential of n-3 PUFA in ophthalmic diseases, allergic conditions and liver diseases is also gradually being recognised(Reference Jump, Lytle and Depner33–Reference Li, Li and Cao35). In ophthalmic diseases, n-3 PUFA alleviate dry eye symptoms by improving the ocular lipid layer and slowing down tear evaporation(Reference Downie, Ng and Lindsley36); n-3 PUFA also slow the progression of age-related macular degeneration by promoting the survival and repair of retinal nerve cells, as well as protecting the retina from oxidative damage(Reference Jiang, Shi and Fan37); In addition, n-3 PUFA have shown some potential in controlling myopia(Reference Pan, Zhao and Xie38). In allergic diseases, n-3 PUFA can effectively alleviate symptoms of allergic rhinitis by inhibiting the production of pro-inflammatory cytokines. Additionally, n-3 PUFA have a relieving effect on airway inflammation, helping to improve lung function in asthma patients and thereby effectively alleviating their symptoms(Reference Heras, Gomi and Young39,Reference Wake and Kobayashi40) . In liver diseases, n-3 PUFA can reduce hepatic fat accumulation, improve liver function, regulate inflammatory responses and slow down liver damage(Reference Jump, Lytle and Depner33,Reference Shama and Liu41) .
Additionally, although the potential of n-3 PUFA in cancer treatment has garnered widespread attention, their therapeutic efficacy remains somewhat controversial(Reference Manson, Cook and Lee42,Reference Lee, Seong and Kim43) . Some studies indicate that n-3 PUFA exert their effects through multi-target mechanisms(Reference D’Angelo, Motti and Meccariello44,Reference Liput, Lepczyński and Ogłuszka45) , such as inhibiting cell proliferation, promoting apoptosis, suppressing angiogenesis, reducing inflammation, lowering metastasis and regulating epigenetic abnormalities. These actions may inhibit the growth of various types of tumours, including breast cancer(Reference Fabian, Kimler and Hursting46), colon cancer(Reference D’Angelo, Motti and Meccariello44) and prostate cancer(Reference Liang, Henning and Grogan47).
In addition to the therapeutic effects in the aforementioned diseases, the application of n-3 PUFA in the field of orthopaedics is also gaining increasing attention(Reference Li, Lu and Qi48–Reference Huang, Vi and Zong52) (Fig. 2). With the changes in modern lifestyle, the incidence of orthopaedic diseases, particularly osteoarthritis (OA)(Reference Allen, Thoma and Golightly53,Reference Bijlsma, Berenbaum and Lafeber54) and osteoporosis (OP)(Reference Ayers, Kansagara and Lazur55), has been rising year by year, significantly affecting patients’ quality of life and posing a major challenge to the healthcare system. Therefore, exploring the application of n-3 PUFA in the field of orthopaedics holds important research value and practical significance.
Application of n-3 PUFA in the field of orthopaedics.
Research progress of n-3 PUFA in osteoarthritis
In recent years, research on the application of n-3 PUFA in OA has gradually increased(Reference Cordingley and Cornish56,Reference Shawl, Geetha and Burnett57) .OA(Reference Bijlsma, Berenbaum and Lafeber54) is a chronic and degenerative disease, commonly linked with joint inflammation and cartilage degeneration. The clinical manifestations primarily include joint pain, stiffness, swelling and limited mobility, significantly affecting patients’ quality of life. n-3 PUFA play a role in the prevention and treatment of OA through various mechanisms(Reference Calder58–Reference Gutiérrez, Svahn and Johansson62), primarily including altering the lipid composition of cell membranes, inhibiting pro-inflammatory signalling pathways, promoting the production of anti-inflammatory mediators and regulating immune responses.
Alteration of the lipid composition of cell membranes
n-6 PUFA, especially arachidonic acid (AA), occupy a crucial position in the phospholipid components of cell membranes(Reference Das63). AA is converted into prostaglandin E2 (PGE2) via the cyclooxygenase-2 pathway and into leukotriene B4 via the 5-lipoxygenase pathway. Both of these pro-inflammatory molecules regulate immune responses by chemotactically attracting immune cells to the site of injury, promoting the onset and maintenance of inflammation(Reference Pang, Liu and Zhao64–Reference Hoxha66). Therefore, the excessive accumulation of n-6 PUFA in the body is closely associated with many inflammation-related diseases.
