Vitamin D deficiency linked to high blood pressure

"Vitamin D supplements could help high blood pressure," The Independent reports. The paper reports on new research into genetic variations associated with low vitamin D levels and their relationship with blood pressure.

Researchers pooled 35 studies of nearly 100,000 people with a European background. They found that the lower the vitamin D levels, the higher blood pressure was.

But they did not look at whether vitamin D supplements or exposure to sunlight would lower blood pressure. And, similarly, they also did not look at whether a lowering of blood pressure improves health outcomes.

These findings are also limited by the fact the study only included people with a European background. It is therefore unclear whether a similar association would be found in other ethnicities.

It is important to have adequate vitamin D levels, as deficiency can cause symptoms such as tiredness, general aches and pains, and, if more severe, rickets in children and osteomalacia in adults.

Read our special report about the alleged health benefits of vitamin D.

Where did the story come from?

The study was carried out by researchers from universities across the UK, Ireland, Norway, Germany, the US, Finland, Sweden, Denmark, Croatia, Austria, the Netherlands and Australia.

It was partly funded by the British Heart Foundation, the UK Medical Research Council, the National Institute for Health Research, the NHS, and the Academy of Finland.

Other funding sources for the studies included in this research were drug companies, food manufacturers and lifestyle companies across Europe.

The study was published in the peer-reviewed medical journal The Lancet: Diabetes and Endocrinology. It has been published on an open access basis, so it is free to read online.

The quality of the media's reporting of the study was mixed. While the overall findings of the study were reported accurately, many news sources jumped to the conclusion that supplements or sunlight exposure could lower blood pressure. For example, the Daily Express claimed that, "Sunshine is the best way to get your daily dose". It is unclear what evidence, if any, this claim is based on.

What kind of research was this?

This research was a meta-analysis that pooled the results of 35 cohort studies. It aimed to further knowledge about any association between vitamin D levels and high blood pressure.

Previous observational studies have shown an association between low vitamin D and an increased risk of hypertension (high blood pressure) and cardiovascular disease.

It has proved difficult to measure vitamin D levels over time, however. It would be unethical to design a study where one group of people were made to have low levels of vitamin D because of the risk of adverse effects, such as bone damage.

The researchers therefore had to find an alternative way of comparing people with different levels of vitamin D. They did this by looking at variations in four genes involved in the production and metabolism (breakdown) of vitamin D in a large population sample.

The researchers hypothesised that these genes should have an influence on lifelong vitamin D levels, so they could be used to look for an association with blood pressure and hypertension.

This type of study cannot prove causation – that is, it cannot prove that low vitamin D causes high blood pressure. A randomised controlled trial making use of vitamin D supplements would be required to do this.

What did the research involve?

The researchers pooled results from 35 cohort studies of people with European ancestry from Europe and North America. This included 31 adult studies (99,582 people) and four adolescent studies (8,591).

Genetic analysis recorded the status of four genes, two of which affect the production and two that affect the metabolism of vitamin D. Variations in these genes (called single-nucleotide polymorphisms, or SNPs) have previously been associated with lower levels of vitamin D.

Actual vitamin D levels were available for participants in 19 of the studies (51,122).

Blood pressure measurements were provided for all of the studies. They defined hypertension (high blood pressure) as a systolic blood pressure reading of 140mmHg or higher, a diastolic reading of 90mmHg or higher, or current use of antihypertensive drugs. This definition would be considered reasonable by most experts.

If people were taking antihypertensive drugs, they added 15mmHg to the systolic reading and 10mmHg to the diastolic reading to account for the likely effect of the drugs.

The researchers then analysed the results, looking for any associations between:

• actual vitamin D levels and high blood pressure
• actual vitamin D levels and variations in each of the four genes 
• genetic variations in combination with vitamin D levels and high blood pressure

The results were adjusted for age, body mass index (BMI), sex and geographical region. Blood samples for vitamin D concentrations were adjusted for the month when the sample was taken to account for increased levels related to sun exposure, as well as laboratory and total cholesterol and triglycerides.

