Study population
The JECS protocol has been described elsewhere [15, 16]. In short, the JECS is a nationwide government-funded birth cohort study that aims to determine the associations of various environmental factors with child health and development. JECS participants are women residing in 15 regions of Japan who were enrolled during the first trimester of pregnancy between January 2011 and March 2014 [15, 16]. Follow-ups were conducted during the second or third trimester, at childbirth, and at 1 month postpartum during scheduled in-hospital checkups. Subsequent follow-ups were conducted at 12 and 36 months postpartum by mail.
The present study analyzed the jecs-ta-20190930 dataset released in October 2019, which comprises 104,062 records obtained from a questionnaire-based survey of the participants. We excluded 3,758 cases that resulted in miscarriage or stillbirth and 1,891 cases of multiple births to focus on typical pregnancies (Fig. 1). Additionally, we also excluded 33,790 records because of incomplete responses to the questionnaire and 423 records for children whose sleep duration was recorded as 0, leaving 64,200 questionnaires with all data available for the final analysis.
Ethics approval and consent to participate
The study protocol was approved by the Institutional Review Board on Epidemiological Studies of the Japanese Ministry of the Environment (authorization number: 100910001) and the ethics committees of all participating institutions. The JECS is conducted in accordance with the Declaration of Helsinki and all other national regulations, and written informed consent was obtained from parents/guardians of the participants whose age was below 16.
Data assessment
Exposure
Dietary intake of fermented foods during pregnancy (from the discovery of pregnancy to the second or third trimester) was assessed using a food frequency questionnaire (FFQ) [17]. Fermented foods were foods such as cheese and yogurt, the preparation of which involves fermentation of food ingredients by microorganisms. This FFQ is a semi-quantitative instrument that assesses the average consumption of 171 food and beverage items. The FFQ includes four fermented foods: miso soup (made with miso, a Japanese traditional fermented seasoning), yogurt, cheese, and natto (Japanese fermented soybeans). The FFQ has not been validated specifically for pregnant women but has been validated in a large epidemiological study of adults in the general population and has already been used in a number of the JECS studies [18,19,20]. In this FFQ, participants were asked how often they consumed each food type and how much of it they consumed from learning of the pregnancy to the present. For miso soup, six frequency categories were used to record overall consumption frequency (from almost never to every day), nine frequency categories were used to record the daily consumption frequency (from < 1 time to ≥ 10 times), and five categories were used to report the taste of the miso soup (from very bland to very strong), which was taken to indicate the amount of miso in the soup. The daily intake (g/day) of miso was then calculated by multiplying the overall consumption frequency by the daily consumption frequency by a factor based on the reported taste. For the other three fermented foods—yogurt, cheese, and natto—the standard portion size for each food type was categorized as small (50% smaller than standard), medium (same as standard), or large (50% larger than standard). Nine frequency categories for each item were used to record consumption frequency (< 1 time/month to ≥ 7 times/day).
The daily intake of each of these three fermented foods was calculated by multiplying the consumption frequency by the standard portion size. Then, participants were categorized by quartile of intake amount (g/day) for each of the four fermented foods.
Outcome
To measure child sleep duration at 3 years after childbirth, parents were instructed to indicate when their child slept on the previous day. Parents marked the times when their child was asleep by drawing lines through boxes, indicating 30-min intervals, for the 24-h period beginning from 12:00 am at the start of the previous day.
Sleep duration of 10–13 h in a 24-h period is recommended for 3-year-old children by the United States National Sleep Foundation [21]. Therefore, we selected 10 h as the lower limit of the appropriate sleep duration and defined children sleeping less than this amount as having sleep deprivation.
Covariates
The covariates adjusted for were energy intake during pregnancy as assessed using the FFQ [17], maternal age during pregnancy, previous childbirth, body mass index (BMI) at 1 month after childbirth, maternal education level, annual household income during pregnancy, marital status at 6 months after childbirth, alcohol intake at 1 month after childbirth, smoking status at 1 month after childbirth, employment status at 1 year after childbirth, sex of the child, child attendance at nursery at 1 year after childbirth, the location where the child slept at night at 1 year after childbirth, birth weight, gestational age, consumption of dairy products at 3 years after childbirth, presence of any disease up to 3 years after childbirth, and date (month) of birth. These variables were categorized as in our previous study [13].
Statistical analyses
Unless otherwise stated, data are expressed as the mean ± standard deviation or median. Odds ratios (ORs) and 95% confidence intervals (95% CIs) for the risk of sleep deprivation according to each fermented food intake were calculated using logistic regression analysis, with each lowest quartile used as a reference. Adjusted ORs were calculated using all of the covariates described in the previous section, whereas crude ORs were calculated without adjustment for any covariates. In trend tests, categorical numbers were assigned to the quartile distributions for each fermented food intake and were treated as continuous variables. A two-sided p-value of < 0.05 was regarded as statistically significant. Analyses were performed using SAS version 9.4 (SAS Institute Inc., Cary, NC).
Additional analysis
To determine the association between overall fermented food intake during pregnancy and the sleep of their children at 3 years of age, we calculated the total score for quartiles of each of miso, yogurt, cheese, and natto during pregnancy, where the first quartile counted as 1 point, the second quartile as 2 points, and so forth. Thus, the score for the overall intake of fermented foods ranged from 4 to 16 points. The total score was also further categorized into quartiles. Analysis was likewise calculated using logistic regression analysis to obtain ORs and 95% CIs, setting the lowest quartile as the reference group.