In contrast, n-3 PUFA, particularly DHA and EPA, can competitively bind to cell membrane phospholipids with n-6 PUFA, thereby increasing the content of n-3 PUFA in the membrane and maintaining a healthy balance of lipid signalling in the body. This shift reduces the proportion of AA, thereby decreasing the production of pro-inflammatory substances such as PGE2 and leukotriene B4 derived from n-6 PUFA and mitigating the extent of the inflammatory response(Reference Peña-de-la-Sancha, Muñoz-García and Espínola-Zavaleta67) (Fig. 3).
n-3 PUFA reduce inflammatory responses by competitively binding to cell membrane phospholipids with n-6 PUFA.
Inhibition of pro-inflammatory signalling pathways
The NF-κB signalling pathway is a central pathway in many inflammatory responses (Fig. 4), playing a crucial role in inflammation-related diseases such as OA(Reference Yao, Wu and Tao68). The NF-κB signalling pathway can activate the synthesis of pro-inflammatory cytokines, including TNF-α and IL-1β, promoting the initiation and persistence of inflammation(Reference Zhong, Liang and Zhang69). DHA can inhibit the degradation of key proteins in the NF-κB pathway, thereby preventing the nuclear translocation of NF-κB, reducing its activity in the nucleus and decreasing the release of pro-inflammatory cytokines, ultimately alleviating the inflammatory response(Reference Liu, Zhang and Yang59,Reference Jiang, Zeng and He70) . This mechanism is of significant importance in orthopaedic diseases such as OA. Jin et al. (Reference Jin, Dong and Sun71) found through animal models that a diet rich in n-3 PUFA inhibits the expression of the NF-κB signalling pathway, demonstrating anti-inflammatory and anti-OA effects. Zhang et al. (Reference Zhang, Dai and Zhang72) also found that edible oils with a low n-6/n-3 PUFA ratio can delay the progression of OA by inhibiting the NF-κB pathway.
Classical NF-κB signalling pathway. This figure illustrates the main process of the classical NF-κB signalling pathway: pro-inflammatory cytokines and pathogen-associated molecular patterns bind to cell surface receptors, activating the IκB kinase complex. The IκB kinase complex phosphorylates IκB proteins, leading to their degradation and the release of NF-κB (p65/p50). Subsequently, NF-κB translocates to the nucleus, where it binds to specific DNA sequences and initiates the transcription of target genes. These target genes include matrix metalloproteinases, ADAMTS4/5, Runx2 and HIF2α, which are involved in important biological processes such as inflammation, extracellular matrix degradation, osteogenesis and hypoxic response.
Promotion of anti-inflammatory lipid mediator production
After metabolism, n-3 PUFA not only reduce the production of pro-inflammatory molecules but also promote the generation of active anti-inflammatory lipid mediators such as resolvins, protectins and maresins(Reference Dyall, Balas and Bazan73,Reference Chiang and Serhan74) (Table 2). These active anti-inflammatory lipids have shown potential in inhibiting cartilage degradation and promoting joint tissue repair, particularly playing an important role in inflammation resolution and tissue regeneration(Reference Oppedisano, Bulotta and Maiuolo75–Reference Shih, Tao and Gilpin77). Park et al. (Reference Park, Roh and Pan78) found through animal models that resolvins can reduce the release of inflammatory factors (such as TNF-α, IL-1β, etc.) by binding to specific receptors on inflammatory cells in both neurogenic and inflammatory pain models, thereby alleviating the inflammatory response. Zhao et al. (Reference Zhao, Wang and Wang79) demonstrated through a rat model that protectins possess pro-resolving properties, promoting autophagy to accelerate the clearance of damaged cells, thereby exerting anti-inflammatory effects. Lu et al. (Reference Lu, Feng and Zhang80) similarly found through a rat model that maresins activate the PI3K/Akt pathway and inhibit the NF-κB pathway, reducing the secretion of matrix metalloproteinase-13, which increases type II collagen in cartilage, thereby exerting anti-inflammatory effects and protecting cartilage.
Active anti-inflammatory lipid mediators derived from n-3 PUFA metabolism
MMP13, matrix metalloproteinase-13.
