A cross-sectional study examined the eating habits of 80,000 people in Britain finding happiness and mental health are highest among those eating 7 portions of fruit and vegetables/day. Only a tenth of the population achieve this, whilst a quarter eat just one or no portions/day.
David G. Blanchflower, Andrew J. Oswald, Sarah Stewart-Brown. Is Psychological Well-being Linked to the Consumption of Fruit and Vegetables? Social Indicators Research, 2012.