This study examined associations between the consumption frequency of plant foods and EWB among 19–39-year-old employees of the City of Helsinki, Finland. The main finding of the study was that more frequent consumption of most of the plant food items was associated with better EWB. Overall, the strongest association was found between the consumption of fresh vegetables and EWB among both women and men. Associations remained after adjusting for socioeconomic circumstances and working conditions, although household income and current economic difficulties attenuated the associations modestly.
Our results are in line with previous studies, conducted among different populations and in varying settings, which have suggested a higher level of fruit and vegetable consumption to contribute to better mental health outcomes. A meta-analysis of eighteen studies found that both fruit and vegetable intake separately as well as combined were associated with a lower risk of depression in cohort and cross-sectional studies [9]. A Canadian longitudinal study showed fruit and vegetable consumption to be inversely associated with later depression and psychological distress in two-year cycles [30]. A recent prospective study among young Australian women found a higher fruit (≥ 4 servings) and vegetable (≥ 5 servings) intake to be associated with lower odds of depressive symptoms in comparison to one serving or less per day [31]. The previous studies have, however, not focused especially on employees but rather examined general adult populations. Thus, our results provide novel insights examining an employee population, that is, a population from which the poorest and least healthy have been left out due to the so-called healthy worker effect [32].
In this study, all of the examined plant foods showed some positive associations with EWB. The strongest associations were found for fresh vegetables among both women and men and also for fruit among women. Compared to many of the earlier studies, which have mostly examined fresh vegetable and fruit consumption only, this study included a slightly wider range of plant foods. We examined the different food items separately–instead of total consumption–to detect possible differences. There are some earlier results on differences between various categories of plant foods. A small survey among young adults from the United States and New Zealand found raw fruit and vegetables to be associated with reduced depressive symptoms and more positive mood, while fruit and vegetables that were cooked or canned did not show such associations [33]. A study on the American NHANES data found that besides total fruit and vegetable intake, the consumption of berries, tomatoes, green vegetables and dried fruit showed inverse associations with depressive symptoms [34]. Wholegrain consumption, which showed modest associations with EWB in this study, has seldom been studied in relation to mental health, but some earlier evidence exists. Among Chinese adults a higher consumption level of wholegrain foods was associated with a lower level of depressive symptoms [35]. A review study suggested that a diet high in fiber from wholegrains among other plant foods could benefit mental health [36]. It should be noted that in our study, information was available concerning wholegrain bread only, and thus, other wholegrain foods, such as porridge or brown rice, were not covered.
This study examined the consumption frequency as categorised into daily versus non-daily consumption. It should be noted that this is a broad measure of frequency and does not include information on the quantity of consumption. Direct comparisons cannot thus be made with the national and international dietary recommendations, but it is likely that even among participants in the “good” category, that is daily use, the consumption is to a large part lower than what is recommended in Finland and internationally [13]. However, studies with different kinds of dietary assessment methods have produced largely parallel results on the associations between dietary factors and mental health. Concerning plant foods, previous studies have found associations with mental health outcomes utilising varying frequency or quantity measures capturing shorter or longer term food consumption. In a Swiss study, a recommendation of “5-a-day” was used and consuming the five portions of vegetables and fruit per day was associated with better mental health indicated by lower psychological distress [37]. An Iranian study converted consumption frequencies reported by participants into a quantity in grams per day, and found the highest consumption level, compared to the lowest, to be associated with lower odds of depression and distress [38]. A Canadian longitudinal study utilised a daily fruit and vegetable consumption score which showed inverse associations with depressive symptoms [30]. Overall, our results confirm the findings of the previous studies, which have, regardless of the exact measurements and methods, shown positive associations with mental health and well-being for even a relatively low level of plant food consumption.
We examined women and men separately due to previous studies having shown sex differences in both plant food consumption and mental health [23, 39, 40]. With regard to the associations between plant food consumption and mental health outcomes, very few studies have previously considered differences between women and men. Among Iranian adults, women but not men with high intake of fruit and vegetables were found to have lower odds of depression [38], whereas among Swiss general population, sex differences were not found for associations between fruit and vegetable consumption and psychological distress [37]. In this study, we found fewer associations to be statistically significant among men than women. This difference could be due to the smaller number of men in the study which likely affects the statistical power. However, the associations between plant food consumption and EWB had the same direction, and the differences in the estimates that were observed between daily and non-daily consumers of the various food items were mostly larger among men than among women.
