European Journal of Clinical (2013) 67, 1234–1242 & 2013 Macmillan Publishers Limited All rights reserved 0954-3007/13 www.nature.com/ejcn

MINI REVIEW Omega-3 fatty acids: a review of the effects on adiponectin and leptin and potential implications for obesity management

B Gray1,2,3, F Steyn4, PSW Davies1,3 and L Vitetta2,3

An increase in adiposity is associated with altered levels of biologically active proteins. These include the hormones adiponectin and leptin. The marked change in circulating concentrations of these hormones in obesity has been associated with the development of insulin resistance and metabolic syndrome. Variations in dietary consumption have also been shown to impact obesity. Specifically, omega-3 fatty acids have been correlated with the prevention of obesity and subsequent development of chronic disease sequalae. This review explores animal and human data relating to the effects of omega-3 fatty acids (marine ) on adiponectin and leptin, considering plausible mechanisms and potential implications for obesity management. Current evidence suggests a positive, dose-dependent relationship between omega-3 fatty acid intake and circulating levels of adiponectin. In obese subjects, this may translate into a reduced risk of developing , metabolic syndrome and diabetes. In non-obese subjects, omega-3 is observed to decrease circulating levels of leptin; however, omega-3-associated increases in leptin levels have been observed in obese subjects. This may pose benefits in the prevention of weight regain in these subjects following calorie restriction.

European Journal of Clinical Nutrition (2013) 67, 1234–1242; doi:10.1038/ejcn.2013.197; published online 16 October 2013 Keywords: adiponectin; leptin; adipokines

INTRODUCTION Human leptin consists of a 146 amino-acid protein with a 2 Adipocytes produce a number of different proteins, known as molecular mass of 16.0 kDa. Expression and secretion of this 15,16 adipokines, that have specific biological activities.1,2 Adipose hormone is positively correlated with mass and adipocyte 17 18 tissue can regulate energy homeostasis through the action of the size. Cortisol and insulin both stimulate leptin expression. secreted adipokines. These include the hormones adiponectin and Leptin was originally considered an anti-obesity hormone due to leptin.3 Altered levels of adiponectin and leptin are correlated with its experimental effects on metabolism and food intake, in that it obesity and cardiovascular disease (CVD).4,5 Adiponectin is was observed to decrease food intake and increase energy 19 negatively correlated with obesity, with lower levels associated expenditure in rodents. These findings were expanded with with increased risk of death or myocardial infarction (MI).6 reports that plasma leptin concentrations are directly proportional 20,21 Conversely, leptin levels are positively correlated with obesity,7 to mean food intake over a period of days or weeks. Thus, a with high levels identified as an independent risk factor for CVD.8 decrease in plasma leptin concentration serves as a short-term 20 Adiponectin is a 244 amino-acid protein that has many adaptation to starvation or fasting. Correspondingly, leptin levels regulatory actions on energy homeostasis, modulating glucose decrease significantly following dietary or lifestyle-induced weight 22,23 and lipid metabolism, promoting fatty acid oxidation and loss. enhancing insulin sensitivity in the liver and within skeletal In non-obese individuals, leptin acts via hypothalamic receptors 24 muscle.1,9–11 Increased adiponectin levels were initially associated to inhibit feeding and increase thermogenesis. Reduced levels with increased mortality in CVD; however, more recent findings result in decreased central sympathetic nervous outflow and suggest that this may be as a result of a protective effect in mobilization of energy stores via stimulation of glucocorticoid 25 cardiovascular conditions, leading to compensatory upregulation secretion. Obese individuals have higher plasma leptin in this hormone.12 The marked difference in plasma adiponectin concentrations, though in this case, elevated leptin does not concentrations between non-obese and obese individuals induce the expected reduction in food intake and increase in stimulated additional interest in the molecule and the formation energy expenditure. This suggests that obese individuals become of numerous hypotheses regarding its role in the hastened resistant to the effects of endogenous leptin.25,26 Hyperleptinemia development of vascular disease associated with obesity, has been associated with alterations in cytokine signalling,8 metabolic syndrome and type 2 diabetes mellitus (T2DM).5 including increased production of pro-inflammatory cytokines Decreased circulating levels of adiponectin in obesity have such as tumour necrosis factor-alpha (TNFa)27 and C-reactive been associated with an increased risk of coronary artery protein (CRP),28 and it has also been shown to promote platelet disease9,13 and MI.14 aggregation and thrombosis.29,30

