Lifestyle and Dietary Factors Associated with Serum 25-Hydroxyvitamin D Levels in Korean Young Adults

Abstract

Inadequate vitamin D status is highly prevalent in the Korean population, especially among young adults. Nonetheless, correlates of suboptimal vitamin D levels in young adults are not well defined. This study aimed to investigate potentially modifiable determinants of vitamin D levels in young adults in Korea. This cross-sectional study was based on health check-up data from 3,450 healthy male and female university students aged 18-29 yr in Seoul between April and May 2013. Serum 25-hydroxyvitamin D [25(OH)D] levels were determined using chemiluminescent immunoassay. Anthropometric data were measured, and lifestyle, dietary, and sociodemographic factors were obtained through self-administered questionnaires. General linear regression was used to assess correlates of serum 25(OH)D levels. The mean serum 25(OH)D level was 11.1 ng/mL, and the prevalence of 25(OH)D levels less than 10 ng/mL was 44.7% (39.5% in men, 50.2% in women). In a final multivariable model, significant positive correlates of serum 25(OH)D were older age, male sex, increased physical activity, sunlight exposure ≥ 30 min/day, eating breakfast regularly, consumption of dairy and fatty fish, and use of vitamin D-containing supplements. In contrast, significant inverse correlates were obesity (body mass index, BMI ≥ 25 kg/m(2)) or underweight (BMI < 18.5 kg/m(2)), abdominal obesity, increased sedentary time, and frequent consumption of instant noodles and sugar-sweetened beverages. In conclusion, many modifiable lifestyle and dietary factors were associated with low serum 25(OH)D levels in Korean young adults. Further studies on potential mechanisms of the correlates and optimal strategies to improve vitamin D status in this vulnerable subpopulation are warranted.

Keywords: Breakfast; Instant Noodle; Life Style; Obesity; Sugar-sweetened Beverage; Thinness; Vitamin D; Young Adult.

Fig. 1. Distribution of serum 25-hydroxyvitamin D (25(OH)D) concentrations among 3,450 Korean university students aged 18 to 29 yr, 2013 spring. Unadjusted frequency distribution of serum 25(OH)D levels in the study participants (n = 3,450). The graph was smoothed using normal-smoothing density for normal distribution. One subject with a serum 25(OH)D level greater than 50 ng/mL was excluded from the figure.

Fig. 2. Multivariable-adjusted mean serum 25-hydroxyvitamin D [25(OH)D] levels by sex and lifestyle factors (A-F). BMI, body mass index (kg/m²): underweight (< 18.5), normal (18.5-<23), overweight (23-<25), and obese (≥ 25). Multivariable-adjusted mean serum 25(OH)D levels were calculated via PROC GLM after adjustment for the variables in the figure and age, sex, body mass index (underweight/normal/overweight/obese), abdominal obesity (no/yes), physical activity (low/moderate/high), sedentary time (tertile), and sunlight exposure (< 30 min/d, ≥ 30 min/d). ^*P < 0.05 and ^†P < 0.01 with the leftmost category as a reference.

Fig. 3. Multivariable-adjusted mean serum 25-hydroxyvitamin D [25(OH)D] levels by dietary factors (A-H). Multivariable-adjusted mean serum 25(OH)D levels were calculated via PROC GLM after adjustment for the variables in the figure and dairy intake (≤ 2, 3-4, 5-6, ≥7/wk), fatty fish intake (≤ 1/mon, 2-3/mon, 1-2/wk, ≥3/wk), instant noodle intake (≤ 1/mon, 2-3/mon, 1-2/wk, ≥ 3/wk), sugar-sweetened beverage intake (≤ 1/mon, 2-3/mon, 1-2/wk, ≥ 3/wk), breakfast eating (≤ 3, 4-6, 7 d/wk), and vitamin D-containing supplements use (yes/no). ^*P < 0.05 and ^†P < 0.01 with the leftmost category as a reference except for B (normal category as a reference).