Using data from twin studies conducted over many years, researchers have found a link between levels of depression and the amount of fruit and vegetables people eat each day.
Twin studies are powerful tools for untangling the roles that genetics and environment play in various traits, behaviors and conditions. Because identical twins share essentially 100% of their genes and fraternal twins share, on average, 50%, most of the differences between them are due to the experiences one twin had but not the other.
In a new international study led by the University of New South Wales (UNSW) Sydney’s Center for Healthy Brain Aging (CHeBA), researchers examined the association between fruit and vegetable intake and depression symptoms in twins over the age of 45.
“Twin pairs share 50-100% of their genetic background and, when raised together, share the same family environment,” said Dr Karen Mather, leader of CHeBA’s Genomics and Epigenomics Group and the study’s senior author. “One of the advantages of the twin design is that it can help address the issue of unwanted factors, such as socioeconomic status early in life, influencing the results.”
‘Eat more fruit and vegetables!’ It has been the catchcry of parents for decades. It’s something that has been confirmed by science, unlike more dubious parental claims, such as that eating bread crusts will make your hair curly. While evidence is growing about the effect that diet has on mental health, including depression, few studies have focused specifically on how fruit and veggie intake affects depression in over-45s. Fewer still have availed themselves of the unique opportunity that twin studies offer to reduce the influence of genetic and environmental confounding variables.
So, for the present study, the researchers examined the association between baseline fruit and vegetable intake and depressive symptoms in 3,483 twins aged over 45. They collected data from four longitudinal twin studies that ran for up to 11 years, undertaken in Australia, Denmark, Sweden, and the US.
Self-reported fruit and vegetable intake was collected and categorized into ‘low,’ ‘moderate,’ and ‘high’ for both fruit and veggies. For fruit, low intake was, on average, 0.3 serves a day, and high was 2.1 serves a day. For vegetables, low intake was 0.5 serves a day and high was 2.0. Participants’ depressive symptoms were measured at baseline and followed up using validated measures of assessing depression.
A high intake of both fruit and vegetables was associated with lower depressive symptoms over time compared to a low intake. A moderate intake of veggies, but not fruit, was also associated with lower depressive symptoms versus low intakes.
“The findings present another argument for increasing fruit and vegetable intake in adults over 45 years of age,” said Dr Annabel Matison, a postdoctoral research fellow and lead author of the study.
The researchers did note, however, that ‘high’ fruit and vegetable intake still fell well below the dietary recommendations of most countries. It made them wonder what sort of results they’d have seen if people were eating at least the daily recommended intake.
“We found that fruit and vegetable consumption in the two large Scandinavian studies were particularly low, with the average for both being less than half the World Health Organization recommended intake of at least five serves per day,” Matison said. “We are unclear what the reduction in depression scores would be if intakes were increased to recommended levels.”
The researchers suggest that the effect that fruit and vegetables have on depression is at least partly linked to the microbiome, which has become the focus of a lot of recent research.
“The importance of the gut microbiome and its potential influence on depression as a result of inflammation, both systemic and neuroinflammation, is becoming increasingly well understood,” said Matison.
The study had limitations. The researchers were unable to adjust for physical activity, as the data was not available across all studies. Another was that the studies used different self-report questionnaires to record dietary intake. Yet another was that depressive symptoms were self-reported, whereas clinical diagnosis is the gold standard for depression assessment. Against this is the strength, particularly, of the twin design, which reduced the influence of a range of potential confounders such as socioeconomic status, physical activity and chronic health conditions.
Future studies should address these limitations by using comprehensive, standardized tools to collect data about fruit and vegetable intake, in addition to clinical assessment for depressive symptoms.
The researchers say that, based on their findings, healthcare providers should consider monitoring the fruit and vegetable intake of their patients, particular those with depressive symptoms.
The study was published in the journal Scientific Reports.
Source: CHeBA