Are you an indoors or outdoors person? Me, I find it hard to sit for hours on the sofa. Even though I spend half my life at a desk, a whole day at home without going out leaves me feeling somehow dirty.
I need the cleansing power of fresh air.
So I was surprised and dismayed to discover a year ago, following a routine blood test, that all the readings were normal – except for my vitamin D. It was on the threshold between "low" and "insufficient", and a long way below where it ideally should be.
My GP recommended supplements. A sober man who, like me, spends his holidays tramping the hills and, like me, considers most supplements worthless for most people, he nevertheless makes an exception for Vitamin D. He takes it himself – the bottle was on his surgery desk in front of him.
As it happened, I had a bottle of my own at home – a gift from a friend who is a leading advocate for Vitamin D. I had set the pills aside thinking that I, an outdoorsy sort, surely did not need them. I was wrong.
It turns out that I am not alone. This week, new guidance from the National Institute for Health and Care Excellence (NICE) said that as many as one in five people in Britain may be deficient in the vitamin.
It recommended that free supplements be given out more widely, especially to the elderly, who may be at increased risk of osteoporosis and to children, threatened by the bone deformity rickets.
Cases of rickets – a disease associated with the Victorian era – have risen sharply in recent years and Dame Sally Davies, England's Chief Medical Officer, warned recently that children who spend too much time playing on computers and not enough outside in the sunshine could be in danger from the disease.
Vitamin D is the only vitamin we make for ourselves – through the action of sunlight on the skin. Although some comes from the diet – fish, Marmite and fortified breakfast cereals are good sources – few people realise that even a balanced diet cannot supply enough on its own.
Sunshine is necessary. If you can't get enough of the real thing, then supplements – bottled sunshine – are a good substitute. In summer, 15 minutes' sun exposure of hands and face each day should be all that is needed to provide adequate levels.
But in winter, it is a different story. The gloomy weather and low light in countries north of 30 degrees latitude means that a large part of the UK population is deficient between October and March.
I was tested in February 2013 – which could explain my low level. I started taking supplements immediately, stopped last summer and started again in October. At my most recent blood test, in March, my level was normal and a mini heatwave at the end of the month, which had us eating in the garden, persuaded me to stop again.
Next October, I will be reaching for the bottle of pills once more.
I shall not, however, be taking the mega-doses some have recommended. Vitamin D is essential for healthy bones. But it has been promoted in recent years as a kind of panacea, with studies claiming it protects against heart disease, cancer, diabetes, high blood pressure, schizophrenia and multiple sclerosis, among others.
It may indeed have a protective effect in these diseases – but so far, the proof is lacking. In a report in 2010, the authoritative US Institute of Medicine reviewed more than 1,000 studies and concluded the vitamin had been oversold. The high levels some doctors recommended were unnecessary and could even be harmful. Doses above 4,000 international units a day were inadvisable, the IOM said.
When next winter comes around, I shall be taking half that amount.
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