Dr Joseph Firth, an Honorary Research fellow at The University of Manchester and Research Fellow at NICM Health Research Institute at Western Sydney University, says existing research has been unable to definitively establish if dietary improvement could benefit mental health.
But in a new study published in Psychosomatic Medicine, Dr Firth and colleagues brought together all existing data from clinical trials of diets for mental health conditions.
And the study provides convincing evidence that dietary improvement significantly reduces symptoms of depression, even in people without diagnosed depressive disorders.
Dr Firth said: "The overall evidence for the effects of diet on mood and mental well-being had up to now yet to be assessed.
"But our recent meta-analysis has done just that; showing that adopting a healthier diet can boost peoples' mood. However, it has no clear effects on anxiety."
The study combined data from 16 randomised controlled trials that examined the effects of dietary interventions on symptoms of depression and anxiety.
Sixteen eligible trials with outcome data for 45,826 participants were included; the majority of which examined samples with non-clinical depression.
The study found that all types of dietary improvement appeared to have equal effects on mental health, with weight-loss, fat reduction or nutrient-improving diets all having similar benefits for depressive symptoms.
"This is actually good news" said Dr Firth; "The similar effects from any type of dietary improvement suggests that highly-specific or specialised diets are unnecessary for the average individual.
"Instead, just making simple changes is equally beneficial for mental health. In particular, eating more nutrient-dense meals which are high in fibre and vegetables, while cutting back on fast-foods and refined sugars appears to be sufficient for avoiding the potentially negative psychological effects of a 'junk food' diet.
Dr Brendon Stubbs, co-author of the study and Clinical Lecturer at the NIHR Maudsley Biomedical Research Centre and King's College London, added: "Our data add to the growing evidence to support lifestyle interventions as an important approach to tackle low mood and depression.
"Specifically, our results within this study found that when dietary interventions were combined with exercise, a greater improvement in depressive symptoms was experienced by people. Taken together, our data really highlight the central role of eating a healthier diet and taking regular exercise to act as a viable treatment to help people with low mood."
Studies examined with female samples showed even greater benefits from dietary interventions for symptoms of both depression and anxiety.
Dr Firth added: "We're not yet sure why not know why some of our data showed significantly greater benefits from diets for women.
"So more research is needed on this. And we also need to establish how the benefits of a healthy diet are related to improvements in physical health
"It could be through reducing obesity, inflammation, or fatigue -- all of which are linked to diet and impact upon mental health.
"And further research is still required to examine the effects of dietary interventions in people with clinically-diagnosed psychiatric conditions."
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