What is vitamin D?
The natural form of vitamin D is produced in the skin when energy from ultraviolet B rays (UVB) in sunlight converts cholesterol in the cells of the skin into vitamin D3, a steroid hormone.
The natural form of vitamin D from sunlight or fatty animal sourced foods such as oily fish and egg yolks is called Vitamin D3 (cholecalciferol), while vitamin D from plant material (such as mushrooms grown in sunlight or exposed to other sources of ultraviolet (UV) radiation) is referred to as vitamin D2 (ergocalciferol). Vitamin D3 is the preferred form of vitamin D, as studies have shown that vitamin D3 is nearly twice as effective at raising vitamin D blood levels than vitamin D2
Whether vitamin D originates from exposure to sunlight, or from food or supplements, the first stop is the liver where it picks up extra oxygen and hydrogen molecules to become 25-hydroxyvitamin D, also known as calcidiol, indicated as 25(OH)D. Health practitioners test for this chemical as the unit of measurement to determine vitamin D levels in the bloodstream. From the liver the 25(OH)D travels to the kidneys, where it is converted to calcitriol and acquires a final pair of oxygen and hydrogen molecules in order to become the active form of vitamin D, indicated as 1,25(OH)2D.
Why is vitamin D important?
Vitamin D plays such an important role in the body that it is the only vitamin that the body makes by itself, although only when the skin is exposed to sufficient sunlight.
Vitamin D deficiency can lead to thin, brittle or misshapen bones, or even osteoporosis (“thin bone” disease) in older adults. Vitamin D deficiency in children can result in a painful condition called rickets (softening or weakening of the bones).
Vitamin D is a fat-soluble vitamin that the body can store for a long time and acts as facilitator for the intestinal absorption of calcium, which is essential to form and maintain strong bones. Its major function is to maintain normal blood levels of calcium. With normal levels of vitamin D, the body absorbs about 30% to 40% of dietary calcium, but only about 10% to 15% when vitamin D levels are low.
Ongoing research indicates that vitamin D does much more than protect bones. Vitamin D receptors are not only found in the intestines, but many of the body’s tissues also contain vitamin D receptors, such as the brain, the prostate, the heart, blood vessels, muscles, and endocrine glands.
Vitamin D is vital for facilitating the optimal functioning of the immune system. Sufficient levels of vitamin D are required to trigger T-cells (the protective “killer” cells of the immune system) into action, otherwise these cells can remain dormant and inactive.
According to Harvard there is growing evidence that, although vitamin D may not prevent cancer, it appears to help reduce the risk of death from cancer when people have been on vitamin D supplementation trials for a year or longer. “This might mean vitamin D could make tumours less aggressive, less invasive, or less likely to metastasize” is Harvard’s conclusion.
Normal neuromuscular function requires adequate levels of vitamin D, while vitamin D helps to lower insulin resistance, which plays a major part in heart disease. Studies have shown that a normal intake of vitamin D can reduce the risk of death if you have cardiovascular disease, while deficiency can raise the risk of a heart attack by up to 50%.
Vitamin D influences the working of the thyroid gland, which secretes a hormone that regulates the blood calcium levels, which in turn helps to regulate blood pressure.
Vitamin D acts as a steroid hormone in cells, where it binds to vitamin D receptors in the cell nucleus of almost every cell in the body, where it functions as a transcription factor to modulate gene expression.
Insufficient levels of vitamin D – resulting in reduced levels of vitamin D activity in cells – are also associated with cognitive decline. Vitamin D plays a role in activating the genes in the cells that create and maintain brain synapses, but when the right genes are not activated due to suboptimal levels of vitamin D, it is one of the factors that can contribute to cognitive decline and Alzheimer’s.
How much vitamin D do you need?
Thirty minutes of exposure (without sunscreen) to the face, back or legs at least twice a week should provide ample levels of vitamin D, but might also expose you to dangerous levels of cancer-causing UV radiation.
Modern lifestyles have led to inadequate exposure to sunlight, while the use of sunblock further inhibits the conversion to vitamin D. Ageing is also linked to a higher risk of vitamin D deficiency.
A simple blood test will reveal if you, like many other people worldwide, have insufficient levels of vitamin D in your blood. You may need to take daily supplements to maintain healthy levels of vitamin D.
On the other extreme, mega-doses (more than 30 000 IU per day) of vitamin D supplements over extended periods can result in vitamin D toxicity, which is a very rare but serious condition that causes a build-up of calcium in the blood, which can cause nausea, vomiting, frequent urination, weakness, and may progress to bone pain and the forming of calcium stones in the kidneys. (Mega-doses of vitamin D are usually prescribed by medical practitioners over a very short term in the event of severe deficiency, in order to bring vitamin D up to normal levels.)
After researching local and international guidelines, the South African pathology laboratory, Ampath, has decided to report vitamin D levels, measured in nanogram per millilitre (ng/ml), as follows:
• Vitamin D deficiency: Below 12ng/ml.
• Vitamin D insufficiency: 12 – 19 ng/ml.
• Vitamin D sufficiency: 20 ng/ml and above.
• Safe upper limit: 50 – 60 ng/ml.
Earlier guidelines for the recommended daily supplements of vitamin D, such as these typical (2017) guidelines from the Mayo Clinic (USA), are 400 international units (IU) for children up to age 12 months, 600 IU for ages 1 to 70 years, and 800 IU for people over 70 years.
A study conducted in Saudi Arabia on the treatment of 135 men and women with vitamin D deficiency, has shown that once normal levels of vitamin D have been reached, a maintenance dose of 2 000 IU daily was insufficient to maintain normal levels.
Ampath recommends that the “maintenance tolerable upper limit of vitamin D” is 4 000 IU per day for healthy adults and it should not be exceeded without medical supervision.
The ideal vitamin D levels should be between 40 ng/ml and 70 ng/ml and the correct amount of vitamin D is 5 000 IU per day for adults for the rest of their lives, says the Vitamin D Council (USA), a scientist-led group that promotes vitamin D deficiency awareness. An international panel of scientists at the GrassrootsHealth Nutrient Research Institute (USA) says that it is important to remember that the appropriate measure of vitamin D is not intake, but the achieved vitamin D blood levels, which should be in the region of 40-60 ng/ml.
In cases where insufficient levels of vitamin D may play a role in cognitive decline, vitamin D blood levels of 50-80 ng/ml is suggested by Dr Dale Bredesen, who has been involved in Alzheimer’s research over a number of decades.
It should be kept in mind that vitamin D is being used continuously in the body and insufficient levels may take a long time to increase and reach sustainable levels from daily supplements. A study on athletic performance has indicated that at levels below 40 ng/ml the body diverts most or all of the ingested or sun-derived vitamin D to immediate metabolic needs and this can lead to deficiency.
As you age, your skin is less able to produce vitamin D and after the age of 65 people only generate a quarter of the amount of vitamin D that they did in their 20’s.
As few people have adequate exposure to sunlight and natural diets usually do not contain adequate quantities of vitamin D, most people may not be aware that they need to take vitamin D supplements. The safe option is to have vitamin D levels checked annually, or more regularly in the case of a known deficiency. All it takes is a simple blood test
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Transcription/DNA transcription. Published online in Scitable, by Nature Education. (www.nature.com)