https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3309636/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5241405/ Vitamin D and Calcium: https://www.ncbi.nlm.nih.gov/pubmed/30180273 https://www.everydayhealth.com/colu.../are-vitamin-supplements-really-bioavailable/