Dr Mark Porter: Should we all be taking vitamin D?

New research showing that vitamin D may protect against cancer of the colon is the latest in a long line of studies suggesting that there is far more to this vitamin than its traditional role in helping to maintain healthy bones. And although this new study involved more than 500,000 people from ten different European countries, it is pertinent to the UK because so many of us have low levels of vitamin D — especially at this time of year.

So should more of us be considering supplements?

Vitamin D is required to help the body make proper use of calcium — the basic building block of bones — which is why interest in it has traditionally centred on rickets, a condition that results from severe deficiency. But we now understand that it does far more than maintain the skeleton, and that milder degrees of deficiency may predispose to a range of other illnesses including diabetes, lung disease, osteoporosis, multiple sclerosis (MS), rheumatoid arthritis, various cancers, and even Alzheimer’s disease.

The vitamin is manufactured in the skin when we’re exposed to sunlight but our northern latitude and short summers mean that for six months of the year UV levels are so low that most of us don’t make enough of the vitamin to meet basic requirements.

The problem is compounded by the use of sunscreens, which further reduce natural UV exposure during the summer months when vitamin D stores are replenished, and the popularity of supplements containing vitamin A (it is thought that as many as 1 in 5 of the population take one), which antagonise some of the D’s actions.

As well as manufacturing the vitamin in our skin, we also take it in from our diet. It occurs naturally in oily fish, such as tuna and herring, and liver, milk and eggs. In most industrialised countries it is added to margarine, some powdered milks, bread and breakfast cereals.

The scale of the potential problem is nicely outlined by researchers from the University of Sheffield, who looked at vitamin D levels in the cord blood of babies born at the hospital at the end of spring — the time when their mothers’ vitamin D stores were likely to be at their lowest. Seven out of ten of the babies had lower than ideal levels of vitamin D, 90 per cent of whom were white, dispelling the myth that deficiency is principally a problem in ethnic minorities with darker skin (the extra pigment reduces UV penetration even further).

Meanwhile in Finland researchers tracking a group of people born in the Sixties turned up evidence suggesting a link between low levels of vitamin D and Type 1 diabetes later in life. Children brought up in the gloomy winters of Finland are more than a hundred times more likely to develop the condition than their peers in sunny Venezuela. Since then UK research has shown that children given vitamin D supplements are around a third less likely to develop diabetes.

Vitamin D is a steroid and there are receptors on almost every cell in the body. Its link with diabetes could be explained by an action on receptors in the pancreas and on the various cells in the immune system, but there are similar vitamin D specific targets in almost every other organ, including the brain, heart, muscles, kidneys and, of course, the skeleton. Little wonder then that there is growing interest in the role of the vitamin in diseases, ranging from MS to Alzheimer’s.

Should we all be taking a supplement? It’s too early to tell whether mass supplementation is the way forward, but it is now widely agreed that the following key groups are those most likely to benefit:

Women who are pregnant or breastfeeding
Children under 5 (particularly breastfed infants)
The over-fifties
Vegetarians who don’t eat oily fish
Anyone who is housebound or who covers up when outside
Those who are dark-skinned — particularly Asians.

The standard dose for an adult is 10mcg day and trials looking at the safety of long-term supplementation have reported no adverse effects at intakes of more than double that. Much higher doses (50mcg a day or more) can weaken bones and increase the risk of calcium deposits (such as kidney stones) forming elsewhere.

During the summer most people will manufacture enough vitamin D by exposing their face and arms to the sun for an hour across the week. You don’t have to sunbathe, just accumulate around ten minutes of exposure while going about your normal business — assuming you are not wearing a sunscreen, or a moisturiser that contains one.

http://www.timesonline.co.uk/tol/life_and_style/health/expert_advice/article7008426.ece