Vitamin D is not a vitamin at all but is actually a hormone precursor that is converted to active vitamin D, in the form of calcitriol, and then used throughout the body for a multitude of chemical processes and biological functions.
Vitamin D is unique as it is the only vitamin that the body can make; all others must be consumed. Vitamin D is made when skin is exposed to sunlight, causing a series of chemical reactions that turns cholecalciferol (a derivative of cholesterol) into active vitamin D. Whilst vitamin D is present to some extent in animal foods, specifically in fish, eggs full-fat dairy and liver, this does not contribute significantly to the body’s vitamin D stores. We must therefore rely on our natural production to maintain healthy vitamin D status. Unfortunately for those of us living in the UK, and other similar latitude countries, it is not possible to make vitamin D between October and March as the wavelength (or strength) of the sun is too low, so we must either rely on optimal stores built up over the summer or consider supplements containing vitamin D.
Safe sun exposure (ensuring you don’t burn) through the summer months is important to make sure your body’s stores are full and can last the winter. Many of us, however, do not get enough sun even in the summer months as we are unable to achieve the recommended exposure to the sun, perhaps because of clothing, daylight working hours and fear of skin cancer. If this is the case it is important to supplement with vitamin D to look after your health and protect against bone mineral loss. There are two forms of vitamin D that have biological activity, vitamin D3 and vitamin D2, with vitamin D3 considered the best form to take.
Vitamin D is most known for its role in bone health and protecting against low bone density conditions such as rickets and osteoporosis. Vitamin D is essential for calcium absorption and recycling; it follows, therefore, that the amount of vitamin D we have directly determines how much calcium is available in the blood to carry out its biological functions. In addition to its importance for healthy bones, calcium is also fundamental to cell signalling; as a result, lack of vitamin D is now implicated in a wide range of health conditions affecting the muscles and central nervous system, the immune system, brain, heart and lungs. Cardiovascular disease, autoimmunity, depression and Alzheimer’s are just some of the conditions increasingly linked to low vitamin D status.