Alice Branton explains that Vitamin D is critical for the functioning of every cell in the body and compares it to the electricity that powers a house. Symptoms of Vitamin D deficiency can include poor immunity, lack of energy, bone pain, and even correlations to diabetes and cancer. Brayton notes that despite the ability to get Vitamin D from the sun, studies show a high percentage of people worldwide are deficient, possibly due to a decreased ability to convert sunlight to Vitamin D. She dispels the myth that Vitamin D is only for bone health and states that supplements often aren't bioavailable.