Validation of a food frequency questionnaire to determine vitamin D intakes using the method of triads.

BACKGROUND: Dietary sources of vitamin D (both natural and fortified) are increasingly contributing to consumers' vitamin D intake and status. Therefore, the present study aimed to validate a vitamin D food frequency questionnaire (FFQ) for the assessment of habitual vitamin D intake. METHODS: A total of 49 apparently healthy consenting adults (aged 18-64 years) from the local community were sampled at the end of winter. Dietary intakes were recorded using a 4-day weighed food record (4d-WFR) and a 17-item FFQ based on foods known to contribute to dietary vitamin D intake. Fasting vitamin D status was quantified by serum 25-hydroxyvitamin D [25(OH)D] using liquid chromatography tandem mass spectrometry. The method of triads was applied using these three measurements to determine the overall validity of the FFQ. RESULTS: Vitamin D intakes from 4d-WFR ranged between 0.42 and 31.65 µg day(-1), whereas intakes determined from the FFQ ranged from 1.03 to 36.08 µg day(-1). Serum 25(OH)D concentrations ranged between 12.89 and 279.00 nmol L(-1). The mean (SD) difference between the FFQ and 4d-WFR was +1.62 ( 3.86). There were strong correlations between the vitamin D intake estimated by the FFQ and that from the 4d-WFR (r = 0.562) and also with serum 25(OH)D concentrations (r = 0.567). Vitamin D intake estimated from the 4d-WFR was also strongly correlated with serum 25(OH)D concentrations (r = 0.411). The overall validity coefficient calculated using the method of triads was high (0.881). CONCLUSIONS: The vitamin D FFQ has been validated for use in future studies aiming to assess habitual vitamin D intake.

Vitamin D, Muscle Function, and Cardiorespiratory Fitness in Adolescents From the Young Hearts Study.

CONTEXT: Vitamin D insufficiency is common among the adolescent population and may have implications for health outcomes in later life. Few studies have investigated the role of vitamin D status in muscle function and cardiorespiratory fitness (CRF) during adolescence. OBJECTIVE: The aim of this study was to investigate the association between vitamin D status and fat-free mass (FFM), muscle strength, muscle power, and CRF in a representative sample of adolescents from Northern Ireland. DESIGN, SETTING, AND PARTICIPANTS: This was a cross-sectional study involving 1015 adolescents (age 12 and 15 y), who had 25-hydroxyvitamin D [25(OH)D] data available as part of the Young Hearts Study 2000. MAIN OUTCOME MEASURES: The main outcome measures of this study were FFM (kg) and FFM corrected for height [fat-free mass index; FFM (kg)/height (m(2))], muscle strength (kg), peak muscle power (kW) and VO2 max (CRF; mL/kg/min). RESULTS: Multinomial regression analyses, controlling for environmental and lifestyle factors, demonstrated that boys age 15 years in the highest tertile of standardized serum 25(OH)D concentration (> 51 nmol/L) had significantly higher muscle strength (ß = 3.90; P ≤ .001) compared with those in the lowest tertile (< 32 nmol/L). These results were not evident in any other age-sex group and vitamin D status was not significantly associated with muscle power or CRF in any of the four age-sex groups. CONCLUSION: These results support a role for vitamin D in muscle function in adolescent males and suggest the need for more research in this vulnerable age group.