This retrospective study of a Japanese general population investigated the association between unhealthy dietary habits and proteinuria onset. We found that late dinner and skipping breakfast were associated with higher risks for proteinuria onset. This result was significant even after adjusting for potential confounders, such as BMI and WHtR.
There are a variety of definitions of eating habits, which must be considered when comparing them with previous studies. For example, the participants in our study showed lower proportions of late dinner and skipping breakfast than those in previous studies. A study of South Asian Canadians defined late dinners as after 8 p.m., which was found in 37% of the subjects [20]. In another study of Americans, skipping breakfast was defined as when the meal was taken and the proportions were 17% for men and 24% for women [21, 22]. Not only the differences in the questionnaire, the higher mean age in our study might also have resulted in the lower proportions of unhealthy dietary habits than those in other studies.
Although lifestyle is an important factor in reducing residual risk, compared with established interventions such as antihypertensive therapy and smoking cessation, each lifestyle is characterized by a close relationship between them. An observational study conducted by Katsuma et al. reported that late dinner and skipping breakfast might have interaction for risks of proteinuria [10]. In our study, a status of having both late dinner and skipping breakfast habits showed no increased risk for proteinuria. The reason for this difference is unclear, but it might be due to the different questionnaire or the definitions of unhealthy dietary habits.
Dietary habits, such as late dinner and skipping breakfast, were reported to be associated with obesity or increased waist circumference, which are possible intermediate factors of proteinuria onset. For instance, an observational study conducted by Sakurai et al. reported that skipping breakfast was closely associated with annual changes in BMI and waist circumference among men [23]. In another longitudinal study, late dinner was associated with obesity [24]. It was uncertain why BMI and WHtR were not increased in people with unhealthy dietary habits in our study; however, it implied that they might not be intermediate factors in our study setting. The possibility that weight loss may not serve as an appropriate surrogate for the reduction in the risk of CKD is a point of caution when providing dietary guidance to high-risk individuals.
In this study, we confirmed the importance of these dietary habits and proteinuria onset in a large-scale Japanese general population. Previous studies have reported about dietary habits and risks for kidney injury. A cross-sectional study conducted by Fujibayashi et al. reported that eating irregular meals was associated with 1.40 times higher risks for proteinuria [25]. Another observational study reported that people having two unhealthy lifestyles have 2.04 times higher risks for CKD [26]. The effect sizes associated with dietary habits in our study were slightly small compared to those associated with dietary habits in the previous studies, but they are important for contributing to the reduction of residual risk.
In our study setting, the precise mechanism for the development of proteinuria was uncertain, but unobserved hypertension may be the possible cause. For instance, physiological suppression of cortisol at night is less observed after late dinner [27], and nocturnal cortisol elevation has been reported to be associated with morning hypertension [28]. These mechanisms might be involved in the relationship between late dinner and proteinuria onset. With regard to skipping breakfast and hypertension, Witbracht et al. reported that skipping breakfast can increase blood pressure due to hunger stress [29]. Conversely, Ahuja et al. reported that eating breakfast can reduce the rise in blood pressure [30]. A study of observing night-time blood pressure and blood pressure in the presence or absence of breakfast might be useful for examining the relationship between late dinner, skipping breakfast, and proteinuria onset.
Our study has several limitations. First, we could not include the changes in unhealthy dietary habits during the follow-up period. Second, nutrient intake and nutrient balance that were not in the questionnaire could not be assessed. Third, it is a well-known fact that renin–angiotensin system inhibitors suppress proteinuria [31], but we were unable to include classes of antihypertensive agents in the model. Fourth, the subjects might have received nutritional guidance during the study period, which was not recorded in our study.