Vitamin D is a hormone that is primarily synthesized in the skin upon exposure to sunlight. It is important for maintaining bone health as it aids in calcium absorption from the intestines. Apart from this, vitamin D also plays a role in immune function and is associated with various chronic diseases and mental health. In the UK, many people have low levels of vitamin D, which can cause symptoms such as muscle weakness, fatigue, and mood swings. People with dark skin and those who do not spend much time outdoors are particularly at risk of low vitamin D levels. Vitamin D can also be obtained from food, especially oily fish, eggs, and fortified foods. However, food sources may not be sufficient to correct a deficiency in vitamin D.