N-3 Polyunsaturated Fatty Acids through the Lifespan: Implication for Psychopathology Open Access

Abstract

Objective:

The impact of lifetime dietary habits and their role in physical, mental, and social well-being has been the focus of considerable recent research. Omega-3 polyunsaturated fatty acids as a dietary constituent have been under the spotlight for decades. Omega-3 polyunsaturated fatty acids constitute key regulating factors of neurotransmission, neurogenesis, and neuroinflammation and are thereby fundamental for development, functioning, and aging of the CNS. Of note is the fact that these processes are altered in various psychiatric disorders, including attention deficit hyperactivity disorder, depression, and Alzheimer’s disease.

Design:

Relevant literature was identified through a search of MEDLINE via PubMed using the following words, “n-3 PUFAs,” “EPA,” and “DHA” in combination with “stress,” “cognition,” “ADHD,” “anxiety,” “depression,” “bipolar disorder,” “schizophrenia,” and “Alzheimer.” The principal focus was on the role of omega-3 polyunsaturated fatty acids throughout the lifespan and their implication for psychopathologies. Recommendations for future investigation on the potential clinical value of omega-3 polyunsaturated fatty acids were examined.

Results:

The inconsistent and inconclusive results from randomized clinical trials limits the usage of omega-3 polyunsaturated fatty acids in clinical practice. However, a body of literature demonstrates an inverse correlation between omega-3 polyunsaturated fatty acid levels and quality of life/ psychiatric diseases. Specifically, older healthy adults showing low habitual intake of omega-3 polyunsaturated fatty acids benefit most from consuming them, showing improved age-related cognitive decline.

Conclusions:

Although further studies are required, there is an exciting and growing body of research suggesting that omega-3 polyunsaturated fatty acids may have a potential clinical value in the prevention and treatment of psychopathologies.

Introduction

Despite the improved pharmacological approaches that moderately buffer the worldwide burden of poor mental health, a recent finding suggests that mental illnesses will continue to rise worldwide during the coming decades (Baxter et al., 2013). The general transition to more calorically dense, processed diets and reduced physical activity have had a significant impact on the overall health of individuals in developed nations and are associated with an increased incidence of psychopathologies such as anxiety and depression (Logan and Jacka, 2014). Accumulating translational evidence implicates the quality of diet as a crucial and common determinant for mental disorders (McNamara et al., 2015). Within the brain, omega-3 polyunsaturated fatty acids (n-3 PUFAs) have been under the spotlight for decades. However, only recently has research looked into the critical role of n-3 PUFAs in brain function and structure throughout the lifespan. n-3 PUFAs constitute key regulating factors of neurotransmission, neurogenesis, cell survival, and neuroinflammation and are thereby fundamental for development, functioning, and aging of the CNS (Mischoulon and Freeman, 2013). Importantly, these processes are altered in various psychiatric disorders, including attention deficit hyperactivity disorder (ADHD), schizophrenia, major depression, and Alzheimer’s disease (Sinn et al., 2010). Despite this evidence, the concept of n-3 PUFAs as a clear therapeutic compound for mental disorders still needs to be clarified. The purpose of this review is to further explore the role of n-3 PUFAs, which have been gaining increasing credence as potential targets for the development of novel strategies for the maintenance of mental health in the prevention and amelioration of psychopathology (Figure 1).

n-3 PUFAs in Early-Life: From Embryogenesis to Adolescence

Brain development is a sequential anatomical process characterized by specific well-defined stages of growth and maturation. It has become more evident that this process is influenced by n-3 PUFAs. Specifically, the levels of docosahexaenoic acid (DHA), the most abundant component of the n-3 PUFAs family, increase sharply along the perinatal period. In rats, the first important step of acquisition of DHA takes place in the embryonic phase and in the first 3 postnatal weeks of life, whereas in humans this period goes between the last trimester of gestation and the first 6 to 10 months after birth (Clandinin et al., 1980a, 1980a). At these stages of life, the foetal metabolic capability to convert the precursor of the n-3 PUFAs family, α-linolenic acid (ALA), to DHA is extremely limited (<0.2% in children). Indeed, it is the mother that guarantees an adequate delivery of DHA to the fetus through the placenta. (Innis, 2007).

The Perinatal Lipid Nutrition Project and The Early Nutrition Programming Project have recently developed consensus recommendations concerning dietary fat intake for pregnant and lactating women. They recommend a minimum DHA intake of 200mg/d (Koletzko et al., 2007). Interestingly, supplementation of n-3 PUFAs during pregnancy not only increases breast milk DHA content but also results in slightly longer gestation as well as reduced risk of preterm delivery (Singh, 2005). Accordingly, infants born prematurely show less accumulation of DHA in the brain (Barcelo-Coblijn and Murphy, 2009). However, the increased supply of DHA to the developing fetal nervous system leads to a progressive depletion of maternal plasma DHA (Smuts et al., 2003). Of relevance, in a cross-national ecological analysis, reduced levels of maternal milk DHA and lower seafood consumption correlated with higher rates of postpartum depression (Hibbeln, 2002). Therefore, diets enriched in n-3 PUFAs are recommended to both restore the depletion of DHA in the mother as well as to look after the needs of the fetus and suckling infant.

