Vitamin D is very important for calcium absorption. It also helps boost immunity by fortifying the white blood cells, which defend against bacteria. In addition, it has anti-inflammatory properties. A link has been made between influenza and vitamin D deficiency in countries that experience long winters with low levels of sunlight. Depression, mental health problems and Seasonal Affective Disorder have also been linked to low vitamin D levels in patients. Other diseases which can be caused by a deficiency in vitamin D include high blood pressure, diabetes, autoimmune diseases, excess fat accumulation around vital organs, different types of cancer, atherosclerosis (thickening of arteries), osteoporosis (deterioration of bone mass) and osteopenia (thinning of bones).