Vitamin D is an extremely important vitamin that 2/3 of people have inadequate levels of. Vitamin D’s structure is complex, which is why it plays several roles in health maintenance. It plays roles in calcium and magnesium absorption from the gut, bone strength and maintenance, fat release from adipose stores, immune strength, tooth integrity, and energy and vigor. Dr. Sue's Vitamin D is supplied as D3, which is the active form. After years of difficulty raising D levels to normal with an oral prescription, Dr. Sue found that liquid D works best. This supplement is taken sublingually, which absorbs more efficiently than taken by mouth. Additionally, high quality sublingual Vitamin D is extremely difficult to find on store shelves. Vitamin D deficiency is a common health problem, as most of us work indoors, wear sunscreen, and live in northern climates. Signs of deficiency include muscle cramps, achiness that can be severe, and tenderness in the sternum and front of legs. Despite its high incidence, deficiency is easily corrected through supplementation.