LONDON (Reuters) - High blood sugar levels cause about three million deaths worldwide each year linked to heart disease and strokes as well as diabetes, researchers said on Friday.

Scientists in the United States and New Zealand have calculated that in addition to the 960,000 diabetes deaths worldwide each year, raised blood sugar levels are linked to 1.5 million deaths from heart disease and 700,000 from strokes.

"A lot of people are dying as a result of their blood glucose being elevated even though many may well be below the clinical threshold of what we call diabetes," Dr Majid Ezzati, of the Harvard School of Public Health in Boston, Massachusetts, said in an interview.

Blood sugar level is the amount of glucose in the blood. People with higher than optimum levels fall below the limit of being diagnosed as diabetic but can still suffer damage to blood vessels and have an increased risk of developing cardiovascular disease.

Ezzati and his team analyzed the impact of higher than optimum glucose levels on deaths in 2001 from heart disease and strokes by gathering data from 52 countries around the world.

Their findings, published in The Lancet medical journal, put the total mortality figure from high blood sugar at 3.1 million.

It is more than the 2.4 million deaths attributed to being overweight or obese but less than 4.8 million caused by smoking and 3.9 million by high cholesterol, according to the researchers.

"Even people who are in pre-diabetes level have blood glucose levels that from a cardiovascular perspective has some risk associated with it," said Ezzati.

"The small risks are accumulating and causing a lot of deaths even though they don't fall into any clinical classification," he added.