Vitamin D is a fat-soluble vitamin which exists in various forms. Most of its forms are relatively inactive in the human body, however calciferol is the most active form and is converted by the liver and kidney into an active hormone. This physiologically active hormone communicates with the intestines, getting them to increase the absorption of phosphorus and calcium and maintain normal blood levels of these minerals.
The result of this interaction between your intestines and vitamin D’s active hormonal form is increased bone strength, promoting bone mineralisation and helping to prevent weak or brittle bones and certain diseases that result from having thin bones. Examples of these diseases include rickets and osteomalacia.
Vitamin D has also been linked to maintaining a healthy immune system and regulating cell growth and differentiation, where cells are told what function they perform in the body.
So, where can I get my vitamin D from?
Vitamin D is present in some foods such as oily fish (tuna, salmon, mackerel and sardines), egg yolk and beef liver. Cod liver oil is also an excellent source of this essential vitamin. Many manufacturers are also fortifying certain products, in particular breakfast cereals, juice and dairy products, with vitamin D.
But the best source of vitamin D available to each and every one of us is sunlight. Sunshine acts as a trigger with the ultraviolet rays activating vitamin D synthesis in the skin.
Over the last few years, with the advent of health campaigns designed to protect our skins from harmful radiation from the sun, concern has mounted over potential vitamin D deficiency in the population, especially in children. Most people do not consume adequate vitamin D in their diets and, consequently, some exposure to sunlight is essential.
There are many factors which can affect the level of vitamin D synthesis in the skin from sunlight. These include:
• Cloud cover – complete cloud cover halves the energy of UV rays;
• Shade – shade can reduce the suns energy by up to 60%;
• Sun cream / screen – creams with an SPF rating of 8 or above will block the UV rays enough to prevent any vitamin D synthesis.
So, how can I get enough?
Exposure to sunlight is important, with 15 minutes or so on your hands, face, arms or legs, a few times a week providing adequate opportunity for the synthesis to occur. However, don’t forget that the middle of the day is the hottest and, subsequently, the most damaging time for unprotected skin. Try to get your sun cream-free exposure in the morning or mid-late afternoon, applying cream and other protection such as a wide-brimmed hat, at all other times.
By also eating a diet high in oily fish, dairy products, eggs and fortified cereals, vitamin D deficiency should not be an issue for you.
Babies and infants
Breastmilk is not a good source of vitamin D, consequently a regular exposure to sunlight is essential in preventing the development of rickets. However, it’s even more vital that you take great care to protect your baby from the harmful effects of UV rays, only exposing him or her early or late in the day.
A baby’s diet can also be supplemented with vitamin D-fortified formula if sunlight exposure is difficult (such as in extremely hot weather or tropical latitudes).
Vitamin D deficiency commonly manifests as the conditions known as rickets, resulting in soft bones and deformities generally in children, and osteomalacia, resulting in weak bones and muscle weakness. People who are at risk of developing a deficiency are:
- Those with limited exposure to sunlight;
- The lactose or milk intolerant;
- Breastfed babies;
- Strict vegetarians or vegans.
Darker skinned people and people over the age of 50 also have an increased risk as their skin does not synthesise sunlight as effectively as lighter skinned or younger people.
Vitamin D is a fat-soluble vitamin and, consequently, people with a condition such as Crohn’s disease, Cystic Fibrosis, Celiac Disease and certain forms of liver disease, that prevents them from absorbing fat efficiently, can also be at higher risk of developing a vitamin D deficiency.
Other benefits of adequate vitamin D
There is some evidence to suggest that vitamin D can help to protect against some forms of cancer. Preliminary studies have shown significant correlations between calcium/vitamin D intake and a reduced risk of colon and colorectal cancers. Further trials are needed to determine the validity of this link.
Can I have too much?
Yes. Too much vitamin D can result in vomiting, lack of appetite, constipation and weight loss. It can also raise the levels of calcium in the blood resulting in irregular heart rhythms and mental confusion. However it’s unlikely that vitamin D toxicity results from an excess of sunlight or food containing the nutrient. In general, toxicity comes from over-consumption of supplements or too much cod liver oil. Long-term excessive consumption of vitamin D increases the risk of suffering side effects.