Harvard's School of Public Health reports that an estimated 1 billion people worldwide are deficient in vitamin D. This report also cites several studies that link vitamin D levels with bone strength, muscle retention, heart health, and the immune system. Studies have also shown that Vitamin D can boost elasticity and collagen production in the skin while also reducing the appearance of lines, dark spots and acne. Clinical trials surmised vitamin D supplements can be particularly beneficial to older women. Source: "Vitamin D and Health." Harvard School of Public Health. Robbins JA, et al. "Women's Health Initiative clinical trials: interaction of calcium and vitamin D with hormone therapy." Menopause. February 2014. PubMed. NCBI. NLM. NHI.