"This is a significant breakthrough for patients. It is the first evidence that vitamin D3 can improve heart function of people with heart muscle weakness - known as heart failure," said Klaus Witte from University of Leeds in the UK.

Vitamin D3 can be boosted by exposure to sunlight, but heart failure patients are often deficient in it even during the summer because older people make less vitamin D3 in response to sunlight than younger people, researchers said.

Vitamin D3 production in the skin is also reduced by sunscreen, they said. The study involved more than 160 patients who were already being treated for their heart failure using proven treatments including beta-blockers, ACE-inhibitors and pacemakers.

Participants were asked to take vitamin D3 or a dummy (placebo) tablet for one year. Those patients who took vitamin D3 experienced an improvement in heart function which was not seen in those who took a placebo.

Changes in heart function were measured by cardiac ultrasound. Heart specialists measure heart function by taking an ultrasound scan of the heart (known as an echocardiogram) and measuring how much blood pumps from the heart with each heartbeat, known as ejection fraction.

The ejection fraction of a healthy person is usually between 60 percent and 70 percent. In heart failure patients, the ejection fraction is often significantly impaired - in the patients enrolled into the study, the average ejection fraction was 26 per cent, researchers said.

In the 80 patients who took Vitamin D3, the heart's pumping function improved from 26 percent to 34 percent. In the others, who took placebo, there was no change in cardiac function, they said.

This means that for some heart disease patients, taking vitamin D3 regularly may lessen the need for them to be fitted with an implantable cardioverter defibrillator (ICD), a device which detects dangerous irregular heart rhythms and can shock the heart to restore a normal rhythm, researchers said.

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