Vitamin D is sometimes called the ‘sunshine’ vitamin and is also known as cholecalciferol. It can be synthesised in the body when the skin is exposed to UV radiation and consumed in selected foods.
Pure vitamin D is a white crystalline substance which is soluble in fat rather than water. Vitamin D is actually made up of 10 compounds and these include D1, D2, D3 and so on. However, the most important of these is D2 (ergocalciferol), which is derived from plant sources. As noted vitamin D can be synthesised by the body in response to bright sunlight, however this process is hampered by clothing and of course poor weather. Pigments and proteins in the skin can also mediate this process, hence the need for dietary sources if we don’t get enough or absorb enough via sunlight. Our livers and kidneys convert vitamin D to form its physiologically active form.
Why do we need it?
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Vitamin D plays a dual role as a vitamin and as a hormone. As a vitamin it helps to regulate calcium and bone metabolism and as a hormone it functions by sending messages to the intestines to promote the absorption of calcium and phosphorous. This is important for us in terms of bone formation and having strong teeth. Without vitamin D, for example, our bones can become brittle and thin and even misshapen. Research also indicates that vitamin D can also help promote a healthy immune system and regulate cell growth and differentiation (the process which determines what type of cell it becomes when it matures).
How much vitamin D do we need?
Because vitamin D can be synthesised in the skin on exposure to sunlight there’s no general UK Dietary Reference Value (DRV) for adults. However, around 10-15 minutes of sun exposure at least twice a week to the face, arms, back and hands without sunscreen is unusually enough to get enough vitamin D. However, it is very important for people with limited sun exposure to include good sources of vitamin D in their diets. In this respect the Reference Nutrient Intake (RNI) is set at 10mcg per day (1 mcg = one thousandth of a milligram). For pregnant and lactating women 10mcgs of vitamin D supplementation is recommended on a daily basis. For infants and children under two, up to 7mcgs is recommended. Men and women over the age of 65 should also supplement with 10mcgs.
What happens if I don’t get enough?
Because vitamin D is required for bone metabolism a shortage can result in bone softening (rickets in infants and children, osteomalacia in adults), tooth decay, reduced bone healing after fractures, defective bone growth, inadequate absorption of calcium, muscular weaknesses and impaired immunity.
Who should take the most care to regulate vitamin D intake?
Anybody who spends a lot of time indoors, lives in a cloudy climate or very northerly latitude. Dark skinned people living in such areas have even more of this need. Strict vegetarians are also at risk of not getting enough vitamin D.
Can I get too much vitamin D?
Yes, vitamin D is fat soluble and not really eliminated, so it can accumulate in the body’s tissues if too much is consumed. This can lead to calcium deposition in deep soft tissue and possible kidney damage. The UK Food Standards Agency recommends that vitamin D supplementation should not exceed 25mcg per day.