This winter-based placebo-controlled, single-dose vitamin D randomized controlled trial (RCT) aims to examine the impact of various levels of habitual calcium intake on dietary vitamin D requirements in older adults stratified by calcium intake. This will provide new data on the impact of different levels of calcium intake, ranging from low/moderate to high, on winter serum 25(OH)D levels, and their utilization and catabolism in adults.

The aim of this study is to perform a randomised controlled vitamin D3 intervention study in apparently healthy, free-living adults (aged 50+ y) to investigate whether different levels of habitual calcium intake, ranging from low-moderate to high, influence serum 25(OH)D concentrations and indices of vitamin D activation and catabolism during winter, when vitamin D intake is adequate versus inadequate.

This research will provide new data and scientific understanding in relation to the impact of different levels of dietary calcium intake on vitamin D requirements in the older adult population. As such, this new data will inform dietary reference values for vitamin D.

Inclusion Criteria:

Consenting white men and women aged ≥ 50 y

Exclusion Criteria:

• Unwilling to discontinue consumption of vitamin D-containing supplements 4 wks before the initiation of the study and throughout the study.
• Planning to take a winter vacation (during the course of the 15-wk intervention) to a location at which either the altitude or the latitude was predicted to result in significant cutaneous vitamin D synthesis from solar radiation (e.g., a winter sun coastal resort or a mountain ski resort)
• Use of tanning facilities of any type.
• Pregnancy or planning to become pregnant during the study
• A severe medical illness,
• hypercalcaemia,
• Known intestinal malabsorption syndrome,
• Excessive alcohol use,
• Taking medications known to interfere with vitamin D metabolism