Animal studies reveal n-3 PUFAs as guarantors of proper brain development in early postnatal life (Lei et al., 2013). n-3 PUFA supplementation has been shown to protect from neuronal loss and decreased neurogenesis in the cerebral cortex and hippocampus of neonates (Lei et al., 2013). Furthermore, this effect can be long-lasting and show further benefits in neurocognitive function later in adulthood (Lei et al., 2013). In humans, Jensen et al. (2005) have shown a higher Psychomotor Development Index in breastfed infants of mothers who underwent DHA administration (200mg/d) for a period of 4 months. Psychomotor development, eye-hand coordination, and visual acuity were all improved after DHA algal-oil treatment. However, these improvements were limited to infants 30 months old and no further advantages on mental development were found. Although of relevance, this study has limitations in terms of treatment duration and assessment instruments. A different study measured IQ in children whose mothers underwent maternal supplementation with cod liver oil (1183mg/10mL DHA, 803mg/10mL eicosapentaenoic acid [EPA]) started from week 18 until 3 months after delivery (Helland et al., 2003). The scoring of the Mental Processing Composite of the Kaufman Assessment Battery for Children was increased by 4 points in 4-year-old children who were born to mothers who had taken cod liver oil enriched in DHA and EPA. However, only 84 children of the 590 pregnant women enrolled in the study completed the Kaufman Assessment Battery for Children at 4 years of age. Another similar study has shown higher mental processing scores and high degree of stereopsis and stereo acuity in children 3.5 years old whose mothers received a DHA-rich diet (Williams et al., 2001). Given the importance to maintain proper DHA levels during the neonatal period of life, the content and kind of fatty acids introduced in the body are of importance. In fact, infants fed formula with 0.4% or 2.4 % energy from ALA had 2.3±0.2 and 2.2±0.3g/100g fatty acids as DHA in plasma, respectively, despite the large difference in ALA intake, whereas infants fed formula with only 0.12 % energy from DHA had plasma DHA levels of 5.2±0.2g/100g (Innis, 2007).

n-3 PUFAs maintain their importance in brain development and functioning during adolescence (Table 1). At this stage of life, rats fed with an n-3 PUFA-deficient diet tend to have decreased n-3 PUFA mass (82% less) compared with control (Bondi et al., 2014). Such animals show less exploratory behavior, weaker memory performance, and increased tyrosine hydroxylase expression (dopamine [DA] precursor) in the dorsal striatum.

In humans, Kuratko et al. (2013) provided a systematic review based on 15 publications regarding the influence of DHA in learning and behavior in healthy children. The studies differed in purpose and design, and some did not achieve a consistent conclusion regarding DHA’s effect on specific cognitive tests. However, the analysis found benefits of DHA supplementation in brain activity and school performance from over one-half of the considered studies. A landmark study showing improved brain activity after DHA supplementation dates back to 2010 (McNamara et al., 2010a). McNamara et al. showed for the first time regulation of cortical metabolic function and cognitive development exerted by DHA concentrations in the grey matter of healthy boys during sustained attention (McNamara et al., 2010a). The study was conducted in 33 subjects (9 years old) who were assigned to receive placebo or 1 of the 2 doses of DHA (400mg/d; 1200mg/d) for 8 weeks. A longitudinal study in arctic Quebec has revealed a relation within school-age children with higher DHA cord plasma concentration and memory function (Boucher et al., 2011). Despite this interesting finding, a previous study conducted in DHA-fed healthy children in the UK did not show any improvement on cognitive performance and learning (Kennedy et al., 2009). However, other studies have observed benefits of n-3 PUFAs, particularly in malnourished subjects. For instance, improvements in learning and cognitive performance have been shown in n-3 PUFA-supplemented, malnourished children 7 to 9 years old in South Africa (Dalton et al., 2009) and 8- to 12-year-old Mexican children (Portillo-Reyes et al., 2014), although, no benefits were found in 6- to 10-year-old malnourished children from India (Muthayya et al., 2009) and Indonesia (Osendarp et al., 2007). The discordance between these studies may be accounted for by differences in the extent of malnutrition. Moreover, further discrepancies can be due to differences in the experimental plan such as supplements used, different dosages, and duration of the trials. Overall, the current research supports the view that adequate intake of DHA since prenatal life may have a positive impact on brain activity, learning, and cognition in healthy children. Indeed, being components of cell membranes, n-3 PUFAs are involved in important phases of brain development such as neurogenesis, myelination, synaptogenesis, and dendritic arborisation, which are in continuous flux during development and learning (Wurtman, 2014). Thus, this raises the possibility that n-3 PUFAs may have a beneficial effect on neurodevelopmental psychiatric disorders (McNamara and Carlson, 2006).