What were the basic results?

Without taking the genetic component into account, increased vitamin D concentrations were associated with reduced systolic blood pressure and reduced risk of hypertension. There was no association with diastolic blood pressure.

These results did not differ after accounting for age, sex, the method of blood pressure measurement, geographical region or BMI.

All four vitamin D-related single-nucleotide polymorphisms (SNPs) in the genes involved in the production and metabolism of vitamin D were strongly associated with vitamin D concentrations.

In other words, the results confirmed previous reports that genetic variations of these genes are associated with a reduction in the concentration of vitamin D.

Grouping people by the two genes involved in the production of vitamin D, every 10% increase in vitamin D concentration was associated with:

• reduced systolic blood pressure of 0.37mmHg (95% confidence interval [CI] 0.003 to 0.73)
• reduced diastolic blood pressure of 0.29mmHg (95% CI 0.07 to 0.52)
• 8.1% reduced odds of hypertension (odds ratio [OR] 0.92, 95% CI 0.87 to 0.97)

When analysing each of the four genes without taking the vitamin D concentration into account:

• only one of the genes involved in the production of vitamin D showed an association with reduced diastolic blood pressure and risk of hypertension
• there was no association if both genes involved in vitamin D production were looked at together, unless the results were combined with other larger studies, where they were associated with a reduced risk of hypertension
• there was no association between the two genes involved in vitamin D metabolism and any blood pressure outcome

How did the researchers interpret the results?

The researchers concluded that, "The results suggest that people who have genetic variants associated with low endogenous production of 25(OH)D [low vitamin D] have an increased risk of hypertension, emphasising the need for further, well-designed randomised controlled trials to assess causality and the potential clinical benefits of vitamin D supplementation.

The go on to say that, "In view of the costs and side effects associated with antihypertensive drugs, the possibility of preventing or reducing hypertension with vitamin D supplementation is very attractive.

"However, because we cannot exclude the possibility that the findings from this study were caused by chance, they need to be replicated in an independent, similarly powered study."

Conclusion

This study has found an association between increased vitamin D levels and reduced blood pressure and risk of hypertension. This was also found when the vitamin D concentration level was taken in combination with a normal genetic ability to produce vitamin D.

However, this study cannot prove that lower levels of vitamin D cause high blood pressure, or that taking vitamin D supplements would reduce high blood pressure – other factors could account for the results.

The researchers did try to limit this, however, by accounting for some obvious confounding factors, such as age, sex, triglycerides and total cholesterol.

The study's strengths include the large number of participants, although they were all of European descent, so it is not known if the results could be directly applicable to other ethnicities.

Limitations with this type of study design include using the genetic variations as a proxy for lifetime vitamin D levels. The genetic variations may have led to biological adaptations to compensate.

It is also not known whether the level of sun exposure would actually have a greater effect on vitamin D levels than these genetic variations.

In addition, it may be the case that the genetic variations influence other metabolic pathways independently of their effects on vitamin D levels and therefore on blood pressure.

Either way, it is important to have adequate levels of vitamin D, as deficiency can cause symptoms such as tiredness, general aches and pains, and, if more severe, rickets in children and osteomalacia in adults.

Most people don't need to take supplements to get the recommended amount of vitamin D. You can get vitamin D in two ways: through your diet and moderate exposure to sunlight.

Foods rich in vitamin D include:

• oily fish, such as salmon, sardines and mackerel
• eggs
• fortified fat spreads
• fortified breakfast cereals
• powdered milk

Regularly going outside for a few minutes in the middle of the day without wearing sunscreen between April and October should provide enough exposure to create sufficient vitamin D.

You certainly don't need to get a suntan, let alone risk sunburn. Overexposure to the sun in this way can increase your risk of skin cancer.