1Children’s Nutrition Research Centre, Queensland Children’s Medical Research Institute, The University of Queensland, Royal Children’s Hospital, Herston, QLD, Australia; 2Centre for Integrative Clinical and Molecular Medicine, School of Medicine, The University of Queensland, Princess Alexandra Hospital, Woolloongabba, QLD, Australia; 3School of Medicine, The University of Queensland, St Lucia, QLD, Australia and 4The School of Biomedical Sciences, The University of Queensland, St Lucia, QLD, Australia. Correspondence: B Gray, Children’s Nutrition Research Centre, The University of Queensland, Old Kitchen, Royal Children’s Hospital, Crn Fourth Avenue and Back Road, Herston, QLD 4029, Australia. E-mail: [email protected] Received 21 April 2013; revised 13 August 2013; accepted 8 September 2013; published online 16 October 2013 Effects of omega-3 fatty acids B Gray et al 1235 The quality and quantity of dietary lipid consumption has also Omega-3 and omega-6 fatty acids are fundamental been shown to contribute to the current obesity epidemic.31–34 By components of the phospholipids in cell membranes. They are influencing the production of adipokines, dietary lipids released mainly through the action of phospholipase A2 and then subsequently impact upon satiety and adiposity.31,33 In metabolised to signalling molecules known as .46 The particular, higher plasma levels of omega-3 fatty acids are partial substitution of omega-3 in place of omega-6 fatty acids associated with a lower body mass index (BMI), waist favours the synthesis of generally weaker prostanoids (for circumference and hip circumference, suggesting that these example, thromboxane A3, which, contrary to omega-6-derived fatty acids may contribute to a healthy weight status and the thromboxane A2, has minimal platelet-aggregating and vaso- prevention of abdominal adiposity.35 Both animal and human constrictive potency).47 The results of these changes in studies have reported lower insulin levels in the presence of production are vasodilatation as well as inhibition of platelet marine lipid-derived omega-3 fatty acids,36,37 as well as decreased aggregation and inflammation. mortality and cardiovascular incidents.38–41 The beneficial effects of omega-3 PUFA are brought about not An estimated $33.9 billion is spent annually on alternative just by the modulation of the amount and types of eicosanoids medicine in United States of America alone.42 Of this, $14.8 billion produced47 but also the regulation of intracellular signalling was accounted for by nutritional supplements.42 Omega-3 pathways, transcription factor activity48 and gene expression.49 supplements (including fish ) were reported as the most The combination of these mechanisms result in the regulation of common nonvitamin/nonmineral product taken by adults,43 inflammation, platelet adhesion, blood pressure regulation, heart accounting for 37% of respondents who had utilised natural rhythm and .50 products in the last 30 days according to the National Health Interview Survey (2007).43 Research supports the benefits of these fatty acids in a number of the co-morbidities of obesity, including Omega-3 fatty acids in health. Clinical evidence supports the use CVD39 and T2DM.44 It may therefore be asked whether intake of of omega-3 fatty acids in a number of inflammatory conditions, including many of the co-morbidities of obesity, such as CVD,39 omega-3 fatty acids will also have a role in the prevention of the 51,52 44 53 development and progression of obesity itself. , T2DM and dyslipidemia. In 1999, the GISSI-Prevenzione trial recruited 11 324 people with a recent history of MI for treatment with 1 g per day of marine Essential fatty acids in health omega-3 fatty acids. During the 3.5 years of supplementation, Essential fatty acids (for example, omega-3 and omega-6) are results showed reduced occurrences of all causes of mortality necessary for the general maintenance of optimal health.45 Marine (20%), cardiovascular death (30%) and sudden cardiac death lipids (for example, , ) are the richest dietary (45%).38 This is in agreement with other reviews indicating source of omega-3 fatty acids, including the polyunsaturated fatty decreased mortality and cardiovascular incidents associated with acids (PUFA), docosaexaenoic acid (DHA) and eicosapentaenoic regular consumption of fish and marine lipid supplements.40,41 acid (EPA). Humans are unable to synthesise sufficient quantities Systemic inflammation, characterised by increased levels of of these fatty acids.39 Acquisition is primarily from dietary or inflammatory markers such as CRP, interleukin (IL)-6 and TNFa,is supplementary sources. an indicator for increased risk of CVD.46,54 Increased circulating levels of these inflammatory markers have been correlated Omega-3 vs omega-6 fatty acids. In Western diets, omega-6 fatty with cardiovascular risk factors, including hypertension and acids are the predominant form of polyunsaturated . Dietary metabolic syndrome.55,56 Australia’s National Heart Foundation sources of omega-6 (linoleic acid and arachadonic acid) include recommendations currently include supplementation with , eggs, nuts, , whole grains and most oils. omega-3 fatty acids for decreasing the risk of heart disease, as Figure 1 demonstrates the metabolically distinct and opposing well as for individuals with documented CVD and elevated physiological function of omega-3 and omega-6 fatty acids.46 triglycerides (up to 4000 mg/day).57 Research continues to identify

