The Keio Twin Research Center (KoTReC) was established in 2009 at Keio University to combine two longitudinal cohort projects — the Keio Twin Study (KTS) for adolescence and adulthood and the Tokyo Twin Cohort Project (ToTCoP) for infancy and childhood. KoTReC also conducted a two-time panel study of self-control and psychopathology in twin adolescence in 2012 and 2013 and three independent anonymous cross-sectional twin surveys (ToTcross) before 2012 — the ToTCross, the Junior and Senior High School Survey and the High School Survey. This article introduces the recent research designs of KoTReC and its publications.

The aim of this study was to estimate the associations of the first occurrence of pathogen-specific clinical mastitis (CM) with milk yield and milk composition (somatic cell count (SCC), lactose, fat, protein content in milk and milk urea nitrogen (MUN)). We studied 3149 dairy cows in 31 Hokkaido dairy farms in Japan. Five pathogen groups were studied: Streptococcus spp.; Staphylococcus aureus (S. aureus); coagulase-negative staphylococci (CNS); coliforms; and fungi. Test-day milk data and clinical records were collected from June 2011 until February 2014. Mixed models with an autoregressive correlation structure were fitted to quantify the effects of CM and several other control variables (herd, calving season, parity, week of lactation, and other diseases). Primipara (first lactation) and multipara (second and later lactations) were analysed separately. All pathogens, particularly S. aureus and fungi, were associated with significant milk losses in multipara. In this study, S. aureus and CNS infections were not associated with significant milk loss in primipara. All pathogens, in particular S. aureus and fungi, significantly increased SCC in both parity groups. All pathogens, especially CNS (in primipara) and S. aureus (in multipara), decreased lactose content. All pathogen groups except for fungi were associated with significant changes in fat, protein and MUN. Some pathogens such as Streptococcus spp. and coliforms seemed to be associated with long-term fat, protein and MUN changes. These findings provide estimates that could be used to calculate precise costs of CM, and also provide better indicators of pathogen-specific mastitis.

An easily understandable index that measures the quality of carbohydrate may aid people in adopting dietary habits that improve their glucose tolerance. We aimed to evaluate the relationship between a ratio of dietary fibre to carbohydrate intakes (fibre:carbohydrate ratio (F:C-R)) and glucose tolerance cross-sectionally and longitudinally. Subjects were 190 Japanese men and women without type 2 diabetes (mean age 55·4 years) who participated in a 5-month diet and exercise programme. We compared baseline anthropometric, dietary and metabolic profiles between those with higher F:C-R and those with lower ratios. Multivariable regression analyses were performed to examine the associations between the F:C-R and homoeostasis model of assessment for insulin resistance (HOMA-IR) and HbA1c at baseline and between changes in the F:C-R and changes in HOMA-IR and HbA1c over the 5-month period. At baseline, the higher F:C-R group had significantly lower body weight, lean body mass, fasting insulin level and HOMA-IR as compared with the lower F:C-R group. The two groups had similar intakes of carbohydrate and fat, whereas protein intake was greater in the high F:C-R group. Baseline F:C-R was not significantly associated with HOMA-IR or HbA1c at the beginning of the study in multivariable models. Increases in the ratio during the 5-month programme was associated with reductions in HbA1c (P<0·001). These findings highlight the potential utility of the F:C-R in strategies aimed at type 2 diabetes prevention.

Maternal nutritional status during pregnancy is an important determinant of fetal growth. Although the effects of several nutrients and foods have been well examined, little is known about the relationship of overall maternal diet in pregnancy to fetal growth, particularly in non-Western populations. We prospectively examined the relationship of maternal dietary patterns in pregnancy to neonatal anthropometric measurements at birth and risk of small-for-gestational-age (SGA) birth among 803 Japanese women with live-born, singleton, term deliveries. Maternal diet in pregnancy was assessed using a validated, self-administered diet history questionnaire. Dietary patterns from thirty-three predefined food groups (g/4184 kJ) were extracted by cluster analysis. The following three dietary patterns were identified: the ‘meat and eggs’ (n 326), ‘wheat products’, with a relatively high intake of bread, confectioneries and soft drinks (n 303), and ‘rice, fish and vegetables’ (n 174) patterns. After adjustment for potential confounders, women in the ‘wheat products’ pattern had infants with the significantly lowest birth weight (P = 0·045) and head circumference (P = 0·036) among those in the three dietary patterns. Compared with women in the ‘rice, fish and vegetables’ pattern, women in the ‘wheat products’ pattern had higher odds of having a SGA infant for weight (multivariate OR 5·2, 95 % CI 1·1, 24·4), but this was not the case for birth length or head circumference. These results suggest that a diet high in bread, confectioneries, and soft drinks and low in fish and vegetables during pregnancy might be associated with a small birth weight and an increased risk of having a SGA infant.

