Having too little vitamin D has been linked to an increased risk of heart disease, a US study has found.

The Harvard Medical School team said the risk was particularly high for those who also had high blood pressure.

Writing in the journal Circulation, they said correcting vitamin D deficiency could be beneficial.

A British Heart Foundation spokeswoman said more information was needed on how the vitamin affected heart health.

Blood pressure link

Vitamin D is mainly obtained from exposure to the sun, as well as from certain foods such as oily fish and eggs.

There are concerns that many people, including the elderly, pregnant women and those who wear all-concealing clothing do not get enough of the vitamin.

A lack of the nutrient, already known to cause weakened bones, has also been linked to multiple sclerosis and rheumatoid arthritis.

The US study, which began in 1996, looked at over 1,700 people with an average age of 59.

Their vitamin D levels were tested and they were then monitored for up to seven years.

Those with low levels of vitamin D in their blood, below 15 nanograms per millilitre (ng/ml), had twice the risk of a heart attack, heart failure or a stroke compared to those with higher levels (above 15ng/ml).

The highest incidence of cardiovascular disease was seen in those with high blood pressure and low vitamin D levels.

Overall, 28% of individuals had levels of vitamin D below 15 ng/ml. Only 10% had levels above 30 ng/ml - considered ideal for bone health.

The researchers say that because receptors for vitamin D are found in heart muscle and blood vessel lining, low levels could be a contributing factor in heart disease.

Uncertainty

But Dr Thomas Wang, who led the research, added: "What hasn't been proven yet is that vitamin D deficiency actually causes increased risk of cardiovascular disease.

"This would require a large randomised trial to show whether correcting the vitamin D deficiency would result in a reduction in cardiovascular risk."

Other studies looking at using different vitamins to benefit heart health have not shown positive results.

But Dr Wang said: "Just because other vitamins haven't succeeded doesn't preclude the possibility of finding vitamins that might prevent cardiovascular disease.

"Vitamins are easy to administer and in general have few toxic effects."

June Davison, cardiac nurse at the British Heart Foundation said: "This study suggests an association with Vitamin D deficiency and an increased risk of heart and circulatory disease.

"However, the reasons as to why this happens are uncertain and further research is needed to understand the mechanisms behind this."