Figure 1. Metabolism of omega-6 () and omega-3 () by and lipoxygenase pathways. PGE, prostaglandin; PGI, prostacylin; TX, thromboxane. Adapted from Simopoulos, 2002.46,47,50,117

& 2013 Macmillan Publishers Limited European Journal of Clinical Nutrition (2013) 1234 – 1242 Effects of omega-3 fatty acids B Gray et al 1236 and explore the novel mechanisms by which omega-3 exerts its non-significant increase and two studies reported minimal benefit in CVD. changes. Study cohorts varied, including studies of subjects Although a review published by Rizos et al.58 found no within healthy weight range, as well as those with genetic or significant reduction in all-cause mortality or cardiac death dietary-induced obesity. A number of these studies also noted associated with omega-3 intake, these results should be additional benefits following omega-3 fatty acids interpreted with caution given that the mean does of omega-3 supplementation. These included reversed diet-induced insulin from the 20 studies included was 1.51 g/day and only 10 these resistance and dyslipidemia, improved adiposity70 and changes in studies utilised a dose X1 g/day of omega-3. TNFa.37 Increased adiponectin levels associated with omega-3 Many properties ascribed to omega-3 fatty acids are applicable fatty acids was demonstrated independently of significant not just to patients with CVD but also to diabetic patients. changes in BMI, waist circumference, systolic blood pressure, These include improved triglycerides,44 anti-inflammatory, fasting plasma glucose, total , high-density lipoprotein- antithrombotic and antiatherogenic effects of fish .59 Of cholesterol or low-density lipoprotein-cholesterol between the further interest, a lower prevalence of diabetes60–64 and glucose supplemented and control groups.37 intolerance65 have been reported in populations known to have a In summary, the majority of studies reported omega-3 intake very high omega-3 fatty acid intake (for example, Greenland, induced statistically significant increases in adiponectin Alaska, Faroe Islands). An inverse correlation between omega-3 levels in both animal68–71 and human models.37,73–75 These intake and conditions such as diabetes and glucose intolerance included studies with subjects in normal weight range has been observed in an elderly Dutch population.66 However, a (18pBMIp25 kg/m2),73 overweight (25pBMIp30 kg/m2)74 and Cochrane review concluded that there is insufficient evidence to obese (BMIX30 kg/m2).37,75 Further, results were consistent demonstrate impaired glycaemic control associated with use of between healthy individuals37,73 and those investigating omega-3 in patients with T2DM, showing no statistically hyperlipidaemic patients with 2TDM74 or recent history of MI significant changes in fasting glucose, fasting insulin or body (insignificant increase in adiponectin levels).76 weight.67 Thus, further clinical and mechanistic studies regarding the effects of omega-3 intake on insulin sensitivity are necessary. This review aims to explore animal and human data pertaining Leptin to the effects of omega-3 fatty acids (marine lipids) on plasma Limited studies were available reporting the effects of omega-3 adiponectin and leptin levels, considering plausible mechanisms fatty acids on circulating levels of leptin that included both human of action and potential implications for obesity management. (Table 2) and rodent models.36,68,69,77,78 Of these studies utilising participants with a stable weight (no concurrent weight loss or caloric restriction),36,68,69,76,79,80 the majority demonstrated either RESULTS minimal change or a reduction in leptin levels associated with Adiponectin omega-3. Available studies assessing the effect of omega-3 fatty acids Perez-Matute et al.69 demonstrated that when introducing a fat- on adiponectin include both human (Table 1) and rodent rich hyperenergetic diet with subsequent continuous weight gain models.36,37,68–72 Three studies reported significant increases in in rats, additional supplementation with omega-3 fatty acids adiponectin following omega-3 supplementation, one reported resulted in a further increase in leptin levels. In vitro studies

