The research project, funded by the Medical Research Council, involved 160 patients who were already being treated for heart failure using proven treatments and medicines.
Vitamin D3 can be boosted by exposure to sunlight, but older heart failure patients are often deficient in it.
Half the participants were asked to take vitamin D3, in the form of the blackcurrant-flavoured chewable tablet, while the other half took a placebo tablet.
Dr Klaus Witte, from the university’s school of medicine and a consultant cardiologist at Leeds Teaching Hospitals NHS Trust, led the VINDICATE study which found doses boosted the heart’s pumping function.
He said: “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.
“These findings could make a significant difference to the care of heart failure patients.”
Changes in heart function were measured by cardiac ultrasound, using scans to see how much blood pumped from the heart with each heartbeat, known as ejection fraction.
The ejection fraction of a healthy person is usually between 60 and 70 per cent. In heart failure patients, the ejection fraction is often far lower – those enrolled on the study had an average of 26 per cent.
In the 80 patients who took vitamin D3, the heart’s pumping function improved to 34 per cent. In the others, who took placebo, there was no change in cardiac function.
The findings from the VINDICATE study were presented at the American College of Cardiology 65th Annual Scientific Session & Expo in Chicago yesterday.
Heart failure affects about 900,000 people in the UK and more than 23 million worldwide.
The condition can affect people of all ages, but it is more common in older people – more than half of all people globally with heart failure are over the age of 75.