NZ First’s vitamin D review an expensive waste of time

The general election looms and political parties have turned their minds to introducing new, attractive health policies. New Zealand First has released a 15-point health plan, one of which is to “fund a review of vitamin D and determine optimal levels for maintaining health”. Yet after decades of research into vitamin D supplementation, there is little evidence that more reviews or testing would improve health outcomes. At a time when health resources are stretched, is this the best use of public money?

Vitamin D was identified in the 1920s as the cause and cure for rickets, a condition causing bone deformity in children. More recently, it has become widely used in some countries as a supplement and touted as a remedy for a huge range of medical conditions.

While some vitamin D comes in the diet, most is produced in our skin as a result of exposure to sunshine. Thus, vitamin D deficiency is primarily a problem of not getting enough sun, typically occurring in frail elderly individuals who seldom get outside, or in veiled women.

Vitamin D deficiency has been a particular problem when people with dark skin, which is less efficient in manufacturing vitamin D, move to relatively sun-deprived areas. There have been epidemics of rickets in the children of dark-skinned immigrants to Britain and northern Europe, for instance, a problem now prevented by the widespread use of vitamin D supplements in those at-risk populations.

There is good awareness of vitamin D deficiency among medical practitioners in New Zealand. Many frail older people, particularly those in institutions, routinely receive vitamin D supplements to compensate for their lack of exposure to sunshine. There is also an awareness that vitamin D deficiency can occur in babies, particularly those who have prolonged breastfeeding or whose mothers are at risk of vitamin D deficiency.

As a result, people at risk of Vitamin D deficiency are routinely given supplements, and serious illness caused by deficiency is now uncommon in New Zealand. So it’s unclear why vitamin D should be a priority in New Zealand First’s health policies. Possibly this concern has travelled here from countries where vitamin D supplementation has been more widespread, such as in the United States.

Very many studies over the last 30 years have found that patients suffering from a broad range of medical conditions (affecting the heart, respiratory system, diabetes, cancer, nervous system etc) have low blood vitamin D levels. This led to the hypothesis that vitamin D deficiency caused a large number of medical conditions, and which led to the widespread use (in some countries) of vitamin D as a tonic that would treat or prevent these conditions.

It also led to a lot of clinical trials to determine whether vitamin D supplements could prevent or improve such illnesses. This has been a massive undertaking, with studies involving as many as 25,000 participants being followed for up to five years.

Overwhelmingly, these studies failed to show vitamin D supplements improved bone density, or reduced risk of fractures, falls, cancer or cardiovascular disease. The hardening consensus internationally is that the use of vitamin D supplements in the healthy general population is without benefit. Indeed, over-vigorous supplementation can produce adverse effects. Some studies suggested it could increase the risks of falls and fractures.

If low vitamin D is found in many people with illnesses yet vitamin D supplements don’t have a positive impact on those illnesses in trials, we can only conclude that many illnesses cause low vitamin D levels, not the reverse. Possibly they have low levels because people who are unwell, for whatever cause, spend less time outdoors.

Other contributions to the relationship between Vitamin D and illness are that some medical conditions are associated with obesity and obesity itself results in lower blood levels of vitamin D; the vitamin is taken up into fat cells and therefore isn’t measured in the circulation. Also, inflammatory conditions are associated with tissue damage which leads to loss of vitamin D or the protein that it binds to from the blood, again leading to lower measured levels.

This is how science is supposed to work. Researchers see a pattern (that many people with a range of illnesses have low vitamin D levels), develop a hypothesis (that low vitamin D causes these medical problems), and then test the hypothesis (through a series of randomised controlled clinical trials). This means a definitive conclusion can be reached; that vitamin D deficiency results from a wide range of medical conditions but it is not the cause of most of them.

A concerning aspect of the New Zealand First proposal is the wish to determine the optimal vitamin D levels for the New Zealand community. In countries where there has been a focus on optimal vitamin D levels, this has led to the widespread measurement of vitamin D at great expense to the health budget with no tangible medical benefit.

Because of this experience overseas, most New Zealand doctors provide supplements to those who are lacking in sunlight exposure but they avoid measurements, except in unusual circumstances where there is significant diagnostic doubt about a patient’s medical problems.

After decades of research and large clinical trials, our conclusion about Vitamin D is quite straightforward; the major conditions caused by severe vitamin D deficiency are rickets in a small number of children and some under-mineralisation of bone in elderly people who don’t spend enough time in the sun.

Vitamin D supplements should be and are being targeted to these at-risk groups, but routine testing and supplementation across the wider population would be costly and unlikely to improve public health.