n-3 PUFAs: Implication for Childhood Disorders

Deficits in n-3 PUFAs have been associated with higher risk of development of childhood disorders such as ADHD (Richardson and Ross, 2000). In a cross-sectional study, Burgess et al. (2000) observed that 40% of recruited ADHD subjects (53 ADHD 6- to 12-year-old boys) had significantly lower proportions of plasma DHA and EPA and greater frequency of n-3 PUFAsdeficiency syndrome such as thirst, frequent urination, and dry hair. Importantly, such physical signs linked with deficit in n-3 PUFAs are consistent in ADHD (Richardson et al., 2000). Moreover, behavioral and learning problems, such as anomalous visual, motor, attention, and language processing, have been linked to lower n-3 PUFAs plasma levels (Richardson, 2004). The reason for lower n-3 PUFAs levels in ADHD is unknown. Evidence suggests that testosterone can impair the biosynthesis of n-3 PUFAs while estrogens are positively related with the conversion of DHA from ALA both in rodents (Marra and de Alaniz, 1989) and humans (Childs et al., 2008). This may explain the higher prevalence of ADHD in males than in females.

In animal studies the spontaneously hypertensive rat is generally used as a model of ADHD (Meneses et al., 2011). Spontaneously hypertensive rat fed n-3 PUFA (EPA-DHA)-enriched diet (n-6:n-3 PUFAs ratio of 1:2.7), from gestational phase until postnatal day 50, partially ameliorated ADHD-like behavior by improving reinforcer-controlled activity, impulsiveness, and inattention (Dervola et al., 2012). Moreover, such a diet increased the DA and serotonin turnover ratio together with decreased levels of glutamate in the striatum of the same animals (Dervola et al., 2012). These neurotransmitters are thought to be altered in ADHD animal models (Gainetdinov et al., 2001) and ADHD young subjects (Paloyelis et al., 2012).

In humans, supplementation with PUFAs (480mg DHA, 80mg EPA, 40mg arachidonic acid [AA], 96mg γ-linolenic acids, and 24mg α-tocopheryl acetate) improved ADHD children’s oppositional defiant behavior from a clinical to a nonclinical range (Stevens et al., 2003). In this study, both parents and teachers rated improvement in conduct problems and attention difficulties after 4 months of treatment. However, more than one-half of the patients were prescribed medication for ADHD, making it difficult to attribute the observed improved symptoms to the n-3 PUFA treatment. A similar PUFA mixture (480mg DHA, 186mg EPA, 96mg γ-linolenic acids, 42mg AA, and 60 IU vitamin E per day), supplemented for 12 weeks, showed improvement in anxiety/shy tests, cognition, inattentiveness, hyperactivity/impulsiveness, total DSM-IV index, and Conners total global index in children with learning difficulties (Richardson and Puri, 2002). However, none of the subjects were formally diagnosed with ADHD and the sample size was small (41 children, 8–12 years old). Although the studies reported above seem to be encouraging, 3 recent systematic reviews have reported only minor n-3 PUFA effect in reducing ADHD symptoms. Grassman et al. (2013) reported mild behavioral and cognitive improvement in ADHD children after treatment with low doses of n-3 PUFAs. However, the studies included in their analysis were heterogeneous, had small sample sizes, and only a limited number were placebo controlled. Bloch and Qawasmi (2011) have reported a modest effect in the treatment of ADHD after treatment with high doses of n-3 PUFAs (especially EPA) in comparison with current pharmacotherapies such as psychostimulants, atomoxetine, or α(2) agonists. Gillies et al. (2012) reported improvement after combined n-3 PUFA and n-6 PUFA supplementation only in a minority of the studies that met the inclusion criteria of their review. Given its evidence of modest efficacy, it may be reasonable to investigate n-3 PUFAs as a supplement to traditional pharmacologic interventions. Future studies need to be adequately powered and placebo controlled and use adequate dosage, (Table 2).