Table 1. Omega-3 fatty acids and adiponectin in humans

Study (year) Study design and subjects Sample size Study Study dose Change in duration adiponectin

Gammelmark et al.75 Randomised, double- 50 (25 control, 25 n-3) 6 weeks 2 g /day vs 2 g Increase in the n-3 blinded, placebo controlled fish oil/day group compared clinical trial. Overweight and (equivalent 1.1 g n-3) with control* obese subjects Itariu et al.86 Randomised, open label 55 (28 control, 27 n-3) 8 weeks 20 g Butterfat vs 3.36 g Control decreased clinical trial. Severely obese, n-3 (caloric (PX0.05), minimal non-diabetic subjects equivalent) change in the n-3 group. Itoh et al.37 Randomised, single-blinded 52 (26 control, 26 n-3) 3 months Control diet vs Control m Adiponectin** clinical trial. Obese Japanese diet with 1.8 g EPA/ subjects day marine lipid Koh et al.87 Randomised, single-blinded, 99 (49 control 50 n-3) 2 months Unspecified placebo Minimal change in placebo controlled, parallel vs 2 g/day n-3 the n-3 group. Kondo et al.73 Observational study. 17 n-3 8 weeks Minimum 3 g/day n-3 m Adiponectin* Healthy, non-obese, intervention, derived from fish (for Japanese subjects 4 week example, Pacific saury, follow-up ) Nomura et al.74 Observational study. 126 (76 diabetic, 6 months 1800 mg EPA/day Non-diabetic: m Hyperlipidaemic subjects 50 non-diabetic) Adiponectin (PX0.05). Diabetic: m Adiponectin** Patel et al.76 Randomised pilot study. 35 (19 control, 16 n-3) 3 months Usual care control vs m Adiponectin Males, post MI (BMI o20 1 g marine lipid/day (PX0.05) and 432 kg/m2) (EPA 465 mg and DHA 375 mg)

Abbreviations: BMI, body mass index; DHA, docosaexaenoic acid; EPA, eicosapentaenoic acid; MI, myocardial infarction; n-3, omega-3. *Po0.05; **Po0.01, minimal change in adiponectin is defined as the difference of p0.5 mg/ml before and after supplementation.

European Journal of Clinical Nutrition (2013) 1234 – 1242 & 2013 Macmillan Publishers Limited Effects of omega-3 fatty acids B Gray et al 1237 Table 2. Omega-3 fatty acids and leptin in humans

Study (year) Study design and subjects Sample size Study Study dose Effect on leptin duration

