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Back in 1913, Sir Edward Mellanby, a medical doctor discovered that that the disease rickets (softening and weakening of bones) was extremely high in the United Kingdom especially in Scotland.¹ He determined that cod liver oil seemed to be curing the disease and hypothesised that Vitamin A, a constituent of cod liver oil was responsible for the prevention of rickets.¹ However, biochemist Elmer McCollum (who incidentally discovered Vitamin A) decided to test this hypothesis and correctly concluded that the factor that cured rickets was a new vitamin, which he called vitamin D.¹ Why call it ‘D’ you wonder? Because it was the next free letter of the alphabet for naming vitamins.
WHY DO WE NEED IT?
Vitamin D is a fat-soluble vitamin which exists in two forms; ergosterol (vitamin D2) and cholecalciferol (vitamin D3) and is responsible for regulating calcium levels in blood to promote muscle and bone health in order to help prevent conditions such as rickets and osteoporosis.² ³
HOW CAN WE GET VITAMIN D?
It is obtained mainly through the direct action of sunlight (90%) or through dietary intake (such as dairy products, eggs and fish).² It is important to highlight that it is almost impossible to obtain sufficient vitamin D from the diet alone.⁴
The Australian Recommended Daily Intake (RDI) of Vitamin D for adults aged 19-50 is 5 micrograms/day (200 International units (IU)).⁵ However, there is evidence that at least 20 micrograms (800 IU) is required for adequate benefit.⁶
In addition to sunlight and diet, supplements can also be taken. Vitamin D3 supplementation (cholecalciferol) is recommended since D3 is used more effectively in the body. Cholecalciferol (vitamin D3) 1000 IU or 25 micrograms is the supplement most commonly used.⁶
WHAT BIOLOGICAL PROCESSES ARE INVOLVED?
Vitamin D2 is found naturally in sun-exposed mushrooms whereas D3 is synthesised by humans from UVB sunlight.⁷ Humans cannot make vitamin D2 and fish such as salmon, mackerel, and herring contain vitamin D3.⁷
Ultraviolet (UV) radiation from the sun is necessary to produce vitamin D3 in the skin and is the best natural source of vitamin D.⁷ However, UV radiation from the sun is also the main cause of skin cancer not to mention that Australia has one of the highest rates of skin cancer in the world.⁸ ⁹
As Vitamin D3 from the skin or diet is biologically inert, a chemical reaction needs to occur in the liver to form calcidiol (25(OH)D) (which is a reliable indicator of vitamin D status).⁷ However, calcidiol requires another reaction in the kidneys to form the biologically active form of vitamin D, calcitriol (1,25(OH)2D) which then stimulates intestinal calcium absorption.⁷
VITAMIN D DEFICIENCY
Vitamin D insufficiency affects almost 50% of the population worldwide.¹⁰
Vitamin D deficiencies can cause demineralisation of bones, resulting in an increased susceptibility to fractures.¹¹ Low vitamin D levels can cause bone and muscle pain resulting in rickets in children and osteomalacia in adults.¹²
There is no definitive value that indicates that an individual is vitamin D deficient, however most sources generally consider levels lower than 50 nmol/L as suboptimal for adults.¹³ The Australian Health Survey 2011–12 found that 23% of Australian adults had Vitamin D levels below 50 nmol/L.¹⁴
Vitamin D deficiency is also associated with the development of cardiovascular diseases¹⁵ and various types of cancer and autoimmune disorders.¹⁶
VITAMIN D AND IMMUNE HEALTH
Although the classical actions of vitamin D are to promote calcium homeostasis and bone health, Vitamin D is also thought to play a role in maintaining the immune system.³ ¹⁷
Early evidence of vitamin D’s stimulant properties on innate immunity came from reports about tuberculosis treatment with cod liver oil.³ More current studies have elucidated that calcitriol (active form of Vitamin D) enhances the antimicrobial effects of cells, which fight against Mycobacterium tuberculosis (the bacteria which causes tuberculosis).³
A double-blind placebo study using nasopharyngeal swab culture and a therapeutic dose of vitamin D showed that vitamin D administration resulted in a statistically significant (42%) decrease in the incidence of influenza infection.¹⁸
Additionally, a recent review of the role of vitamin D and influenza concluded that the evidence of vitamin D’s effects on the immune system suggest that it should reduce the risk of influenza, but that more studies are required establish a causal relationship.¹⁹
THE TAKE-AWAY
Vitamin D is responsible for regulating calcium levels in blood to promote muscle and bone health with insufficient intakes resulting not only in the classical deficiency diseases of rickets and osteomalacia but also in increased bone metabolism and fracture risk.
Vitamin D is also emerging as a promising vitamin for immune health with accumulating evidence warranting further research.
Make no bones about it, Vitamin D is an essential nutrient which is vital for everyday health.
