"fruits and vegetables predicted improvements in positive affect the next day, suggesting that healthy foods were driving affective experiences and not vice versa. Meaningful changes in positive affect were observed with the daily consumption of approximately 7-8 servings of fruit or vegetables... Eating fruit and vegetables may promote emotional well-being..."
White, B. A., Horwath, C. C., Conner, T. S. (2013). Many apples a day keep the blues away - Daily experiences of negative and positive affect and food consumption in young adults. Br J Health Psychol