Naming Vitamin D a vitamin is not entirely accurate as it is acts more like a hormone than a vitamin. The fact that we can produce most of it from cholesterol and exposure to ultraviolet sunlight limits its classification as a vitamin, which we would usually rely on diet alone for adequate levels of. The role of Vitamin D is not fully understood but it is one very important active compound in our bodies.
It is that we can produce our own and that vitamin D has specific target organs upon which it acts and which are distant from its site of production that better suits the description of a hormone. From sterols we produce vitamin D3 which is transported to the liver and converted to cholecalciferol (25-hydroxyvitamin D). This is then activated to calcitriol (1,25-dihydroxyvitamin D) in the kidneys which is the vitamin/hormone that is the most active and important form.
Calcitriol, along with parathyroid hormone and calcitonin, regulates calcium and phosphate metabolism. Together they control the absorption of calcium and phosphate from the gut and the associated homeostasis of these minerals through bone metabolism. It is usually this role in bone health where we are first introduced to the power of vitamin D. It was discovered many moons ago when puppies with rickets (aaahh..) were successfully treated with cod liver oil. The active compound was later isolated from the oil and as it was the fourth “vitamin” discovered it was creatively knighted Vitamin D. The deficiency of the vitamin was shown to cause bone irregularities such as rickets in this case, a form of osteomalacia (bone softening) and is also associated with osteoporosis, the loss of bone mineral density.
More recently we have begun to understand many other roles that vitamin D plays in the body. These include cell differentiation and growth in many tissues such as the skin, muscles, pancreas, nerves and the immune system. Vitamin D is also involved in the regulation of blood pressure and insulin production. A deficiency can show up though muscle weakness and tremors and low levels of Vitamin D have been correlated with heart disease, depression and some cancers. It has also become more evident that a larger number of people have inadequate levels than originally thought. It is a difficult nutrient to get in from dietary sources and this fact combined with risk factors such as dark skin, the use of sunscreen, living at higher latitudes (further from equator and the sun) and indoor or early/late sport and training has resulted in this increased prevalence.
What can we do?
Firstly make sure you are getting at least a little sun daily, 10-15 minutes without SPF cream is advised. For the happier parts of the world (like Cape Town) this is easier than certain other somewhat duller parts of the globe. If you are in a greyer city you can consider the occasional use of a UVB sun bed.
Foods high in vitamin D should be included in a balance diet from sources such as oily fish (herring, mackerel, salmon, tuna, cod and pilchards), eggs, liver and fortified products (milk or orange juice). The current DRI for Vitamin D is 5-10 μg/day (200-400 IU/day) but this might be a conservative value that research is suggesting is too low.
Supplementation can be considered where blood levels confirm inadequate circulating calcitriol. It is worth having your vitamin D status measured with a simple blood test should a deficiency concern exist. If supplementation is recommended by your health care provider make sure it is in the form of vitamin D3.
In sportsmen and athletes inadequate levels of Vitamin D can be damaging to health and performance and in cases where inadequate levels have been confirmed supplementation has been shown to be beneficial for strength, power, balance and reaction time. According to the Australian Institute of Sport it is a Category A Supplement that is supported by research and warrants use in certain situations which include those mentioned above.