immunity, cell proliferation, insulin secretion, and blood pressure. In low and middle income countries, foods fortified with vitamin D are "practically nonexistent." Most people in the world depend on the sun to get vitamin D, and elderly populations in low UVB countries experience higher rates of cancer. There are not many foods that naturally have vitamin D. Examples are cod liver oil and oily fish. If people cannot get sunlight, then they will need 1,000 IU of vitamin D per day to stay healthy. A person would have to eat oily fish three or four times per week in order to get enough vitamin D from that food source alone. People with higher levels of vitamin D tend to have lower rates of diabetes, heart disease, and stroke and tend to have lower blood pressure. However, it has been found that vitamin D supplementation does not improve cardiovascular health or metabolism, so the link with vitamin D must be in part indirect. People who get more sun are generally healthier, and also have higher vitamin D levels. It has been found that ultraviolet radiation (even UVA) produces nitric oxide (NO) in the skin, and nitric oxide can lower blood pressure. High blood pressure increases the risk of stroke and heart disease. Although long-term exposure to ultraviolet contributes to non-melanoma skin cancers that are rarely fatal, it has been found in a Danish study that those who get these cancers were less likely to die during the study, and were much less likely to have a heart attack, than those who did not have these cancers. People in certain situations, such as people with intellectual disabilities and neurodevelopmental disorders who stay inside most of the time have low vitamin D levels. Getting enough vitamin D can help stave off "autoimmune diseases, cardiovascular disease, many types of cancer, dementia, types 1 and 2 diabetes mellitus, and respiratory tract infections." Fetuses and children who do not get enough vitamin D can result in "growth retardation and skeletal deformities." Lower prevalence of multiple sclerosis Multiple sclerosis (MS) is least prevalent in the sunniest regions. Exposure to the ultraviolet-B radiation of sunlight appears to be most important and this may operate via vitamin D synthesis. Risks to skin Ultraviolet (UV) irradiation present in sunlight is an environmental human carcinogen. The toxic effects of UV from natural sunlight and therapeutic artificial lamps are a major concern