Regulation of immune responses
n-3 PUFA can also modulate immune responses and alleviate chronic inflammation in OA(Reference Poggioli, Hirani and Jogani81,Reference Coniglio, Shumskaya and Vassiliou82) (Fig. 5). Dietary supplementation with n-3 PUFA effectively increases cell membrane fluidity, thereby modulating immune cell function, particularly the function of T cells and macrophages(Reference Gutiérrez, Svahn and Johansson62). Studies have found that T cells play a crucial role in immune suppression and tissue repair. n-3 PUFA can alter the ratio of T cell subsets, promote the activity of anti-inflammatory T cells (such as Th2, Treg) and inhibit the activation of pro-inflammatory T cells (such as Th1, Th17), helping to reduce immune-mediated inflammatory responses in OA(Reference Hou, McMurray and Chapkin83–Reference Perez-Hernandez, Chiurchiù and Perruche85). In addition, n-3 PUFA can modulate macrophage polarisation, promoting their transformation into anti-inflammatory M2 macrophages(Reference Schwager, Bompard and Raederstorff86,Reference Videla, Valenzuela and Del Campo87) . Numerous studies have shown that M2 macrophages play a key role in immune suppression and tissue repair. They alleviate inflammatory damage to joints and cartilage tissue and slow the progression of OA by secreting anti-inflammatory cytokines such as IL-10 and TGF-β (Reference Lu, Zhang and Pan88–Reference Zhou, Yang and Shi90).
The regulatory role of n-3 PUFA in immune responses.
Research progress of n-3 PUFA in rheumatoid arthritis
Rheumatoid arthritis (RA)(Reference Gravallese and Firestein91) is a chronic, systemic autoimmune disease characterised by chronic inflammation, swelling, pain and dysfunction of the joints. RA typically affects symmetrical joints, particularly those in the hands, feet, knees, wrists and elbows. As the disease progresses, RA can lead to joint structural damage, resulting in joint deformities and loss of function(Reference Brown, Pratt and Hyrich92). The current treatment strategies for RA mainly rely on immunosuppressants, anti-inflammatory drugs and biologics(Reference Hwang, Rim and Nam93,Reference Smolen94) . However, in recent years, an increasing number of studies have found that n-3 PUFA, as natural anti-inflammatory agents, demonstrate potential therapeutic value in the treatment of RA(Reference Nikiphorou and Philippou95,Reference Vadell, Bärebring and Hulander96) .
Analogous to its effects in OA, n-3 PUFA can also exert anti-inflammatory effects through multiple mechanisms, alleviating the inflammatory response in RA(Reference Poggioli, Hirani and Jogani81,Reference Coniglio, Shumskaya and Vassiliou82) (Table 3). Raad et al. (Reference Raad, Griffin and George97) demonstrated through clinical samples that n-3 PUFA can significantly reduce the production of pro-inflammatory cytokines, such as TNF-α and IL-6, thereby helping to alleviate symptoms such as joint swelling, pain and morning stiffness caused by inflammation. Similarly, Navarini et al. (Reference Navarini, Afeltra and Gallo Afflitto98) also demonstrated through animal experiments and clinical trials that n-3 PUFA reduce the levels of arachidonic acid (AA) in immune cells, inhibiting the production of pro-inflammatory mediators like PGE2. Moreover, they promote the production of anti-inflammatory cannabinoids and cytokines, such as IL-10, while suppressing the levels of pro-inflammatory cytokines (e.g. TNF-α, IL-1β and IL-6), thus overall modulating immune responses and reducing the inflammatory symptoms of RA. Additionally, Jin et al. (Reference Jin, Sun and Ling99) first demonstrated in a collagen antibody-induced arthritis model that protectin DX (PDX), produced from n-3 PUFA metabolism, significantly inhibits the production of Th17 cells and pro-inflammatory mediators through the miR-20a-NLRP3 inflammasome pathway. This promotes Treg cells and anti-inflammatory cytokines, slows joint damage and improves the progression of RA. Furthermore, as persistent inflammation and joint destruction are major features of RA(Reference Deng, Zhang and He100), Su et al. (Reference Su, Han and Choi101) found that lipid mediators produced from DHA exhibit significant anti-inflammatory effects. In the collagen antibody-induced arthritis mouse model, lipid mediators significantly alleviated arthritis, cartilage erosion and bone destruction by downregulating osteoclast-related gene expression, inhibiting the NF-κB pathway, reducing the production of pro-inflammatory cytokines and increasing IL-10 levels, thereby demonstrating potential for mitigating RA symptoms.
Mechanisms of anti-inflammatory effects of n-3 PUFA in rheumatoid arthritis
AA, arachidonic acid.
Research progress of n-3 PUFA in gout
Gout(Reference Dalbeth, Gosling and Gaffo102) is a condition caused by abnormal purine metabolism, typically characterised by joint inflammation, pain and swelling. It is triggered by hyperuricaemia, which results in high levels of uric acid in the blood. Uric acid crystals accumulate in the joints and surrounding tissues, particularly in areas such as the toes, ankles and knees, leading to acute inflammatory responses(Reference Danve, Sehra and Neogi103,Reference Zhang, Chen and Yuan104) (Fig. 6).