n-3 PUFAs in Adulthood: Stress Response

In adulthood, full brain development is already achieved. However, at this stage of life stressful life events can alter mood states and cognition, increasing susceptibility to psychopathologies (Cattaneo et al., 2015). n-3 PUFAs seem to play a role in the regulation of the stress-response influencing the activity of the hypothalamic–pituitary–adrenal (HPA) axis. In humans (7 volunteers), the stimulation by 30 minutes of mental stress (mental arithmetic’s and Stroop’s test) of plasma epinephrine, cortisol, and energy expenditure were all significantly blunted by 3 weeks of 7.2g/d fish oil administration (Delarue et al., 2003). However, the subjects were tested before and after 3 weeks of n-3 PUFA supplementation; thus, a possible effect of acclimatization due to the repetition of the testing procedure over time should be taken into account. Similarly, ACTH and cortisol plasma levels were blunted by 4 weeks of 7g/d fish oil supplementation prior lipopolysaccharide (2ng/kg)-induced neuroendocrine response (Michaeli et al., 2007). In contrast with the n-3 PUFA antiinflammatory effect (Grimm et al., 2002), high levels of cytokines were not reversed by n-3 PUFAs, perhaps due to a too low content of EPA (17%) and DHA (11%) in the fish oil supplement. Indeed, 2.5g/d of n-3 PUFAs (2g EPA, 0.35g DHA) supplementation showed reduction of stimulated interleukin 6 (IL-6) and tumour necrosis factor plasma levels in healthy medical students. Interestingly, this was associated with reduced anxiety symptoms in healthy students without an anxiety disorder diagnosis (Kiecolt-Glaser et al., 2011). This evidence acquires more interest, since stress can exert a pivotal role in memory and cognition (McEwen, 2007). Researchers at the University of Pittsburgh have determined that n-3 PUFAs can enhance cognition in young individuals. Healthy young adults (18–25 years of age) after 6 months of n-3 PUFA supplement, mostly DHA (750mg/d) and EPA (930mg/d), experienced improvement in working memory (Narendran et al., 2012). Working memory assessment was performed using a verbal n-back task that used 3 loads of working memory (1-, 2-, and 3-back) (Abi-Dargham et al., 2002). In addition to that, prior to supplementation of n-3 PUFAs, higher red blood cell DHA levels were significantly correlated with working memory in a group of young adults. Interestingly, a similar association between high DHA plasma levels and improved cognitive function was previously observed in a sample of 208 healthy subjects (30–54 years of age; Muldoon et al., 2010). DHA, but not other n-3 PUFAs, was associated with better scores on tests of nonverbal reasoning, mental flexibility, working memory, and vocabulary. Another study conducted in Australia showed 6 weeks of 6g/d fish oil intake (1.5g DHA, 360g EPA) were enough to ameliorate the Perceiving Stress Scale scoring in stressed university staff (Bradbury et al., 2004). Due to a lack of investigation, further research is warranted to elucidate the mechanisms by which n-3 PUFAs enhance cognitive performance in healthy individuals (Table 3).

n-3 PUFAs and Psychopathologies

Major Depressive Disorder

Since the 19th century, the incidence of major depressive disorder (MDD) in Western countries seems to have increased. In contrast, the dietary intake of n-3 PUFAs has dramatically declined in favor of n-6 PUFA intake (Molendi-Coste et al., 2011). Joseph Hibbeln was one of the first investigators to draw attention to the importance of n-3 PUFAs in psychiatric disorders. In 1998, Hibbeln showed a cross-national significant negative correlation between worldwide fish consumption and prevalence of MDD (Hibbeln, 1998). Moreover, higher n-6/n-3 ratios, such as the AA/EPA ratio, has been detected in blood samples (Lin et al., 2010) and red blood cell phospholipids (Logan, 2003) of depressed patients. Accordingly, n-3 PUFA concentrations in the blood reflect an accurate, although not identical, representation of n-3 PUFAs levels in the brain (Horrobin, 2001). Lower DHA levels have been found in the postmortem orbitofrontal cortex of MDD patients (McNamara et al., 2007). Moreover, lower n-3PUFA levels have been found in chronic hepatitis C viral infection patients that developed interferon (IFN)-induced depression after IFN-α intervention. This finding identifies both n-3 PUFA-related genotypes and n-3 PUFA levels as risk factors for IFN-induced depression (Su et al., 2010). However, these findings do not show that fish consumption can cause differences in the prevalence of MDD or that eating fish or fish oils is useful in treatment. One double-blind, placebo-controlled study investigated the effect of EPA as adjunct to antidepressant therapy in a group of 20 MDD diagnosed patients. Although this was a small study (17 women, 3 men), the addition of 2g of EPA to standard antidepressant medication showed highly significant benefits by week 3 of treatment. Primarily, EPA showed effects on insomnia, depressed mood, and feelings of guilt and worthlessness (Nemets et al., 2002). In 2002, Peet and Horrobin observed that a specific EPA dosage (1g/d) was effective in ameliorating depressive symptoms in subjects with persistent depression despite ongoing treatment with antidepressant. In this 12-week, randomized, double-blind, placebo-controlled trial, 53% of the subjects who received EPA (17 subjects) achieved a 50% reduction on the Hamilton Depression Rating Scale score. In addition, the EPA had a broad-spectrum positive effect leading to improvements in anxiety, sleep, lassitude, libido, and suicidal ideation (Peet and Horrobin, 2002). Moreover, a 2-week, double-blind, placebo-controlled trial conducted in a group of 162 patients showed n-3 PUFA efficacy in the prevention of IFN-induced depression in hepatitis C virus patients (Su et al., 2014). Both DHA (1.75g/d) and EPA (3.5g/d) significantly delayed the onset of IFN-induced depression after 24 weeks of IFN-α treatment. Despite that, only EPA-treated patients showed lower incident rates of IFN-induced depression (10% vs 30% for placebo [oleic oil, 4g/d], P=.037). Although clinical outcomes on the effect of EPA on major depression seem to be promising, trials using DHA are inconclusive. Thirty-six subjects with major depression assigned to receive DHA (2g/d) for 6 weeks did not show differences in the score of the Montgomery-Asberg Depression Rating Scale compared with the placebo-treated group (Marangell et al., 2003). A recent meta-analysis focused on the hypothesis that EPA represents the key compound of the n-3 PUFA family having effects in the treatment of major depression. Fifteen trials (916 total participants) using n-3 PUFAs as either a mono or adjunctive therapy were analyzed. Studies were selected based on prospective, randomized, double-blinded, placebo-controlled study design, if depressive episode was the primary complaint with or without comorbid medical conditions and, if appropriate outcome measures were used to assess depressed mood. This meta-analysis concluded that n-3 PUFA supplements with >60% of EPA (in a dose range of 200 to 2200mg/d in excess of DHA) ameliorated the clinical condition. However, doses containing primarily DHA or <60% EPA were not effective against primary depression. Moreover, trial duration (4–16 weeks) was not a predictor of outcomes, suggesting that EPA improvements may not be limited to the initial treatment period (Sublette et al., 2011). Albeit, it is not possible to recommend n-3 PUFAs as either a mono or adjunctive therapy in major depression as yet, though the current research is strong enough to justify further studies.

