We have compared the effects of EPA and placebo on cognitive and clinical symptoms of ADHD and on mechanistically-related biomarkers. Overall, EPA improves (more than placebo) focused attention at the CPT test, one of our main outcome measures. Moreover, youth with low endogenous EPA levels show even stronger effects of EPA in a further measure of attention (HRT) and in a measure of vigilance (HRTISIC), both improving more than in the placebo group. To our knowledge, this is the first study to report the effects of EPA monotherapy in ADHD, and the first study ever to use endogenous baseline PUFAs levels to stratify subjects in a PUFAs clinical trial. Further analyses indicate that EPA treatment in those with high endogenous EPA levels may actually have detrimental effects.

Our main findings are consistent with (at least some) previous studies investigating n-3 PUFAs (usually, combinations of EPA and DHA) in ADHD. Our recently-published meta-analysis1 has shown that PUFAs improves attention and impulsivity in children with ADHD, but the results of the individual studies have been inconsistent. For example, Sinn et al.29 found an improvement in attention, but Voigt et al.30 did not; similarly, Vaisman et al.31 found an improvement in impulsivity, but Voigt et al.30 found that, much like this present study, placebo did better than PUFAs for impulsivity. While it is difficult to draw firm conclusions on the reasons behind these discrepant findings, it is worth mentioning that both our study and Voigt et al.30 have around half of the sample with comorbid ODD. Interestingly, subgroup analyses in our data show that the improvement in attention does occur in the children with comorbid ODD, thus suggesting that specific classes of symptoms (impulsivity vs. attention, in this case) may respond differently to EPA also based on the presence of comorbid psychiatric conditions.

To test whether low endogenous PUFAs levels predict response to PUFAs, we use a similar approach to that used by two previous ‘personalised psychiatry’ studies, i.e., Raison et al.17 and Rapaport et al.18 : they both found antidepressant effects by an anti-TNF-alpha medication and by n-3 PUFAs, but only in patients with high baseline inflammation, i.e., those with biological evidence of abnormalities in the mechanism targeted by the interventions (in these two studies, inflammation). Consistent with this framework, we find that youth with the lowest levels of baseline endogenous EPA show the largest improvement in cognitive function following EPA. Although there are no other similar studies, previous research, in healthy children, as well as children and adults with ADHD and other neurodevelopmental disorders, has shown that PUFAs improve cognitive function more in the presence of low “n-3 PUFAs status”, i.e., in those with evidence of low dietary intake of fish, or if they present symptoms of the aforementioned EFA deficiency32.

Interestingly, and much like our study, Raison et al.17 and Rapaport et al.18 find not only that high inflammation predicts a better response to the anti-inflammatory intervention, but also that low inflammation predicts a better response to placebo, i.e., lowering inflammation in people who already have low inflammation actually has adverse effects. We also find that children with the highest baseline EPA levels perform better on placebo than on EPA, in impulsivity and in parental and teachers’ reports of ADHD and emotional symptoms. A previous meta-analysis in depression has also shown a “J-shaped curve” in the protective effects of PUFAs, increasing for doses of up to 1.8 g/day of n-3 PUFAs (or 0.6 g/day of EPA + DHA intake), and then decreasing for higher doses33; this suggests that very high doses of PUFAs (or, by extensions, normal doses in people with high endogenous PUFAs levels), may have adverse effects. However, it is important to emphasise that this J-shaped curve has been described for the intake of PUFAs from the diet, rather than supplementation33, and hence some potential mechanisms to explain these negative effects (high dietary intake of omega-6, or environmental contaminants like mercury from fish) are not present in our study. Reassuringly, most studies of children with ADHD (conducted largely in Western countries) have shown average endogenous EPA levels that are lower than those in our study1, probably because of the low-fish and high-omega-6 dietary intake in non-Asian countries34; thus, it is unlikely that non-Asian children would normally reach endogenous PUFAs levels that are similar to those in the highest tertile of our sample. Nevertheless, with fish intake and other natural sources of omega-3 being constantly advocated as part of a healthy diet, it is important to be aware that supplementing those who already have high levels of EPA may be detrimental.

The youth receiving EPA in our study show an increase in blood erythrocytes EPA and total n-3 PUFAs levels, and a decrease in the n-6/n-3 ratio, at the end of the 12 weeks. This is an expected finding, and it confirms the compliance of the youth with treatment. However, we also found that DHA levels do not increase in youth who received EPA, which is surprising since EPA is physiologically converted into DHA. However, this finding is consistent with a previous study showing that 1 g/day EPA in patients with depression only increases plasma EPA levels but not DHA levels35. Similarly, studies in patients with dyslipidemia and healthy subjects found that 3-4 g/day EPA increases plasma EPA but not DHA levels36,37. In humans, there is a poor enzymatic conversion of EPA to DHA37, and genetic variants in fatty acid desaturase 2 gene (FADS2), the key enzyme responsible for the conversion from EPA to DHA, further reduce this conversion38. Interestingly, and of specific interest for our study, a single nucleotide polymorphism (SNP) in FADS2 has been significantly associated with ADHD39, suggesting that the genetic profile of ADHD may have further effects on PUFAs metabolism. While we wanted specifically to test whether EPA alone was enough (following from the evidence in depression16), most of the other studies included in our previous meta-analysis1 have combined EPA + DHA interventions, and, indeed, in a previous cross-over clinical trial, an increase in erythrocyte EPA and DHA after a combined EPA + DHA intervention was associated with improved attention and behavioural symptoms in children with ADHD40. Thus, taken together, our data do seem to suggest that, for ADHD clinical symptoms, combined EPA and DHA may have a broader impact than EPA alone on clinical symptoms, possibly because affecting both EPA and DHA levels.