Pathogenesis and progression of gout. The pathogenesis and progression of gout begin with increased uric acid intake and decreased renal excretion capacity, leading to elevated blood uric acid levels and resulting in hyperuricaemia. As the uric acid concentration continues to rise, urate crystals accumulate in the joints and surrounding tissues, triggering an immune response and causing acute arthritis. Without treatment, this process can persist, ultimately leading to chronic gout, causing joint damage and long-term inflammation.
It is generally believed that seafood may have an adverse impact on individuals with hyperuricaemia(Reference Clebak, Morrison and Croad105,Reference Li, Yu and Li106) . Because seafood is rich in purine compounds, which are metabolised into uric acid in the body. Excessive uric acid accumulation is a major trigger of gout. However, Zeng et al. (Reference Zeng, You and Ye107), analysing data from 12 505 participants in the 2007–2016 NHANES database, found that seafood with low n-3 PUFA content is associated with a higher risk of gout, while seafood rich in n-3 PUFA does not carry this risk and may even counteract the negative effects of purines on gout through its anti-inflammatory properties. Similarly, Zhang et al. (Reference Zhang, Zhang and Terkeltaub108) confirmed this finding in a clinical study of 724 gout patients. Those who consumed at least two servings of n-3 PUFA-rich fish had a 26 % reduced risk of gout flare-ups compared with those who had not consumed such fish in the past 48 h.
Besides their excellent anti-inflammatory effects, n-3 PUFA have also been shown by Saito et al. (Reference Saito, Toyoda and Takada109) through cell models to inhibit renal urate transporter 1, which is responsible for the reabsorption of urate into the bloodstream. By inhibiting urate transporter 1, n-3 PUFA increase uric acid excretion and thereby reduce serum urate levels, alleviating gout symptoms.
In conclusion, moderate consumption of seafood rich in n-3 PUFA or supplementation with n-3 PUFA through dietary supplements may provide an effective dietary intervention strategy for gout patients, helping to alleviate symptoms and significantly reduce the frequency of gouty arthritis flare-ups.
Research progress of n-3 PUFA in osteoporosis
OP(Reference Wen, Xu and Zhang110) is a chronic metabolic bone disease characterised by a significant decrease in bone density and destruction of bone microstructure, resulting in fragile bones and an increased risk of fractures. It is often referred to as a ‘silent disease’ because patients typically do not exhibit noticeable clinical symptoms until a fracture occurs(Reference Zhou, Huang and Chen111). The pathogenesis of OP is closely related to multiple factors, including an imbalance in bone remodelling, loss of minerals (particularly calcium) and abnormal regulation of the processes of bone formation and bone resorption(Reference Ayers, Kansagara and Lazur55,Reference Wen, Xu and Zhang110) . n-3 PUFA, in addition to their anti-inflammatory effects, regulate bone metabolism through various mechanisms, promoting bone health(Reference Sharma and Mandal112,Reference Abshirini, Ilesanmi-Oyelere and Kruger113) (Table 4).
Application of n-3 PUFA in osteoporosis
BMSC, bone marrow stromal cells; OPG:RANKL, osteoprotegerin: receptor activator of NF-κB ligand.
Promotion of bone formation
With the deepening of bone metabolism research, scholars have gradually revealed the regulatory mechanisms of n-3 PUFA on the osteogenic process. Chen et al. (Reference Chen, Wang and Wang114) found that in growing animals with rapid bone modelling, n-3 PUFA promote the differentiation of osteoblasts by enhancing the expression of osteoblast-related genes and proteins such as β-catenin, RUNX2 and osterix, thereby accelerating the process of bone formation. Hao et al. (Reference Yue, Bo and Tian115) also found through cell experiments that n-3 PUFA promote osteogenic differentiation and inhibit adipogenic differentiation of bone marrow stromal cells by upregulating the Wnt/β-catenin signalling pathway and promoting the expression of osteogenic transcription factors. Gao et al. (Reference Gao, Hong and Zhan50) demonstrated through cell experiments that n-3 PUFA might promote osteogenic differentiation of bone marrow stromal cells through the miR-9–5p/extracellular signal-regulated kinase (ERK)/alkaline phosphatase (ALP) signalling pathway, providing support for bone repair and regeneration. Zhang et al. (Reference Zhang, Tian and Wang116) demonstrated through cell experiments that n-3 PUFA enhance bone formation by promoting the transdifferentiation of chondrocytes into osteoblasts in the growth plate, which contributes to the improvement of OP.
