So how do you know if you are getting enough?
Text books will tell you that you should be getting between 5mcg (200iu) and 10mcg (400 iu) a day, some of which we can get from food – notably fatty fish and cod liver oil – and the rest from sunlight. A few minutes of exposure of the face and hands a few times a week should be enough. That’s enough to prevent the bone softening that shows up as rickets.
But experts like Robert Heaney, professor of Medicine at Creighton University, Omaha believe that the blood levels that this amount provides means you should actually be classed as deficient. Heaney, for instance, calculates that to get most of the population up to a reasonably healthy level would require a daily supplement for everyone of ten times the RDA of 5mcg (200iu)– that’s 50mcg (2000iu) - on top of whatever they were getting already from diet and sunlight.
And this raises another even more controversial issue – toxicity. Along with a low RDA vitamin D traditionally comes with a big warning of the dangers of the harm it can do if you take too much. Over 2000 IU, according to some UK sources, puts you at risk of absorbing too much calcium, leading to damage to liver and kidneys. Other side-effects of overdosing are said to include increased thirst, nausea, vomiting, and the deposit of calcium in blood vessel walls.
But even among academics there is little agreement on this. The Food and Nutrition Board of the American Institute of Medicine has set the guaranteed safe level per day at 50mcg (2000 iu), while many of the North American experts believe much more is safe. Dr John Cannell takes 125mcg (5,000 iu) himself daily in the winter and advises people to take 50mcg (2000iu) for each kilo of their body weight daily for three days at the first sign of an infection.
He’s supported by Reinhold Veith, at the Department of Laboratory Medicine and Pathobiology, University of Toronto who believes 1000mcg (40,000iu) could be a toxic dose but only if taken over a long time. These campaigners regularly point out that a fair-skinned person with about 80% of their skin exposed to the sun for 20 minutes at mid-day can produce about 250mcg (10,000iu) with no harm at all.
Further support for a higher level being both necessary and safe came from a major review of Vitamin D published in the prestigious New England Journal of Medicine  which concludes that: “As long as (someone’s) combined total is 30ng per millilitre, the patient has sufficient Vitamin D.” Now that’s an amount that translates to about 65mcg (2600iu) a day. And as for the dangers, the review comments that: “Doses of 10,000 IU per day for up to 5 months do not cause toxicity.”
And Dr Hypponnen’s study  , published in the American Journal of Clinical Nutrition, seems to suggest that most people aren’t getting enough. It’s long been known that people in some immigrant groups are more likely to be deficient. She describes as “alarming” her discovery that most white middle-aged people, the ones thought to be OK, are also seriously deficient. “Only 40% of them had enough vitamin D in their blood to make their bones as strong as possible, even in the summer,” she says.
As yet it’s too early to say who is right about all of this; the one thing everyone agrees on is that these new ideas about vitamin D need further testing. But modern day living does seem designed to reduce our vitamin D intake to a minimum. We’re positively Dracula-like when it comes to sunlight, spending our days in doors and when we do venture out, we are urged to slap on the sun block. As for our diet, the low-fat mantra discriminates against foods with vitamin D, most of which come with high doses of fat and cholesterol.
“Most people living in America and Northern Europe have levels of vitamin D that are low, based on the latest evidence,” declares Professor Meir Stampfer, an epidemiologist at Harvard Medical School. “And on top of that as a population we are older and fatter than we used to be and both those things reduce our ability to absorb Vitamin D from sunlight.”
Since our food is unlikely to be fortified any time soon, should you be taking a supplement? No one can tell you for certain but the answers to the questions below may help you decide. And you can take a home test to check your levels.
Can’t we get enough from sunlight?
To make Vitamin D you need sunlight that contains UV B rays which can be blocked by cloud and fewer and fewer of them reach the ground the further north you go. (It’s the UV A rays that age you the most, and having enough vitamin A in the skin protects you from this.) In the States certain disorders such as multiple sclerosis and colon cancer became much more common the further you move up from the sunbelt in the south.
Here in the UK, Liverpool is about level with Edmonton in Canada (53 degrees north) where you can make little vitamin D for up to six months of the year. According to Dr Hypponnen’s study, Scottish people were twice as likely to be vitamin D deficient as those south of the border. Even in Boston – 42 degrees north – you won’t be able to make vitamin D from sunlight for four months from November to February and Boston is a bit south of Marseilles.
