Eating more fruits and vegetables rich in micronutrients has been associated with better mental health, but a new study suggests that eating them raw is best for mood.
"Our research has highlighted that the consumption of fruit and vegetables in their 'unmodified' state is more strongly associated with better mental health compared to cooked/canned/processed fruit and vegetables," lead author Tamlin Conner, PhD, Department of Psychology, University of Otago, Dunedin, New Zealand, said in a statement.
"This research is increasingly vital, as lifestyle approaches such as dietary change may provide an accessible, safe, and adjuvant approach to improving mental health," she said.
The study was published online April 10 in Frontiers in Psychology.
To investigate whether raw fruit and vegetable intake (FVI) is more strongly associated with a range of mental health outcomes than processed FVI, the researchers conducted a cross-sectional survey of 422 adults aged 18 to 25 years living in New Zealand and the United States (66% women, 67% white).
"We contrasted consumption of raw fruit and raw vegetables with relatively more processed forms of these foods (cooked, frozen, canned or tinned, as a group) in order to investigate the benefits of FV in an unmodified state (raw), compared to FV that has undergone a level of processing that may cause changes to the nutrient quality and quantity," they write.
Six aspects of mental health were measured to capture both negative and positive aspects of the illness/wellness continuum: depressive symptoms, anxiety, negative mood, positive mood, life satisfaction, and flourishing.
Raw FVI had the strongest positive associations with most of the mental health measures. After controlling for demographic and health covariates, raw FVI was associated with fewer depressive symptoms and higher positive mood, life satisfaction, and flourishing. Intake of raw fruits was also associated with reduced negative mood. By contrast, processed FVI was only positively associated with mood; it was not positively associated with any of the other mental health variables.
The top 10 raw foods related to better mental health were carrots; bananas; apples; dark leafy greens, such as spinach; grapefruit; lettuce; citrus fruits; fresh berries; cucumber; and kiwifruit.
Cooking and processing fruits and vegetables "likely limits the delivery of nutrients that are essential for optimal emotional functioning," said Conner.
Limitations and Caveats
"Although this was only a correlational design, the patterns were robust when controlling for demographic and health covariates, and they paralleled the findings of recent intervention research showing significant effects of increasing fresh FVI on mental well-being," the researchers write.
However, given the study's correlational design, "we cannot be sure that food consumption is directly and causally driving improvements in mental health," they point out. These preliminary results suggest that more controlled experimental research, that would investigate directionality, is warranted, they add.
Two researchers from the Food and Mood Center, Deakin University, Victoria, Australia, who reviewed the study for Medical News, said the study "confirms what is widely acknowledged within the literature. Higher vegetable and fruit consumption is positively associated with mental and other health outcomes. However, this is the first study to compare raw vs processed fruit and vegetable intake and mental health."
However, Meghan Hockey, PhD candidate, and Anu Ruusunen, PhD, RD, cautioned that use of a nonvalidated food recall measure "may limit the validity of results within this cross-sectional study."
Another limitation, acknowledged by the authors, is that fruits and vegetables that were prepared using various cooking methods were grouped together. "This may be erroneous, as steaming has been proven to retain greater nutrients compared to boiling vegetables," Hockey and Ruusunen wrote in an email to Medical News.
"Further, the beneficial effects of nutrients in vegetables may be negated by methods such as deep frying. This should be taken in consideration when interpreting results," they said.
Also, by not using comprehensive methods to assess diet, the study was unable to control for confounding effects of overall diet quality. "It may be argued that greater intakes of processed fruits and vegetables are proxies for unhealthier dietary patterns and indicative of greater intakes of processed foods. This may explain poorer mental health outcomes observed in those who consumed predominantly processed fruits and vegetables," they point out.
Despite these limitations and caveats, the results are "suggestive that raw fruits and vegetables are superior to processed fruits and vegetables for mental health outcomes," Hockey and Ruusunen said.
It is worth noting, they add, that processed fruits and vegetables "may be more accessible, affordable, and practical for some individuals. Given the low consumption rates of vegetables within the population, it is important to promote the message that consuming fruits and vegetables, raw or processed, is superior to consuming none. Considerations should be given to feasibility, accessibility, and affordability of fruit and vegetables when making these recommendations."
The study was funded by the Department of Psychology at the University of Otago. The authors, Meghan Hockey, and Dr Ruusunen have disclosed no relevant financial relationships.
Front Psychol. Published online April 10, 2018. Full text