In summary, numerous studies indicate that n-3 PUFA regulate osteoblast differentiation and function through various signalling pathways, promote osteogenic differentiation of bone marrow stromal cells, inhibit adipogenic differentiation and enhance bone formation by promoting the transdifferentiation of chondrocytes into osteoblasts, suggesting their potential therapeutic role in bone repair, regeneration and the improvement of OP.
Inhibition of bone resorption
The abnormal activity of osteoclasts is the main cause of excessive bone resorption(Reference Xue, Luo and Wang117,Reference McDonald, Khoo and Ng118) . Several studies have shown that the role of n-3 PUFA in bone metabolism is not only reflected in promoting bone formation but also in inhibiting bone resorption. It achieves this by inhibiting the key factor for osteoclastogenesis, receptor activator of NF-κB ligand, thereby reducing osteoclast activity and decreasing bone resorption, which helps maintain bone health(Reference Abshirini, Ilesanmi-Oyelere and Kruger113,Reference Song, Jing and Hu119) . Zhan et al. (Reference Zhan, Tian and Han120) discovered that n-3 PUFA can lower PGE2 levels and the expression of EP4, thereby increasing the ratio of osteoprotegerin to receptor activator of NF-κB ligand, inhibiting the NF-κB signalling pathway and ultimately suppressing osteoclastogenesis. Similarly, Wang et al. (Reference Wang, Wu and Li121) also reached a similar conclusion, further confirming the role of n-3 PUFA in inhibiting bone resorption in OP.
Research progress of n-3 PUFA in fracture
Fracture(Reference Tai and Chen122) refers to the rupture or breakage of bone continuity, typically caused by direct or indirect external forces. It not only affects the structural integrity of the bone but may also be accompanied by soft tissue damage, haematoma and nerve injuries(Reference Tai and Chen122–Reference Casas and Gonzalez124). The treatment for fractures varies depending on the nature, location and severity of the fracture(Reference Steinmetz, Brgger and Chauveau125–Reference Herterich, Baumbach and Kaiser131). The preventive effect of n-3 PUFA on fractures primarily involves promoting bone formation and inhibiting bone resorption as part of anti-OP therapy. Several studies have verified this preventive effect(Reference Sadeghi, Djafarian and Ghorabi132,Reference Martyniak, Wei and Ballesteros133) . In fracture treatment, the application of n-3 PUFA is generally used as an adjunctive therapy. Research by Kafadar et al. (Reference Halil Kafadar, Yalçın and Çakar134) indicated that the combined use of n-3 PUFA and vitamin D3 had a positive effect on fracture healing.
In mechanistic studies, Chen et al. (Reference Chen, Cao and Sun135) found that in a mouse femur fracture repair model, n-3 PUFA promote fracture healing by enhancing the expression of insulin-like growth factors, reducing the formation of PGE2, increasing calcium absorption and decreasing the release of inflammatory factors from osteoclasts. Huang et al. (Reference Huang, Vi and Zong52) demonstrated through animal experiments that treatment with n-3 PUFA-derived MaR1 significantly improved tibial fracture healing, which was manifested by reduced cartilage formation and increased bone deposition, thereby enhancing the stiffness of the bone structure. In the early stages of treatment, MaR1 reduced the number of pro-inflammatory macrophages in the callus and lowered the levels of inflammatory biomarkers. Subsequently, it promoted osteoblast differentiation and enhanced the osteogenic activity of bone marrow stromal cells.
In addition, n-3 PUFA also have certain effects on fracture-related complications. Zhang et al. (Reference Zhang, Terrando and Xu136) found through clinical studies that specialised pro-resolving mediators derived from DHA can promote the resolution of acute inflammation and effectively inhibit neuropathic pain caused by tibial fractures. Zheng et al. (Reference Zheng, Jia and Li137) also demonstrated through clinical research that daily supplementation of n-3 PUFA can reduce the risk of pulmonary embolism and symptomatic deep vein thrombosis after fracture surgery, without increasing the risk of bleeding.
