The double-blind, randomised trial found that a high dose of omega-3 fatty acid EPA (or eicosapentaenoic acid) might work better than a placebo, but only for kids who have ADHD and an omega-3 deficiency.
"The omega-3 supplements only worked in children that had lower levels of EPA in their blood, as if the intervention was replenishing a lack of this important nutrient," explains biological psychiatrist Carmine Pariante from King's College, London.
Fish oil supplements have been explored as a treatment for ADHD in the past - with mixed results. So much so, that they are not currently approved by the US Food and Drug Administration (FDA) as a way to manage ADHD.
ADHD is a disorder marked by ongoing inattention, hyperactivity or impulsivity. Such behaviour is thought to reflect differences in brain development and activity, and fish oil has often been linked to improved cognitive function.
Furthermore, children with ADHD are sometimes known to have insufficient fatty acid levels, presenting with dry skin, eczema and dry eyes. Recent studies on animals have also connected omega-3 fatty acids in the brain's frontal cortex with hyperactivity.
Fish oil is one of the most popular sources of polyunsaturated fatty acids, and both EPA and docosahexaenoic acid (DHA) appear to be lower than normal in children with ADHD.
But while some studies have noted improvements in ADHD symptoms when this supplement was taken, others have not. In fact, some researchers suspect it might even make things worse. Differences in dosages and the ratio of fatty acids only makes comparing these results all the more difficult.
In a 2013 systematic review of 10 randomised, placebo-controlled trials, only two showed a statistically significant benefit; while 6 showed no benefit at all, and 2 more showed benefits on only some ADHD rating scales.
A later review on fish oil pills in 2017 showed greater promise. Of the 16 studies examined, 13 showed modest benefits in ADHD symptoms, including hyperactivity, impulsivity, attention, visual learning, word reading, and memory.
Yet even in cases where results appear positive, the effects are often modest and can sometimes take high dosages for months on end to kick in.
The latest research took place over a mere 12 weeks, and involved 92 children with ADHD aged 6 to 18. After being given a small daily dose of EPA, the participants with omega-3 deficiencies were found to have a few mild cognitive benefits, while those who already had normal levels of the fatty acid showed no benefits whatsoever.
"On the other hand, it is possible to have too much of a good thing," says child psychiatrist Jane Chang from China Medical University. For those kids with high levels of omega-3 already, any additional dosage may actually have detrimental effects, and Chang urges parents to consult with their child's physician before they try anything new - even a supplement.
Meanwhile, a review in 2018 warned that health care providers should not reinforce the idea that fatty acids can replace treatments that have far more robust evidence.
The authors argue that while fish oil supplements may have small beneficial effects on children with ADHD, further high-quality research is needed before we can suggest them as an additional treatment - let alone a substitute for more serious approaches.
The study was published in Translational Psychiatry.
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