Applied nutritional investigationDietary fiber intake and depressive symptoms in Japanese employees: The Furukawa Nutrition and Health Study
Introduction
Depression is a highly prevalent mental disorder in the general population [1]. It is a major public health issue in terms of reduced productivity, lowers quality of life, and increases mortality [1]. Experimental and epidemiologic evidence suggests that dietary factors may play an important role in the development of depression [2], [3], [4], [5], [6]. Recent research suggests that the gut–brain axis, a bidirectional communication between the gut microbiota and the brain, functions as a pathway for the gut microbiota to modulate brain function, possibly through endocrine, immune, and neural pathway [7]; the hypothalamus–pituitary–adrenal axis regulates cortisol secretion and cortisol can affect immune cells, alter gut permeability and barrier function, and change gut microbiota composition [7]. Conversely, the gut microbiota and probiotics can alter the levels of cytokines, which may influence brain function [7]. In addition, the vagus nerve and tryptophan, the precursor of serotonin, are involved in relaying the influence of the gut microbiota to the brain [7]. Diet modification, especially fiber intake from fruits, vegetables, and other plants, can alter the profile of intestinal microbiota [8]. These findings draw attention to the need for further examination of the association between dietary fiber intake and depressive status.
There is a paucity of epidemiologic evidence for the association between dietary fiber intake and depressive symptoms in free-living people. Oishi et al. [9] found an inverse, albeit not statistically significant, association between fiber intake and depressive symptoms among older Japanese adults. Woo et al. [10] reported a statistically significant inverse association between fiber intake and depression score in an older Chinese population. These studies [9], [10], however, did not adjust for potentially important confounders including dietary intake of other nutrients, sleeping habits, and physical activity [2]. In addition, short-chain fatty acids (SCFAs), which are produced by fermentation of dietary fiber by intestinal microbiota, have been suggested to inhibit inflammation [11], a possible mediating factor for depression [12]. The fermentation of fiber by intestinal microbiota varies depending on its sources [13], [14], [15], [16] and types [16], [17]; vegetable and fruit or soluble fiber is more readily fermented than cereal or insoluble fiber [13], [14], [15], [16], [17]. Yet, no study has examined the association of depressive symptoms with sources (vegetables, fruits, and cereals) and types (soluble and insoluble) of fiber intake. To address these issues, we examined the association between dietary fiber intake and depressive symptoms in a Japanese working population.
Section snippets
Study design and participants
Data were derived from the Furukawa Nutrition and Health Study. Details of the study procedure were described elsewhere [6], [18], [19]. A nutritional epidemiologic survey was conducted undergoing periodic health examination in April 2012 (factory A) and May 2013 (factory B) among employees of a manufacturing company and its affiliates in the Kanto region of Japan. Of 2828 health examination attendants (women, 11%), 2162 (1930 men and 232 women aged 18–70 years) agreed to participate in the
Results
Participant characteristics by tertiles of energy-adjusted total fiber intake are presented in Table 1. Compared with participants with low total fiber intake, participants with higher total fiber intake were older and more likely to be women and physically active during leisure time, and drank green tea more frequently, but they were less likely to have job strain and be in low-ranking job positions, night or rotating shift workers, smokers, alcohol drinkers, and engaged in physical activity
Discussion
In this cross-sectional study, we found that higher intake of dietary fiber derived from vegetables and fruits was associated with a lower prevalence of depressive symptoms in Japanese workers, even after adjusting for a wide variety of potentially important dietary (including folate, vitamin B6, vitamin B12, n-3 polyunsaturated fatty acids, magnesium, zinc, coffee, and green tea) and non-dietary confounders. In contrast, dietary intake of total, soluble, insoluble, and cereal fiber was not
Conclusion
Higher intake of dietary fiber derived from vegetables and fruits was associated with significantly lower odds of depressive symptoms in Japanese employees even after adjustment for a variety of potential confounders including depression-related nutrients. The present finding based on cross-sectional observation requires confirmation in prospective studies. Further investigation into gut microbiota composition and SCFAs would contribute to the understanding of mechanisms whereby dietary fiber
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Cited by (0)
T. Mizoue designed the research; M.E., K. Kurotani, T.K., K. Kuwahara, R. I., H.T., I. Kabe, and T. Mizoue conducted research; T. Miki performed statistical analysis, wrote the manuscript, and had primary responsibility for final content; and all authors were involved in revision and approved the final version of the manuscript. None of the authors had any conflicts of interest. M.E., T.K., R. I., H.T., and I. Kabe are health professionals at Furukawa Electric Corporation. The authors declare no conflicts of interest. T.K., M.E., H.T., R. I., and I. Kabe are health professionals at the Furukawa Electric Corporation. We thank Fumiko Zaizen (Furukawa Electric Corporation) and Ayami Kume, Sachiko Nishihara, Yuho Mizoue, Saeko Takagiwa, and Yuriko Yagi (National Center for Global Health and Medicine) for their help in data collection.