Research progress of n-3 PUFA in sarcopenia
Sarcopenia(Reference Cruz-Jentoft and Sayer138) is a chronic condition characterised by the loss of muscle mass and strength, which is age-related and commonly observed in elderly populations. Patients with sarcopenia often experience mobility difficulties, which can significantly affect their daily lives. Early identification and active interventions, such as exercise, nutritional support and pharmacological treatments, can significantly improve the progression of sarcopenia and enhance the quality of life for the elderly(Reference Sayer and Cruz-Jentoft139–Reference Sayer, Cooper and Arai141). In recent years, n-3 PUFA have garnered increasing attention in the treatment and prevention of sarcopenia. Their anti-inflammatory properties, ability to promote muscle synthesis and enhance muscle strength make them an effective adjunctive therapy(Reference Jimenez-Gutierrez, Martínez-Gómez and Martínez-Armenta142–Reference Bird, Troesch and Warnke144). The specific mechanisms of action are summarised in Table 5.
Mechanisms of n-3 PUFA in sarcopenia
mTOR, mammalian target of rapamycin.
Anti-inflammatory effect
The anti-inflammatory effects of n-3 PUFA have been widely recognised(Reference Calder58,Reference Jin, Dong and Sun71,Reference Zhang, Dai and Zhang72,Reference Lu, Zhang and Pan88,Reference Zhou, Yang and Shi90) . As individuals age, their immune system declines, leading to a phenomenon known as immunosenescence, which results in an increase in pro-inflammatory cytokines. Pro-inflammatory factors such as TNF-α and IL-6 can directly or indirectly affect muscle metabolism through multiple mechanisms, both promoting muscle breakdown and potentially inhibiting muscle synthesis(Reference Suzuki145–Reference Vinel, Lukjanenko and Batut147). However, it is noteworthy that some studies suggest IL-6 may play a regulatory role in skeletal muscle hypertrophy and satellite cell activity, indicating a bidirectional function(Reference Serrano, Baeza-Raja and Perdiguero148,Reference McKay, De Lisio and Johnston149) .
The expert consensus report by Serhan et al. (Reference Serhan, Bäck and Chiurchiù150) highlights that n-3 PUFA have anti-inflammatory properties, which help maintain muscle mass and serve as precursors to specialised pro-resolving mediators. These mediators play a crucial role in immune modulation, tissue repair and the effective resolution of inflammation, while also preventing damage to host defense mechanisms. Additionally, Matthew et al. (Reference Johnson, Lalia and Dasari151) demonstrated through an aged mouse model that EPA can improve muscle protein quality, particularly by reducing mitochondrial protein carbamylation induced by inflammation, which alleviates age-related mitochondrial dysfunction and improves mitochondrial protein quality. Lalia et al. (Reference Lalia, Dasari and Robinson152) similarly demonstrated through samples from elderly individuals that n-3 PUFA reduce the production of mitochondrial oxidants, attenuate the inflammatory response, promote muscle protein synthesis and enhance the anabolic response to exercise in older adults, thereby improving overall muscle function.
Mammalian target of rapamycin activation
The mammalian target of rapamycin (mTOR) signalling pathway is a key regulator of cell growth, proliferation and metabolism, playing an important role in the inhibition of skeletal muscle autophagy(Reference Zeng, Liang and Wu153,Reference Hsu, Wang and Chao154) . Research by Azzolino et al. (Reference Azzolino, Bertoni and De Cosmi155) showed that n-3 PUFA not only exert independent effects but also synergize with amino acid intake. By activating the mTOR signalling pathway, n-3 PUFA increase the efficiency of protein synthesis, promoting muscle protein synthesis and enhancing muscle mass.
However, it is worth noting that in a randomised controlled trial, López et al. (Reference López-Seoane, Jiménez and Del Coso156) found that while n-3 PUFA supplementation was beneficial for muscle protein synthesis rates, no effects on mTOR, protein kinase B or skeletal muscle gene expression were observed when measuring changes in skeletal muscle volume and mass. This suggests that the specific molecular mechanisms by which n-3 PUFA may enhance muscle mass and protein synthesis through the mTOR pathway still require further investigation and validation.
Improvement of insulin sensitivity
Insulin is a key hormone that promotes muscle protein synthesis. Insulin resistance reduces insulin signalling, leading to an increase in protein catabolism and a decrease in protein synthesis in skeletal muscle. Chronic insulin resistance exacerbates this imbalance, resulting in muscle mass loss, which can eventually develop into sarcopenia(Reference Liu and Zhu157–Reference Marcotte-Chénard, Oliveira and Little160). PPAR-γ is a key molecule involved in regulating skeletal muscle glucose homeostasis and insulin sensitivity. Its abnormal expression is closely related to skeletal muscle IR, particularly in obese or type 2 diabetic patients(Reference Azzolino, Bertoni and De Cosmi155).
