 Thousands of papers have been published on the important topic of what determines people's happiness and psychological health, but what about the potential influence of the different kinds of foods that people eat? The rising prevalence of mental ill health is causing a considerable burden, and so inexpensive and effective strategies are required to improve the psychological well-being of our population. And now we have a growing body of literature suggesting that dietary intake may have the potential to influence psychological well-being. Dietary intake of what? Well, given the strong evidence base for the health benefits of fruits and vegetables, researchers started there. Cross-sectional studies from all over the world support this relationship between happiness and fruit and vegetable intake. Those eating fruits and vegetables each day at a higher likelihood of being classified as very happy, suggesting a strong and positive correlation between fruit and vegetable consumption and happiness, perhaps feelings of optimism, too. The largest such study was done in Great Britain, where a dose-response relationship was found between daily servings of fruits and vegetables, and both life satisfaction and happiness, meaning more fruits and veggies meant more happiness. People who got up to seven or eight servings a day reported the highest life satisfaction and happiness. And these associations remain significant even after controlling for factors such as income, and illness, and exercise, smoking, and body weight, suggesting fruit and vegetable consumption wasn't just acting as a marker for other healthy behaviors. But how could eating plants improve happiness on their own? Well, many fruits and veggies contain higher levels of vitamin C, which is a cofactor in the production of dopamine, the zest-for-life neurotransmitter, and the antioxidants reduce inflammation, which may lead to higher levels of eudaimonic well-being. Aristotle's notion of eudaimonia described as the highest of all human goods, the realization of one's true potential, which was the aim of this study. They wanted to know whether eating fruits and vegetables was associated with other markers of well-being beyond happiness and life satisfaction, like greater eudaimonic well-being, a state of flourishing characterized by feelings of engagement, meaning, and purpose in life. So a sample of about 400 young adults followed for about two weeks, and indeed young adults who ate more fruits and veggies reported higher average eudaimonic well-being, more intense feelings of curiosity, greater creativity, and they could follow this on a day-by-day basis, greater well-being on the days they ate healthier. These findings suggest that food and vegetable intake is related to other aspects of human flourishing beyond just feeling happy. Not so fast, though. Instead of eating good food leading to a good mood, maybe the good mood led to eating good food. Experimentally, if you put people on a good mood, they rate healthy foods like apples higher than indulgent foods like candy bars. Given a choice between M&M's and grapes, individuals in a positive mood were more likely to choose the grapes. The results of these studies led support to a growing body of research that suggests that positive mood facilitates resistance to temptation. Who needs comfort foods when you're already comforted? It's like which came first, the stricken or the egg? Yes, eating eggs may increase our likelihood of chronic disease, but maybe chronic disease also increases our likelihood of eating unhealthy foods. Which came first, the mood or the food? What we need is a study like this, but instead of looking at well-being and diet on the same day, you see if there's a correlation between what you eat today and how you feel tomorrow. But we didn't have a study like that until now. They found the same strong relationships between daily positive mood and fruit and vegetable consumption, but lagged analyses showed that fruit and vegetable consumption predicted improvements in positive mood the next day, not vice versa. On days when people ate more fruits and vegetables, they reported feeling calmer, happier, more energetic than they normally do, and they also felt more positive the next day. So eating fruits and vegetables really may promote emotional well-being. Look, single bouts of exercise can elevate one's mood. Why not the same thing with healthy food? How many fruits and vegetables? Seems we need to consume approximately 7.2 daily servings of fruit, or 8.2 servings of vegetables to notice a meaningful change.