Even in summer in the UK you only really get enough UVB rays between about 10am and 2pm; just exposing your hands face and arms during that time gives you about 5 to 10mcg (200 - 400iu), which isn’t nearly enough for the new levels. To get those the American experts are suggesting you need to expose 50% to 80% of your skin for about 20 minutes.
There’s a lot of debate about the benefits of tanning beds. They are strongly opposed by those concerned about skin cancer; another criticism is that they don’t supply the right balance of UV B and UV A rays. So some experts have suggested trying sun lamps, which maintain the natural balance. There’s not enough evidence to make a firm judgement.
How do I know if I’ve got enough?
The vitamin D that you make from sunlight or get from either a supplement or your diet gets stored in your blood in a form known as 25(OH)D. A level of 27.5nmol/L (nanomoles per litre) of 25(OH)D is what you’d make with around 10mcg (400iu) a day. “That’s enough to prevent rickets but not enough for your health,” says Dr. Heaney.
In fact in the latest studies it’s a level that counts as deficient; 50nmol/L is considered as just about acceptable and in Hypponnen’s study the optimum level was 80nmol/L and over. This is the level that most other studies find benefit but to get that you need about 40mcg (1600iu).
Most people’s vitamin D blood level is below 50nmol/l. You want to get this up to 80nmol/l for maximum protection. While 15mcg, which is what's in in the multivitamin I take, is enough for most of the year, if your vitamin D level is average you want to boost this during the winter by taking the equivalent of 30mcg daily.
However TAKING VITAMIN D ONCE A WEEK works just as well.
Here’s what I do and why: if I’m already taking 15mcg every day I need an additional 15mcg x 7 = 105mcg a week. I take a vitamin D emulsion that gives 25mcg(1000iu) per drop. So, I just take 4 drops once a week.
A recent study of teenagers confirms this. The researchers found they needed up to 30mcg of vitamin D during the winter to boost vitamin D levels to a healthy level.
How much you need also depends on your age. An average 70-year-old, according to Stampfer, can make only about a quarter of the vitamin D from the sun as a 20-year-old. American experts recommend anyone taking a large supplement should have their blood level tested and if you want to check your vitamin D level you can order a simple home test kit.
Could I be harmed if I take too much?
Although vitamin D undoubtedly can be toxic in very large doses taken for a long time, just how much we can take without running into trouble is very uncertain. Certainly there are lots of examples of people who have taken large doses with no ill effect. For instance there’s journal report of wheelchair-bound patients with severe weakness and fatigue who were given very high doses – totalling 1250mcg (50,000iu) a week; they suffered no problems and were walking after 6 weeks.
Another described how a group of adolescents with a severe deficiency were given single monthly doses of 2,500mcg (100,000iu) with no ill effect. Heaney points out that light-skinned people working out of doors often show up as having 200 nmol/L in their blood at the end of summer.
All vitamins are team players and one way of avoiding problems if you are taking a high dose supplement of vitamin D might be to make sure have good levels of minerals such as calcium and magnesium that normally interact with it. A recent feature in the journal Medical Hypotheses suggested that additional amounts of vitamin K at the same time could well reduce the danger . This is an area that needs further research.
Can it fight off infections?
The link between vitamin D and infections comes via an anti-microbial peptide called cathelicidin which our immune system uses to fight off bacteria, viruses and fungi. A report in the online journal ScienceNews describes how the gene that makes cathelicidin becomes more active when vitamin D is present. The peptide works by punching holes in the outside wall of a bacterium causing its insides to leak out.
One research group has found that if you are infected with TB there is a rise in the active form of vitamin D, but for that to happen it has to be available in the stored form. In one study those with low levels of stored vitamin D produced 63% less cathelicidin when infected with TB than those with more. This could explain why dark skinned people in northern countries are more susceptible to TB – their skin needs longer exposure to the sun to make the same amount as a pale person.
Perhaps raising your blood levels of vitamin D would be a good idea before going into hospital where infections such as MRSA are a real danger.
What is the cancer connection?
Several teams of researchers have found that adequate levels of vitamin D lower the chances of developing breast cancer. This latest research comes from a team of cancer prevention specialists at Moore’s Cancer Centre at University of California San Diego, published in the Journal of Steroid Biochemistry and Molecular Biology.
They found that women with the highest level of vitamin D in their blood (up to 120 nmol/L) had the lowest risk of breast cancer, while those with the lowest level (25nml/L) had the highest risk.
“The results were very clear,” said co author Dr Cedric Garland “the higher your level, the lower the risk.” To have a blood level that would cut your risk by 50%, the researchers said that you would have to take 50mcg (2000iu) daily and also spend ten to fifteen minutes in the sun.
