Specifically, they were asked to report the number of servings eaten of fruit (excluding fruit juice and dried fruit), vegetables (excluding juices), and several categories of unhealthy foods like biscuits/cookies, potato chips and cakes and muffins.
Researcher Caroline Horwath said the results showed a strong day-to-day relationship between more positive mood and higher fruit and vegetable consumption, but not other foods.
"On days when people ate more fruits and vegetables, they reported feeling calmer, happier and more energetic than they normally did.''
Dr Conner and her team ran additional analyses and found that eating fruits and vegetables predicted improvements in positive mood the next day, suggesting that healthy foods might improve mood.
These findings held regardless of the body mass index of the individuals.
"After further analysis we demonstrated that young people would need to consume approximately seven to eight total servings of fruits and vegetables per day to notice a meaningful positive change.
"One serving of fruit or vegetables is approximately the size that could fit in your palm, or half a cup.''
Dr Conner said to get the amount needed to increase positivity, people should make half their plate at each meal vegetables and snack on whole fruit, like apples, during the day.
While this research showed a promising connection between healthy foods and healthy moods, further research was necessary and the researchers recommend the development of random control trials evaluating the influence of high fruit and vegetable intake on mood and wellbeing.