The studies included a total of 101,950 participants age 16 to 72 years old across the USA, Australia, Europe and the Middle East and recorded the presence of depression or depressive symptoms in all participants using self-reports, medical diagnoses and/or antidepressant use.
All participants also completed questionnaires about their diet and were given a score of how inflammatory the diet was.
The findings, published in the journal Clinical Nutrition, showed that in all of the studies, participants who ate a more pro-inflammatory diet, such as those high in cholesterol, saturated fats and carbohydrates, were around 40% more likely to have depression or depressive symptoms.
The results were also consistent in both males and females and across all ages, and over both short and long follow-up periods, with some participants followed for up to 13 years.
“These results have tremendous clinical potential for the treatment of depression, and if it holds true, other diseases such as Alzheimer’s which also have an underlying inflammatory component,” said study author Dr. Steven Bradburn.
“Simply changing what we eat may be a cheaper alternative to pharmacological interventions, which often come with side-effects.”
“It should be stressed, however, that our findings are an association, rather than causality. Further work is needed to confirm the efficacy of modulating dietary patterns in treating depression with relation to inflammation.”
Inflammation occurs when the body releases proteins, antibodies and increased blood-flow to areas affected by infections, injuries, and toxins in an effort to defend and protect itself.
An anti-inflammatory diet contains plenty of fiber, vitamins — especially A, C, D — and unsaturated fats. The popular Mediterranean diet, which includes olive oil, tomatoes, green vegetables and fatty fish, is a good example, with the researchers suggesting that this diet could be followed to potentially lower the risk of depression.
A US study published earlier this year also found that the DASH diet, which is similar to the Mediterranean diet, and is high in fruits, vegetables and whole grains and encourages nuts and beans, lean meats, fish and poultry rather than saturated fats and sugar, may also reduce the risk of depression. Those who adhered to the diet most showed an 11% lower risk than those who adhered the least.
In contrast, those who followed a Western diet most closely, which is high in saturated fats and red meats and low in fruits and vegetables, were more likely to develop depression.