Although several nutrients and foods are suggested to be preventive against postpartum depression, all previous studies have primarily focused on single nutrients or foods. In contrast, studies on dietary patterns, namely the measurement of overall diet by considering the cumulative effects of nutrient, may provide new insights into the influence of diet on postpartum depression. We prospectively examined the association between dietary patterns during pregnancy and the risk of postpartum depression among 865 Japanese women. Diet was assessed with a validated, self-administered diet history questionnaire. Dietary patterns from thirty-three predefined food groups (energy-adjusted food (g/d)) were extracted by factor analysis. Postpartum depression was defined as present when the subjects had an Edinburgh Postnatal Depression Scale score of ≥ 9 at 2–9 months postpartum. A total of 121 women (14·0 %) were classified as having postpartum depression. Three dietary patterns were identified: ‘Healthy’, ‘Western’ and ‘Japanese’ patterns. After adjustment for potential confounders, neither the ‘Healthy’ nor the ‘Japanese’ pattern was related to the risk of postpartum depression. Compared with the first quartile of the ‘Western’ pattern, only the second quartile was independently related to a decreased risk of postpartum depression (multivariate OR 0·52, 95 % CI 0·30, 0·93), although no evident exposure–response associations were observed (P for trend = 0·36). The present study failed to substantiate clear associations between dietary patterns and the risk of postpartum depression. Further studies with more accurate measurements are warranted to confirm the relationship between dietary patterns and the risk of postpartum depression.

Increased homocysteine levels might accelerate dopaminergic cell death in Parkinson's disease (PD) through neurotoxic effects; thus, increasing intake of B vitamins involved in the regulation of homocysteine metabolism might decrease the risk of PD through decreasing plasma homocysteine. However, epidemiological evidence for the association of dietary B vitamins with PD is sparse, particularly in non-Western populations. We conducted a hospital-based case–control study in Japan to examine associations between dietary intake of folate, vitamin B6, vitamin B12 and riboflavin and the risk of PD. Patients with PD diagnosed using the UK PD Society Brain Bank criteria (n 249) and controls without neurodegenerative diseases (n 368) were recruited. Dietary intake during the preceding month was assessed at the time of study recruitment using a validated, self-administered, semi-quantitative, comprehensive diet history questionnaire. After adjustment for potential dietary and non-dietary confounding factors, intake of folate, vitamin B12 and riboflavin was not associated with the risk of PD (P for trend = 0·87, 0·70 and 0·11, respectively). However, low intake of vitamin B6 was associated with an increased risk of PD, independent of potential dietary and non-dietary confounders. Multivariate OR (95 % CI) for PD in the first, second, third and fourth quartiles of vitamin B6 were 1 (reference), 0·56 (0·33, 0·94), 0·69 (0·38, 1·25) and 0·48 (0·23, 0·99), respectively (P for trend = 0·10). In conclusion, in the present case–control study in Japan, low intake of vitamin B6, but not of folate, vitamin B12 or riboflavin, was independently associated with an increased risk of PD.

Background. An ecological analysis found that the docosahexaenoic acid content in mother's milk and seafood intake were inversely correlated with postpartum depression. This prospective study investigated the relationship of consumption of selected high-fat foods and specific types of fatty acids with the risk of postpartum depression.

Method. The subjects were 865 Japanese women. Dietary data were obtained from a self-administered diet history questionnaire during pregnancy. The Edinburgh Postnatal Depression Scale (EPDS) was used for the evaluation of postpartum depression. Adjustment was made for age, gestation, parity, cigarette smoking, family structure, family income, education, changes in diet in the previous month, season when data at baseline were collected, body mass index, time of delivery before the second survey, medical problems in pregnancy, baby's sex and baby's birthweight.

Results. The percentage of women with high depression scores was 14·0%. No evident dose–response associations were observed between intake of fish, meat, eggs, dairy products, total fat, saturated fatty acids, monounsaturated fatty acids, n-3 polyunsaturated fatty acids, n-6 polyunsaturated fatty acids, linoleic acid, α-linolenic acid, arachidonic acid, eicosapentaenoic acid or docosahexaenoic acid and the ratio of n-3 to n-6 polyunsaturated fatty acids and the risk of postpartum depression. However, there was an inverted J-shaped relationship between intake of n-3 polyunsaturated fatty acids and docosahexaenoic acid and the risk of postpartum depression.

Conclusions. This study failed to substantiate a clear inverse relationship between fish and n-3 polyunsaturated fatty acid intake and postpartum depression. Further investigations are needed to determine whether fish and n-3 polyunsaturated fatty acid consumption is preventive against postpartum depression.