I was working in Otago last year and it was noticeable how many patients down there are on regular Vitamin D.
In Auckland we tend to prescribe this only for the traditional high-risk groups – those with dark skin or who cover their skin most of the time, or aged and disabled people who don’t get much sun exposure.
But studies have shown that many people living in the southern part of NZ are deficient in Vitamin D over the winter months, hence its common use.
This vitamin has a fascinating history, illustrating both the achievements and limitations of medical science. Even though the bone disease rickets was recognised in antiquity, the discovery that it was due to Vitamin D deficiency was made only in the early 20th Century.
Treatment with supplements such as cod liver oil and exposure to sunlight was very successful, giving the impression that rickets was “licked” as a public health problem. But at various times, and in various groups, it has made a comeback.
Also, this emphasis on bone disease obscured the fact that Vitamin D has lots of other functions, including fighting infection. And when this vitamin is short, the body will preferentially use it to support bone strength at the expense of the immune system. So, the traditional recommended levels were set a bit low – enough to prevent rickets but not enough for its other functions. Subsequent studies have shown an association between low Vitamin D levels and various cancers, cardiovascular disease, diabetes, autoimmune conditions and even mental illness. Also, obesity can adversely affect its levels. But this is where it gets tricky. Are these associations causal or coincidental? Because this question wasn’t answered for a long time, the gap was filled by unsupported claims for Vitamin D as a prevention and cure for just about everything. This was compounded by genuine uncertainty about optimal blood levels and doses.
In addition, the move to avoid sun exposure due to skin cancer risk has made it harder for the general population to maintain good Vitamin D levels. Fortunately, some good quality research has made things a bit clearer – optimal blood levels have been identified (greater than 50 nmol/l). However, this might not help much, as you can’t get a test done in our local lab unless you pay for it! Doctors are discouraged from testing because it is expensive. If a patient is considered at risk then the policy is to just treat them (usually with one tablet a month, which is adequate for most). But if you are really keen you can pay the $80 and then discuss the results with your doctor.
There seems no point in giving extra Vitamin D to people who have normal levels. A recent report in Nature concluded from several large studies that Vitamin D supplementation to people with normal levels did not prevent any of the above diseases known to be associated with deficiency. So, because something is good for you it doesn’t mean that more is better. And there is a small risk of toxicity with high doses. For an excellent summary of this advice see www.consumer.org.nz
As always, if this has raised questions for you, seek medical advice from your GP or other health professional.