Serum 25-hydroxyvitamin D levels and risk of all-cause and cause-specific mortality: A 14-year prospective cohort study.

BACKGROUND & AIMS: The circulating vitamin D level that is optimal for health is unknown. This study aimed to examine the association between circulating vitamin D level and risk of all-cause and cause-specific mortality. METHODS: This prospective cohort study included 18,797 Korean adults aged 40 years or older, living in rural areas, with no history of cancer or cardiovascular disease (CVD) at baseline. Serum 25-hydroxyvitamin D (25(OH)D) levels were measured at baseline. Participants were followed-up from the survey date (2005-2012) until December 31, 2021. Cox proportional hazards regression was used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for mortality by baseline vitamin D level. Restricted cubic splines were used to explore the nonlinearity. RESULTS: The median (interquartile range) of 25(OH)D level was 55.8 (40.8-71.8) nmol/L. During a median follow-up of 14.3 years, 2250 deaths were recorded. Compared with participants with a 25(OH)D level <30 nmol/L, higher vitamin D levels (30 to < 50, 50 to < 75, and ≥75 nmol/L) were associated with a lower risk of all-cause mortality: HR (95% CI) of 0.82 (0.69-0.98), 0.74 (0.62-0.88), and 0.69 (0.57-0.84), respectively. A nonlinear relationship between vitamin D level and all-cause mortality was observed, with the risk plateauing between 50 and 60 nmol/L (p for nonlinearity = 0.009). The association was more pronounced for cancer-related mortality. HR 0.55 (95% CI: 0.39-0.77) for a 25(OH)D level ≥75 nmol/L compared with <30.0 nmol/L. Low vitamin D levels were associated with increased CVD mortality in men. CONCLUSIONS: Vitamin D level was inversely associated with all-cause and cause-specific mortality in middle-aged and older adults. Maintaining a serum 25(OH)D level of approximately 50-60 nmol/L may contribute to longevity and warrants further investigation.

Korea Nurses' Health Study and the health of reproductive-aged women: a cohort profile.

The Korea Nurses' Health Study (KNHS) is an ongoing, large-scale, prospective cohort study of women nurses, focusing on the effects of occupational, environmental, and lifestyle factors on the health of women. The first KNHS survey was performed in 2013-2014 (n=20,613). As of December 2023, 11 follow-up surveys have been conducted. Participants who were pregnant were asked to participate in the early pregnancy survey (n=2,179) and postpartum survey after giving birth (n=2,790). The main variables included socio-demographic, work-related, lifestyle, physical, mental, and women's health factors. Blood, urine, and toenail samples were collected from a participant subgroup of the first survey (n=1,983). The subgroups of the second survey completed a food frequency questionnaire in 2019 (n=300) and 2021 (n=871). In 2020, a subgroup of the first survey answered a coronavirus disease 2019-related survey (n=975). To examine various health-related factors in young adults, new participants were added to the KNHS cohort in the 11th (n=1,000) and 12th (n=1,002) surveys. The KNHS cohort will help identify health and illness determinants in Korean women. Data can be accessed at https://coda.nih.go.kr/frt/index.do.

Folate, vitamin B12, and homocysteine status in the Korean population: data from the 2013-2015 Korea National Health and Nutrition Examination Survey.

OBJECTIVES: We aimed to assess the serum folate, vitamin B12, and homocysteine status in Korean adolescents and adults using national data. METHODS: Blood samples were collected from participants aged ≥10 years in the Korea National Health and Nutrition Examination Survey 2013-2015. The stored serum samples were used to measure folate, vitamin B12, and homocysteine concentrations. A total of 8,016 participants were included in this analysis. Unweighted descriptive statistics and adjusted geometric means of the B vitamins and homocysteine concentrations were estimated. RESULTS: Females had higher serum folate and vitamin B12 concentrations and lower serum homocysteine concentrations than males. Folate deficiency (<6.8 nmol/L) and hyperhomocysteinemia (>15 µmol/L) were found in 8.6% and 11.8% of males, respectively. Approximately 3% of males had low or marginally low vitamin B12 status (≤221 pmol/L). Folate and vitamin B12 deficiencies and hyperhomocysteinemia were found in <2% of females. Suboptimal folate status was prevalent among adolescents and young adults, while suboptimal vitamin B12 status and hyperhomocysteinemia were relatively higher in older adults. Adjusted mean homocysteine concentrations were sharply decreased from the first to second decile of serum folate in males. CONCLUSIONS: In the Korean population, the proportion of males who achieved desirable folate and homocysteine concentrations were lower than those of females. Although most Koreans have adequate vitamin B12, a suboptimal folate status is common, particularly among adolescents and young adults. These findings could establish a foundation for public health initiatives aimed at improving folate levels in the Korean population.