Should people reach for a carrot rather than a candy bar to help protect against depression? Does a diet rich in fruits and vegetables actually make you happier? These are questions I ponder when not writing guest blogs for my friends.
The answer to these questions seems to be a growing yes.
There is compelling evidence that a healthier diet protects against depression. Multiple studies published in the last decade – mainly in nutrition journals – have found reduced risk for depression among people with a higher intake of fruits and vegetables. This link holds when controlling for socioeconomic status, education, physical activity, smoking, unhealthy food consumption, and pretty much any other variable you can think of that might explain this pattern. And, the evidence suggests that poor dietary intake precedes the onset of depression and not the other way around.
My research adds another flavor to this unfolding story. For the last six years, I have been tracking people’s daily intake of fruit and vegetables to see how consumption is related to changes in daily emotion. In each of these studies, I have found a striking and strong association between daily fruit and vegetable consumption and a variety of positive emotions like happiness. In my first study, I asked 281 young adults ages 18 – 25 to report their mood each day for three weeks, and to report the number of servings eaten that day of fruits and vegetables (canned, frozen or fresh, but not fried and not juices), and several types of unhealthy foods like cookies, potato chips, and desserts. I found that on days when people ate more fruits and vegetables, they reported feeling much happier. Unhealthy foods like chips and desserts had little to no association with mood. Also, by tracking people on a daily basis across those three weeks, I was able to address which came first – eating fruit and vegetables or feeling happy. I found that eating more fruit and vegetables predicted improvements in happiness the next day – but happiness did not predict eating more fruits and vegetables the next day – suggesting that diet preceded changes in happiness and not the other way around.
Recently, I published another study that replicated and extended this finding by showing that fruit and vegetable consumption predicted other positive emotional states even more strongly than happiness—states like how engaged and inspired people felt that day, how interested and curious they were in their environment, and, how creative they felt. In fact, these patterns were almost twice as strong as the patterns found with happiness. What is going on here? There may be a connection between fruit and vegetables and the motivation or drive to engage in daily life – known in scientific parlance as approach motivation.
There are biologically plausible pathways for why fruit and vegetables could promote engagement and drive. Fruit and vegetables contain a range of vitamins and minerals including vitamin C, folate, vitamin B-6, iron, and selenium. Vitamin C might be a key pathway here. It is an important co-factor in the production of dopamine, which is critical to motivational drive. A recent study found increased vitamin C levels in the blood following kiwifruit consumption with corresponding improvements in emotional vitality. B-vitamins and complex carbohydrates in fruit and vegetables also promote the synthesis of dopamine and serotonin.
The absence of approach motivation – called motivational anhedonia – is one of the key features of depression. This raises the possibility that a sustained lowered intake of fruit and vegetables could contribute to motivational anhedonia, which could raise the risk for depression. However, in my research studies of young adults, lower fruit and vegetable consumption related to lower approach motivation but not to higher depression.
What are the next steps? Intervention research. Before getting too excited about these findings, intervention studies are needed to test for causal effects of fruit and vegetables on approach motivational states. This requires getting people to eat more fruits and veggies, testing the consequences for psychological outcomes, and measuring the potential biochemical pathways that could account for such changes.
So, at this stage, I cannot say that eating carrots will make you more creative or that fruit will help you flourish, but the evidentiary base for this is growing. My best advice is to hedge your bet and eat at least five servings per day (two fruit and three or more veggies). You can do this by including fruit and vegetables at each meal, and the servings will add up across the day. Bottom line? Opt for carrots rather than candy.