The effectiveness and benefits of vitamin D are greatly undermined in the absence of adequate levels of magnesium in the body, according to Dr. Carolyn Dean, MD, ND, magnesium expert and medical director of the non-profit Nutritional Magnesium Association. Magnesium acts with and is essential to the activity of vitamin D, and yet most Americans do not get their recommended daily allowance (RDA) of this important mineral. Extensive research has shown that vitamin D deficiencies play a major role in the development of dozens of diseases, including breast cancer, prostate cancer and colon cancer, as well as diabetes, heart disease, arthritis, osteoporosis, psoriasis and mental illness.