A 3-week double-blind randomised controlled trial by Moradi et al. (Reference Moradi, Alivand and KhajeBishak161) showed that supplementation with n-3 PUFA significantly enhanced PPAR-γ activity, improving insulin sensitivity, thereby inhibiting muscle protein catabolism and promoting muscle health. Liu et al. (Reference Liu, Lin and Tzeng162), using a diet-induced diabetic rat model, demonstrated that supplementation with n-3 PUFA-rich fish oil controlled weight loss in diabetic rats and repaired impaired glucose tolerance. At the same time, n-3 PUFA improved insulin sensitivity, enhanced glucose metabolism and protected against muscle atrophy induced by diabetes. Therefore, n-3 PUFA have the potential to improve muscle mass and quality of life in diabetic patients.
Research progress of n-3 PUFA in spinal degenerative diseases
Spinal degenerative diseases (SDD)(Reference Zhao, Ma and Han163)refer to a group of conditions involving the gradual degeneration of the spine and its components, including intervertebral discs, vertebrae and spinal joints. These diseases typically develop with age and may be influenced by genetic, environmental and lifestyle factors. The symptoms of SDD vary depending on the specific location and severity of degeneration but usually include pain, stiffness and limited range of motion. As the disease progresses, nerves may be compressed, leading to radicular symptoms. In severe cases, compression of the spinal cord may occur, causing symptoms such as sensory loss, limb weakness, incontinence and even paralysis(Reference Bumann, Nüesch and Loske164–Reference Witiw and Fehlings167).
Currently, research on the application of n-3 PUFA in SDD is relatively limited. However, some preliminary studies suggest that n-3 PUFA may have a positive effect on the treatment of intervertebral disc degeneration. A study by NaPier et al. (Reference NaPier, Kanim and Arabi168) found through a rat model that n-3 PUFA dietary supplements can reduce systemic inflammation by lowering the AA:EPA ratio in serum, potentially offering protective effects against the progression of intervertebral disc degeneration. Similarly, Shang et al. (Reference Shang, Ma and Zhang169) verified this effect through cell experiments and found that the protective role of DHA on the intervertebral disc may be exerted by regulating the expression of long non-coding RNA nuclear-enriched abundant transcript 1, thereby exerting its anti-inflammatory effects and reducing extracellular matrix degradation in nucleus pulposus cells caused by oxidative stress, ultimately slowing the pathological progression of intervertebral disc degeneration.
The dorsal root ganglion is a key pathway for sensing external stimuli. In the case of lumbar disc prolapse, degenerated nucleus pulposus tissue compresses the nerve root, leading to increased and prolonged infiltration of macrophages in the dorsal root ganglion. This can alter the function of neurons, causing excessive neural activation, which ultimately results in persistent pain or sensory abnormalities(Reference Naratadam, Mecklenburg and Shein170,Reference Lu, Chen and Jiang171) . A study by Manzhulo et al. (Reference Manzhulo, Ogurtsova and Lamash172) using an animal model concluded that DHA can effectively alleviate neurogenic pain in SDD by inhibiting the response of satellite glial cells in the dorsal root ganglion, reducing the expression of the pro-apoptotic protein p53 and promoting neuroprotective effects in the dorsal root ganglion. Additionally, a study by Wang et al. (Reference Wang, Li and Wang173) using an lumbar disc prolapse rat model showed that mechanical and thermal hypersensitivity, increased inflammatory cytokines IL-1β and IL-18 levels and nerve root pain induced by activation of the NLRP3 inflammasome caused by nucleus pulposus exposure could all be significantly reversed by MaR1 produced from n-3 PUFA metabolism. This indicates that n-3 PUFA not only alleviate the early damage caused by lumbar disc prolapse-induced changes but also promote long-term nerve repair and functional recovery.
In summary, the application of n-3 PUFA in SDD holds great promise, particularly in alleviating inflammation, oxidative stress and promoting nerve repair. These potential benefits highlight the therapeutic value of n-3 PUFA in the management of SDD and related conditions.
Discussion
This article introduces the main types of n-3 PUFA (ALA, EPA, DHA) and their important roles in human health, with particular emphasis on their application in orthopaedic diseases such as OA, RA, gout, OP, fractures, sarcopenia and SDD. It also describes in detail the effects of n-3 PUFA in anti-inflammatory responses, improving bone metabolism, enhancing muscle strength and protecting nerves. Particularly in terms of anti-inflammation, n-3 PUFA exert their effects by altering the lipid composition of cell membranes, inhibiting pro-inflammatory signalling pathways, promoting the production of anti-inflammatory lipid mediators and regulating immune responses. These mechanisms effectively alleviate inflammation related to orthopaedic diseases and promote the repair of bones and joints.