In this study, adjusting for socioeconomic circumstances, and to a lesser degree also for working conditions, had some attenuating effects on the observed associations between plant food consumption and EWB. Especially, adjustments for household income and current economic difficulties affected the associations. Previous studies examining associations between plant food consumption and mental health have mostly considered only single socioeconomic factors, mainly education, and the findings have been inconsistent [9, 37, 41]. However, many earlier studies have shown that a lower socioeconomic position and more disadvantageous socioeconomic circumstances are associated with a lower level of plant food consumption [14, 16,17,18]. In our recent study, especially lower income and current economic difficulties showed clear associations with belonging to lower long-term fruit and vegetable consumption trajectories among ageing employees [14]. Based on the results of this study, further, more detailed studies on the mechanisms of the associations among different employee groups are clearly warranted.
Overall, in the light of previous evidence, it could be suggested that the results of this study, in their part, confirm the importance of plant food consumption, highlighting the need for intervention studies related to the dietary habits and mental health among employees. Since mental health problems have been shown to be frequent among employees, it would be crucial to develop efficient measures and policies to promote employees’ mental well-being in the long term. Increasing fruit, vegetable and other plant food consumption could have such potential, and different kinds of workplace intervention studies on the ways to increase plant food use have already been conducted internationally [42,43,44]. The potential benefits of an increased plant food consumption are also supported by trial studies which have shown that a dietary pattern with a high level of various plant foods could have positive influence and even therapeutic effects on mental health [11, 45, 46]. However, further intervention studies are warranted in order to obtain information concerning mental health specifically as well as to find cost-effective ways to implement the information among different employee groups and working environments.
In addition to interventions, longitudinal studies with follow-up data over a longer time period would provide the most useful information. Future studies should also consider register-based mental health outcomes in particular, as well as other employee groups including municipal employees from different age groups as well as employees elsewhere in the public and private sectors. Specifically, employees in manual work would be a group in which there could be the largest potential in terms of increasing the overall consumption level of plant foods as a way to promote employee mental health and work ability.
Methodological considerationsThis cross-sectional study was based on survey data among young and midlife municipal employees. According to a non-response analysis, the study population can be considered as representative of the target population, the female and male 19- to 39-year-old employees of the City of Helsinki, Finland [21]. The proportion of female employees was high among the respondents, 78.5%, corresponding to the overall sex distribution among the employees of the City of Helsinki. In general, the results are, with caution, generalisable to young and midlife employees in the municipal sector in Finland, and possibly also elsewhere in the public sector. To some degree, the results might also be generalisable to similar employee populations in other high-income countries, although caution should be applied due to possible cultural differences in, for example, food habits. With regard to the measure of food consumption, the 14-item FFQ, short FFQs have in previous studies been shown to be suitable as measures of frequently consumed foods [47].
The limitations include the characteristics of the survey data and the study population consisting of municipal employees which should be taken into account in the interpretation of the results and when making generalisations. In Finland, a large proportion of the municipal employees are employed in education, social and healthcare sectors, due to which the majority of both the target population, the City of Helsinki employees, and the study participants were women [21]. The lower number of male participants causes restrictions to the statistical power among men in our data. The data also included participants who were not working at the time of the survey due to for example parental or sick leave (N = 512). Therefore, we tested whether adjusting for the current employment status would affect the results and found negligible effects (no data shown). With regard to the EWB measurement, it should be noted that the category of ‘poor well-being’ was based on the lowest quartile of the score and the outcome is thus relative, comparing participants with lower and higher scores. The survey measures in general cause limitations. The FFQ was not validated and it did not include portion sizes or information on whether the consumed items were part of a main meal or snacks, for example. Consumption quantities could thus not be assessed. It has been shown, however, that the contribution of portion size questions in FFQs to the food intake variance may be negligible [48]. In addition, the general cautions warranted when interpreting results obtained with self-reported data, due to, for example, under- or over-reporting, apply to this study. Especially, over-reporting is a possibility with regard to the consumption of plant foods. Also, it should be noted that causality cannot be determined with cross-sectional survey data. Reverse associations are also possible, that is, poorer mental health could influence food habits, leading to a lower level of consumption of plant foods [11, 49].
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