We investigated inflammation as one mechanism that might explain the actions of EPA on cognitive function in ADHD. EPA has been shown to have anti-inflammatory action via antagonising membrane arachidonic acid (AA) formation and inhibition of the synthesis of pro-inflammatory mediators41. However, EPA did not affect hs-CRP levels in our study. Of course, the average hs-CRP levels of youth with ADHD in our study (just above 2 mg/L) were below the threshold for even low-grade inflammation (3 mg/L); thus, the presence of only very mild levels of inflammation might have made impossible to detect an anti-inflammatory action of EPA. Moreover, it is also important to highlight that a previous study found a significant decrease in plasma CRP levels in children with ADHD treated with combined 900 mg PUFAs per day (DHA 165 + EPA 635 mg)42; thus, it is also possible that EPA monotherapy is ineffective to induce an anti-inflammatory action. In a previous study we found that EPA pre-treatment is able to prevent inflammation-induced depression following IFN-alpha treatment16; however, we did not measure CRP or indeed other immune biomarkers in that study, and thus we cannot exclude that the beneficial effects of EPA were due to changes in other mechanisms. Thus, the present study does support the possibility that both EPA and DHA are needed to exert a clear anti-inflammatory action.

The levels of neurotrophins, BDNF, were also not affected by EPA in this study. Previous studies have demonstrated that treatments for ADHD, such as methylphenidate43 and atomoxetine44, modulate levels of BDNF, although BDNF levels were increased after 6 weeks treatment of methylphenidate43 and decreased after a 3-month treatment of atomoxetine44. Studies of the associations between n-3 PUFAs intake and BDNF levels have also been inconsistent: for example, a positive association has been reported between n-3 PUFAs consumption and serum BDNF levels in adolescents45, but clinical trials do not find an effect of n-3 PUFAs on BDNF levels in adults with distress following trauma46 or in adults with diabetes mellitus and depression47. Again, preclinical evidence indicates that neuroplasticity effects of PUFAs may require both EPA and DHA48, suggesting an explanation for our negative findings.

Our study, although with several strengths, is not without possible limitations. First the ADHD population is heterogeneous, with age ranging 6–18 years and comorbidity with ODD; however, these characteristics make our sample more similar to the ADHD population in clinical settings, which reinforces the generalisability of our study. A second limitation is the duration of our study, 12 weeks, while most of the studies included in our previous meta-analysis1 have longer durations (mean of 15.6 weeks). Perhaps a longer duration of the trial would have been more successful in eliciting effects of EPA on clinical symptoms in ADHD. However, longer trials risk high drop-out rates, and we wanted to test an intervention that was easily deliverable in non-specialist settings; our drop-out rate of around 10% testifies to the success of this approach. It is also interesting to note that those who discontinued the study had a younger age than those who completed the study; this may be due to the fact that parents of young children with ADHD might feel less distressed by the symptoms and thus less keen in pursuing treatment, as suggested by previous studies49. Finally, we did not perform multiple comparison corrections in our analysis; we felt that this would have been too stringent, and might have generated false-negative findings. Instead, we have relied on the presentation of effect size differences throughout the papers, so that clinical significance, rather than statistical, is used to convey the strengths of the results; improvement in the overall group was at least moderate in its effect size (.38), and the improvements in the youth with the lowest EPA levels were big in their effect sizes (>0.8), thus we trust that these are true positive findings.

In conclusion, our study shows some benefits of EPA monotherapy on cognitive symptoms of ADHD. As amply discussed, it is possible that a combined EPA + DHA strategy might have been more beneficial, and as such we support the recent recommendation by a panel of ADHD experts that patients who prefer omega-3 supplements over stimulants should take a combination of DHA and EPA at doses ≥750 mg per day for at least 12 weeks50. However, we additionally recommend that this strategy should be even more strongly advocated for children with evidence of low endogenous PUFAs levels, as indicated by direct measurement, dietary habits or symptoms of EFA deficiency. Conversely, in the cases where high endogenous levels of PUFAs might already be present because of a dedicated diet or previous supplements, PUFAs levels should be investigated before trialling this strategy, to limit any potential negative effects. In this way, we can start bringing the benefits of ‘personalised treatment’ to children with ADHD.