Bipolar Disorder and Schizophrenia

To date, the link between n-3 PUFA levels and bipolar disorder/schizophrenia is poorly understood. n-3 PUFAs can have a slight beneficial effect on depressive symptoms when added to an existing psychopharmacological maintenance treatment for bipolar disorder. For instance, 30 patients with bipolar disorder were randomized to receive 9.6g/d of n-3 PUFAs (EPA 6.2g, DHA 3.2g) or placebo (olive oil) in addition to their ongoing usual treatment. The n-3 PUFAs patient group showed a significantly longer period of remission than the placebo group as assessed by the Kaplan-Meier survival analysis (Stoll et al., 1999). In a second study, patients with bipolar disorder were randomized to receive either 1to 2g/d ethyl-EPA (n = 24–25, respectively) or placebo (paraffin oil, n = 26) in addition to their ongoing usual treatment (Frangou et al., 2006). The EPA treatment, without apparent benefit of 2g over 1g EPA, significantly improved the Hamilton Rating Scale for Depression together with the Clinical Global Impression Scale. Nevertheless, current data on the efficacy of DHA and EPA in the treatment of bipolar disorder are insufficient for us to draw definite conclusions that can guide clinical practice.

Despite the large paucity of data, it seems that n-3 PUFAs may be of help in reducing psychotic-like symptoms (Schlogelhofer et al., 2014). Eighty-one participants with subthreshold psychosis were followed for 12 months to investigate the rate of progression to first-episode psychotic disorder. Previously, one-half of the group underwent a 12-week intervention period of 1.2g/d n-3 PUFAs or placebo. Only 5% of the n-3 PUFA group had transitioned to psychotic disorder in contrast to a 27.5 % in the control group. Moreover, n-3 PUFA intake ameliorated positive, negative, and general symptoms assessed by the Positive and Negative Syndrome Scale (Amminger et al., 2010). Overall, well-designed and executed randomized controlled trials in this field are clearly lacking, and the need for such high-quality primary research is acute. Study duration should be long enough to ensure that the n-3 PUFAs can be fully absorbed into brain cell membranes and therefore should ideally be 3 months at a minimum. Finally, the dose and composition of the n-3 PUFA supplement should be modelled on current evidence, which suggests that 1 to 2g/d of EPA or majority EPA supplement may be the most effective form of the treatment, although further research into the efficacy of varied compositions and doses of n-3 PUFA treatment is also necessary (Table 4).