However, this review also has several limitations. First, as a narrative review, the literature selection did not follow the strict criteria of a systematic review, which may have resulted in the omission of relevant studies or selection bias. Second, the included studies exhibit considerable heterogeneity, covering different experimental models, dosage regimens and study populations, which complicates the comparison and synthesis of results.
Nonsteroidal anti-inflammatory drugs are currently the most widely used anti-inflammatory drugs in clinical practice, which primarily exert their anti-inflammatory, analgesic and antipyretic effects by inhibiting the activity of two cyclooxygenases, cyclooxygenase-1 and cyclooxygenase-2. Their effects are rapid, typically providing pain relief within a few hours after oral administration(Reference Leow, Zheng and Shi174–Reference Eisenstein, Hilliard and Pope176). However, chronic or excessive use may lead to side effects such as gastrointestinal discomfort, renal damage, liver damage, elevated cardiovascular risks and allergic reactions(Reference Bindu, Mazumder and Bandyopadhyay177–Reference Minaldi and Cahill181). In contrast, n-3 PUFA, as a therapeutic approach with both nutritional and medicinal properties, have fewer side effects and a higher clinical safety profile(Reference Stonehouse, Benassi-Evans and Bednarz182–Reference Mori, Murasaki and Yokoyama186). Therefore, its anti-inflammatory potential warrants further investigation and holds promise for widespread clinical application.
Despite extensive research demonstrating the anti-inflammatory effects and potential therapeutic value of n-3 fatty acids in orthopaedic diseases, several limitations remain in the current body of research. First, most studies are based on animal models or in vitro cell experiments, and their translation to clinical applications requires further validation through large-scale, high-quality human clinical trials. Second, there is considerable variability in the dosage, administration methods and intervention durations of n-3 fatty acids across studies, with a lack of standardised protocols, which limits the comparability and generalisability of results; the optimal dosage and administration regimen have yet to be established.
Moreover, the complexity and multifactorial nature of orthopaedic diseases make it difficult to fully assess the effects of a single nutritional intervention, necessitating comprehensive multidisciplinary and multifactorial studies to explore the underlying mechanisms and efficacy in depth. Regarding long-term clinical use, evidence on the efficacy and safety of n-3 PUFA remains insufficient, especially concerning efficacy differences among various populations and individualised treatment strategies.
Lastly, as a dietary supplement, research on the interactions between n-3 PUFA and conventional drugs (such as nonsteroidal anti-inflammatory drugs and immunomodulators) is limited. Long-term combined use may introduce uncertainties in drug responses, affecting safety and therapeutic outcomes. The precise mechanisms of action and clinical feasibility of n-3 PUFA in certain orthopaedic-related diseases remain unclear, particularly in terms of signalling pathways and molecular targets, which require further investigation.
To address these issues, future research should prioritise large-scale, high-quality randomised controlled trials to validate the efficacy and safety of n-3 PUFA, especially under conditions of long-term use and combination therapy. Additionally, integrating molecular biology and systems biology approaches to deeply explore molecular mechanisms and targets, supplemented by pharmacokinetic and pharmacodynamic studies, will clarify the specific impacts of various dosages and administration routes on humans. Such research will provide a more robust theoretical foundation and scientific guidance for the clinical application of n-3 PUFA.
In conclusion, the above studies not only deepen our understanding of the mechanisms of action of n-3 PUFA in orthopaedic diseases but also provide a solid scientific foundation for their clinical application. Looking ahead, n-3 PUFA is expected to become an important complement or alternative to traditional drugs and holds promise for synergistic use with other modern therapeutic approaches, such as gene therapy and stem cell therapy. Particularly in the complex treatment of orthopaedic diseases, n-3 PUFA may serve as a key adjunctive therapy, offering more scientific, precise and personalised therapeutic options, ultimately improving the clinical outcomes for patients.
Acknowledgements
Not applicable.
The authors’ responsibilities were as follows: H. M. conceptualised the manuscript; H. M. and G. L. drafted the initial subsections of the manuscript, with further input from L. Z., M. Z., L. L., S. Z., J. L., X. S., Y. L., M. M., J. H., J. Y. and L. Z.; H. M. and G. L. edited the manuscript; and all authors read and approved the final manuscript.
Scientific and Technological Innovation Project of China Academy of Chinese Medical Sciences (CI2021B010).
The authors declare no conflicts of interest.
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