Itariu et al.86 Randomised, open label 55 (28 control, 27 n-3) 8 weeks 20 g Butterfat vs 3.36 g n-3 Control clinical trial. Severely (caloric equivalent) increased obese, non-diabetic (PX0.05); subjects Minimal change in the n-3 group Mori et al.79 Randomised clinical trial. 63 (16 control, 17 n-3, 4 months Control daily fish meal (F); weight- F: mLeptin Overweight, treated 16 weight- reduction, reduction regimen (W); two (PX0.05) W: k hypertensive men and 14 combined regimes) regimens combined (FW) (daily fish Leptin women meals contained an average of 3.65 g (PX0.05) FW: of n-3 fatty acids/day; unspecified k Leptin** EPA/DHA content) Patel et al.76 Randomised pilot study. 35 (19 control, 16 n-3) 3 months Usual care control vs 1 g marine k Leptin Males, post MI (BMI o20 lipid/day (EPA 465 mg and DHA (PX0.05). and 432 kg/m2) 375 mg) Winnicki et al.80 Comparative Population 608 (329 control, 279 NA Primarily vegetable/ k Leptin** Study. Two related dietary n-3) consumption vs 300–600 g fish/day homogeneous tribal populations of Tanzania (similar BMI values) Abbreviations: BMI, body mass index; DHA, docosaexaenoic acid; EPA, eicosapentaenoic acid; MI, myocardial infarction; NA, not applicable; n-3, omega-3. *Po0.05; **Po0.01; minimal change in leptin is defined as the difference of p1.0 pg/ml before and after supplementation. demonstrated an EPA-induced increase in leptin mRNA gene brings into question the clinical relevance of leptin resistance in expression,77 a result that was less pronounced in the presence of relation to weight loss outcomes. insulin.78 Perez-Matute et al.78 also demonstrated in vitro that EPA- induced increases in leptin secretion were highly correlated with Dosage of omega-3 fatty acids increased glucose utilisation. The study showed an inverse Of the human studies investigating the effects of omega-3 on relationship with anaerobic glucose metabolism in both the these adipokines, the amount of supplementation varied presence and absence of insulin. Lipogenesis was decreased by significantly.37,73,74,76,79,80,86,87 Not all studies specified the EPA in the absence of insulin, whereas lipolysis was unaffected. amount of marine lipid (mg/day) and/or the amount of omega-3 The authors therefore suggested that EPA, like insulin, stimulates (EPA/DHA) they contained. In studies where omega-3 fatty acids leptin production by increasing the oxidative metabolism of were derived from dietary sources, Mori et al.79 and Flachs et al.36 glucose. reported the addition of a daily fish meal clarifying the average There are many inconsistencies within and between the results amount of omega-3 but not EPA and DHA. Winnicki et al.80 from human and animal models pertaining to the effects of specified the amount of fish, though not omega-3, EPA or DHA, omega-3 intake on plasma leptin concentrations.68,69,76,79 Insulin which makes dose calculations difficult as EPA and DHA content resistance and obesity have been posited to lead to can vary from 177 mg/100 g in catfish through to 2648 mg/100 g inappropriately high plasma leptin levels for a given fat in salmon40 and will also vary according to the region from which mass.81,82 Further research indicates that this may occur as a it is sourced. Of further interest, an additional study in overweight consequence of desensitisation of the leptin signal, a and obese participants, greater decreases were observed in phenomenon referred to as leptin resistance.4 In situations adiponectin levels through supplemented intake of omega-3 by where weight and diet are stable, leptin is thought to signal a comparison to equivalent increase in dietary intake of omega-3,88 set point at which energy intake and energy expenditure are suggesting that differences in results between studies may be balanced.83 This set point reflects a level of leptin sensitivity, related not just to the dose but also the form of omega-3 (dietary which might vary with genetic, nutritional and/or environmental or supplemented). factors.81 Oral supplementation dosages (where specified) ranged from 1 A comparative study between two tribes from Tanzania, with to 3.6 g per day of Omega-3 PUFA.73,76,79,86,87 EPA ranged from similar BMIs but differing diets, showed that in the tribe who 465 to 1800 mg/day74,76 and DHA was only specified in one study regularly consumed fish (mean BMI 20.4), leptin levels were lower with a value of 375 mg/day.76 Study comparisons included either as compared with those in the tribe who did not consume fish baseline levels of subjects or a control group (no supplement, regularly (mean BMI 20.1 kg/m2).80 These results were, however, unspecified placebo or supplementation with alternative lipid not replicated in a subsequent clinical study.79 These studies source). The majority of studies utilising doses of omega-3 that differed in study design (comparative population study vs clinical exceeded 1 g/day were more likely to report significant changes in study), sample size (608 vs 35 participants respectively), mean BMI adipokine levels.37,73,74,79 of study population (non-obese vs overweight) and existence of previously established medical conditions (non-specified vs post MI). Implications of increasing adiposity Further, data demonstrate that following a weight reduction of Increasing adiposity (as approximated by BMI) correlates with 10% in a combination of obese and non-obese subjects, low-dose decreased plasma adiponectin and increased plasma leptin infusion of leptin (replicating levels before weight reduction) levels.4 Hence, it is interesting to further consider the effects of normalises appetite and energy expenditure84 that would omega-3 fatty acids on these adipokines across different BMI otherwise adjust to promote weight regain.85 This observation groups. Although the available data are limited, it appears that

