The first thing that we should clear up is that vitamin D is not a vitamin. It is actually a hormone, related to other steroid hormones such as testosterone, oestrogen and progesterone, amongst others. The body makes vitamin D from cholesterol by the action of ultraviolet B radiation in sunlight on your skin, but can also acquire the hormone from the diet. Foods such as eggs and fatty fish contain vitamin in a form called D3 (cholecalciferol). Vitamin D2 (ergocalciferol) is a different form and is present is plants and fungi. Milk and bread tends to be fortified with either D2 or D3 artificially. The active form of vitamin D in the body is called 25-hydroxyvitamin D (25-OH)D, and this is produced in the liver from either dietary sources, or the vitamin D3 produced in the skin from sunlight.
The active form of vitamin D is most well known for its role in calcium metabolism and good skeletal health. However, recently vitamin D has been receiving growing interest from scientists and nutritionist because of its vital functions in the modulation of the immune system. Growing evidence is linking vitamin D deficiency to autoimmune diseases such as type I diabetes, multiple sclerosis, rheumatoid arthritis and heart disease. In addition, many studies are also starting to uncover the role of vitamin D deficiency in the formation of various sorts of cancer in humans. Because of these important functions of vitamin D, there is clearly a case to be put forward to maintaining optimum levels in the blood. And to do this it looks like supplementation for many people may be the only answer.
Now you might think that supplementation of vitamin D would be unnecessary. Many ‘experts’ have this viewpoint, but science shows they are wrong. A growing body of research is now starting to question if the amount of vitamin D we get in our diets is enough for vitamin D sufficiency. Under normal circumstances we produce huge amounts of vitamin D from exposure to the sun. It is estimated that ten to fifteen minutes of whole body exposure in peak summer sun at lower latitudes releases 20,000 IU of vitamin D3 into the circulation from the skin. When you consider that the government recommended supplementation level for adults in the UK is 200 IU, you can start to see the problem. The fact is that these low levels of the vitamin were conceptualised from data in children in order to prevent rickets. They have little to do with optimal health.
Factors such as sunscreen usage, lack of sun at higher latitude and lifestyle habits such as spending longer times inside during daylight hours are preventing many people from acquiring the sun exposure necessary to boost their levels of vitamin D within the blood. Too many people are relying on dietary intakes which are starting to be identified as inadequate to maintain optimal vitamin D circulation. For example, a paper published in the Journal of Nutrition in 20061 concluded that the low government recommended levels of vitamin D are actually harmful. Another paper in the Journal of Nutrition in 20052 stated that 2000 IU of vitamin D may be needed daily in adults in order to maintain blood levels at an adequate level in absence of sun exposure. This is five times the current recommended level set by the government in the United Kingdom.
The ideal solution would be to allow the body to produce the vitamin naturally by reaction of UV light on the skin. Those of us who live in hotter climates can use this method to maintain optimum levels of this vital hormone. The reality is that people who live in equatorial regions are likely to have healthy levels of vitamin D in the blood. Those of us who live at higher latitude, who for large periods of the year do not receive any intensity in the sun, or those that avoid sun exposure, may need to consider supplementation as the answer. It would be nice to be able to receive adequate levels of vitamin D from food. However, it would be impossible to incorporate the necessary levels in a healthy diet. A serving of fatty fish only provides around 400 IU per serving.
So if the only option is to supplement, how much should you take? Well this depends really on biochemical individuality. The only way to really know is to have a blood test and monitor the blood concentration as you take a vitamin D supplement. The ideal concentration in the blood should be around 40 ng/mL (~100 nmol/L) of 25-hydroxyvitamin D. Blood test are available from health care professionals and are the preferred option. This is important as it will ensure that the levels in the blood remain within a safe level. Toxicity of vitamin D can occur, but it is often overstated by doctors. Toxicity symptoms can include elevated calcium in the urine and blood, which can cause abdominal pain, constipation, vomiting, muscle weakness, thirst, itching and high blood pressure. These discontinue as soon as the supplement is withdrawn.
In reality, it is unlikely that if a sensible dose is taken that toxicity will ever be an issue, and many more people have ill health because of a deficiency of vitamin D than from toxicity. For those who will not take a blood test, supplementation with around 2000 IU of vitamin D a day during times of little sun exposure will probably cover most people and allow them to maintain vitamin D sufficiency without any toxicity. Supplements tend to come in 10 or 25 microgram tablets and will be listed on the ingredients and either vitamin D2 or vitamin D3. You should ensure that your supplement is in the D3 form. A study in the American Journal of Clinical Nutrition published in 20063 reported that vitamin D2 is not as effective at raising blood levels and is estimated to be only about 20-40% as potent. See you in the sun.