A quick Google search for the side effects for vitamin D deficiency reveals that they are: getting sick or infected often, fatigue and tiredness, bone and back pain, depression, impaired wound healing, bone loss, hair loss, and muscle pain. One of the others not mentioned is weight gain. Some of the common risk factors for becoming deficient are: having dark skin, being elderly, being overweight or obese, not eating much fish or dairy, living far from the equator where sunlight is limited, always using sunscreen, and staying indoors.