n-3 PUFAs in Elderly

Clinical studies have been carried out to elucidate the role of n-3 PUFAs in healthy older subjects (Table 5). One of the largest randomized, controlled trials to date recruited 867 cognitively healthy subjects (70–79 years old) and did not reveal improved cognitive functioning in the California Verbal Learning Test after 24 months of treatment (200mg EPA plus 500mg DHA) (Dangour et al., 2010), even though at the end of the study, the n-3 PUFA serum levels were higher compared with the placebo group (olive oil) and the n-3:n-6 PUFA ratio was relatively high in both groups. Moreover, all the recruited subjects showed a high cognitive functioning at the beginning of the study, as assessed by the Mini-Mental State Examination. This, the authors suggest, is the reason for the negative findings. In another study, higher administration of n-3 PUFAs (900mg DHA) for 24 weeks showed improvements in verbal recognition memory and visuospatial learning in elderly subjects with low habitual intake of DHA (Yurko-Mauro et al., 2010). Accordingly, higher concentration (1.3g DHA and 0.45g EPA) and longer duration (12 months) of n-3 PUFAs showed improvement in different cognitive domains of a neuropsychological battery (Lee et al., 2013). In this study, the choice of subjects could have been critical. Thirty-five healthy elderly women with mild cognitive impairment (MCI) from a low socioeconomic background were recruited. Moreover, in this group the habitual intake of n-3 PUFAs was inadequate for financial reasons. These findings are in agreement with a double-blind, randomized control trial conducted by Sinn et al. (2012) in which 40 MCI subjects (over 65 years old) with low fish intake were divided into 3 experimental groups to receive a supplement rich in EPA (1.67g EPA plus 0.16g DHA), DHA (1.55g DHA + 0.40g EPA), or LA (2.2g). After 6 months of n-3 PUFA supplementation, depressive symptoms, assessed by the Geriatric Depression Scale, verbal fluency (Initial Letter Fluency), and self-reported physical health,were improved, especially in the DHA group. It is likely that MCI and low n-3 PUFA consumption offer the best prospect of cognitive improvement. Interestingly, DHA seems to be more of benefit in both memory and cognition than EPA, or their combination, as observed in adulthood. This discrepancy could be due to the phospholipid degradation occurring at this last stage of life. Since DHA constitutes the most abundant fatty acid in the brain and due to its importance in the formation and functionality of the CNS, it is plausible to think that DHA can ameliorate cognitive performance more than other PUFAs in older subjects.

n-3 PUFAs and Alzheimer’s Disease

Postmortem analysis of AD subjects shows lower n-3 PUFAs levels in the hippocampus and frontal lobe together with decreased hippocampal size (Soderberg et al., 1991; Yehuda et al., 2002). Moreover, reduced risk of senile dementia and AD has been reported to be related to higher fish consumption (Morris et al., 2003). However, no cognitive improvements were observed in subjects affected by moderate AD after 24 weeks of n-3 PUFAs (EPA 1080mg plus DHA 720mg) (Chiu et al., 2008). Neither has a longer trial, up to 1 year of n-3 PUFA supplementation, shown neuropsychiatric improvements in AD subjects (Freund-Levi et al., 2008). On the other hand, Yehuda et al. (1996) have shown improvements in mood, cooperation, appetite, sleep, ability to navigate in the home, and short-term memory after only 4 weeks of an ALA:LA mixture (1:4). Furthermore, within 174 AD subjects that underwent n-3 PUFA administration for a total period of 1 year, only 15% of them showed reduction in cognitive decline (Freund-Levi et al., 2006). The impact of n-3 PUFAs on mental illnesses in elderly are reported in Table 6.

n-3 PUFAs: Mechanisms of Action

Inflammation

As outlined in the previous sections, n-3 PUFAs have multiple implications in several conditions. Therefore, they are likely to act via multiple mechanisms (Figure 2). The antiinflammatory effect exerted by n-3 PUFAs represents one of the most investigated mechanisms. Moreover, chronic inflammation is now considered to be central to the pathogenesis of stress-related disorders such as depression (Barnes et al., 2016). Primarily EPA and in part DHA metabolize into antiinflammatory compounds such as leukotrienes (5 series), prostaglandins (3 series), resolvins, lipoxins, and neuroprotectin D1. Competing with n-6 PUFAs for metabolism, both EPA and DHA may interfere with the production of the n-6 PUFA arachidonic acid-derived proinflammatory eicosanoids such as prostaglandins (series 2), leukotrienes (series 4), and thromboxanes (series 2) (Das, 2006). DHA and EPA have been also shown to inhibit the release of proinflammatory cytokines, such as interferon-γ, tumour necrosis factor-α, IL-1β, IL-2, and IL-6, directly acting on the transcriptional factor NF-kB (Kang and Weylandt, 2008).