Similar blood levels and vitamin D intake were found to protect against colon cancer in a study that followed 1500 people for 25 years. At 80nml/L the rate was cut by 50%, while over 100nmol/L reduced it by 66% according to the American Journal of Preventative Medicine. This team did say that 50% of the skin should be exposed to the sun.
And the same seems to be true of prostate cancer. A team led by a researcher from Harvard Medical school followed 15,000 men and found that those who had below average levels of the vitamin in their blood had a “significantly increased risk of aggressive prostate cancer.” In this study, published in PLOS Medicine, 50 nmol/L counted as deficient and even 80nmol/L was described as “sub-optimal”.
Cancer experts in this country are still cautious about accepting studies like these. They point out that that although they show people with higher levels of vitamin D in their blood have a lower risk, that doesn’t definitively show that it was a lack of the vitamin that caused it. People who keep their vitamin levels up, they argue, may do all sorts of other healthy things as well.
If you’d like to find out more about cancer and vitamin D read my Special Report.
The cholesterol connection
Vitamin D rich foods include many of the ones we no longer eat – intestines, organ meat, skin and fat as well as shell fish and kippers and herrings. Just to make matters worse the animals themselves need to have been regularly exposed to sunlight and intensively reared animals aren’t. So getting the higher amounts from food, especially on a largely vegetarian diet, is hard. You’d have to have oily fish at least a couple of times a week plus cod liver oil.
But something else besides our Dracula lifestyles and lack of Vitamin D rich foods may be having an effect and that’s the widespread use of the cholesterol lowering drugs statins to prevent heart disease. In order to manufacture vitamin D the skin needs cholesterol. So could statins be reducing production? No one knows the long-term effect because there have only been three studies – the longest lasted just three months; they found no effect.
What else can vitamin D do?
The possible effect of the vitamin D winter famine isn’t confined to flu. People are generally sicker in winter; they are more likely to have heart attacks, patients diagnosed with cancer in winter die sooner, there’s a rise in depression and weight gain. All of which have been linked to a lack of vitamin D in the blood.
The good health associated with the Mediterranean diet, particularly its ability to stave off heart disease may have as much to do with the Mediterranean sun as with the regional food. Diabetes also seems to benefit from good levels of vitamin D.
A study of 10,000 young children by the Institute of Child Health in London found that giving a very high vitamin D supplement of 50mcg (2000iu) reduced the risk of insulin-dependent type 1 diabetes by 88%. Other reports have found it can stabilise blood glucose levels. The FDA says that more research needs to be done before vitamin D can be conclusively linked with a raised risk of diseases other than calcium and bone problems.
How Much Do You Need Each Day?
In the pursuit of optimum nutrition we recommend eating oily fish three times a week (a serving of salmon or mackerel provides around 8mcg, eating six free-range eggs a week (an egg provides 0.5mcg, exposing your skin to the sun every day (you’ll make around 10mcg with 20 minutes exposure between 10 and 2pm), and supplementing at least 15mcg (600iu). That will give you around 30mcg (1200iu) a day. During the winter months (November to April in the UK) it’s probably worth supplementing an additional 25mcg(1,000iu) and possibly more, especially if you are older and live further North.
Jerome Burne and Patrick Holford
 Oliver Gillie (Editor) SUNLIGHT, VITAMIN D & HEALTH - A report of a conference held at the House of Commons in November 2005, organised by the Health Research Forum Available as a free download from http://www.healthresearchforum.org.uk
 Hyppönen E. and Power C, ‘Hypovitaminosis D in British adults at age 45 y: nationwide cohort study of dietary and lifestyle predictors Am J Clin Nutr. 2007 Mar;85(3):860-8
 Heaney R.P., ‘The case for improving vitamin D status’, The Journal of Steroid Biochemistry and Molecular Biology, 2007 Mar; 103(3-5):635-641
 ScienceDaily - Vitamin D Backed For Cancer Prevention In Two New Studies - February 8 2007 Available online from www.sciencedaily.com
 Cannell J.J. et al., ‘Epidemic influenza and vitamin D. - Epidemiol Infect. 2006 Dec;134(6):1129-40.
 Holick M.F., ‘Vitamin D deficiency’, New England Journal of Medicine Jul 2007; 357(3):266-281
 Masterjohn C., ‘Vitamin D toxicity redefined: Vitamin K and the molecular mechanism’, Med Hypotheses. 2007;68(5):1026-34.