& 2013 Macmillan Publishers Limited European Journal of Clinical Nutrition (2013) 1234 – 1242 Effects of omega-3 fatty acids B Gray et al 1238 omega-3 fatty acid supplementation further increases adiponectin obesity and CVD such as cholesterol. Mean BMI also differed levels in obese vs lean individuals (Figure 2a). between cohorts (20.1 vs 27.4 kg/m2). The effect of omega-3 fatty Data pertaining to omega-3 and leptin are less clear (Figure 2b). acid supplementation in overweight individuals may differ in an This figure illustrates changes in the magnitude of the omega-3 overweight but otherwise healthy cohort. fatty acid-induced changes in plasma leptin levels across different Given the limitations in the available data, further research in studies. Observations are presented according to BMI. In non- this area is warranted, specifically larger-scale trials investigating obese subjects, leptin levels decreased following supplementa- the effects in both healthy individuals with differing BMI as well as tion.80 Conversely, in studies incorporating severely obese trials investigating the effect of omega-3 PUFA on these subjects, leptin levels increased. There were, however, some adipokines in different pathological conditions. inconsistencies within these results, for example, Patel et al.76 inducted subjects who had recently experienced an MI. This may Limitations in available literature have been associated with alterations in other risk factors for Adipokines are influenced by dietary fat and energy intakes.89 Thus trials involving placebos that contain other fatty acids can be problematic in this area of research. This was clearly demonstrated by Iariu et al.86 where greater changes in adipokine levels were observed in the control group (supplemented with 20 g butterfat; caloric equivalent to 3.36 g omega-3 supplement in the treatment group) than in those supplemented with omega-3. Other studies included within this review compared omega-3 groups with either standardised diet or the same standardised diet with the inclusion of other lipids. Primary limitations within currently available literature include differences in the method of dose reporting (as previously mentioned, this makes comparison between the trials difficult) and the lack of large-scale, controlled trials with a suitable comparator diet or inactive supplement.

Plausible mechanisms Mitochondrial function in obesity. Adipocytes undergo two stages of development: differentiation and hypertrophy.90 Early stage maturation (differentiation) presents with adipocytes that have a higher level of metabolic activity and fuel consumption. During this stage, cells are relatively small and insulin sensitive and demonstrate increased adiponectin expression. By contrast, cells from aged tissue have increased size (hypertrophy) and have lost most of their functional activities, including adiponectin synthesis.90,91 In obesity, the number of hypertrophied fat cells also increases,91 with subsequent reduction in the mitochondrial protein content.92 This correlates with data from pharmaceutical therapies known to either increase or decrease mitochondrial protein content and/or function also noted to alter adiponectin levels. For example, the thiazolidinediones class of drugs are known to stimulate adipocyte differentiation and prevent hypertrophy.93 This occurs as a result of increased mitochondrial Figure 2. (a) Relationship between BMI and adiponectin levels protein content of adipocytes.92 Further, thiazolidinediones before and after omega-3 fatty acid supplementation. Data for 94,95 available studies assessing adiponectin levels before and after increase adiponectin synthesis. Conversely, anti-HIV drugs induce peripheral fat wastage and central adiposity, with the latter omega-3 supplementation were sorted according to mean BMI 96 classification. This included normal,73 overweight,74,76,87 obese37 or characterised by mitochondrial defects and hence are associated 97 severely obese ranges.86 For each of these groups, the average of with reduced adiponectin synthesis. As adiponectin synthesis is mean adiponectin levels in omega-3 treatment groups were linked to mitochondrial function in adipocytes,91 it is plausible that calculated both before and after supplementation. In the case of the mitochondrial dysfunction associated with obesity98 is a observational population studies, before and after supplementation contributing factor in the concurrent decrease observed in levels were considered to be mean adiponectin levels for the adiponectin levels (Figure 3). population groups without and with consumption of marine lipids, respectively. The mean values for each set of studies in a given BMI range were then graphed along with approximated polynomial Omega-3 fatty acids and inflammation. Omega-3 fatty acids have an integral role in the prevention and treatment of CVD,57 arthritis trends before and after supplementation. (b) Relationship between 1 BMI and leptin levels before and after omega-3 fatty acid and other inflammatory or autoimmune disorders. Studies supplementation. Data for available studies assessing leptin levels specifically assessing anti-inflammatory effects of omega-3 have before and after omega-3 supplementation were sorted according reported differing results. Pot et al.99 demonstrated no significant to mean BMI classification. This included normal,80 overweight,76 changes in inflammatory markers associated with omega-3 intake 79 86 obese or severely obese ranges. For each of these groups, the in healthy individuals. By contrast, other studies have average of mean leptin levels in omega-3 treatment groups were demonstrated an omega-3 fatty acid-induced inhibition of calculated both before and after supplementation. In the case of nuclear transcription factor kappa B (NFkB), a key transcription observational population studies, before and after supplementation factor in cytokine gene expression, cellular adhesion and levels were considered to be mean leptin levels for the population 100–102 groups without and with consumption of marine lipids, respectively. inflammation. Omega-3 has also been shown to suppress The mean values for each set of studies in a given BMI range were monocyte synthesis of pro-inflammatory mediators, including then graphed along with approximated polynomial trends before IL-1,4,103,104 IL-6105 and TNFa.103,106,107 Due to the correlation and after supplementation. between inflammatory markers such as IL-6, BMI and percentage

