Prevalence and determinants of profound vitamin D deficiency (25-hydroxyvitamin D <10 nmol/L) in the UK Biobank and potential implications for disease association studies

BACKGROUND: 25-hydroxyvitamin D (25OHD) is the principal biomarker of vitamin D status. Values below the assay detection limit (<10 nmol/L) are often reported as missing. Thus the most severely deficient participants are excluded from research which can lead to inaccurate findings such as underestimated prevalence of deficiency, overlooked risk factors, and biased evaluation of disease associations. METHODS: In total 369,626 individuals from the UK Biobank cohort were included in this study. Data on 25OHD concentration and relevant demographic and lifestyle factors such as age, supplement intake, diet, and time spent outdoors were used in the analyses. Ambient UVB radiation was approximated for each participant. 25OHD was evaluated as a categorical outcome and we reintroduced participants with 25OHD values < 10 nmol/L (conventionally reported as missing values) back to the dataset. Adjusted regression models were used to investigate the determinants of profound (25OHD <10 nmol/L) and severe (10-25 nmol/L) vitamin D deficiency and to assess disease associations (with 25-50 nmol/L as the reference category). RESULTS: 1,784 (0.48 %) individuals were profoundly deficient and a further 47,226 (12.78 %) individuals were severely vitamin D deficient. The proportions of profoundly and severely deficient were highest among Asians, 9 % and 47 %, respectively. Ambient UVB radiation was the second strongest predictor: comparing the lowest vs. highest quartile, the risk of profound deficiency was 17-fold increased and that of severe deficiency 7.5-fold increased. Use of vitamin D supplements substantially reduced risk of profound (4.4-fold) and severe (2.5-fold) deficiency, as did fish intake (5- and 1.9-fold, respectively). Profound deficiency was more strongly associated with chronic illness, diabetes, and emphysema compared to severe deficiency. CONCLUSION: The prevalence of profound and severe vitamin D deficiency among Asian and Black ethnicities in the UK is high and requires targeted action. Solar radiation is potent in protecting against profound and severe vitamin D deficiency. Studies evaluating the relationship between vitamin D status and other health outcomes may be biased if profoundly deficient participants are excluded.