"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."
While the study looked at young adults, Dr Conner said the results would probably be the same for all ages, although more research would need to be conducted.
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.
Greengrocer Hari Sukhalal, from Aro Fruit Supply in Wellington, has been selling fruit and vegetables for more than 30 years, and said the study, which has been published in the British Journal of Health and Psychology, did not surprise him.
He had seen a shift in people's buying patterns and they seemed more positive in their outlook.
"People want to be healthy and being healthy makes them happier," he said.
Clinical psychologist Debbie Newlove said people who bought fruit and vegetables were often more healthy and could have a more positive outlook.
"It could be related to a whole range of factors, not just fruit and veges, but it could be right."
Some bosses have already worked out the benefits of supplying free fruit to their staff.
Employers and Manufacturers Association chief executive Kim Campbell said that people were generally unhappy if they felt sick, and eating healthier foods helped ward off illness.
"Giving your staff some, of course it will make them happy."
Healthy fare keeps worker from nibbling on junk
Caroline Wells takes full advantage of the free fruit offered to staff at Wellington intellectual property firm AJ Park.
"There's usually quite a good range. Lately I've been loving the nectarines and plums."
She works in the marketing team and said she would be upset if the programme were to end.
"Snacking on fruit makes me feel happier and healthier throughout the day because then it means that I'm not snacking on junk, which otherwise I might be."
The company's human resources manager, Sue Quinnell, said it had provided fresh fruit for three or four years. "We know people like it, because every day the fruit baskets are empty and yet the chocolate biscuit jars aren't.
"A quick poll revealed that our younger people are happier having fresh fruit," she said. "So people definitely enjoy the fruit and are making healthy eating choices."
The study
*281 participants with mean age of 20 completed a daily food diary for 21 days.
*Those with a history of an eating disorder were excluded.
*Participants rated how they felt using nine positive and nine negative adjectives.
*They reported the number of servings eaten of fruit (excluding fruit juice and dried fruit), vegetables (excluding juices), and several categories of unhealthy foods such as biscuits/cookies, potato chips and cakes and muffins.
