A study has revealed that the risk of strokes and heart disease is doubled by diabetes - but scientists still do not know why.
Diabetes is heavily associated with disorders linked to the health of blood vessels.
Around 10% of deaths from heart and artery diseases - around 325,000 a year in all the industrialised countries put together - are now believed to be caused by diabetes.
However only a tiny proportion of the harmful effects can be explained by known risk factors including obesity, high levels of blood fats and high blood pressure, according to scientists.
Researchers also found that higher-than-average blood sugar levels in people without diabetes were only weakly related to cardiovascular disease. High blood sugar is the key danger of diabetes.
Cambridge University scientists led an international group of researchers who analysed data from 102 studies in 25 countries involving 700,000 participants.
They found having diabetes roughly doubled the risk of developing heart disease or suffering different kinds of stroke.
Dr Nadeem Sarwar, one of the Cambridge scientists, said: "Our findings highlight the need for better prevention of diabetes coupled with greater investigation of the mechanisms by which diabetes increases the risk of cardiovascular disease."
The research is published in The Lancet medical journal.