European Journal of Clinical Nutrition (2013) 1234 – 1242 & 2013 Macmillan Publishers Limited Effects of omega-3 fatty acids B Gray et al 1239

Figure 4. Proposed relationship between omega-3 fatty acids, Figure 3. Proposed relationship between adiponectin and mito- adipokines and mitochondrial function. Metabolised by the liver, chondrial function in white adipose tissue. Non-obese individuals omega-3 fatty acids are associated with decreased levels of have a higher proportion on immature adipocytes. In obese inflammatory markers, such as CRP, TNFa, NFkB, IL-1 and IL-6. subjects, there is generally a higher proportion of mature Circulating levels of adiponectin have been inversely correlated with adipocytes. Comparatively, mature adipocytes have lost much of many of these markers. Conversely, leptin levels have been their functional activities, including the expression of adipo- positively correlated with CRP and TNFa. Subsequently, it is nectin.90–92,94–97 proposed that changes in circulation of these hormones associated with omega-3 supplementation result in improved function of the skeletal muscle mitochondria function, with increased glucose body fat, it has been suggested that these inflammatory markers 27,28,37,69,94,110,112,114,118,119 may have a role in the link between obesity and insulin uptake and fatty acid oxidation. resistance.108 Esposito et al.109 investigated the effects of weight loss and lifestyle changes on vascular inflammatory markers in obese intake and adiponectin levels. This is of particular interest given women. After 2 years, BMI decreased more in the intervention that decreased circulating levels of adiponectin in obesity are group (weight loss and lifestyle modification) than in control associated with increased risk of and MI subjects, as did serum concentrations of IL-6, IL-18 and CRP. whereas increased levels are associated with enhanced insulin Adiponectin levels increased significantly in correlation with sensitivity. decreased levels of these inflammatory markers. Omega-3- Conversely, the magnitude of omega-3-induced changes in associated changes in adiponectin have been linked to plasma leptin levels appear to be partially dependent on BMI. By concurrent changes in pro-inflammatory cytokines, such as summarising available data, omega-3 supplementation appears to TNFa.37 The reverse has also been demonstrated, with TNFa decrease plasma leptin levels in non-obese subjects, yet increase shown to dose-dependently reduce circulating measures of levels in severely obese subjects. In non-obese individuals, leptin adiponectin.94 Therefore, it is hypothesised that the ability of serves to decrease food intake and increase energy expenditure. omega-3 fatty acids to decrease adiponectin, at least in part, may In obese subjects, where weight is stable, a resistance to the be due to their ability to downregulate TNFa and other endogenous effects of leptin develops. These increased levels are inflammatory markers. also associated with the development of CVD. However, in obese Circulating leptin concentrations have been positively subjects on a calorie-restricted diet, infusion of leptin to replicate correlated with levels of CRP,110 as well as TNFa,111,112 levels of leptin observed before weight reduction is able to reverse independently of BMI.113 Leptin and TNFa both increase with the expected changes in appetite and energy expenditure that weight gain, decrease with weight loss, and are commonly would otherwise promote weight regain. Omega-3-associated elevated in obesity.111 The elevated expression of TNFa in obesity increases in leptin levels observed in obese subjects may pose is thought to be a contributing factor to the subsequent benefits in the prevention of weight regain during calorie- development of hyperleptinemia.114–116 restricted diets in this group. It is proposed that the decrease in inflammatory cytokines (NFkB, TNFa, IL-1 and IL-6), associated with omega-3 fatty acid supplementation, induces an increase in adipocyte production CONFLICT OF INTEREST of adiponectin, along with improved leptin sensitivity and a LV received funding from FIT Bioceuticals for the scholarship support of BG. These corresponding decrease in leptin production (Figure 4). sponsors had no involvement in the collection, analysis or interpretation of the data; writing the report; or the decision to submit the paper for Publication. 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