Researchers in the U.K. used data from a longitudinal study to assess the relationship between fruit and vegetable intake and mental well-being. The result? “We find that well-being rises in an approximately dose-response way with both the number of portions of fruits and vegetables consumed, and the number of days in a given week an individual consumes either fruits or vegetables.”

The study, titled “Lettuce be happy: A longitudinal UK study on the relationship between fruit and vegetable consumption and well-being,” by Neel Ocean, Peter Howley, and Jonathan Ensor, was published inSocial Science & Medicine. The researchers used data collected repeatedly from the same individuals between 2010 and 2017 in the U.K. Household Longitudinal Survey. The use of data from such a large study (over 45,000 individuals) over such a long period of time allowed the researchers to establish causality in a way that a shorter or smaller study never could.

The paper compared the effects of eating vegetables and fruits against other known effects—for instance, marriage—and found that if an individual who consumed vegetables daily stopped consuming them altogether, they would suffer a greater estimated loss in mental well-being than becoming widowed, or approximately 57% of the loss of someone who went from being employed to being unemployed. On the other hand, upping consumption from never to 4-6 days per week generates approximately the same increase in life satisfaction as being married.

The researchers repeatedly emphasized that it is necessary to eat fruits and vegetables both in large quantities and often. Eating nothing but fruits and vegetables for one day a week and only one day a week, therefore, is not as useful as eating a serving of fruits or vegetables with one or two meals per day, every day.

The researchers suggested that this information may help people adhere more closely to ‘five-a-day’ guidelines: “Positive effects for mental well-being accrue relatively rapidly, whereas the benefits in terms of physical health accrue in the medium to long-term.” Emphasizing the short-term results of healthy eating can help people start and stick to a healthy lifestyle.

The researchers acknowledged that the relationship between fruit and vegetable intake and mental well-being could be reversed: That is, that mentally healthy people are more likely to eat fruits and vegetables. They suggested that additional research using longitudinal data is still necessary, as is supplementation with randomized-control trials and quasi-experimental methods designed to explore the impact of different kinds of diets on well-being.