HPA Axis

n-3 PUFAs seem to play a role in the regulation of the stress-response influencing the activity of the HPA axis. It has been proposed that n-3 PUFA deficiency may induce a chronic stress state by disruption of glucocorticoid receptors (GR)-mediated negative feedback (Larrieu et al., 2014, 2016). Controversially, the same authors observed that supplementation of n-3 PUFAs (9 weeks) prevented detrimental chronic social defeat stress-induced emotional and neuronal impairments by attenuating HPA dysfunction (Larrieu et al., 2014). Similarly, n-3 PUFA (EPA 12%, DHA 18%; 16 weeks) supplementation decreased stress-induced high plasma corticosterone levels and decreased anxiety- and depressive-like behaviors, as assessed by the elevated plus maze and the forced swim test, while increasing cognition as assessed by the Morris water maze in rats that underwent restrain stress (Ferraz et al., 2011). Accordingly, a study from our laboratory recently revealed n-3 PUFA (EPA 80%, DHA 20%; 12 weeks)-induced hippocampal GR activation correlated with cognitive and mood state improvements in adult female rats assessed by the novel object recognition, elevated plus maze, and forced swim test (Pusceddu et al., 2015b) (Table 7). Another study from our laboratory showed the protective effects of DHA against corticosterone-induced neuronal death as well as astrocyte overgrowth in cortical primary cultures (Pusceddu et al., 2015c). Furthermore, in the same study, we observed that the DHA was able to reverse the corticosterone-induced downregulation of GR expression levels in neurons. The regulation of GRs may represent a novel mechanism exerted by n-3 PUFAs which is worth to investigate further.

BDNF

DHA regulation of BDNF protein levels has gotten the attention of several researchers. This is of importance, since stress-related pathologies are strongly associated with decreased levels of BDNF (Lee and Kim, 2010). Accordingly, decreased BDNF levels are found in rats fed a diet containing inadequate n-3 PUFAs (Rao et al., 2007). Indeed, DHA supplementation to cortical astrocytes cells increased cAMP response element-binding protein and BDNF protein levels via a p38 mitogen-activated protein kinase-dependent mechanism (Rao et al., 2007). Likewise, DHA-induced hippocampal calcium–calmodulin protein kinase II-cAMP response element-binding protein-BDNF pathway activation enhanced synaptic plasticity, memory, and learning in rats (Wu et al., 2008).

Monoaminergic System

Associations between monoaminergic dysfunction in stress-related pathologies and lower dietary levels of n-3 PUFAs have been observed. Male rats with a 61% decrease in brain DHA, induced by feeding a diet deficient in n-3 PUFAs from birth, exhibited lower expression of the serotonin-synthesizing enzyme tryptophan hydroxylase in the midbrain and higher serotonin turnover in the prefrontal cortex compared with controls (McNamara et al., 2010b). Controversially, feeding a diet containing ALA reversed the effect on serotonin turnover (McNamara et al., 2010b). Consistent with this, adult rats supplemented with DHA and EPA exhibited increased concentrations of serotonin in the frontal cortex and hippocampus (Vines et al., 2012). Similarly, in mice, the decrease in brain serotonin levels induced by unpredictable chronic mild stress was reversed by an n-3 PUFA diet supplementation (Vancassel et al., 2008). Likewise, the role of n-3 PUFAs in modulation of noradrenergic neurotransmission has recently received attention. Studies in cultured SH-SY5Y neuroblastoma cells suggest that either brief exposure to, or incorporation of, DHA increased basal, but not KCl-evoked release of [3H]- norepinephrine by a mechanism involving enhanced exocytosis (Mathieu et al., 2010). DHA treatment also increased the density of β-receptors on rat astrocytes in primary culture (Joardar et al., 2006). Also, the DA system is affected by variation in dietary n-3 PUFA content. Virgin females with lower tissue DHA levels had altered abundancy of D1 and D2 DA receptors in the caudate nucleus relative to virgin females with normal DHA (Davis et al., 2010). These receptor alterations have been found in rodent models of depression and are consistent with the proposed hypodopaminergic basis for anhedonia and motivational deficits in depression (Kram et al., 2002). The effects of n-3 PUFAs on adult rodents’ behavior are reported in Table 7.

Glutamatergic System

It has been observed that n-3 PUFAs can regulate the functionality of the glutamatergic system, which can undergo dysregulation during aging. Indeed, n-3 PUFAs are involved in the regulation of the post-synaptic 2-amino-3-propionic acid and N-methyl-d-aspartate receptors (Nishikawa et al., 1994). This suggests that n-3 PUFAs may play an important role in the genesis of long-term potentiation, which is involved in memory formation and restoration of synaptic plasticity (Dyall et al., 2007; Lynch et al., 2007; Kelly et al., 2011). Excess glutamate can lead to excessive release of AA, which initiates a proinflammatory cascade of events involving the production of eicosanoids via the activation of inducible cyclooxigenase and lipoxigenases and the production of proinflammatory cytokines (Bazan, 2007; Farooqui et al., 2007).

