An apple a day does more than keep the doctor away, it may also keep the blues at bay, according to new research.

A team of psychologists at the University of Otago in New Zealand daily servings of fruit and vegetables also appeared to make younger people calmer, happier and more energetic.

In the latest study, published in the journal British Journal of Health Psychology, researchers recruited a total of 281 participants with an average age of 20 years old.   Researchers asked participants to complete an online-based food diary for 21 days.

Participants were also asked to rate how they felt using nine positive and nine negative adjectives for each day of the 21-day trial period.

At the end of the 21 days, the study results showed that participants who ate plenty of fruit and vegetables reported feeling calmer, happier and more energetic.

Researchers said that the findings showed a strong day-to-day relationship between more positive mood and higher fruit and vegetable consumption. However, researchers did not find this association with other foods.

"On days when people ate more fruits and vegetables, they reported feeling calmer, happier and more energetic than they normally did," researcher Dr. Tamlin Conner said in a statement.

Upon further analysis, the study found that eating fruit and vegetables predicted improvements in mood the next day, regardless of the body mass index of the participants in the study.

However, researchers noted that people would need to eat about seven to eight total servings of fruits and vegetables a day to notice "a meaningful positive change" in their mood.

"One serving of fruit or vegetables is approximately the size that could fit in your palm, or half a cup," Connor said.  "My co-author Bonnie White suggests that this can be done by making half your plate at each meal vegetables and snacking on whole fruit like apples."

Researchers note that while the latest study shows a "promising connection" between healthy foods and positive moods, additional studies are needed to confirm the influence of high fruit and vegetable consumption on mood and well-being.