Why is vitamin D’s role in heart disease ignored?
Our clinical columnist Dr Malcolm Kendrick wonders why an easily treatable risk marker for CVD doesn’t appear in any guidelines.
Our clinical columnist Dr Malcolm Kendrick wonders why an easily treatable risk marker for CVD doesn't appear in any guidelines.
We spend billions lowering blood pressure, glucose and cholesterol levels. Yet, there is a risk factor for cardiovascular disease and premature mortality that may be the most important of all. But it is not even routinely measured, let alone treated. The risk factor is a low level of vitamin D.
Caucasians and other pale-skinned races have light skin for a very important reason: to synthesise enough vitamin D to keep them healthy. As dark-skinned people moved further from the equator, they were no longer exposed to enough sunlight – specifically UVB radiation – to convert cholesterol into vitamin D within the epidermis. So, to counteract this, their skin became light.
But for those who spend most of the day indoors, even having light skin is not enough to provide enough vitamin D during the winter months. Perhaps more critical is that dark-skinned people who emigrate further from the equator will find it almost impossible to maintain vitamin D levels – or, to be more accurate, plasma levels of 25-hydroxy-vitamin D (25(OH)D), which is the active substance.
A study in Germany of the immigrant population of children found that 30% of them had severe to moderate vitamin D deficiency1.
More than 90% had 25(OH)D concentrations below 75nmol/l (levels above 75nmol/l are defined as optimal regarding various health outcomes). Boys with a Turkish or Arab-Islamic background had double the risk of having suboptimal concentrations as non-immigrants.
For children of Asian background, the risk of vitamin D deficiency was increased sevenfold. With children of African background, the risk of low vitamin D concentrations was increased eightfold.
The authors recommended that, especially in immigrant children, supplementation of vitamin D beyond infancy, especially in high-risk groups, or fortification of food should be considered.
Low levels, high risk
This is an important health issue – and not just for immigrants. Everyone is aware of the vital role of vitamin D in bone health for protecting against osteomalacia, rickets and osteoporosis. But it is becoming increasingly clear that a lack of vitamin D, at less extreme levels, is also a significant risk factor for many other diseases.
For example, a study done on more than 18,000 health professionals over a 10-year period found that those who were relatively vitamin D deficient were more than twice as likely to develop heart disease2 as those who were not. In the Framingham study, the risk of cardiovascular events was 65% higher in those with the lowest vitamin D levels3. And a study of more than 3,000 patients who were to undergo angiography found low vitamin D levels were associated with a doubled risk of all-cause and cardiovascular mortality4.
But it's not just heart disease. There is powerful evidence that mortality from diabetes and cancer is also significantly higher. This association was first noted in North America, where it was found that mortality from several potentially life-threatening chronic health conditions such as cancer, CVD and diabetes rises with increasing latitude – that is, increasingly distant from the equator.
Other studies have shown that the survival of patients with cardiovascular disease or with some cancers – such as lung, colorectal or breast cancer – was greater if the diagnosis was made during the summer as compared with the winter.
With regard to cardiovascular health, a number of different mechanisms of action link low vitamin D levels to adverse events. For example, low vitamin D leads to upregulation of the rennin-angiotensin-aldosterone system, leading to hypertension. There is also increased insulin resistance and inflammation – resulting in higher C-reactive protein and interleukin-10 levels.
And it does appear that supplementation with the vitamin can reduce cardiovascular and all-cause mortality. A meta-analysis looked at almost 20 randomised trials with 57,000 individuals. The main finding was that a vitamin D intake averaging 500 international units reduced all-cause mortality, mainly by reducing cardiovascular mortality5. (It has been proposed in The Lancet that the benefits of statins are due to the fact that they are vitamin D analogues6.)
So, we have a vital substance that is at suboptimal levels in many people. This is especially true in the winter months and in the immigrant population, who tend to have worse health outcomes than the surrounding population.
It can easily be measured. It can be very simply, and cheaply, treated. And when treated it has a significant impact on overall and cardiovascular mortality. Yet it appears in no guidelines and receives almost no attention whatsoever.
Dr Malcolm Kendrick is a GP in south ManchesterVitamin D