Aβ Oligomers and Antiapoptosis

In vitro studies have shown DHA preventive effects against Aβ oligomer-induced neurotoxicity both in cortical and hippocampal cultures (Florent et al., 2006; Wang et al., 2010). Moreover, DHA has been found to promote cellular survival and prevent cortical neuronal apoptosis in primary cultures in a physiological condition (Cao et al., 2005) and after chronic corticosterone treatment (Pusceddu et al., 2015c). Moreover, similar results have been found in animal models of AD fed with n-3 PUFA-enriched diet (Lim et al., 2005a; Green et al., 2007). Interestingly, learning memory performance improved in Aβ-infused adult rats supplemented upon DHA intervention (Hashimoto et al., 2002; Hashimoto et al., 2005, 2009) (Table 8). These findings indicate that n-3 PUFAs decrease Aβ levels and have antioxidative stress and antiapoptosis effects, leading to neuron protection and maintenance of learning memory ability.

New Horizons: The Gut Microbiome

Parallel with these findings, we recently shed light on the role of n-3 PUFAs in regulating the gut microbiome, which is involved in a bidirectional communication with and influence over the brain both in rodents and humans (Cryan and Dinan, 2012). Indeed, there is a corpus of evidence to support the view that the gut microbiome plays an important role in stress-related psychopathologies such as depression (Dinan and Cryan, 2016). A first study conducted in our laboratory showed that EPA/DHA combination (1g/kg/d) was able to normalize early-life stress-induced disruption of female rat gut microbiome (Pusceddu et al., 2015a). This microbial disruption seems to be mainly due to a shift in Bacteroidetes:Firmicutes, the alteration of which in human stool specimen has been revealed in depressed individuals (Jiang et al., 2015). The early-life stress-induced microbial disruption also altered abundance of members of the gut microbiome known to have inflammatory effects such as Akkermansia (Stecher et al., 2007) and Flexibacter (Frank et al., 2007), and this was correlated with alteration of the corticosterone response to acute stress. Notably, EPA/DHA administration was beneficial for restoring members of the gut microbiome with immunoregulatory functions, suggesting a possible prevention to an overly robust stress-induced inflammatory response that may contribute to the onset of mental illnesses. Subsequent to this first evidence, a second study revealed a correlation between neurobehavioral outcomes and gut microbiome composition in mice fed with diets containing altered n-3 PUFA status (Robertson et al., 2016). Although the impact of n-3 PUFAs on the gut microbiome is still at its infancy, understanding the mechanisms behind their potential interplay may open up novel attractive strategies to prevent the onset of psychopathologies.

Conclusions

Despite strong evidence pointing out an inverse correlation between n-3 PUFAs levels and quality of life/psychiatric diseases, the introduction of n-3 PUFAs in clinical practice is still in its infancy. The primary reason for this is largely because of inconsistent and inconclusive randomized clinical trials. Inadequate dosing, inadequate duration, and lack of placebo control are major weaknesses in many studies. Use of insensitive and inappropriate clinical rating measures is another cause for concern. Nonetheless, there are some clear recommendations emerging. Low habitual intake of n-3 PUFAs is associated with poorer mental health and, in children, low literacy ability. A major finding is the positive evidence within older healthy adults with age-related cognitive decline. They benefit most from consuming n-3 PUFAs, particularly DHA. However, with the development of senile dementia or Alzheimer’s disease, n-3 PUFAs lose efficacy. While MDD still remains one of the most discussed and investigated forms of psychopathology, a clear identification of n-3 PUFAs as a treatment or adjunct therapy has not yet emerged. The evidence base is still weak and RCTs have been inconsistent with many study design limitations. Hence, a major challenge ahead is to design and conduct rigorous RCTs to provide proper evidence of n-3 PUFA application in clinical conditions, such as MDD. Further studies are also required to definitively demonstrate that n-3 PUFAs reduce the risk of transit from a prepsychotic state to overt schizophrenia. On the other hand, n-3 PUFAs are safe and well-tolerated supplements with only mild transient side effects. Moreover, being considered as a “natural remedy” and due to their relatively low cost, n-3 PUFAs represent an appealing option for individuals. While the evidence is not entirely conclusive to make specific recommendations for dietary intake of n-3 PUFAs, the limited ability of synthesis of n-3 PUFAs de novo and their critical role in brain development and functioning make it reasonable to include n-3 PUFAs in a balanced daily diet.

Statement of Interest

J.F.C. declares research funding: Mead Johnson Nutrition, Cremo, Suntory Wellness Danone-Nutritia, 4D Pharma. Speaker’s Bureau: Yakult, Mead Johnson, Janssen, Boehringer Ingelheim, Research Consultant: Alkermes and Mead Johnson.

Acknowledgments

The APC Microbiome Institute has conducted studies in collaboration with several companies including GSK, Pfizer, Cremo, Suntory Wellness, Wyeth, Nutritica, and Mead Johnson. T.G.D. and J.F.C. have spoken at meetings sponsored by food and pharmaceutical companies.

References

Author notes