The findings showed that happiness increased incrementally for each extra daily portion of fruit and vegetables up to eight portions per day.
People who changed from almost no fruit and vegetables to eight portions of a day showed an increase in life satisfaction. Usually people's motivation to eat healthy food is weakened by the fact that these were predictive of alterations in happiness and satisfaction later in life.
"However, well-being improvements from increased consumption of fruit and vegetables are closer to immediate," Oswald added.
Large positive psychological benefits were found within two years of an improved diet consisting of more fruit and vegetables, the researchers said. "There is a psychological payoff now from fruit and vegetables -- not just a lower health risk decades later," noted Redzo Mujcic, researcher at University of Queensland in Australia.
The results could be used by health professionals to persuade people to consume more fruit and vegetables, particularly in the developed world where the typical citizen eats an unhealthy diet, said the paper to be published in the American Journal of Public Health. For the study, the team followed food diaries of 12,385 randomly selected people. The authors adjusted the effects on incident changes in happiness and life satisfaction for people's changing incomes and personal circumstances.