Vitamin D supplementation and vitamin D biomarkers in relation to sunlight exposure and musculoskeletal health in older adults using two different study designs

Abstract

Vitamin D plays a role in musculoskeletal (MSK) health through genomic and non-genomic pathways. In the UK, SACN used a 25(OH)D concentration of 25 nmol/L as the basis for setting the (Recommended Nutrient Intake) RNI of 10 μg/day based on MSK outcomes (rickets, osteomalacia, falls, muscle strength and function). In setting vitamin D recommendations for North Americans, the IOM used a 25(OH)D concentration of 50 nmol/L as the basis for setting the (Recommended Dietary Allowances) RDA of 15 μg/day, based onthe relationship between 25(OH)D and bone outcomes (rickets, osteomalacia, bone mineral density and calcium absorption). The differing thresholds used to define “vitamin D adequacy” may be due to (1) different MSK outcomes and approaches used to underpin the DRVs (2) geographical differences in vitamin D status and response to vitamin D supplementation and exposure to sunlight. Since vitamin D biomarkers and vitamin D supply required for optimum MSK function are not agreed universally, this thesis is set out to determine the effect of vitamin D supply on muscle function and to determine the association between commonly-used vitamin D biomarkers [25(OH)D and PTH] and muscle function [Grip Strength (GS), Timed-up and Go (TUG)] and bone health [quantitative ultrasound] in older adults using two different studies from the North East of England (55° North). In addition, because sunlight exposure is known to influence vitamin D biomarkers and response to vitamin D supplementation, this thesis also attempted to quantify the impact of sun exposure on vitamin D status in both studies. The first study was a Randomized Control Trial (RCT) of monthly vitamin D supplementation (12000 IU, 24000 IU and 48000 IU vitamin D3) for 1 year on MSK health [Vitamin Din Older People study-VDOP (n=379; age >70 years)]. The second study was a follow-up study of older adults at moderate risk of colon cancer [the Biomarkers of Risk of Colon Cancer Follow-Up study -BFU (n=47, age >49 years)]. Baseline findings from the VDOP study showed that plasma 25(OH)D concentration, <25 nmol/L was associated with lower odds [OR 0.34, 95% CI, 0.166 -0.691] ofhigher muscle function, compared with plasma 25(OH)D concentrations ≥25 nmol/L, after adjusting for the season of blood sampling and relevant confounders.Serum PTH was not associated with any MSK parameter at baseline. Vitamin D supplementation with 12000IU, 24000IU or 48000 IU monthly for one year increased 25(OH)D concentrations in a dose dependent manner, but had no effect on GS or TUG. Using a detailed sun exposure questionnaire to derive a composite sunshine exposure score, personal UV exposure failed to predict post-intervention 25(OH)D concentration in any of the vitamin D treatment groups.Findings from the BFU study showed that serum 25(OH)D concentration was not associated with GS, TUG or QUS in older adults. Serum PTH concentration was associated with QUS but not GS or TUG. Holiday visits was the only sun exposure variable which was associated with serum 25(OH)D concentration (p=0.042) at baseline in VDOP study. Sixteen participants were vitamin D deficient (25 –50 nmol/L) at follow-up which increased from 7 participants at baseline over the 12 year follow-up period since the initialphase of data collection. In summary, low dose vitamin D supplementation improved vitamin D status in older adults such that vitamin D deficiency was prevented but it failed to improve muscle function. In line with SACN recommendations it may be prudent to maintain serum 25(OH)D > 25 nmol/L throughout the year to maintain healthy musculoskeletal functioning.